HomeMy WebLinkAbout1 Alaska Coal Geology, Resources and CoalbedU.S. Department of the Interior
U.S. Geological Survey
Alaska Coal Geology, Resources, and
Coalbed Methane Potential
By Romeo M. Flores, Gary D. Stricker, and Scott A. Kinney
U.S. Geological Survey, Denver, Colorado 80225
DDS–77
U.S. Department of the Interior
Gale A. Norton, Secretary
U.S. Geological Survey
Charles G. Groat, Director
U.S. Geological Survey, Reston, Virginia: 2004
For sale by U.S. Geological Survey, Information Services
Box 25286, Denver Federal Center
Denver, CO 80225
For more information about the USGS and its products:
Telephone: 1-888-ASK-USGS
World Wide Web: http://www.usgs.gov/
Conversion Factors
Multiply By To obtain
Length
inch (in.) 2.54 centimeter (cm)
inch (in.) 25.4 millimeter (mm)
foot (ft.) 0.3048 meter (m)
mile (mi.) 1.609 kilometer (km)
Area
acre 4,047 square meter (m2)
acre 0.4047 hectare (ha)
acre 0.4047 square hectometer (hm2)
acre 0.004047 square kilometer (km2)
square foot (ft2) 9,290 square centimeter (cm2)
square foot (ft2) 0.0929 square meter (m2)
square inch (in2) 6.452 square centimeter (cm2)
square mile (mi2) 259.0 hectare (ha)
square mile (mi2) 2.590 square kilometer (km2)
Weight
metric ton 1.10 ton, short (2,000 lb)
ton, short (2,000 lb) 0.907 metric ton
pound (lb) 453.59 gram (gm)
gram (gm) 0.0022 pound (lb)
Energy
Btu per pound (Btu) 0.0022 mega joules per kilogram
Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:
°F= (1.8x°C) +32
Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:
°C= (°F-32)/1.8
III
Contents
Abstract ..........................................................................................................................................................1
Introduction ...................................................................................................................................................1
Geological Setting ........................................................................................................................................4
Physiographic Regions ...................................................................................................................4
Regional Geology ............................................................................................................................4
Origin of Alaska Coal ....................................................................................................................................6
Coal Metamorphism, Composition, Rank, and Occurrence...................................................................7
Coal Resource Classification ......................................................................................................................8
Distribution of Coal in Alaska....................................................................................................................10
Northern Alaska-Slope Coal Province ....................................................................................................10
Cretaceous Rocks .......................................................................................................................................10
Nanushuk Group............................................................................................................................10
Colville Group .................................................................................................................................13
Cretaceous-Tertiary Rocks........................................................................................................................16
Jago River Formation....................................................................................................................17
Sagavanirktok Formation .............................................................................................................17
Coal Resource Assessment of the Northern Alaska-Slope Coal Province .......................................22
Cretaceous Rocks .........................................................................................................................22
Coal Quality ....................................................................................................................................23
Tertiary Rocks ................................................................................................................................24
Coal Quality ....................................................................................................................................24
Coal Petrology................................................................................................................................24
Central Alaska-Nenana Coal Province ...................................................................................................25
Tertiary Usibelli Group ..................................................................................................................26
Healy Creek Formation .................................................................................................................27
Sanctuary Formation ....................................................................................................................31
Suntrana Formation......................................................................................................................31
Lignite Creek Formation ...............................................................................................................32
Grubstake Formation ....................................................................................................................32
Nenana Gravel ...............................................................................................................................35
Coal Resource Assessment of the Central Alaska-Nenana Coal Province ......................................35
Coal Quality ....................................................................................................................................39
Coal Petrology................................................................................................................................39
Southern Alaska-Cook Inlet Coal Province ............................................................................................40
Tertiary Rocks ................................................................................................................................42
Lower Tertiary Rocks ....................................................................................................................42
Upper Tertiary Kenai Group .........................................................................................................45
Hemlock Conglomerate ................................................................................................................52
Tyonek Formation ..........................................................................................................................54
Beluga Formation ..........................................................................................................................60
IV
Sterling Formation .........................................................................................................................65
Coalfields ........................................................................................................................................68
Matanuska Coalfield .....................................................................................................................68
Susitna-Beluga Coalfield .............................................................................................................74
Broad Pass Coalfield ....................................................................................................................75
Kenai Coalfield ...............................................................................................................................75
Coal Resource Assessment in the Southern Alaska-Cook Inlet Coal Province ...............................76
Matanuska Coalfield .....................................................................................................................78
Susitna-Beluga Coalfield .............................................................................................................78
Broad Pass Coalfield....................................................................................................................79
Kenai Coalfield ............................................................................................................................................79
Coal Quality ....................................................................................................................................79
Coal Petrology................................................................................................................................80
Coalbed Methane Potential ......................................................................................................................80
Northern Alaska-Slope Coal Province .......................................................................................81
Central Alaska-Nenana Coal Province ......................................................................................82
Southern Alaska-Cook Inlet Coal Province ...............................................................................82
Summary ....................................................................................................................................................107
Acknowledgments ....................................................................................................................................112
References Cited ......................................................................................................................................117
Figures
1. Map showing coal ranks in coal basins and coal occurrences in Alaska with emphasis
on the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet
coal provinces ........................................................................................................................................2
2. Map showing the coal rank and land ownership of Alaska ...........................................................3
3. Map showing the geology and structure of Alaska .........................................................................3
4. Map showing the physiographic regions of Alaska ........................................................................5
5. Map showing general tectonic framework of the Cook Inlet Basin, associated subduction
zone, accreted terranes, and volcanic arc.......................................................................................6
6. Stratigraphic column of the Mesozoic and Cenozoic rocks in the Northern Alaska-
Slope coal province...............................................................................................................................9
7. Photograph of a coal bed (about 20 feet thick) in the Nanushuk Group ....................................11
8. Map showing net coal thickness map of the Nanushuk Group in the western part of
the Northern Alaska-Slope coal province .......................................................................................11
9. Cross section showing the Nanushuk progradation sequences.................................................12
10. Sandstone percentage contour map in the Northern Alaska-Slope coal province .................13
11. Paleogeographic maps showing the depositional environments of the Nanushuk Group in the
central Northern Alaska-Slope coal province. (A) early to middle Albian time; (B) middle to late
Albian time; (C) late Albian to Cenomanian (?) time; and (D) Cenomanian time
(maximum regression)...........................................................................................................................14
V
12. Photograph of interbedded coal, sandstone, siltstone, and mudstone of the
Kogosukruk Tongue of the Prince Creek Formation along the lower Colville
River downstream from the mouth of the Anaktuvuk River .........................................................16
13. Paleogeographic map showing the depositional environments of the Jago
River Formation in the Arctic National Wildlife Refuge area .......................................................17
14. Photograph of a coal bed underlain by a sandstone in the Sagavanirktok
Formation ..............................................................................................................................................18
15. Photograph of fluvial-channel sandstone and associated rocks in the
Sagavanirktok Formation....................................................................................................................18
16. Stratigraphic cross section of the Tertiary Brookian sequence in the eastern part of
the National Petroleum Reserve Alaska ..........................................................................................19
17. West to east stratigraphic cross section showing variations of the coal beds,
conglomerates, sandstones, mudstones, and siltstones in the Staines Tongue
of the Sagavanirktok Formation between the National Petroleum Reserve in Alaska and
Arctic National Wildlife Refuge ........................................................................................................20
18. Maps showing the (A) depth to top of coal-bearing interval and (B) net coal thickness
isopach of the Staines Tongue of the Sagavanirktok Formation .................................................21
19. Map showing the coalfields in the Central Alaska-Nenana coal province ...............................25
20. Generalized stratigraphic and lithofacies column of the Usibelli Group in the Central
Alaska-Nenana coal province ...........................................................................................................26
21. Stratigraphic cross section showing the variations of the conglomerates, sandstones,
siltstones, mudstones, and coal beds in the lower part of the Usibelli Group in the Healy
Creek coalfield on the southern part of the Central Alaska-Nenana coal province ................27
22. Paleogeographic maps showing depositional environments of: (A) Healy Creek
Formation, (B) Sanctuary Formation, (C) Suntrana Formation, and (D) Lignite Creek
Formation ..............................................................................................................................................28
23. Crossbed-orientation measurements in fluvial-channel sandstones in the Healy Creek,
Suntrana, and Lignite Creek Formations in Suntrana area ..........................................................28
24. Stratigraphic cross section showing the basal conglomerates and sandstones in the
lower part of the Healy Creek Formation east of Suntrana ..........................................................29
25. Photograph of conglomerates (a few inches to 5 feet thick or a few centimeters to
1.5 meters) and sandstones (6 inches to 8 feet thick or 15.2 centimeters to 2.4 meters)
deposited by braided streams in the lower part of the Healy Creek Formation in east
of Suntrana ...........................................................................................................................................29
26. Stratigraphic cross section showing the middle, coal-bearing part of the Healy Creek
Formation east of Suntrana ...............................................................................................................30
27. Stratigraphic cross section of the uppermost part of the Healy Creek Formation showing
the fluvial-channel sandstones and F coal bed, and overlying Sanctuary Formation east
of Suntrana ...........................................................................................................................................30
28. Photograph of the uppermost part of the Healy Creek Formation, F coal bed, and
overlying mudstones of the Sanctuary Formation in the Lignite Creek coalfield .....................31
29. Photograph of the lacustrine mudstone and lenticular limestone units in the Sanctuary
Formation in the Lignite Creek coalfield ......................................................................................... 31
VI
30. Stratigraphic cross section of the Suntrana Formation showing the Nos. 2, 3, and 4 coal
beds and interbedded fluvial-channel sandstones west of Suntrana........................................32
31. Photograph of the Suntrana showing the thick No. 3 coal bed, clinker bed of No. 4 coal
bed, thin No. 5 coal bed, and interbedded fluvial-channel sandstones and clay
plug-overbank deposits west of Suntrana .......................................................................................33
32. Photograph of the abandoned fluvial-channel mudstone or clay plug deposit........................33
33. Stratigraphic cross section of the Nos. 3, 4, and 6 coal beds of the Suntrana Formation
in the Poker Flats strip mine of Usibelli Coal Mine ........................................................................34
34. Photograph of the Poker Flats strip mine showing the highwall exposing fluvial-channel
sandstones and No. 3 coal bed (lower bench) and No. 4 coal bed (upper bench)...................34
35. Thickness map of the No. 6 coal bed of the Suntrana Formation................................................35
36. Stratigraphic cross section of the lower part of the Lignite Creek Formation showing
interbedded fluvial-channel sandstones, crevasse splay flood-plain deposits, and thin
coal beds west of Suntrana ...............................................................................................................36
37. Photograph of the interbedded fluvial-channel sandstones, flood-plain deposits, and
an interbedded thin coal bed of the Lignite Creek Formation west of Suntrana ......................36
38. Photograph of the coarsening-upward mudstone, siltstone, and sandstone sequence
overlain by thin coal beds of the Lignite Creek Formation west of Suntrana ............................36
39. Stratigraphic cross section showing variation of minable Healy Creek coal beds and
associated sandstones, mudstones, and siltstones in the Jarvis Creek coalfield ...................38
40. Map showing coalfields in the Southern Alaska-Cook Inlet coal province ...............................41
41. Tectonic and volcanic settings of the Cook Inlet Basin .................................................................42
42. Generalized time-transgressive stratigraphy in the Cook Inlet Basin ........................................43
43. Depositional model of the Kenai Group in the Cook Inlet Basin ..................................................44
44. Generalized chronostratigraphic column of the coal-bearing Kenai Group and related
rock units in the Southern Alaska-Cook Inlet coal province ........................................................44
45. A generalized stratigraphic column of the Chickaloon and Wishbone Formations in the
Matanuska coalfield ............................................................................................................................45
46. Photograph of coal beds of the Chickaloon Formation in the Wishbone Hill coal district ......46
47. Photograph of the lenticular fluvial-channel sandstone (20 feet or 6.1 meters thick) and
associated rocks of the Chickaloon Formation in the Wishbone Hill coal district ...................46
48. Photograph of the tabular crevasse splay sandstone and associated flood-plain
deposits of the Chickaloon Formation in the Wishbone Hill coal district ...................................47
49. Vertical and lateral lithofacies variations of the Wishbone Formation in the Wishbone
Hill coal district ....................................................................................................................................47
50. Photograph of the braided-stream-deposited conglomerates and sandstones in the
Wishbone Formation in the Wishbone Hill coal district ................................................................48
51. Stratigraphic cross section of the lower part of the Chickaloon Formation in the
Wishbone Hill coal district .................................................................................................................48
VII
52. Photograph of the lower part of the Chickaloon Formation showing the Midway coal
zone and adjoining fluvial-channel sandstones in the Wishbone Hill coal district ..................49
53. Stratigraphic cross section of the middle part of the Chickaloon Formation in the
Wishbone Hill coal district .................................................................................................................49
54. Photograph of the upper part of the Chickaloon Formation showing the Premier coal zone
(50 feet or 15.2 meters thick), Jonesville coal zone (30 feet or 9.1 meters thick), and
associated fine-grained sediments in the Wishbone Hill coal district .......................................50
55. Stratigraphic cross section of the upper part of the Chickaloon Formation in the
Wishbone Hill coal district .................................................................................................................50
56. Photograph of the upper part of the Chickaloon Formation showing the Jonesville coal
zone overlain by fluvial-channel sandstones (>50 feet or >15.2 meters thick) of the
Wishbone Formation in the Wishbone Hill coal district ................................................................51
57. Paleogeographic map showing depositional environments of the Chickaloon Formation
in the Matanuska coalfield .................................................................................................................51
58. Map showing lines of stratigraphic cross sections of the Kenai Group in the offshore and
onshore Cook Inlet Basin. Map also shows areas of cross sections the Kenai Group in
the Chuitna area, Capps Glacier area, along the west coast of Kenai Peninsula, and
along the north coast of Kachemak Bay .........................................................................................52
59. Offshore north-south cross section (A–A’) of the Kenai Group along the axis of the Cook
Inlet Basin .............................................................................................................................................53
60. Offshore (west) to onshore (east) cross section (B–B’) of the Kenai Group across the
Cook Inlet Basin ...................................................................................................................................54
61. Onshore north-south cross section (C–C’) of the Kenai Group along the western part of
the Kenai Peninsula or eastern margin of the Cook Inlet Basin .................................................55
62. Photograph of conglomerates in the Hemlock Conglomerate in the Katmai National Park ...56
63. Photograph of sandstones in the Hemlock Conglomerate in the Katmai National Park .........56
64. Photograph of thin coal and carbonaceous shale beds in the Hemlock Conglomerate in
the Katmai National Park....................................................................................................................57
65. Photograph of braided stream deposits (conglomeratic lower part) in the Hemlock
Conglomerate ........................................................................................................................................57
66. Net coal thickness isopach map of the Tyonek Formation in the Cook Inlet Basin ..................58
67. Photograph of coal beds and interbedded fluvial-channel sandstones and mudstones
in the Tyonek Formation in the Chuitna River drainage basin ......................................................58
68. Photograph of fluvial-channel sandstones and Chuitna coal bed in the Tyonek Formation
in the Chuitna River drainage basin ..................................................................................................58
69. Net sandstone thickness isopach map of the Tyonek Formation in the Cook Inlet Basin .......59
70. Paleogeographic map (block diagram) showing depositional environments of the
Tyonek Formation in the Cook Inlet Basin........................................................................................59
71. Generalized stratigraphic column of minable coal beds in the Tyonek Formation in the
Chuitna River drainage basin and adjoining areas ........................................................................60
VIII
72. Three-dimensional cross sections (fence diagram) of the Chuitna coal bed and
interbedded erosional-based sandstones deposited by braided streams of the
Tyonek Formation in the Chuitna River drainage basin ................................................................61
73. Stratigraphic cross section of the coal beds, fluvial-channel sandstones, and intertidal
deposits in the Diamond Chuitna lease area east of the Chuitna River drainage basin .........61
74. Stratigraphic lithofacies sequence in the Tyonek Formation showing tidal sandstone
flats facies near Wasilla .....................................................................................................................62
75. Stratigraphic cross section of the coal beds and fluvial-channel sandstones in the
Diamond Chuitna lease area east of the Chuitna River drainage area ......................................63
76. Vertical lithofacies and associated geophysical logs of minable coal beds
(Reds 1, 2, and 3, and Blue) and interbedded fluvial-channel sandstones, and flood
plain claystones and siltstones in the Diamond Chuitna lease area east of the Chuitna
River drainage area .............................................................................................................................63
77. Vertical lithofacies of coal beds (Reds 1, 2, and 3) and interbedded tidal and intertidal
sandstones, siltstones, and mudstones in the Diamond Chuitna lease area east of the
Chuitna River drainage area ..............................................................................................................64
78. Photograph of the Capps Glacier coal bed (50 feet or 15.2 meters thick) and overlying
fluvial-channel sandstones in the Capps Glacier area..................................................................64
79. Photograph of the coal beds and interbedded flood-plain/crevasse splay deposits in the
Capps Glacier area .............................................................................................................................64
80. Structural cross section (north-south) of the Capps Glacier coal bed and associated
rocks of the Tyonek Formation in the Capps Glacier area ............................................................64
81. Stratigraphic cross section (northeast-southwest) of the rocks of the Tyonek Formation
at Barabara Point showing lenticular conglomerates, sandstone, and coal beds ..................65
82. Stratigraphic (structural) cross section of the Capps Glacier coal bed and associated
rocks of the Tyonek Formation in the Capps Glacier area ............................................................65
83. Photograph of the fluvial-channel sandstones (average 60 feet or 18.3 meters thick),
flood-plain mudstone and siltstones, and coal beds of the Beluga Formation along the
coastal bluffs in west Homer, Kenai Peninsula ...............................................................................66
84. Photograph of a coal bed (3.5 feet or 1.1 meters thick) and crevasse splay deposits of the
Beluga Formation along the coastal bluffs west of Homer, Kenai Peninsula ...........................66
85. Stratigraphic cross section of the Beluga Formation showing thick coal beds
(for example, Cooper coal bed), fluvial-channel sandstones, and flood-plain mudstone
and siltstone along the coastal bluffs west of Homer, Kenai Peninsula ....................................67
86. Stratigraphic cross section of the Beluga Formation showing interbedded thin to thick
coal beds (for example, Cooper coal bed), fluvial-channel sandstones, and flood-plain
deposits along the coastal bluffs west of Homer, Kenai Peninsula ............................................67
87. Paleogeographic map (block diagram) showing depositional environments of the Beluga
Formation in the Cook Inlet Basin .....................................................................................................68
88. Photograph of fluvial-channel sandstones and thin coal of the Sterling Formation
along the coastal bluffs in the Clam Gulch area, Kenai Peninsula..............................................69
IX
89. Photograph of fluvial-channel sandstones overlying thin (3 feet [0.9 meter]) to thick
(12 feet [3.6 meters]) coal beds of the Sterling Formation along the coastal bluffs
between the Clam Gulch and Ninilchik, Kenai Peninsula ..........................................................69
90. Stratigraphic cross sections showing variations in fluvial-channel architecture in the
upper part of the Sterling Formation in the Clam Gulch area, Kenai Peninsula: A,
Lower part of Clamgulchian type section; B, Middle part of Clamgulchian type
section; C, Upper part of Clamgulchian type section ..................................................................70
91. Stratigraphic cross section showing coal beds, fluvial-channel sandstones, and
interbedded flood-plain mudstones and siltstones in the lower part of the Sterling
Formation between the Clam Gulch area and Ninilchik, Kenai Peninsula ..............................71
92. Photograph of thin to thick coal beds in the lower part of the Sterling Formation.................72
93. Block diagram showing depositional environments of the Sterling Formation in the
Cook Inlet Basin .................................................................................................................................72
94. Map showing the geology and coal districts in the Matanuska coalfield ...............................73
95. Cross section of the Premier and Jonesville coal zones of the Chickaloon Formation in
the Wishbone Hill coal district ........................................................................................................73
96. Geologic map of the Wishbone Hill coal district showing doubly plunging syncline
disrupted by normal faults ................................................................................................................74
97. Photograph of a 4-foot-thick (1.2 meters) coal bed interbedded with fluvial-channel
sandstones and flood plain mudstones and siltstones in the Sterling Formation in the
Clam Gulch area .................................................................................................................................76
98. Stratigraphic cross section showing interbedded coal beds, fluvial-channel sandstones,
and flood-plain mudstones and siltstones in the lowermost part of the Sterling Formation
along the north shore of Kachemak Bay east and west of McNeil Canyon ............................76
99. Photograph of a coal bed with tonstein partings and related rocks of the Beluga
Formation along the beach bluffs on the northern shore of the Kachemak Bay ....................76
100. Stratigraphic cross section showing interbedded coal beds, fluvial-channel sandstones,
and flood-plain mudstones and siltstones in the uppermost part of the Beluga Formation
west of McNeil Canyon ....................................................................................................................76
101. Stratigraphic cross section showing interbedded coal beds, fluvial-channel
sandstones, and flood-plain mudstones and siltstones in the uppermost part
of the Beluga Formation at the mouth of Fritz Creek ...................................................................77
102. Distribution of surface vitrinite reflectance values at sea level in the Northern
Alaska-Slope coal province .............................................................................................................83
103. Map of the Northern Alaska-Slope coal province showing distribution of bituminous
and subbituminous coals ..................................................................................................................83
104. Map showing surface vitrinite reflectance values in the Northern Alaska-Slope, Central
Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces ............................................84
105. Vitrinite reflectance values for the Meade Quadrangle, National Petroleum Reserve
in Alaska ............................................................................................................................................106
106. Vitrinite reflectance values for the Tunalik No. 1 well, National Petroleum Reserve
in Alaska ............................................................................................................................................106
X
107. Stratigraphic cross section of the Nanushuk Group with superimposed vitrinite
reflectance values ...........................................................................................................................108
108. Coalbed methane potential in the Nanushuk Group coals based on the thickness and
vitrinite reflectance of the nonmarine part of the group in the Northern Alaska-Slope
coal province ....................................................................................................................................109
109. Distribution of surface vitrinite reflectance (Ro) values in the Northern Alaska-Slope
coal province ....................................................................................................................................109
110. Map of the Cook Inlet Basin showing distribution of oil and gas fields offshore and
onshore ..............................................................................................................................................110
111. Facies profile of the lower part of the Sterling Formation and accompanying downhole
logs showing horizons of gas accumulation...............................................................................111
112. Facies profile of the upper part of the Beluga Formation and accompanying downhole
logs showing horizons of gas-perforated intervals ...................................................................112
113. Location map of the Kenai gas field in the Kenai Peninsula....................................................112
114. Basinwide and vertical variations of vitrinite reflectance (Ro) values in the Cook Inlet
Basin ..................................................................................................................................................113
115. Coalbed methane prospect area and depths to vitrinite reflectance values of
0.6 percent superimposed on the thickness isopach of the Tyonek Formation south of
the Castle Mountain fault in the northeastern part of the Cook Inlet .....................................113
116. Downhole geophysical logs, hot wire total gas and methane contents, vitrinite
reflectance values, and illite diagenetic values in the Edna Mae Walker drill hole ............114
117. Stratigraphic cross section of the Kenai Group in the offshore Cook Inlet Basin with
superimposed vitrinite reflectance values .................................................................................115
118. Stratigraphic cross section of the Kenai Group in the onshore Cook Inlet Basin with
superimposed vitrinite reflectance values .................................................................................116
Tables
1. Coal resource estimates for Alaska using the classification system of Wood and others (1983)........8
2. Estimates of hypothetical coal resources for the Cretaceous Nanushuk Group and Tertiary
Staines Tongue in the Sagavanirktok Formation in the Northern Alaska-Slope coal province .........23
3. (a) Coal quality of coal deposits in the Cretaceous Nanushuk Group in the Northern
Alaska-Slope coal province. (b) Coal quality of coal deposits in the Tertiary Staines
Tongue in the Sagavanirktok Formation in the Northern Alaska-Slope coal province ..............24
4. Estimates of coal resources for the Tertiary Usibelli Group in the Central Alaska-Nenana
coal province ..........................................................................................................................................37
5. Coal quality of coal deposits in the Tertiary Usibelli Group in the Central Alaska-Nenana
coal province..........................................................................................................................................40
XI
6. Estimates of coal resources for the Tertiary Kenai Group in the Matanuska, Broad Pass,
Susitna, and Kenai coalfields in the Southern Alaska-Cook Inlet coal province .......................78
7. Range (minimum and maximum values) of quality parameters for Tertiary coal deposits in
the Matanuska, Broad Pass, Susitna, and Kenai coalfields in the Southern Alaska-Cook
Inlet coal province .................................................................................................................................80
8. (a) Vitrinite reflectance values of coals across the surface in the Northern Alaska-Slope,
Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces. (b) Vitrinite
reflectance values of coals across the surface in the Northern Alaska Slope,
Central-Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces ..............(a) 85, (b) 102
9. Properties of sandstone reservoirs and associated gas in the Sterling and Beluga
Formations ............................................................................................................................................110
1
Abstract
Estimated Alaska coal resources are largely in Creta-
ceous and Tertiary rocks distributed in three major provinces.
Northern Alaska-Slope, Central Alaska-Nenana, and Southern
Alaska-Cook Inlet. Cretaceous resources, predominantly bitu-
minous coal and lignite, are in the Northern Alaska-Slope coal
province. Most of the Tertiary resources, mainly lignite to sub-
bituminous coal with minor amounts of bituminous and semi-
anthracite coals, are in the other two provinces. The combined
measured, indicated, inferred, and hypothetical coal resources
in the three areas are estimated to be 5,526 billion short tons
(5,012 billion metric tons), which constitutes about 87 percent
of Alaskaʼs coal and surpasses the total coal resources of the
conterminous United States by 40 percent.
Coal mining has been intermittent in the Central Alaskan-
Nenana and Southern Alaska-Cook Inlet coal provinces, with
only a small fraction of the identified coal resource having
been produced from some dozen underground and strip mines
in these two provinces. Alaskan coal resources have a lower
sulfur content (averaging 0.3 percent) than most coals in the
conterminous United States and are within or below the
minimum sulfur value mandated by the 1990 Clean Air Act
amendments. The identified resources are near existing and
planned infrastructure to promote development, transportation,
and marketing of this low-sulfur coal. The relatively short
distances to countries in the west Pacific Rim make them more
exportable to these countries than to the lower 48 States of the
United States.
Another untapped but potential resource of large mag-
nitude is coalbed methane, which has been estimated to total
1,000 trillion cubic feet (28 trillion cubic meters) by T.N.
Smith, 1995, Coalbed methane potential for Alaska and
drilling results for the upper Cook Inlet Basin: Intergas, May
15 – 19, 1995, Tuscaloosa, University of Alabama, p. 1 – 21.
Introduction
This report is a synthesis of the largely untapped hypo-
thetical coal resources of Alaska, which are estimated to be
as much as 5,526 billion short (or 5.5 trillion) tons (5,012
billion metric tons). The last coal resource assessment in
1974 for the conterminous United States (coal remaining in
the ground) estimated a total coal resource of 3,968 bil-
lion short tons (3,600 billion metric tons) or 4 trillion short
tons (Averitt, 1975). Thus, the Alaska coal resource estimate
surpasses the total coal resources of the conterminous United
States by 40 percent. This report focuses on an assessment of
the coal resources of the three major coal provinces in Alaska:
Northern Alaska-Slope, Central Alaska-Nenana, and Southern
Alaska-Cook Inlet and makes up 87 percent of the total
coal resources of the State (fig. 1). Also, it will concentrate on
the origin, geologic setting, and depositional environments of
the coal, as well as coal rank, quality, and petrology and the
amount of the resources. In addition, this report will sum-
marize the coalbed methane potential and prioritize areas for
exploration and development in these major coal provinces.
The coal resources of Alaska occur in discrete areas
(coalfields) and in isolated, unrelated outcrops (occurrences)
(fig. 1). A coalfield may contain coal beds of various ranks,
shown in figure 2, and different ages and geologic settings,
shown in figure 3. Coal resources and geological settings of
minor coalfields not reported here may be found in Stricker
(1991) and Wahrhaftig and others (1994).
Before the arrival of the European immigrants, native
inhabitants used coal in Alaska (Chapman and Sable, 1960,
p. 159). The Beechey expedition of 1826–1827 reported the
presence of coal in Alaska (Huish, 1836; Dall, 1896). Whal-
ing shippers mined coal from near Cape Beaufort, north of the
Arctic Circle, before the turn of the twentieth century (Con-
well and Triplehorn, 1976). The first coal mine was opened
Alaska Coal Geology, Resources, and
Coalbed Methane Potential
By Romeo M. Flores, Gary D. Stricker, and Scott A. Kinney
2 Alaska Coal Geology, Resources, and Coalbed Methane Potential
in 1855 and closed in 1867 at Port Graham (fig. 2) on the
southwestern part of the Kenai Peninsula (Martin, 1915). The
Russians opened and operated this coal mine before the United
States took possession of the Alaska Territory. The U.S. Con-
gress passed two significant legislative acts in 1914 that led to
development of coal resources of Alaska: (1) the Alaska Coal
Leasing Act promoted opening mines in the Alaskan coalfields
and (2) the Alaska Railroad Enabling Act authorized con-
struction of the railroad from Anchorage to Fairbanks, which
encouraged the use of coal by the locomotives and by the gold
mining operations to power gold dredges and to fuel steam
boilers for thawing the frozen ground.
Many coal mines in the Nenana and Matanuska coalfields
were active after Congress authorized the construction of the
Alaska Railroad, which promoted large-scale production,
transportation, and marketing. The first coal-lease sale was
held in conjunction with an oil-lease sale in 1983. In 1984,
export of Alaskan coal began with shipments to South Korea
from Nenana coalfield. Other developments included con-
struction of a coal terminal at the deep-water port at Seward,
new loading facilities at the Usibelli coal mine, and upgrading
of the Alaska Railroad to handle hauling coal to Seward. In
1985, coal production increased by 60 percent over 1984, with
a gross value on production of 1.4 × 106 short tons
(1.27 × 106 metric tons) valued at $39.7 million (Bundtzen and
others, 1986). Coal production for the year 2000 was
estimated by Szumigala and Swainbank (2001) at 1,473,000
short tons valued at $38.7 million with 708,000 short tons
being exported to Korea. An estimate of total coal in place
is 10.4 × 1012 short tons (9.4 × 1012 metric tons), or about 50
percent of total of conterminous U.S. resources.
The coal resources of Alaska, have been only minimally
exploited or developed. Mined coal is presently utilized for
domestic electric power-generating plants, and approximately
one-half of the production from the Usibelli mine is shipped to
Korea and potentially to other countries bordering the western
Pacific Rim.
Figure 1. Coal ranks in coal basins and coal occurrences in Alaska with emphasis on the Northern Alaska-Slope, Central
Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces. Compiled and modified from Merritt and Hawley (1986); Barnes
(1967a, 1967b); Magoon and others (1976); Plafker (1987).
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NORTH SLOPE
Barrow
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GULF
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ALASKA
Juneau
Angoon
ALASKA COALFIELDS AND
COAL OCCURRENCES
Bituminous
Anthracite
Subbituminous
Lignite
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EXPLANATION
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Alaska Railroad and
George Parks Highway
Figure 2. Map showing the coal rank and land ownership of Alaska
Figure 3. Map showing the geology and structure of Alaska. Modified from S.J. Moll, Scott Bie, Devon Peterson,
D.C. Pray, F.H. Wilson, J.M. Schmidt, J.R. Riehle, T.P. Miller (unpublished data, 1997, U.S. Geological Survey, Reston,
Virginia) after Beikman (1980).
Introduction 3
4 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Geological Setting
The geology of Alaska (fig. 3; Beikman, 1980) is best
described by Plafker and Berg (1994) and by various authors
in the 1994 Geology of Alaska volume of The Geological
Society of America edited by George Plafker and H.C. Berg.
These workers have discussed in detail the physiographic,
geologic, tectonic, and volcanic evolution of Alaska from the
Proterozoic to the present. Thus, this report describes only
those physiographic and geologic settings of Alaska that are
critical for understanding the coal-bearing rocks that were
investigated in this study.
Physiographic Regions
Alaska is divided into four physiographic regions (fig.
4); from north to south, they are (1) arctic coastal plain, (2)
northern Cordillera, (3) interior or intermountain plateau, and
(4) southern Cordillera (Plafker and Berg, 1994; Wahrhaftig
and others, 1994).
The arctic coastal plain region extends from the margin
of the Arctic Ocean on the north to the northern margin of the
Arctic Foothills Range on the south and consists mainly of
Tertiary alluvial deposits. Paleozoic, Mesozoic, and Cenozoic
rocks underlie the alluvial deposits. The major coal deposits
are in the Cretaceous Nanushuk and Colville Groups and
Tertiary Sagavanirktok Formation in this region (Wahrhaftig
and others, 1994); minor coals occur in the Lower Mississip-
pian Kekiktuk Conglomerate of the Endicott Group (see fig.
3; Tailleur, 1965; Conwell and Triplehorn, 1976; Sable and
Stricker, 1987; Wahrhaftig and others, 1994).
The northern Cordillera region is dominated by the
east-west-trending Brooks Range (see figs. 3 and 4; Plafker
and Berg, 1994). During the Pleistocene, ice caps carved the
glacial topography of these mountains. Mainly Cretaceous
coal-bearing Nanushuk and Colville Groups underlie the
Arctic Foothills Range (see fig. 4). The Paleozoic rocks are
mainly exposed in the mountains.
The interior or intermountain plateau is between the
Brooks Range on the north and the Alaska Range on the south
(see figs. 3 and 4; Plafker and Berg, 1994). This region is part
of the intermountain plateau that extends into Canada and
the conterminous United States (Great Basin and Colorado
Plateaus). Quaternary alluvial deposits sporadically cover the
region from the Bering Sea to the Yukon Flats (fig. 3). Else-
where the interior region is composed of plateaus, hills, and
uplands, with numerous domes, ridges, and mountains at the
higher elevations (Plafker and Berg, 1994). The interior region
was generally free of ice during the Pleistocene glaciation.
Beneath the loess and vegetation, the interior region contains
pre-Cretaceous basement rocks that include displaced and
rotated lithotectonic terranes of Proterozoic and Paleozoic age
of miogeocline affinity (Plafker and Berg, 1994). The base-
ment rocks also contain Devonian-Lower Jurassic terranes of
oceanic affinity and Jurassic-Lower Cretaceous intraoceanic
arc terranes. Mid-Cretaceous and younger plutonic and related
rocks, flysch basins, and basalts conceal these rocks. Tertiary
coal-bearing rocks are mainly present in several synclinal
basins in the northern foothills of the Alaska Range (fig. 3)
and are partly or wholly detached from each other by erosion
of coal-bearing rocks from intervening structural highs (Wah-
rhaftig and others, 1994).
The southern Cordillera region is the northernmost extent
of the Pacific Mountain system of North America that rims the
Pacific Ocean margin (Plafker and Berg, 1994). The region
extends from the Alaska Range on the north to the margin of
the Gulf of Alaska on the south (fig. 3). It extends westward to
the Aleutian Range and Aleutian Islands, which are a con-
tinuation of the Alaska Range (fig. 3). Widespread mountain
glaciers and ice fields occur in the mountainous parts of the
southern Cordillera region (fig. 3). Glaciers currently extend
into tidewaters at numerous bays and fiords (Plafker and
Berg, 1994). The southern Cordillera region is underlain by
Proterozoic to Cenozoic accreted intraoceanic arc and plateau
terranes, arc-related accretionary prisms, and flysch basins
(Plafker and Berg, 1994). These terranes were intruded by
mid-Cretaceous to Paleogene postaccretion plutons, which are,
in turn, overlapped by Upper Cretaceous-Tertiary basinal and
volcanic rocks. The Tertiary coal-bearing rocks in this region
are mainly found in these basins as typified by the Cook Inlet
Basin (fig. 3).
Regional Geology
Alaska is composed mainly of three crustal rock types:
(1) continental crust of the Cordillera miogeocline; (2)
amalgamated magmatic arcs, oceanic plateaus, melange, and
flysch; and (3) oceanic (including ophiolite) rocks (Plafker and
Berg, 1994). These crustal rocks were modified by magmatism
and metamorphism, overlapped by Cretaceous and Cenozoic
rocks, and affected by Cretaceous and Cenozoic faulting and
rotation. These tectonic processes produced the structural
trends as expressed by the physiographic features of Alaska
(fig. 3).
The northern and eastern parts of Alaska were formed by
intermittent rifting from the Proterozoic (850 Ma) to the early
Paleozoic (Plafker and Berg, 1994). This event was followed
by subsidence of the continental margin and deposition of
the Proterozoic-Paleozoic rocks, which make up the Cordil-
lera miogeocline (Dover, 1994; Grantz and others, 1994). The
Cordillera miogeocline rocks were affected episodically by
plate tectonism that formed the present Cordillera orogenic
belt. Plate convergence during Jurassic-Cretaceous time along
the continental margin resulted in a complex of intraoceanic
arcs, arcs on rifted continental crust, arc-related accretionary
prisms, flysch basins, oceanic plateaus, and oceanic crustal
rocks. Structural deformation, metamorphism, magmatism,
and erosion, in turn, modified these rock (Dusel-Bacon, 1994).
The oceanic crustal rocks define suture zones of either autoch-
Figure 4. Map showing the physiographic regions of Alaska. Modified from Plafker and Berg (1994).Geological Setting 5
6 Alaska Coal Geology, Resources, and Coalbed Methane Potential
thonous or paratochthonous rocks along which the Alaskan
continental margin rocks are adjacent to noncontinental rocks.
The Alaskan continental margin developed an Andean-
type arc system during mid-Cretaceous to Tertiary time as
imposed by the convergence of the Kula and North American
plate (Plafker and Berg, 1994; Nokleberg and others, 1994).
During this time period, accreted terranes were welded to the
continental margin by numerous arc-related volcanic, plutonic,
and metamorphic events (Brew, 1994). Arc-related volcanic
rocks and terrigenous sediments built much of western Alaska,
and the arc-related accretionary prisms formed seaward of the
volcanic arc (fig. 5). The present structural trends of Alaska
were formed by extensive rotations and translations from
Early Cretaceous to early Tertiary time. For example, during
the Early Cretaceous, the Arctic Ocean basin developed by
counterclockwise rotation of northern Alaska away from the
Arctic Canadian continental margin (Plafker and Berg, 1994;
Grantz and others, 1994). During Late Cretaceous and early
Tertiary time the interior physiographic region was translated
west several hundred kilometers along the Tintina and Denali
faults (fig. 3). These faults, in turn, were followed by coun-
terclockwise rotation that displaced preexisting transcurrent
faults.
Tectonic movements along the transcurrent faults con-
trolled depositional environments of the Tertiary coal-bearing
rocks in the interior and southern Cordillera regions (Flores
and Stanley, 1995). Movements, particularly along the Denali
fault, dammed flow-through fluvial systems that drained
alluvial plains north and south of the Alaska Range, where
peat precursors accumulated in associated mires. Damming of
the fluvial systems created lakes, which in turn drowned peat-
forming mires north of the Alaska Range. In addition, dam-
ming of fluvial systems shortened their headwaters, promoting
erosion, high sediment dispersal, and consequently drowning
of peat mires by detritus south of the Alaska Range.
Origin of Alaska Coal
Coal, containing more than 50 percent by weight and
more than 70 percent by volume of organic matter, is com-
posed of plant remains deposited as peat (Schopf, 1956). The
vegetal remains accumulated under mainly reducing condi-
tions beneath the ground-water table in mires or swamps. The
high acidity of the water killed bacteria and fungi that would
Active volcanoes
Cenozoic fore-arc basin
deposits
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ALASKA
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Figure 5. General tectonic framework of the Cook Inlet Basin, associated subduction zone, accreted terranes, and volcanic
arc. Modified from Swenson (1997).
otherwise have digested and completely oxidized the peat
(Schopf, 1956).
Peat-forming mires developed mainly in alluvial and
coastal plains that were drained by fluvial and deltaic distribu-
tary channels (Flores and Stricker, 1993a and b). Commonly,
the coastal plains were barred seaward by barrier-bar systems
that protected back-barrier mires from active marine sedi-
mentation, permitting accumulation of peat deposits (Flores,
Stricker, and Stiles, 1997; Flores and others, 1999). Aban-
doned areas in the coastal plains, which were far removed
from active sedimentation, also served as platforms for peat-
mire development. In active sedimentation areas of the coastal
plains, dense vegetation along margins of the mires juxtaposed
with fluvial and distributary channels prevented flood water
invasions and permitted peat accumulation. In regions where
high rainfall rates existed, mires developed a raised topog-
raphy that shielded them from sedimentation from adjoining
rivers, particularly during floods. Preservation of these peat
mires resulted from rapid subsidence, which promoted accom-
modation space and initiated burial of the peat deposits by
overlying sediments, which led to subsequent compaction and
metamorphism into coal.
The alluvial- and coastal-plain areas and associated mires
in Alaska were formed from Paleozoic to late Tertiary time.
The peat that accumulated in Alaska mires formed low-sulfur
(average 0.3 percent) coal deposits, whereas in many other
regions of the world, encroachment of the sea over peat-form-
ing environments commonly brought sulfate-bearing sedi-
ments that transformed the peat into high-sulfur deposits.
Additionally, when sediments flood portions of the peat mires,
an increased pH typically enhances microbial activity within
the top of the peat, which concentrates sulfur through degra-
dation of plant material. Flooding of the mires also led to an
increase of ash content of the peat due to settling of water-
borne sediments.
Climate and (or) vegetation types may explain the accu-
mulation of low-sulfur coal unique to Alaska, regardless of
age. That is, the Alaskan coal contains low sulfur because of
accumulation in mires developed in high paleolatitudes and
in temperate paleoclimatic conditions (Affolter and Stricker,
1988, 1990). Alternatively, the vegetation may have evolved
through time such that tropical or low paleolatitudinal plants
became adapted to mires developed in high-paleolatitudinal
regions.
The Alaskan peat-forming environments formed in depo-
sitional basins that were developed in the interior and margins
of the State. Riverine plains, in which mires accumulated eco-
nomic coal deposits, drained the interior basins (for example,
Central Alaska-Nenana coal province; see fig. 1). Fluvial
and deltaic coastal plains with associated mires accumulated
economic coal deposits in margin basins (Northern Alaska-
Slope and Southern Alaska-Cook Inlet coal provinces; see fig.
3). Coal that formed in these basins ranges from Mississip-
pian to Miocene in age. The basins underwent detrital infilling
after accumulation of the peat deposits followed by tectonic
deformation that transformed these deposits into various ranks
of coal.
The rank of a coal is a measure of the metamorphism that
took place since deposition of the peat, due primarily to depth
of burial, temperature, time, and pressure (Teichmüller and
Teichmüller, 1968). The Earthʼs temperature increases with
depth of burial (geothermal gradient), and the temperature
necessary to metamorphose the peat to coal probably does
not exceed 300˚–390˚F (150˚–200˚C). Time also plays an
important role in coal rank because it controls coal composi-
tion. For example, peat coal buried for 50–65 Ma will contain
higher volatile matter (subbituminous rank) than coal buried
for 200 Ma, which contains low volatile matter (bituminous
rank). Thus, Tertiary coals are generally subbituminous, and
Cretaceous and Carboniferous coals are usually bituminous.
This broad generalization, however, is not applicable in many
geologic settings in Alaska. Along ancient plate margins and
volcanic island arcs, where heat was produced either by igne-
ous intrusions and volcanism or by increased pressure caused
by tectonic compaction and compression, can increase coal
rank, such as in the Matanuska coalfield.
Coal Metamorphism, Composition,
Rank, and Occurrence
Metamorphism of peat results in transformation of plant
parts (stems, leaves, and so forth) into macerals in coal. The
plant vascular tissue parts such as cell walls (for example,
cellulose and lignin of wood, leaves, roots, and humic cell
contents) are transformed into a vitrinite (huminite) maceral,
initially high in both oxygen and hydrogen. Plant waxes,
secretions, resins, spores, and algae are converted into an
exinite (liptinite) maceral that is high in hydrocarbons (fats
and oils). Carbonized plant parts, the product of oxidation of
organic matter, are changed into an inertinite maceral—for
example, fossil charcoal or fusinite.
Varying degrees of metamorphism produce different
maceral types. The vitrinite, exinite, and inertinite, which were
formed by intense metamorphism, are unique to high-rank
coal (for example, bituminous and anthracite). Low degree of
coalification by less intense metamorphism yields a differ-
ent physical category of macerals for low-rank coal such as
in subbituminous coal and lignite. The vitrinite and exinite
macerals may be correlated to huminite and liptinite macerals,
respectively, in subbituminous coal and lignite (Stach, 1968;
Neavel, 1981; Stach and others, 1982; Stanton and others,
1989). Economic properties of coals depend on the propor-
tions of macerals, and the classification into various types is
based on these proportions. Rao (1980), Rao and Wolff (1981),
and Rao and Smith (1986) performed several petrographic
studies of Alaskan coals, which show that coal rank and its
suitability for various economic uses depends on moisture
content, ash yield, and sulfur content.
Origin of Alaska Coal 7
8 Alaska Coal Geology, Resources, and Coalbed Methane Potential
The occurrence of different ranks of coal in Alaska may
be related to juxtaposition with deformed belts and igneous
intrusions. For example, the bituminous coal in the North
Slope coalfields, which are juxtaposed to the Brooks Range
deformed belt (synclines, anticlines, and thrust and strike-slip
faults), is higher rank than the subbituminous coal away from
the deformed belt (see fig. 1). In the Southern Alaska-Cook
Inlet coalfields, the coal changes from anthracite to bitumi-
nous to subbituminous from east to west in the northeast part
of the inlet in the Matanuska coalfield (see fig. 1). There, the
Matanuska coalfield is bounded by the Talkeetna Mountains
and flanked by the Castle Mountain Fault on the north and by
the Chugach Mountains on the south (see fig. 3). In addition,
numerous volcanic rocks have intruded into the coal-bearing
rocks.
Coal Resource Classification
Wood and others (1983) defined the terminology used
in this report for coal resource classification and estimates.
The categories of the coal resource classification, arranged in
the degree of decreasing geologic assurance as to nearness to
points of control and the relative quality and quantity of geo-
logic data, are (1) measured, (2) indicated, (3) inferred, and (4)
hypothetical. The sum of the measured and indicated resources
is termed demonstrated resource. The sum of the measured,
indicated, and inferred is termed identified resource. The state
of certainty of the existence of a quantity of resource is also
based mainly on correlations of coal beds and enclosed rocks
in relation to the thickness, overburden, rank, quality, and
areal extent of the coal.
1. Measured coal resources have the highest degree of
geologic assurance. Resource estimates are based partly on
measurements from outcrops, trenches, drill holes, and mine
workings and partly on thickness projection of correlatable
beds, coal rank, and geologic data (not more than a specified
distance and depth). The area of measured coal resources is
within 0.25-mi (0.4-km) radius.
2. Indicated coal resources have a moderate degree
of geologic assurance. Estimates of resources are based on
projection of coal thickness and other geologic data from
outcrops, trenches, mine workings, and drill holes for speci-
fied distance and depth beyond those used for the measured
resources. The area of indicated coal resources is between 0.25
and 0.75 mi (0.4 and 1.2 km) radii.
Table 1. Coal resource estimates for Alaska using the classification of Wood and others (1983). [Resource estimates are in millions of short tons (multiply by 0.9072 to convert to metric
tonnes)]
Resource Classifications
Identified Undiscovered
Demonstrated
Coal province, coalfield, and age
Measured Indicated Inferred Hypothetical
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Total for Northern Alaska-Slope 120,000 3,900,000
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Total for Central Alaska-Nenana 6,400–7,700 10,000
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Total for Southern Alaska-Cook Inlet 2,900–12,000 970,000–1,600,000
Total coal resources for Provinces 129,000–140,000 4,900,000–5,500,000
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3. Inferred coal resources have a low degree of geologic
assurance. Estimates of resource are calculated by projection
of thickness, sample, and geologic data from distant outcrops,
trenches, workings, and drill holes for a specified distance and
depth beyond those used for indicated resources. The area of
inferred coal resources is between 0.75 and 3 mi (1.2 and 4.8
km) radii. Estimates of coal thickness, extent, and quantity are
based on inferred continuity, beyond measured and indicated
resources, for which there is geologic evidence.
4. Hypothetical or undiscovered coal resources have the
lowest degree of geologic assurance of these categories. Esti-
mates of coal thickness, extent, and quantity are based on mea-
surements and continuity of coal beyond parameters used in
the inferred resources. The area of hypothetical coal resources
is beyond a 3-mi (4.8-km) radius.
Table 1 shows the coal-resource classification system of
Alaska using the concepts of Wood and others (1983). The
estimates are determined from previous works (Wahrhaftig
and Hickcox, 1955; Barnes and Cobb, 1959; Barnes, 1967a;
McGee and Emmel, written commun., 1979; Merritt and Belo-
wich, 1984; Merritt and Hawley, 1986; Sable and Stricker,
1987; Stricker, 1991; Wahrhaftig and others, 1994), which
used different resource categories. For the present study, we
analyzed and synthesized the various coal resource assess-
ments of these workers and reconstructed them following the
coal-resource classification system of the U.S. Geological Sur-
vey (Circular 891). The revision of the Alaska coal resource
assessments here presented provides (1) a simplified and uni-
fied account of the coal resources; (2) a uniform application
of the guidelines and principles outlined in Wood and others
(1983); (3) standardized coal resource estimates comparable to
those by different workers using the same data; and (4) some
idea as to economic availability of the coal. Measurements
are reported in English units followed by metric units given in
parentheses.
Coal Resource Classification 9
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EXPLANATION
Conglomerate
Shale
This studySandstone
Mudstone
Shaly sandstone
Figure 6. Stratigraphic column of the Mesozoic and Cenozoic rocks in the Northern Alaska-Slope coal province. Modified from
Sable and Stricker (1987).
10 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Distribution of Coal in Alaska
Wood and Bour (1988) identified 50 coalfields and occur-
rences in Alaska (fig. 1). A coalfield is a discrete area that
contains coal-bearing strata with one or more coal beds. Coal
occurrence is an outcrop that contains one or more coal beds
that are isolated and cannot be correlated to other coal-bearing
outcrops. The bulk (87 percent) of the known coal resources of
Alaska are in the three previously mentioned coal provinces:
(1) the Northern Alaska-Slope, (2) Central Alaska-Nenana,
and (3) Southern Alaska-Cook Inlet coal provinces (modified
from Wahrhaftig and others, 1994; fig. 1). The bulk of the
resources in Northern Alaska-Slope coal province are con-
tained in the Cretaceous and Tertiary coal-bearing rocks. The
coal resources in the Central Alaska-Nenana and the Southern
Alaska-Cook Inlet are contained in Tertiary rocks.
Northern Alaska-Slope Coal Province
The Northern Alaska-Slope coal province (fig. 1) is the
largest in the State. It is situated at lat 69°N. and includes
about 32,000 mi2 (82,880 km2) underlain by coal-bearing
rocks, both near the surface and in the deep subsurface (see
figs. 2 and 3). Rocks in this coal province range in age from
Precambrian to Holocene; a representative columnar section is
shown in figure 6.
The Northern Alaska-Slope coal province (fig. 1) consists
of Cretaceous and Tertiary sedimentary wedges shed to the
north-northeast from the Brooks Range during the Laramide
orogeny. Sedimentary wedges were also shed eastward from
now-collapsed highlands in the present Chukchi basin, into a
deep trough that lay between the Brooks Range and the Bar-
row arch. The coal-bearing rocks were deposited in coastal
plains drained by fluvial and deltaic distributary channels that
flowed into the ancestral Beaufort Sea. Peat coal was depos-
ited mainly in mires in highly constructive (river-dominated),
progradational delta plains (for example, Corwin and Umiat
deltas of Ahlbrandt and others, 1979; Roehler and Stricker,
1979; Huffman and others, 1985). Although these deltas were
highly constructional systems, they were increasingly influ-
enced by marine transgressions resulting in destructive deltas
(Huffman and others, 1985, 1988). Sea-level rise reworked the
deltaic sediments into barrier bars forming back-barrier mires.
Deltaic sediments grade updip toward the Brooks Range uplift
into coarse fluvial sediments (sandstones and conglomer-
ates) that were deposited in alluvial fans and braided and
meandering rivers. The Cretaceous and Tertiary coal deposits,
combined in the Northern Alaska-Slope coal province, are the
largest in Alaska. The remoteness of these coal deposits and
the formidable logistic and environmental problems that come
with their exploitation make them presently uneconomic.
However, because of planned infrastructures (for example,
gas pipeline), these coal deposits may be targeted for future
coalbed methane development.
Cretaceous Rocks
The most important Cretaceous coal-bearing rocks in
the Northern Alaska-Slope province are in the Nanushuk and
Colville Groups (Collier, 1906; Tailleur, 1965; Barnes, 1967b;
Conwell and Triplehorn, 1976; Bird and Andrews, 1979;
Molenaar and others, 1984; Stricker, 1991). It should be noted
that coal deposits also occur in Mississippian rocks, but they
are of minor importance and are not discussed in this report.
Coal at Corwin Bluffs, on the Chukchi Sea north of Cape
Lisburne, was mined as early as 1879 for use in whaling ships
(Schrader, 1904). Various mining companies have carried out
preliminary investigations since 1944. During a fuel shortage
in Point Barrow from 1943 to 1944, at least one small mine
was in operation on the Meade River (Clark, 1973). Although
the Meade River coal mine demonstrated the feasibility of
mining under permafrost conditions, active mining has yet to
materialize in the Northern Alaska-Slope coal province.
Collier (1906) first described the Cretaceous coal depos-
its at Corwin Bluff. Later studies showed that coal occurs in
outcrops in the foothills belt (Chapman and Sable, 1960) and
beneath the Arctic coastal plain (Tailleur and Brosgé, 1976).
Cretaceous coal-bearing rocks probably also exist beneath the
Chukchi Sea (Grantz and others, 1975; Affolter and Stricker,
1987b) and the Beaufort Sea (Affolter and Stricker, 1987b).
Nanushuk Group
The Lower Cretaceous Nanushuk Group includes, from
bottom to top, the Tuktu, Kukpowruk, Grandstand, Chan-
dler, Corwin, and Ninuluk Formations (fig. 6); thickness is
as much as 9,800 ft thick (3,000 m). It consists of a marine
sequence that includes the Kukpowruk, Tuktu, and Grand-
stand Formations and a nonmarine sequence that includes
the Corwin Formation and the Killik Tongue of the Chandler
Formation (Sable and Stricker, 1987). Approximately 150
coal beds, with individual beds ranging from a few inches (a
few centimeters) to 20.2 ft (6.1 m) thick, occur in the middle
and upper parts of the Nanushuk Group (Callahan and Sloan,
1978). Rocks exposed at Corwin Bluffs include coal beds
from 5.5 to 8.8 ft thick (1.7 to 2.7 m) (fig. 7), and those at
Cape Beaufort contain coal beds 11 to 17 ft thick (3.4 to 5.2
m). Along the valley walls of the Kukpowruk River, a coal
bed as much 20.2 ft thick (6.1 m) was described by Sand-
ers (1981). Barnes (1967b) reported as many as 60 coal beds
within a 4,600-ft-thick (1,400-m) sequence in the Koalak area.
Many of these coal beds are of bituminous and subbituminous
rank. A net coal thickness greater than 350 ft (106 m) in the
Nanushuk Group in this area and surrounding western part of
the National Petroleum Reserve-Alaska (NPRA) is shown in
figure 8.
The Nanushuk Group consists of an offlap, postorogenic
molasse sequence deposited on a passive continental margin.
Distributation of Coal In Alaska 11
Figure 7. Photograph of a coal bed (about 20 feet thick) in the Nanushuk Group.
Figure 8. Net coal thickness map of the Nanushuk Group in the western part of the Northern Alaska-
Slope coal province. See figure 9 for line of cross section. Modified from Sable and Stricker (1987).
12 Alaska Coal Geology, Resources, and Coalbed Methane Potential
The progressive progradational relation of the sedimentary
units of the Nanushuk Group is depicted in figure 9. The strata
were deposited by fluvial-dominated systems of the Corwin
delta in the western part and the Umiat delta in the central part
of the Northern Alaska-Slope coal province (Ahlbrandt and
others, 1979; Huffman and others, 1985).
The Kukpowruk Formation in the western part of the
Northern Alaska-Slope coal province consists of delta front-
shoreline deposits that grade upward and intertongue with
the nonmarine, coal-bearing Corwin and Chandler Forma-
tions (fig. 6). The Kukpowruk Formation, composed mainly
of delta-front sandstones, ranges in thickness from 2,000 to
3,900 ft (610 to 1,200 m) in the outcrop belt in the northern
foothills. The Corwin Formation consists of alluvial and delta-
plain shale, sandstone, conglomerate, and coal (Roehler and
Stricker, 1979). This formation, although more than 11,300 ft
thick (3,450 m) at Corwin Bluffs along the Arctic coast, thins
eastward to zero in the subsurface near the Colville River.
In the central Northern Alaska-Slope coal province, the
rock succession consists of complexes of nonmarine and mar-
ginal marine rocks overlying and intertonguing with marine
rocks. The marginal marine to marine (basin shelf-slope)
Tuktu Formation, more than 8,000 ft thick (2,400 m), inter-
tongues with the delta-front and lower delta-plain Grandstand
Formation (fig. 6). The Grandstand Formation is overlain
by, and intertongues with, the Killik Tongue of the Chandler
Formation, which is an upper delta-plain rock unit. In the
upper part of the stratigraphic section, a tongue of the Ninuluk
Formation, which intertongues with the overlying Niakogon
Tongue (fig. 6), overlies the Killik Tongue. Molenaar (1985)
indicated that the Seabee Formation of the Colville Group
interfingers with both the Ninuluk Formation and Niakogon
Tongue of the Chandler Formation.
As depicted in figure 9, the vertical stacking of shoreline
deposits of the Grandstand Formation marks a progradational
sequence that may be correlated with the occurrence westward
(landward) of numerous thick coal beds in the Corwin Forma-
tion. The progradational or regressive phase led to the stack-
ing of shoreline deposits at the regressive maximum, which
was described by Fassett and Hinds (1971), Ryer (1981), and
Flores and Cross (1991). These studies have shown that most
coal beds occur at the top, and landward, of shoreline deposits
Dominantly deepmarineshale
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Dominantly nonmarine facies
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Figure 9. Cross section showing the Nanushuk progradation sequences. Modified from Molenaar, (1985). See figure 8
for location of the cross section.
from progradational events. However, the thickest coal, most
extensive, and greatest volume of coal occur preferentially in
stratigraphic positions where the shoreline deposits of succes-
sive progradational events are stacked vertically (Flores and
Cross, 1991).
The Umiat deltaic sediments reflect a higher degree of
reworking than the Colville deltaic sediments as indicated by
a larger sandstone percentage (fig. 10; Ahlbrandt and oth-
ers, 1979; Huffman and others, 1985). The Umiat delta (fig.
11A–D), probably being derived from a smaller source, also
contains a smaller sediment volume than does the Corwin
delta. Molenaar (1981, 1985) suggested that the Corwin delta
formed earlier than the Umiat delta and that the two merged
during Albian time without specific demarcation (fig. 11A–D).
The Corwin delta continued to be the dominant depositional
feature. The Meade arch, which extended southward from
Point Barrow in Brookian time, probably did not play an
active part in controlling the depocenters of the deltas. Paleo-
geographic interpretations of Nanushuk deposition by Mole-
naar (1981, 1985) and Huffman and others (1985) stressed
the dominant east-northeast progradation of the Nanushuk
prodelta slope sediments. These studies also showed a strong
northwestward concentration of sandstone in the upper part of
the Nanushuk Group, from Umiat toward Point Barrow and
parallel to the paleoshoreline. This concentration also indicates
that northwestward longshore currents probably transported
sand from the Umiat delta along the active shelf of the Corwin
delta front (Huffman and others, 1985). The sand accumula-
tion represents offshore bars that shielded coastal plain-back
barrier mires from detrital sedimentation, resulting in the most
prolific coal-forming Nanushuk deltaic environments in the
western Northern Alaska-Slope coal province.
Spicer (1987) reported that the paleoclimate of the North
Slope during Albian to Cenomanian time was cool temperate
with annual temperature varying 5˚–50˚F (3˚–10˚C). Rainfall
was sufficient to sustain the vegetation of peat mires, result-
ing in thick accumulation of peat deposits. Tree growth rings
on the North Slope indicate a rapid change from summer to
winter conditions during the Albian to Cenomanian (Spicer,
1987). Precipitation was also distributed throughout the year
in a manner to preclude oxidation and loss of organic material
in the peat mires. However, Rao (1980) reported that there was
a drying upward trend in the peat mires.
Colville Group
The Upper Cretaceous Colville Group, a Brookian
sequence younger than the Nanushuk Group, contains, from
bottom to top, the Seabee, Schrader Bluff, and Prince Creek
Formations (see fig. 6). The group is as much as 5,000 ft
(1,525 m) thick, consists of a marine interval (Seabee Formation),
marine interval (Schrader Bluff Formation), and a coal-bearing,
nonmarine interval (Prince Creek Formation).
The coal beds of the Colville Group in the vicinity of
Umiat and Maybe Creek (see fig. 8) vary individually from 13
Distributation of Coal In Alaska 13
Figure 10. Sandstone percentage contour map in the Northern Alaska-Slope coal province. Modified
from Huffman and others (1985).
14 Alaska Coal Geology, Resources, and Coalbed Methane Potential
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Figure 11. Paleogeographic maps showing the depositional environments of the Nanushuk Group in the central North-
ern Alaska-Slope coal province. A, Early to middle Albian time. Modified from Huffman and others (1985).
Figure 11. Paleogeographic maps showing the depositional environments of the Nanushuk Group in the central North-
ern Alaska-Slope coal province. B, Middle to late Albian time. Modified from Huffman and others (1985).
Distributation of Coal In Alaska 15
Figure 11. Paleogeographic maps showing the depositional environments of the Nanushuk Group in the central North-
ern Alaska-Slope coal province. C, Late Albian to Cenomanian (?) time. Modified from Huffman and others (1985).
Figure 11. Paleogeographic maps showing the depositional environments of the Nanushuk Group in the central North-
ern Alaska-Slope coal province. D, Cenomanian time (maximum regression). Modified from Huffman and others (1985).
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16 Alaska Coal Geology, Resources, and Coalbed Methane Potential
to 39 ft (4 to 12 m) thick (Brosgé and Whittington, 1966). On
the lower Colville River, the coal beds of the Colville Group
range from 13 to 39 ft (4 to 12 m) thick (Brosgé and Whit-
tington, 1966). Coal beds in the subsurface are typically less
than 39 ft (12 m) thick with a 19-ft-thick (6 m) zone of coal
interbedded with black shale and bentonite described from
the Square Lake Test Well No. 1 core as a bony coal (Collins,
1959).
Recent work by R.M. Flores and G.D. Stricker and other
geologists from the USGS (Dave Houseknecht and Ken Taka-
hashi) and Alaska Division of Oil and Gas Commission (Mark
Myers, Don Brizzolara, and Tim Ryherd) in July, 2002 of
outcrops of the Kogosukruk Tongue (see fig. 6) of the Prince
Creek Formation at the bluffs along the Colville River below
the mouth of the Anaktuvuk River (see fig. 8) shows coal beds
ranging from a few inches (a few centimeters) to as much
as 9 ft (2.7 m) thick (fig. 12). Upstream from this outcrop,
the Kogosukruk Tongue is underlain by the upper part of the
Sentinel Hill Member of the Schrader Bluff Formation, which
is composed of coarsening-upward bentonitic mudstone,
siltstone, and sandstone; the sandstone thickens and coarsens
upward. The uppermost sandstone bed of the Sentinel Hill
Member, which is rooted at the top, in turn, is overlain by a
3.5 ft-thick coal-carbonaceous mudstone bed of the Kogosuk-
ruk Tongue, with both beds separated by an erosional surface
or probably a sequence boundary. The coarsening-upward
Sentinel Hill Member was probably deposited in a delta front.
The coal-carbonaceous mudstone bed and overlying interbed-
ded coal, bentonitic mudstone and siltstone, and medium-
grained to pebbly conglomeratic, stacked erosional-based
sandstone of the Kogosukruk Tongue were probably deposited
in fluvial environments.
Coal beds of the Colville Group have been studied less
than those of the Nanushuk Group because they have shown
less economic potential; most are thinner, have a high ash
yield, and are of lower rank than those in the Nanushuk
Group. Many of these coals are described as lignites.
Cretaceous-Tertiary Rocks
The contact between the Cretaceous and Tertiary rocks in
the Northern Alaska-Slope coal province was determined to be
gradational or conformable by Molenaar (1983) and Molenaar
and others (1984). This led to difficulty in defining a specific
contact and also contributed to inclusion of the Upper Cre-
taceous formations of the Colville Group with the overlying
Tertiary Sagavanirktok Formation. Molenaar (1983) described
this contact as diachronous resulting from a depositional
continuum from the Cretaceous to Tertiary time. That is, the
Cretaceous deltaic systems (for example, Corwin delta) that
prograded northeastward also continued this advance seaward
during Tertiary time. Thus, the nonmarine and marine deposits
Figure 12. Photograph of interbedded coal, sandstones, siltstone, and mudstone of the Kogosukruk Tongue of the Prince
Creek Formation along the lower Colville River downstream from the mouth of the Anaktuvuk River.
of both deltaic systems blend imperceptibly into one another,
and timelines are parallel to their depositional slopes. For
example, the delta-front sands of these deltas deposited during
the regression cross the sloping timelines and become younger
toward the direction of progradation, which is northeastward.
The Cretaceous-Tertiary rocks in the Northern Alaska-
Slope coal province include the Jago River and Sagavanirktok
Formations (see fig. 6; Gryc and others, 1951; Detterman and
others, 1975; Molenaar, 1983; Buckingham, 1987). The Jago
River Formation was dated as Late Cretaceous to Paleo-
cene based on pollen and plant fossils (Palmer and others,
1979; Detterman and Spicer, 1981; Buckingham, 1985). The
Sagavanirktok Formation was dated as Paleocene to Plio-
cene (and may possibly be as young as Pleistocene) based on
palynomorphs and microfossils (Molenaar and others, 1986).
The Sagavanirktok Formation intertongues with the Canning
Formation of the Colville Group (Molenaar and others, 1986).
Jago River Formation
The Jago River Formation, which was named and
described by Buckingham (1987), is as much as 9,387 ft
(2,861 m) thick. It consists of conglomerates, sandstones,
siltstones, mudstones, carbonaceous shales, and coals. Buck-
ingham (1987) divided the formation into four lithofacies
units; from bottom to top these are (1) a delta-plain lithofa-
cies—mainly sandstones and siltstones, as much as 928 ft
(282 m) thick; (2) a meandering stream lithofacies (lower
part)—mainly sandstones and subordinate conglomerates,
siltstones, and coals, as much as 5,526 ft (1,685 m) thick; (3) a
braided stream lithofacies—predominantly conglomerates and
minor sandstones, as much as 2,228 ft (679 m) thick; and (4)
a meandering stream lithofacies—mainly carbonaceous shales
and minor sandstones conglomerates, as much as 702 ft (214
m) thick. The few coal beds that are in the Jago River Forma-
tion are associated with the meandering stream lithofacies and
are thin and uneconomic.
The lithofacies of the Jago River Formation indicate that
the rocks were deposited in a fluvial-dominated delta that was
formed in close proximity to the ancestral Brooks Range (fig.
13). Through time the delta was prograded by a fan delta,
which was subsequently replaced by a fluvial-dominated
delta. This evolution of fluvio-deltaic systems may have been
controlled by tectonism of the ancestral Brooks Range and (or)
eustatic sea-level rise and fall. These fluvio-deltaic systems
prograded to the north-northwest. Most coal-forming mires are
related to these fluvial-dominated deltaic systems.
Sagavanirktok Formation
The Sagavanirktok Formation consists of a thick
sequence of sandstones, siltstones, mudstones, conglomerates,
carbonaceous shales, and coals (fig. 14). Thickness is as much
as 7,500 ft (2,300 m) (Molenaar and others, 1986). Sandstones
are the most abundant lithology (fig. 15). The formation
is a generally coarsening-upward sequence with the lower
part dominated by shaley tongues of the Canning Formation
(Molenaar and others, 1986). There are at least three
Cretaceous-Tertiary Rocks 17
Figure 13. Paleogeographic map showing the depositional environments of the Jago River Formation in
the Arctic National Wildlife Refuge area. Modified from Buckingham (1987).
18 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Figure 14. Photograph of a coal bed underlain by a sandstone in the Sagavanirktok Formation. Photograph
courtesy of S.B. Roberts. Hammer on the sandstone is 1 foot long for scale.
Figure 15. Photograph of fluvial-channel sandstone and associated rocks in the Sagavanirktok Formation.
Photograph courtesy of S.B. Roberts.
Cretaceous-Tertiary Rocks 19
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Alaska. Modified from Molenaar and others (1985).
20 Alaska Coal Geology, Resources, and Coalbed Methane Potential
GR,GAMMARAYLOGTRACEDEN,DENSITYLOGTRACES,SONICLOGTRACEDEPTH(INFEET)SHOWNBESIDEEACHDRILL-HOLECOLUMN��������������������������������������������������������?��������������������������EXPLANATIONConglomerateandsandstoneMudstoneandsandstoneCoalMikkelsenTongueoftheCanningFormationCoarsening-upwardsequence���������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������Figure 17. West to east stratigraphic cross section showing variations of the coal beds, conglomerates, sandstones, mudstones, and siltstones in the Staines Tongue of the Sagavanirktok Formation between the National Petroleum Reserve in Alaska and Arctic National Wildlife Refuge. Modified from Roberts and others (1992). Line of cross section (west to east) is shown in figure 18.
Cretaceous-Tertiary Rocks 21
��CanningRiverCanningRiverPrudhoeBayPrudhoeBay��������������������������DEPTHTOTOPOFCOAL-BEARINGINTERVAL(INMETERS)ISOPACHOFTOTALCOAL(INMETERS)����������������������������������������������������������������������������������������������������������������������������������������������������������������������ShaviovikRiverShaviovikRiver���������Figure 18. Maps showing the (A) depth to top of coal-bearing interval and (B) net coal thickness isopach of the Staines Tongue of the Sagavanirktok Formation. Modified from Roberts and others (1992).
22 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Sagavanirktok tongues that intercalate with the Canning shales
(fig. 16). The uppermost tongue identified by Molenaar and
others (1986) as the Staines Tongue was studied by Roberts
(1991) and Roberts and others (1992) and varies from 650 to
2,950 ft (200 to 900 m) in thickness (fig. 17). The lower 330
to 1,640 ft (100 to 500 m), of the Staines Tongue is dominated
by coarsening-upward mudstone, siltstone, and sandstone
representing parasequence sets. These parasequence sets are
overlain by two coal-bearing intervals—the lower and upper
coal zones in the middle part of the tongue (Roberts and oth-
ers, 1992)—that are separated by interbedded sandstones and
mudstones, which are as much as 295 ft (90 m) thick. The
lower coal zone is as thick as 850 ft (260 m) and contains 12
coal beds. The upper coal zone is as thick as 360 ft (110 m)
and contains seven coal beds. The individual coal beds are as
much as 23 ft (7.1 m) thick. The uppermost part of the Staines
Tongue is interbedded sandstone and mudstones and is as
much as 260 ft (80 m) thick. This interval is, in turn, over-
lain by mudstones of the Mikkelsen Tongue of the Canning
Formation. Coal beds are distributed over an area of 5,790 mi2
(15,000 km2). Near Prudhoe Bay, a coal-bearing interval as
much as 1,310 ft (400 m) thick contains coal beds 1.9–22 ft
(0.6–6.7 m) thick (Roberts, 1991); one 6.5-ft-thick (2 m) coal
zone has been reported on the lower Shaviovik River (fig. 18;
Roberts and others, 1992). Lignite and coaly shale as thick as
19 ft (6 m) occur in the lowermost part of the formation (Det-
terman and others, 1975; Molenaar and others, 1984).
The Sagavanirktok Formation represents the final infill-
ing of the Colville Basin in the eastern part of the Northern
Alaska-Slope coal province. The Staines Tongue represents
an episode of this infilling by the deposition of fluvio-deltaic
sediments. The lower part of the Staines Tongue is domi-
nated by parasequence sets representing delta front-prodelta
deposits, which grade into the shelf-slope mudstones of the
Canning Formation (figs. 17 and 18). The lower coal zone
was deposited in an alluvial-delta plain in which the coal
beds accumulated in interfluvial and interdistributary mires.
The noncoaly interval between the two coal zones reflects a
landward advance of the paleoshoreline resulting from a minor
transgression or sea-level rise. The coal beds of the upper coal
zone were probably formed in lower delta-plain and back-bar-
rier mires as the paleoshoreline then regressed. The thin nature
of the upper coal zone (fig. 17) and the sandy character of the
uppermost part of the Sagavanirktok, which is in turn overlain
by the Mikkelsen mudstone tongue of the Canning Formation,
indicate a back-stepping paleoshoreline. In this setting, rapid
transgression over the peat-forming mires reworked older
deposits, probably forming barrier-shoreface deposits prior to
a marine flood.
The thick and laterally extensive coal beds of the lower
coal zone of the Staines Tongue probably reflect a peat
accumulation in mires formed during a time of paleoshoreline
stability. This event may correspond to a regressive maximum
that led to vertical stacking of paleoshoreline deposits, closely
similar to that described for the Nanushuk Group. However,
unlike the Nanushuk Group, the Staines Tongue of the
Sagavanirktok Formation was affected by sea-level fluctua-
tions prior to the maximum transgression that deposited the
overlying Mikkelsen Tongue of the Canning Formation.
Sea-level fluctuations and marine flooding interrupted
coal-forming mires, which provided only a brief period of
time for peat accumulation.
Coal Resource Assessment of the
Northern Alaska-Slope Coal Province
The coal resource assessments of different workers
in the Northern Alaska-Slope coal province vary greatly in
magnitude and coal resource categories, resulting in confused
reporting of estimates. As a result we reconstructed these
different coal resource estimates following guidelines of the
coal-resource classification system of Wood and others (1983).
This new reporting system of the coal resources of Alaska
in general, and of Northern Alaska-Slope coal province in
particular, as modified from previous estimates, is summarized
in table 1. Following is a historical account of the various coal
resource assessments in the coal province.
Cretaceous Rocks
In an early resource assessment of Cretaceous rocks in
the Northern Alaska-Slope coal province, Barnes (1967a)
calculated a total of 2.4 × 109 short tons (2.2 × 109 metric tons)
of demonstrated coal resources and 117 × 109 short tons (107 ×
109 metric tons) of undiscovered (hypothetical) coal resources.
Later Tailleur and Brosgé (1976) estimated the coal resources
in the coal province by calculating the product of coal-bearing
area and coal concentration. Using surface data and two oil
and gas test wells, these workers estimated the coal resources
in the Northern Alaska-Slope coal province at 120 × 109 short
tons (109 × 109 metric tons) of identified coal resources plus
114 × 109 to 37 × 1012 short tons (104 × 109 to 34 × 1012 metric
tons) of hypothetical coal resources (see table 1).
Later, Sable and Stricker (1987), using all available
data for the Nanushuk Group, estimated coal resources for
the National Petroleum Reserve in the Alaska portion of the
North Slope. Using the methodology described by Wood and
others (1983) and all available data for the area of the known
Nanushuk Group coal-bearing rocks, Sable and Stricker
(1987) estimated the hypothetical coal resources for the
Nanushuk Group on the North Slope, which are shown in table
2. In summary, there are 1.3 trillion short tons (1.2 trillion
metric tons) of subbituminous coal and 1.9 trillion short tons
(1.7 trillion metric tons) of bituminous coal, for a total of 3.2
trillion short tons (2.9 trillion metric tons) of hypothetical
coal resources for the Nanushuk Group on the North Slope of
Alaska (table 2). Barnes (1967a) estimated about 101 billion
short tons (92 billion metric tons) of identified coal resources
in this group of rocks.
Stricker (1991) indicated that the Nanushuk Group
contains an estimated 3.1 × 1012 short tons (2.9 × 1012 metric
tons) of hypothetical coal resources for onshore northern
Alaska (table 2); of this total, 1.3 × 1012 short tons (1.2 × 1012
metric tons) is subbituminous, and 1.9 × 1012 short tons (1.7 ×
1012 metric tons) is bituminous (Stricker, 1983, 1991). In-situ
speculative Cretaceous Nanushuk coal that lies under the
Chukchi Sea has been estimated at 2.0 × 1012 short tons (1.8
× 1012 metric tons) of lignite A to high-volatile bituminous A
coal (Affolter and Stricker, 1987b).
Coal Quality
The coal beds of the Nanushuk Group in the Northern
Alaska-Slope coal province range in apparent rank from lig-
nite A to high volatile A bituminous coal with a mean of high-
Coal Resource Assessment of the Northern Alaska-Slope Coal Province 23
Table 2. Estimates of hypothetical coal resources for the Cretaceous Nanushuk Group (Stricker, 1991) and Tertiary Staines Tongue of the Sagavanirktok
Formation (Roberts and others, 1992) in the Northern Alaska-Slope coal
province.
[>, greater than]
Unit Rank Attitude Overburden (feet)1
Coalresource
estimate2
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North Slope Total (rounded)�����
24 Alaska Coal Geology, Resources, and Coalbed Methane Potential
volatile C bituminous coal (table 3a). The heating values range
from 9,100 to 12,700 Btu/lb (5,050 to 7,060 kilocalories/kilo-
gram) with an average of 12,300 Btu/lb (6,830 kilocalories/
kilogram) (State of Alaska, 1993). Total sulfur content ranges
from 0.1 to 2.0 percent with a mean of 0.3 percent. The
ash yield has a mean of 11.0 percent (Affolter and Stricker,
1987a). The coal is generally subbituminous A under the
Arctic coastal plain and high-volatile bituminous in the folded
foothills, is low in ash (less than 10 percent) and sulfur (1.4
percent) (Sanders, 1981; Affolter and Stricker, 1987a), and has
low concentrations of elements of environmental concern (As,
Be, Hg, Mo, Sb, and Se) (Affolter and Stricker, 1987a). The
higher rank coal beds in the foothills are probably upgraded in
apparent rank by tectonism.
Tertiary Rocks
Early resource assessment of the coal resources of the
Sagavanirktok Formation by Sanders (1976) and Tailleur
and Brosgé (1976) estimated 50–60 billion short tons (45–55
billion metric tons) of hypothetical resources. Evaluation of
48 geophysical logs penetrating coal beds of the formation
led Roberts and others (1992) to estimate a hypothetical coal
resource of 670 billion short tons (610 billion metric tons)
(table 2), which is 10 times more than the original estimate.
The coal included in the estimate by Roberts and others (1992)
occurs mainly in the onshore Northern Alaska-Slope coal
province, where the overburden varies from 150 to 1,800 ft
(46 to 550 m). As shown in figure 17, the thickest coal is in
the southeast part of the coal province. Affolter and Stricker
(1987b) estimated the offshore (beneath the Beaufort Sea)
hypothetical resources to be 300 billion short tons (270 billion
metric tons).
Coal Quality
A summary of chemical analyses of 55 coal outcrop
samples from the Sagavanirktok Formation, as reported by
Roberts and others (1992), indicated that the apparent rank of
the coal beds ranges from lignite A to subbituminous B coal
(3,340–9,740 Btu/lb) (1,860–5,410 kilocalories/kilogram),
with a mean of subbituminous C coal (7,780 Btu/lb) (4,320
kilocalories/kilogram) (table 3b). Total sulfur content is low,
varying from 0.08 to 2.16 percent with a mean of 0.38 percent.
The ash yield varies from 1.2 to 47.1 percent with mean of
11.1 percent. Moisture content ranges from 16.2 to 33 percent
with mean of 23.6 percent.
Coal Petrology
The petrology of the coal beds in the Corwin Forma-
tion of the Nanushuk Group was studied by Rao (1980) in
the Cape Beaufort region. Forty-eight samples from 14 coal
beds showed that the coal is composed mainly of the macerals
vitrinite (huminite), liptinite, and inertinite. The percentage
of vitrinite varies from 47.1 to 89.5 (average of 74.4), that of
liptinite from 0.4 to 10.4 (average of 2.3), and that of inertinite
from 1.8 to 33.9 (average of 23.3). The macerals vary from
coal bed to coal bed as well as within a coal bed. Many of the
beds are high in inertinites (such as fusinite and semifusinite)
(charcoals), the proportions of which are lowest at the bottom
of the coal bed and increase toward the top. This pattern indi-
cates that mires evolved into a drier setting during accumula-
tion of peat deposits, which promoted forest fires that created
the charcoals (fusinites). This interpretation is supported by
Spicer (1987), whose investigation indicated seasonality of the
generally humid climate during the development of the peat-
forming mires. Platanoid-like leaves are closely associated
with fluvial deposits, and gymnosperm and magnoliid-like
Table 3a. Coal quality of coal deposits in the Cretaceous Nanushuk Group in the Northern Alaska-Slope coal province.
[All analyses except Calorific value (Btu) and Ash-fusion-temperature (°F) are in
percent. Values reported on an as-received basis. L after a value means less
than the value shown and G after a value means greater than the value shown.Data from U.S. Geological Survey, 1997, USGS Coal Quality Database
(USCHEM) [unpublished computer database: Reston, Virginia)]
Range
Unit Parameter
Numberofsamples Minimum Maximum
Arithmetic
mean
Standarddeviation
Proximate and ultimate analysis
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Calorific value
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Forms-of-sulfur
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Ash-fusion-temperatures °F
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Table 3b. Coal quality of coal deposits in the Tertiary Staines Tongue of the Sagavanirktok Formation in the Northern Alaska-Slope coal province.
[All analyses except Calorific value (Btu) and Ash-fusion-temperature (°F) are in
percent. Values reported on an as-received basis. L after a value means less
than the value shown and G after a value means greater than the value shown.Data from U.S. Geological Survey, 1997, USGS Coal Quality Database
(USCHEM) (unpublished computer database: Reston, Virginia)]
Range
Unit Parameter
Number
ofsamples Minimum Maximum
Arithmeticmean Standarddeviation
Proximate and ultimate analysis
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leaves are associated with lacustrine and mire deposits (Spicer,
1987). Vitrinite reflectance values of the 14 coal beds vary
from 0.65 to 0.74 (average 0.70) percent.
Central Alaska-Nenana Coal Province
The Central Alaska-Nenana coal province (fig. 1) is the
smallest, most centrally located, and most thoroughly studied
of the coal provinces on the north side of the Alaska Range.
It has accounted for more than one-half of the coal mined in
Alaska and is the only province in Alaska being currently
mined. This coal province is in the northern foothills of the
Alaska Range, extending from about 50 mi (80 km) west to 50
mi (80 km) east of the Alaska Railroad (see fig 1). It consists
of several synclinal basins partly or wholly detached from
each other by erosion of coal-bearing rocks from intervening
structural highs. These coal-bearing synclinal basins were rec-
ognized as coalfields and include the Jarvis Creek, East Delta,
West Delta, Wood River, Mystic Creek, Tatlanika Creek,
Lignite Creek, Healy Creek, Rex Creek, and Western Nenana.
They extend as a discontinuous belt from 9 mi (14.5 km) wide
to 56 mi (90 km) long (fig. 19).
The Healy Creek, Lignite, and Suntrana coalfields, where
past mining occurred and most current mining occurs lie along
the Alaska Railroad and the Anchorage to Fairbanks high-
way (see fig. 1; George Parks State Highway 1). The railroad
provided the needed transportation for marketing the coal. In
1918, underground coal mining by the Healy River Coal Cor-
poration began at Suntrana, 4 mi (6.4 km) east of the conflu-
ence of Healy Creek and the Nenana River (Usibelli, 1986).
Horse-drawn sleds to the railroad camp in Healy originally
transported coal until a railroad spur was built to the mine
in 1922. The Healy River coal mine accounted for one-half
of the Stateʼs production from 1920 to 1940. The rest of the
production was from the Evan Jones mine in the Matanuska
coalfield (see discussion of the Southern Alaska-Cook Inlet
coal province).
The military buildup in Alaska in the 1940s and after
World War II provided a new market for coal that resulted
in opening more mines to meet the demand (Usibelli, 1986).
Usibelli Coal Mine, Inc. (UCM), opened the first strip mine
in the coal province east of Suntrana in 1943. In 1961, UCM
purchased the Healy River Coal Corporation and continued
mining underground. The Arctic Coal Company opened a
small mine on Lignite Creek and operated it until 1963. The
Vitro Mineral Mine was opened in 1963 east of Suntrana and
Central Alaska-Nenana Coal Province 25
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e
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anikaRiverT
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Figure 19. Map showing the coalfields in the Central Alaska-Nenana coal province.
26 Alaska Coal Geology, Resources, and Coalbed Methane Potential
in 1970 was purchased by UCM. Golden Valley Electric Asso-
ciation opened a mine-mouth powerplant at Healy in 1968.
Since that time UCM has supplied coal to the powerplant and
in 1985 entered the international market by supplying coal to
South Korea. UCM is the only active coal mine in the State
today.
Tertiary Usibelli Group
The Usibelli Group (Wahrhaftig, 1987), a nonmarine
sedimentary sequence of Tertiary age, consists, from bottom
to top, of the coal-bearing Healy Creek, noncoaly Sanctu-
ary, coal-bearing Suntrana and Lignite Creek Formations and
noncoaly Grubstake Formation (fig. 20). It is overlain uncon-
formably by the Nenana Gravel. Detailed discussions of the
group are summarized from Wahrhaftig and others (1969),
Wahrhaftig (1987), Wahrhaftig and others (1994), and Stanley
and others (1992). Sanders (1976) recognized as many as 30
coal beds in the Usibelli Group, which are mainly 2.5 ft (0.7
m) thick but can be as much as 30 ft (9.1 m) thick. The vertical
and lateral stratigraphic variations of the Healy Creek, Sanctu-
ary, and Suntrana Formations, which overlie the lower Paleo-
zoic and Precambrian (?) pelitic and quartzose schist sequence
(Csejety and others, 1992), are displayed in figure 21.
The depositional environments of the Usibelli Group
have been interpreted as fluvial and lacustrine deposits
(Buffler and Tripplehorn, 1976; Selleck and Panuska, 1983;
Merritt, 1986; Stanley and others, 1992; Wahrhaftig and oth-
ers, 1994). Flores and Stanley (1995) proposed that the Healy
Creek Formation was deposited in an incised paleovalley
infilled by sediments of transverse alluvial fans and longitudi-
nal braided streams that flowed southward (fig. 22A–D). The
paleocurrent directions from crossbeds in the sandstones of
the Healy Creek, Suntrana, and Lignite Creek Formations (fig.
23) indicate southward flow of streams. Inactive braid-belt
deposits formed platforms for raised mires on which thin to
thick peat deposits accumulated. Lacustrine sediments of the
Sanctuary Formation succeeded these alluvial environments
being deposited in a lake that resulted from coalescing of
flood-plain lakes and fluvial channels caused by damming of
the downstream extent of the ancestral fluvial system, which
flowed southward into the ancestral Cook Inlet Basin. Either
uplift of the Alaskan Range or movement along the Denali
fault may have caused damming. This tectonic movement
caused base level to rise (Flores and Stanley (1995). The lake
was filled by alluvial fan deltas, which gradually lowered base
level and restored the fluvial systems that continued to flow
southward. This led to formation of low-sinuosity streams
���������������
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Shale,siltstone,and sandstone
Conglomerate and conglomeratic sandstone
Sandstone
Coal and carbonaceous shale
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���������������������������������������Figure 20. Generalized stratigraphic and lithofacies column of the Usibelli Group in the Central Alaska-Nenana
coal province.
and related mires during deposition of the Suntrana Forma-
tion. These streams evolved into high-sinuosity (meandering)
streams and accompanying mires during deposition of the
Lignite Creek Formation. Raised mires were associated with
these streams, forming on abandoned deposits of alluvial belts
and flood basins. These mires were common during the depo-
sition of the Suntrana Formation, and topogenous or low-lying
mires, where thin peats accumulated, were common during the
deposition of the Lignite Creek Formation. Another damming
of the downstream extent of these streams by uplift of the
Alaskan Range created a lake that was infilled by alluvial-fan
delta sediments of the Grubstake Formation. Continued uplift
and lowering of base level resulted in northward-flowing
alluvial fans to be shed from the Alaskan Range, forming the
Nenana Gravel.
Healy Creek Formation
The Healy Creek Formation is the oldest rock unit in the
Usibelli Group (fig. 20). The formation, as much as 445 ft
(136 m) thick, consists of interbedded sandstones, conglomer-
ates, siltstones, and mudstones, including carbonaceous shale
and coal beds. Sandstone is the most common rock type and
coal is the least common. It unconformably overlies the pelitic
and quartzose schist sequence (Csejety and others, 1992) with
erosional relief of as much as a few hundred feet.
In most of the synclinal coalfields, the Healy Creek
Formation is early to middle Miocene (Wolfe and Tanai, 1980;
Wahrhaftig, 1987); but in the Rex Creek coalfield, where the
formation was formerly thought to be as old as late Oligocene
(Wolfe and Tanai, 1980), it is now regarded to be as old as late
Eocene (Wolfe and Tanai, 1987).
The Healy Creek Formation consists mainly of fining-
upward sequences of conglomerates, sandstones, and silty
sandstones (fig. 24). The conglomerates are composed of
sedimentary, igneous, and low- to medium-grade metamor-
phic rock types (Stevens, 1971). The sandstones are mainly
quartz-feldspathic-rich rocks. The lower 130 ft (40 m) consists
mainly of amalgamated, basally scoured, lenticular pebble-
cobble conglomerates and sandstones (Stanley and others,
1992). The lowermost conglomerate beds rest with sharp,
erosional contact on the pelitic and quartzose schist sequence.
Conglomerates are normally graded, clast supported, and
crudely imbricated (fig. 25). Sandstones exhibit abundant
tabular and trough crossbeds in sets generally less than 2.3 ft
(70 cm) in height. Ripple and small-scale cross laminations
are common. Also present are scour surfaces at the bases of
the conglomerates and sandstones with as much as 10 ft (3 m)
of erosional relief.
Interbedded sandstones, siltstones, mudstones, coals,
and carbonaceous shales (fig. 26) overlie the conglomer-
atic and sandy interval of the Healy Creek Formation. The
sandstones exhibit erosional basal surfaces, fine upward,
and are crossbedded (mainly trough and planar crossbeds).
Rooted siltstones and mudstones overlie the sandstones and
are interbedded with coal and carbonaceous shales. Coal beds
have combined thickness of as much as 49–61 ft (15–20 m),
and individual beds persist laterally for more than 0.6 mi (1
km). They commonly pinch out, split, and (or) merge. Also,
Central Alaska-Nenana Coal Province 27
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Conglomerate and conglomeratic sandstone
Conglomerate
Sandstone
Coal and carbonaceous shale
Shale-rich unit
Shale,siltstone,and sandstone
EXPLANATION
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Figure 21. Stratigraphic
cross section showing
the variations of the con-
glomerates, sandstones,
siltstones, mudstones,
and coal beds in the
lower part of the Usibelli
Group in the Healy Creek
coalfield on the south-
ern part of the Central
Alaska-Nenana coal
province. See figure
19 for location of
Healy Creek coalfield.
28 Alaska Coal Geology, Resources, and Coalbed Methane Potential
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Flow direction
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stream
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Figure 22. Paleogeographic maps showing depositional environments of: (A) Healy Creek Formation, (B) Sanctuary
Formation, (C) Suntrana Formation, and (D) Lignite Creek Formation. Adopted from Flores and Stanley (1995).
Figure 23. Crossbed-orientation measurements in fluvial-channel sandstones in the Healy Creek, Suntrana, and Lignite Creek
Formations in Suntrana area. Modified from Flores and Stanley (1995).
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PALEOCURRENTS
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N =Number of measurements
Central Alaska-Nenana Coal Province 29
Trough crossbeds
Pelitic and quartzose
schist sequence
(Precambrian Birch Creek Schist)
Healy Creek Formation
(lower part)
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Tabular crossbeds
Planar crossbeds
Ripple lamination
Sandstone
Conglomerate and conglomeratic sandstone
Coal and carbonaceous shale
EXPLANATION
Figure 24. Stratigraphic cross section showing the basal conglomerates and sandstones in the lower part of the Healy
Creek Formation east of Suntrana. Adopted from Flores and Stanley (1995). See figure 19 for location of cross section.
Figure 25. Photograph of conglomerates (a few inches to 5 feet thick or a few centimeters to 1.5 meters) and sandstones (6
inches to 8 feet thick or 15.2 centimeters to 2.4 meters) deposited by braided streams in the lower part of the Healy Creek
Formation in east of Suntrana.
30 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Sandstone
Mudstone and siltstone
Coal and carbonaceousshale
Trough crossbeds
Tabular crossbeds
Megaforesets
Ripple laminations
Root marks
Healy Creek Formation
(middle part)��
EXPLANATION ��������������
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Figure 26. Stratigraphic cross
section showing the middle, coal-
bearing part of the Healy Creek
Formation east of Suntrana. See
figure 19 for location of cross
section. Adopted from Flores and
Stanley (1995). HealyCreekFormation(upperpart)Conglomeratic sandstone
Sandstone
Mudstone and siltstone
Limestone
Coal and carbonaceous shale
EXPLANATION
Trough crossbeds
Tabular crossbeds
Ripple laminations
Root marks
F Coal
Sanctuary Formation
��������������
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��Figure 27. Stratigraphic cross
section of the uppermost part
of the Healy Creek Formation
showing the fluvial-channel
sandstones and F coal bed, and
overlying Sanctuary Formation
east of Suntrana. See figure
19 for location of cross section.
Adopted from Flores and Stanley
(1995).
they interfinger with carbonaceous shales, mudstones, and
siltstones. The highest coal bed of the Healy Creek Formation,
the F coal bed (figs. 27 and 28), which immediately underlies
the Sanctuary Formation, is the only coal bed of sufficiently
continuous lateral extent to be analyzed for reserve estimates
(Wahrhaftig and others, 1994).
The Healy Creek Formation was interpreted to originate
as braided to high-sinuosity stream deposits (Buffler and
Tripplehorn, 1976; Selleck and Panuska, 1983; Stanley and
others, 1992). The streams may have formed on a wet alluvial
fan or on a proximal braid plain where the cobbles, pebbles,
and sands were likely deposited by migrating longitudinal
gravel bars and sandy transverse bars. The interbedded sand-
stones, siltstones, and mudstones probably were deposited in
low-sinuosity fluvial channels and flood plains. Coal beds and
carbonaceous shales may have accumulated in raised mires or
abandoned mires built atop abandoned fluvial channels and
flood-plain deposits.
Sanctuary Formation
The Sanctuary Formation is composed mainly of 130 ft
(40 m) of gray, thinly laminated, varved mudstone and shale
that weather chocolate brown (fig. 29). Mudstones commonly
exhibit nondescript vertical animal burrows. The formation
also contains minor sandstone, siltstone, and limestone. Sand-
stones are rippled and crossbedded and occur as a coarsening-
upward sequence with the underlying siltstones. Limestones
are found as gray, micritic, lenticular beds. This formation
conformably overlies the Healy Creek Formation and was
assigned by Wolfe and Tanai (1980) to the middle Miocene.
The Sanctuary Formation is interpreted to have accumu-
lated in a large, shallow lake. The lake may have originated
as a series of flood-plain lakes, which coalesced due to rise of
base level either by damming of the streams and (or) by tec-
tonic uplift along the path of the streams downstream. Coars-
ening upward sandstones and siltstones probably represent
lacustrine deltas shed either from the nearby fluvial channels
or from fan deltas.
Suntrana Formation
The Suntrana Formation unconformably overlies the
Sanctuary Formation and is as thick as 1,310 ft (400 m)
(see fig. 21). The formation, as a whole, thickens gradually
southeastward and pinches out in the northwestern part of the
coal province. It consists of interbedded sandstones, siltstones,
mudstones, carbonaceous shales, and coal. Sandstones are
abundant, erosional based, fining upward, mainly trough
and planar crossbedded with crossbeds 3.2–6.5 ft (1–2 m) in
height, and pebbly at the base (figs. 30 and 31). They grade
either into rooted siltstone, mudstones and silty sandstones or
are locally unconformably overlain by these deposits (fig. 32).
Coal beds are interbedded with carbonaceous shales and have
a combined thickness ranging from 1.6 to 65 ft (0.5 to 20 m).
Most of the coal beds can be traced laterally over distances
of as much as 15 mi (25 km) (Wahrhaftig, 1973). Two of the
thicker beds (Nos. 3 and 4) are currently mined in the Usibelli
coal mine at Poker Flats (figs. 33 and 34). Thickness of the
No. 6 coal bed, the highest coal bed, is shown in figure 35.
The Suntrana Formation was assigned by Wolfe and Tanai
(1980) to middle Miocene.
The fining-upward, erosional-based sandstones of the
Suntrana Formation probably were deposited in braided
streams by migrating longitudinal bars and transverse side
channel bars (Stanley and others, 1992; Flores and Stanley,
1995). An upward decrease in grain size reflects decreasing
flow resulting from switching and lateral migration of the
stream channels. The erosional-based siltstones, mudstones,
Central Alaska-Nenana Coal Province 31
Figure 29. Photograph of the lacustrine mudstone and
lenticular limestone units in the Sanctuary Formation in the
Lignite Creek coalfield. Hammer on left for scale is 1 foot
(0.3 meter) long.������������������
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Figure 28. Photograph of the uppermost part of the Healy
Creek Formation, F coal bed, and overlying mudstones of
the Sanctuary Formation in the Lignite Creek coalfield.
Man for scale is 6 feet (1.8 meters) tall.
32 Alaska Coal Geology, Resources, and Coalbed Methane Potential
and silty sandstones that scoured into the fining-upward sand-
stones represent deposits in abandoned fluvial channels.
A thick coal bed commonly overlies the fining-upward sand-
stones, which reflect accumulation of peat on raised mires.
Abandoned fluvial channel deposits served as platforms on
which raised mires could be sustained for a long period of
time without drowning by detritus during floods from streams.
However, when the mires were formed in low topography,
detrital sediments flooded the mires and flood plains by cre-
vasse splays and overbank splays, as indicated by interbedded
mudstones, siltstones and silty sandstones. The thick crevasse-
overbank sequence and associated thin coal beds indicate rapid
sedimentation and local subsidence.
Lignite Creek Formation
The Lignite Creek Formation, which is from 490 to 790 ft
(150 to 240 m) thick, overlies and is conformably gradational
with the Suntrana Formation (see fig. 20). The Lignite Creek
consists of interbedded sandstones, siltstones, mudstones,
carbonaceous shales, and coals; sandstones and mudstones
are the most dominant. The sandstones are fining-upward
pebble to coarse grained in the lower part and fine grained in
the upper part. They have an erosional, pebbly base and are
trough and planar crossbedded. The fining-upward sandstone
is commonly overlain by, and gradational to, interbedded
siltstone, mudstone, and coal at the top (figs. 36 and 37). The
coal beds are thin, generally less than 3 ft (1 m) thick, woody,
and relatively lenticular and interbedded with coarsening-
upward mudstones, siltstones, and silty sandstones (fig. 38);
they pinch out northward. A noncoal-bearing conglomeratic
deposit, as much as 37 ft (11 m) thick, occurs along the north
and west margins of the Nenana coal field. Wolfe and Tanai
(1980) have assigned the Lignite Creek Formation to the late
middle to early late Miocene age.
The dominant mudstones and sandstones in the Lignite
Creek Formation reflect its deposition in a high-sinuosity or
meandering stream setting. Mudstones represent suspended
load from these meandering streams, which overtopped the
banks of the streams during floods. Continuous overtopping
of the streambanks of muds resulted in accumulation of thin
coals, mainly <3.2 ft (1 m) thick, in generally low-lying mires.
Coal beds are platy and appear to contain mats of branches
and twigs unlike the blocky appearance of the coal beds in
the Healy Creek and Suntrana Formations. The difference
in appearance is probably due to lower apparent rank of the
Lignite Creek coal.
Grubstake Formation
The stratigraphically highest formation assigned to the
Usibelli Group is the Grubstake Formation (see fig. 20; Wah-��������������������������
�������
����Trough crossbeds
Tabular crossbeds
Planar crossbeds
Ripple laminations
Root marks
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Sandstone
Mudstone and siltstone
Coal and carbonaceous shale
Conglomeratic sandstone
Clinker
Figure 30. Stratigraphic cross section of the Suntrana Formation showing the Nos. 2, 3, and 4 coal beds and interbedded fluvial-
channel sandstones west of Suntrana. See figure 19 for location of cross section. Adopted from Flores and Stanley (1995).
Central Alaska-Nenana Coal Province 33
��������
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������������Figure 31. Photograph of the Suntrana showing the thick No. 3 coal bed, clinker bed of No. 4 coal bed,
thin No. 5 coal bed, and interbedded fluvial-channel sandstones and clay plug-overbank deposits west
of Suntrana. For scale, the sandstone below No. 3 coal bed is 75 feet (22.8 meters) thick.
Figure 32. Photograph of the abandoned fluvial-channel mudstone or clay plug deposit. Note ero-
sional basal contact of the clay plug deposits west of Suntrana. Man is 6 feet (1.8 meter) tall for scale.
34 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Trough crossbeds
Tabular crossbeds
Megaforesets
Ripple laminations
Root marks
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Sandstone
Mudstone and siltstone
Coal and carbonaceous shale
EXPLANATION
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Figure 33. Stratigraphic cross section of the Nos. 3, 4, and 6 coal beds of the Suntrana Formation in the Poker Flats
strip mine of Usibelli Coal Mine. Here the Nos. 3 and 4 coal beds are mined. See figure 19 for location of cross section.
Adopted from Flores and Stanley (1995).
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Figure 34. Photograph of the Poker Flats strip mine showing the highwall exposing fluvial-channel sand-
stones and No. 3 coal bed (lower bench) and No. 4 coal bed (upper bench). For scale, the sandstone
between the Nos. 3 and 4 coal beds is 100 feet (30 meters) thick. White dashed line is the contact between
the coal and fluvial-channel sandstone.
rhaftig and others, 1994). This formation consists of dark gray
laminated shale and mudstone that is 590–980 ft (180–300
m) thick in the northeastern part of the Nenana coal province
but only 2–6 ft (0.6–1.9 m) thick in the southwestern part. In
the eastern part of the coal province, the Grubstake Formation
interfingers southward with coarse-grained, dark, unconsoli-
dated sandstones similar to those in the overlying Nenana
Gravel. A K-Ar age on rhyolitic glass from an ash layer in the
lower part of the Grubstake Formation is 8.3±0.4 Ma, which
coincides with a late Miocene age based on plant megafossils
(Wahrhaftig and others, 1969; Wolfe and Tanai, 1980; Wah-
rhaftig, 1987).
The Grubstake Formation probably accumulated in a
lake closely similar to that of the Sanctuary lake. The lake was
formed by the damming of southward-flowing Lignite Creek
paleostreams by the rising Alaska Range and may be the result
of coalesced flood-plain lakes and fluvial channels due to the
rise of base level caused by tectonic damming.
Nenana Gravel
The Nenana Gravel consists of poorly consolidated, buff
to red, pebble- to boulder-size conglomerates overlying the
Usibelli Group. It ranges in thickness from 3,940 ft (1,200 m)
at the south edge of the Nenana coal province to 980–1,310
ft (300–400 m) along the north edge of the Alaska Range
foothills. Gravel detritus was shed northward from the rising
Alaska Range that blocked the southward-flowing tributary
to the Cook Inlet-Susitna Lowland (Wahrhaftig, 1970). Its
age is bracketed between 8.3 and 2.75 Ma, so is contempo-
raneous with the Sterling Formation in the Cook Inlet area.
The Nenana Gravel is much more widely distributed than the
Usibelli Group, which is primarily confined to synclinal basins
deformed early in the orogeny that later deposited the Nenana
Gravel. Along much of its outcrop length, the formation rests
on rocks older than the Usibelli Group, and detritus from the
Usibelli Group can be recognized in the Nenana Gravel.
Coal Resource Assessment of the
Central Alaska-Nenana Coal Province
The coal resource assessments of different workers in the
Central Alaska-Nenana coal province differ in magnitude and
coal resource categories, which result in varying estimates.
We reconstructed these different coal resource estimates fol-
lowing guidelines of the coal-resource classification system
of Wood and others (1983). This new reporting system of the
coal resources of the Central Alaska-Nenana coal province,
modified from previous estimates is summarized in table 1.
Following is a historical account of the diverse coal resource
assessments in the coal province.
Coal Resource Assessment of the Central Alaska-Nenana Coal Province 35
Measured and indicated coal(<1,000 ft overburden)
Inferred coal (<1,000 ft overburden)
Measured and indicated coal(1,000 ft overburden)
Inferred coal (1,000-3,000 ft overburden)
Trace of outcrop of coal bed No.6
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Coal isopach line
EXPLANATION
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Contour interval 5 feet (1.5 meters)
(queried where uncertain)
T
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RiverCal
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aC
reek
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C r e ek
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�TatlanikaRiverFigure 35. Thickness map of the No. 6 coal bed of the Suntrana Formation. Modified from Wahrhaftig and
others (1994).
36 Alaska Coal Geology, Resources, and Coalbed Methane Potential
�������������
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Trough crossbeds
Tabular crossbeds
Planar crossbeds
Megaforesets
Convolute laminations
Ripple laminations
Root marks
Sandstone
Mudstone and siltstone
Coal and carbonaceous shale
Conglomeratic sandstone
EXPLANATION
Figure 36. Stratigraphic cross section of the lower part of the Lignite Creek Formation showing interbedded
fluvial-channel sandstones, crevasse splay flood-plain deposits, and thin coal beds west of Suntrana. See figure
19 for location of cross section. Adopted from Flores and Stanley (1995).
����
Figure 37. Photograph of the interbedded fluvial-channel sand-
stones, flood-plain deposits, and an interbedded thin coal bed
of the Lignite Creek Formation west of Suntrana. For scale, the
upper coal bed is 3 feet (0.9 meter) thick.
Figure 38. Photograph of the coarsening-upward mudstone,
siltstone, and sandstone (tabular shape) sequence overlain by thin
coal beds of the Lignite Creek Formation west of Suntrana. Ham-
mer on lower left for scale is 1 foot (0. 3 meter) long.
Coal was first mined in the Nenana coal province in 1918
when the Alaska Railroad reached the town of Lignite near
Lignite Creek. The Suntrana Mine was an underground opera-
tion in the Healy Creek coalfield from 1922 to the mid-1950ʼs.
Strip mining by Usibelli Coal Mine (UCM) in the Healy Creek
coalfield began in 1944, eventually replacing the underground
mining. Several 10- to 65-ft-thick (3–20 m) coal beds within
the Suntrana Formation and Healy Creek Formation are sepa-
rated by 32–195 ft (10–60 m) of poorly consolidated sand-
stone and are overlain at the surface by sand and gravel with
an overburden: coal ratio of < 5:1. Present technologies and
economics indicate that essentially all of the strippable coal
in the Healy Creek coalfield has been mined. Production from
the strip and underground mines in this coalfield since January
1, 1959, was 6.6 million short tons (5.9 million metric tons)
(Barnes, 1967a). Approximately 18 million short tons (16 mil-
lion metric tons) of coal has been produced from the Lignite
Creek coalfield by the UCM from 1977 to 1992. In 1985, 48
percent of this production was consumed in Alaska; the rest
was exported to South Korea (Green and Bundtzen, 1989).
Coal production has come from proven reserves in the Healy
Creek and Suntrana Formations. Lignite Creek Formation con-
tains no currently minable reserves within the mine leases.
Healy Creek and Lignite Creek coalfields contain most of
the coal resources in the Central Alaska-Nenana coal province
(table 4), with more than 5.9 × 109 short tons (5.4 × 109 metric
tons) of inferred, measured, and indicated resources (Wah-
rhaftig, 1973). Wahrhaftig and others (1994) revised this esti-
mate to 6.2 × 109 short tons (5.6 × 109 metric tons). About 5.47
billion short tons (4.9 billion metric tons) are in beds more
than 2.5 ft (0.76 m) thick (Barnes, 1967a). Coal resources are
distributed as follows: 1 billion short tons (0.91 billion metric
tons) identified and 2 billion short tons (1.8 billion metric
tons) hypothetical for Healy Creek coalfield; 4.9 billion short
tons (4.4 billion metric tons) identified and 7 billion short
tons (6.4 metric tons) hypothetical for Lignite Creek coal-
field (Stricker, 1991). Measured, indicated, and inferred coal
resources in beds more than 2.5 ft (0.76 m) thick and from
0 to 3,000 ft (915 m) below the surface, are 4.9 billion short
tons (4.4 billion metric tons) (Barnes, 1967a). About 1.36 bil-
lion short tons (1.2 billion metric tons) of total coal resources
from 0 to 3,000 ft (915 m) below the surface was estimated by
Barnes (1967a) in Healy Creek coalfield and 4.1 billions short
tons (3.7 billion metric tons) for Lignite Creek coalfield.
Summaries of the estimates of coal resources of other
coalfields such as Jarvis Creek, Wood River, Rex Creek,
Coal Resource Assessment of the Central Alaska-Nenana Coal Province 37
Table 4. Estimates of coal resources for the Tertiary Usibelli Group in the Central Alaska-Nenana coal province. [Resource estimates are in millions of short tons (multiply by 0.907 to obtain metric tons)]
Coalfield Source Classification Resource estimate
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38 Alaska Coal Geology, Resources, and Coalbed Methane Potential
and Tatlanika Creek coalfields in the Central Alaska-Nenana
coal province are in table 4. In the Jarvis Creek coalfield, the
coal-bearing rocks are as much as 1,970 ft (600 m) thick and
underlie an area of about 40 mi2 (103 km2). These coal-bear-
ing rocks were correlated with the Healy Creek Formation of
the Healy Creek coalfield by Wahrhaftig and Hickcox (1955).
Thirty thin, discontinuous coal beds are present throughout
the coal-bearing sequence, but most of these beds are less than
2.5 ft (0.75 m) thick. Wahrhaftig and Hickcox (1955) calcu-
lated 13 × 106 short tons (12 × 106 metric tons) of indicated
and inferred resources of coal in seven beds exposed along
the south and east sides of the Jarvis Creek coalfield. These
workers suggested that the coalfield might contain as much
as 62 × 106 short tons (57 × 106 metric tons) of additional coal
resources for which no outcrop evidence is available. Metz
(1981) reported that drilling had discovered about 1.1 × 106
short tons (1 × 106 metric tons) of stripping coal in part of the
Jarvis Creek coalfield. Barnes (1967a) originally estimated
indicated and inferred coal resources based on coal beds
greater than 2.5 ft (0.75 m) thick in the Jarvis Creek coalfield
to be about 51 million short tons (46 million metric tons)
between 0 and 1,000 ft (0–305 m) of overburden and about 26
million short tons (24 million metric tons) between 1,000 and
2,000 ft (305–609 m) of overburden.
A small coal strip mine was opened in the center of the
Jarvis Creek coalfield in 1958, and mining continued spo-
radically for many years; more than 1 million short tons (0.9
million metric tons) was estimated as strippable reserve in a
40-acre site (16 hectares) near the old mine by Metz (1981).
At the mine, individual coal beds range from 1 to 10 ft (0.3 to
3 m) thick, and seven beds exceed 6 ft (1.8 m), four of which
were mined at the surface (Belowich, 1987). The minable
coal beds are plotted in figure 39, which shows an 11-ft-thick
(3.4 m) coal separated by a 4-ft-thick (1.2 m) carbonaceous
shale parting that, west and east of RGD1, splits this coal into
two separate beds (B and C), each averaging 7–8 ft (2.1–2.4
m) thick (Belowich, 1987). The intervening strata are fluvial
channel sandstones and flood plain overbank deposits.
In the Wood River coalfield (fig. 19), Merritt (1987)
mapped the Usibelli Group from the Healy Creek Formation
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Figure 39. Stratigraphic cross section showing variation of minable Healy Creek coal
beds and associated sandstones, mudstones, and siltstones in the Jarvis Creek coal-
field. Modified from Belowich (1987). See figure 19 for location of cross section.
to the Grubstake Formation. However, he identified minable
coal beds only within a 600-ft-thick (183 m) coal-bearing
interval of the Suntrana Formation. The coal-bearing Usibelli
Group underlies a >40-mi2 (103-km2) area that makes up the
Wood River coalfield, where individual Suntrana coal beds
are as much as 12 ft (3.7 m) thick. The coal beds considered
as minable include an aggregate coal thickness of 50 ft (15
m) thick of which 30 ft (9.1 m) is recoverable, and overbur-
den was limited to less than 500 ft (150 m). Utilizing all coal
beds greater than or equal to 2.5 ft (0.75 m), Merritt (1987)
estimated indicated coal resources (high assurance) to be 65
million short tons (59 million metric tons) and hypothetical
coal resources (low assurance) to be as much as 200 million
short tons (181 million metric tons). Barnes (1967a) originally
estimated coal resources based on coal beds greater than 2.5
ft (0.75 m) thick in the Wood River coalfield as: 15 million
short tons (13.6 million metric short tons) of measured with
0–1,000 ft (0–305 m) of overburden; 12 million short tons (11
million metric tons) of indicated with 0–1,000 ft (0–305 m)
of overburden; and 241 million short tons (218 million metric
short tons) of inferred with 0–1,000 ft (0–305 m) of overbur-
den. Indicated coal resources under 1,000 to 3,000 ft (305–914
m) of overburden were estimated by Barnes (1967a) to be 33
million short tons (30 million metric tons).
Barnes (1967a) estimated the indicated and inferred
coal resources in the Rex Creek coalfield (see fig. 19) based
on coal beds greater than 2.5 ft (0.75 m) thick to be 9.5 and
113.5 million short tons (8.6 and 103 million metric tons)
with 0–1,000 ft (0–305 m) of overburden, respectively. Total
coal resources, based on coal beds greater than 2.5 ft (0.75
m) thick, are about 123 million short tons (111 million metric
tons).
Indicated and inferred coal resources of the Tatlanika
Creek coalfield (see fig. 19) were estimated by Barnes (1967a)
to be about 117 and 77 million short tons (106 and 70 mil-
lion metric tons) with 0–1,000 ft (0–305 m) of overburden,
respectively. Inferred coal resources from 1,000 to 2,000 ft
(305 to 610 m) of overburden are about 76 million short tons
(69 million metric tons). Total coal resources based on coal
beds greater than 2.5 ft (0.75 m) thick are about 270 million
short tons (245 million metric tons) with 0–2,000 ft (0–610 m)
of overburden.
Coal resources for the 10 coalfields of the Central
Alaska-Nenana coal province were estimated by Merritt and
Hawley (1986) to be 8 billion short tons (7.2 billion metric
tons) identified and 15 billion short tons (14 billion metric
tons) hypothetical. Barnes (1967a) estimated about 6.2 billion
short tons (5.6 billion metric tons) of identified coal resources
in this coal province (table 4).
Coal Quality
Coal in the Central Alaska-Nenana coal province ranges
from lignite to subbituminous but is mainly subbituminous
C (table 5). In the Usibelli Coal Mine the coal is subbitumi-
nous with 7,570 –9,430 Btu/lb (4,210–5,240 kcal/kg) on an
as-received basis, 17.8 percent moisture content, 3.5–13.2
percent ash yield, and 0.1–0.3 sulfur (Barnes, 1967a). The
sulfur content of the Usibelli coal ranks among the lowest of
any United States coal (Rao and Wolff, 1981; Affolter and oth-
ers, 1981). Affolter and others (1994) reported that the Usibelli
mine coal contains high concentrations of lead and selenium
and low concentrations of beryllium and mercury, all of which
are designated as hazardous air pollutants (HAPs) by the 1990
Clean Air Act Amendment.
In general, a typical coal in the Healy Creek and Lignite
Creek coalfields ranges from 6,130 to 9,210 Btu/lb (3,410 to
5,120 kcal/kg) with a mean of 7,780 Btu/lb (4,320 kcal/kg);
ash yield is from 5.2 to 34.5 percent (mean is 9.9 percent);
sulfur is from 0.1 to 1.49 percent (mean is 0.27 percent); and
moisture content ranges from 14.8 to 32.7 percent (mean is
24.7 percent) (Affolter and others, 1994).
Coal in the Jarvis Creek coalfield (see fig. 19) ranges
mainly from 7,820 to 9,420 Btu/lb (4,340 to 5,230 kcal/kg);
ash yield is from 5.2 to 13.1 percent; sulfur content is from 0.3
to 1.4 percent; and moisture content is from 20 to 23 percent
(as-received basis; Barnes, 1967a). However, coal beds in the
vicinity of the Jarvis Creek coal mine range, on an as-received
basis, from 6,550 to 10,000 Btu/lb (3,640 to 5,560 kcal/kg);
ash yield from 2.56 to 32.44 percent; sulfur content from 0.30
to 1.83 percent; and moisture content from 15.90 to 27.62
percent (Belowich, 1987). In addition, the coal beds contain
low concentrations of trace elements (for example, chromium,
beryllium, cadmiun, and cobalt) recognized as HAPs in the
1990 Clean Air Act Amendment.
In the Wood River coalfield (see fig. 19), coals range
from 7,240 to 9,380 Btu/lb (4,020 to 5,210 kcal/kg) (as-
received basis); ash yield from 1.81 to 16.31 percent (as-
received basis); sulfur content from 0.19 to 0.73 percent
(as-received basis); and moisture content from 18.03 to 27.57
percent (as received basis) (Merritt, 1987).
Coal Petrology
Petrology of the coal beds in the Jarvis Creek and Wood
River coalfields exhibits three main maceral compositions:
huminite (vitrinite), liptinite, and inertinite. At Jarvis Creek,
the huminite varies from 62.0 to 88.8 percent, liptinite from
10 to 18.5 percent, and inertinite from 2.2 to 6.5 percent
(Belowich, 1987). In contrast, in the Wood River coalfield the
huminite varies from 77.7 to 93.9 percent, liptinite from 4.7
to 20.3 percent, and inertinite from 0.2 to 9.1 percent (Merritt,
1987). The abundant woody materials preserved as huminites
in the coal beds of the Wood River coalfield suggest that the
coal formed mainly from trees. The variable inertinite compo-
sition of the coal beds in the Wood River coalfield, indicates
that the woody mires were not much affected by forest fires
but more by fluctuating ground-water levels and fungal attack
(Belowich, 1987). The higher huminite content of the coal
beds in the upper part of the coal-bearing interval in the Wood
Coal Resource Assessment of the Central Alaska-Nenana Coal Province 39
40 Alaska Coal Geology, Resources, and Coalbed Methane Potential
River suggests that the mire vegetation evolved through time
with the increase of coniferous trees relative to deciduous trees
(Merritt, 1987).
Southern Alaska-Cook Inlet Coal
Province
The Southern Alaska-Cook Inlet coal province is a large
coal-bearing region that is as much as 100 mi (161 km) wide
and 225 mi (362 km) long and covers an area about 22,500
mi2 (58,275 km2), half of which is beneath the waters of Cook
Inlet (fig. 1). Many of the Tertiary coal-bearing rocks in the
Southern Alaska-Cook Inlet Basin lie beneath the Cook Inlet,
Susitna Lowland, Broad Pass Depression, Matanuska Valley,
and Kenai Peninsula. In this coal province, Barnes (1967a)
identified four coalfields containing Tertiary coal depos-
its—the Broad Pass, Susitna-Beluga, Matanuska, and Kenai
coalfields (fig. 40). Although these Tertiary coal-bearing
coalfields occur in onshore areas bordering the Cook Inlet, this
report will also describe equivalent Tertiary coal-bearing rocks
offshore in the Cook Inlet.
The Southern Alaska-Cook Inlet coal province is centered
on the deep trough in the arc-trench gap between the Aleutian
volcanic arc and the Aleutian Trench (Fisher and Magoon,
1978). The Cook Inlet Basin, which includes the onshore
coalfields and offshore Cook Inlet, lies in the northwestern-
most part of this arc-trench gap. The basin, which contains
the Southern Alaska-Cook Inlet coal province, is a subsiding,
fore-arc basin that lies on the site of a middle Mesozoic open
shelf between a volcanic arc and an ancient Pacific oceanic
crust (fig. 41; Wahrhaftig and others, 1994). The Lower Juras-
sic Talkeetna Formation and the Middle Jurassic Talkeetna
batholith on the north of the basin represent the volcanic arc
(fig. 3). The Kenai and Chugach Mountains represent the
ancient Pacific oceanic crust south and east of the basin. Thick
Tertiary coal-bearing rocks (Paleocene to Pliocene) overlie
Table 5. Coal quality of coal deposits in the Tertiary Usibelli Group in the Central Alaska-Nenana coal province.
[All analyses except Calorific value (Btu) and Ash-fusion-temperature (°F) are in
percent. Values reported on an as-received basis. L after a value means less
than the value shown]. Data from U.S. Geological Survey, 1997, USGS Coal
Quality Database (USCHEM) (unpublished computer database: Reston,
Virginia)]
Range
Unit Parameter
Numberof
samples Minimum Maximum
Arithmetic
mean
Standarddeviation
Proximate and ultimate analysis
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Forms-of-sulfur
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a thick, Middle Jurassic to Upper Cretaceous, terrigenous,
epiclastic sequence, which accumulated on this shelf (Kirsch-
ner and Lyon, 1973; Fisher and Magoon, 1978). The McHugh
Complex and the Valdez Group, which are oceanic crust and
deep-sea turbidite sequences, were accreted to southern Alaska
during Late Cretaceous time to form the Chugach and Kenai
Mountains. This accreted terrane widened the arc-trench gap,
which is now about 280 mi (450 km) wide. Irregular subsid-
ence of the fore-arc basin began in latest Cretaceous time
and continued sporadically throughout Cenozoic time. Basin
subsidence, which was interrupted by mild uplift and erosion,
was greatest during Neogene time in a 155-mi-long (250-km)
segment of Cook Inlet in much of central and southern Alaska.
Swenson (1997) proposed, based on studies by Richard
Curry, David Doherty, and Joseph McGowen (Atlantic-Rich-
field Company, oral commun., 1998), that the Hemlock, Tyo-
nek, Beluga, and Sterling Formations of the Kenai Group and
the West Foreland Formation are regionally time-transgressive
units (fig. 42). In addition, these workers suggested that the
rock units are laterally equivalent facies related to a dynamic
nonmarine depositional basin. That is, the coarsest facies (con-
glomerates and sandstones) were deposited proximal to the
source by an alluvial fan system, which transported sediments
from the uplifted Aleutian volcanic arc and accretionary com-
plex margins (Joseph McGowen, Atlantic-Richfield Company,
Southern Alaska-Cook Inlet Coal Province 41
Figure 40. Map showing coalfields in the Southern Alaska-Cook Inlet coal province.
42 Alaska Coal Geology, Resources, and Coalbed Methane Potential
oral commun., 1998). The distal depositional system in the
basin center consisted of an axial-fluvial system that reworked
the alluvial fan deposits and migrated across the basin floor
in relation to uplift and sediment input (fig. 43). Mires, where
plant material accumulated, developed within the axial fluvial
system.
The Tertiary coal-bearing rocks in the Southern Alaska-
Cook Inlet coal province accumulated in the subsiding Cook
Inlet Basin, which was probably drained by a large, fluvial,
trunk-tributary and alluvial fan system that flowed into the
Pacific (Kirschner, 1988). Alluvial fans drained the basin
margins, and the trunk (axial) stream drained a broad alluvial
plain now occupied by the Cook Inlet. Two major tributary
streams of the trunk river extended northward through the
present Susitna Lowland and Broad Pass Depression and east-
ward through the present Matanuska Valley. A Susitna-Broad
Pass tributary stream probably extended along the north side
of the Alaska Range and drained the Central Alaska-Nenana
coal province (Flores and Stanley, 1995). The Yukon-Tanana
Upland may have been in headwaters of this tributary stream.
Thus, all the coal deposits in the Central Alaska-Nenana and
Southern Alaska-Cook Inlet coal provinces are thought to have
accumulated in mires related to this large, integrated fluvial
drainage system.
Tertiary Rocks
The bulk of the coal in the Southern Alaska-Cook Inlet
coal province is of Oligocene to early Pliocene age (fig. 44).
These late Tertiary coals are distributed in the Susitna-Beluga,
Broad Pass, and Kenai coalfields. However, early Tertiary
(Paleocene and early Eocene) coal occurs in the Matanuska
coalfield.
Lower Tertiary Rocks
The lower Tertiary rocks include the Paleocene-Eocene
Chickaloon Formation and Eocene Wishbone Formation (fig.
45). The Chickaloon Formation is a 3,280- to 4,920-ft-thick
(1,000–1,500 m) Paleocene to lower Eocene sequence of
mudstones, siltstones, and sandstones, with minor conglomer-
ates and coal beds (figs. 46, 47, and 48; Triplehorn and others,
1984; Flores and Stricker, 1993a). The formation rests uncon-
formably on the Cretaceous Matanuska Formation, which is
a sequence of marine sandstone and shale (Barnes and Payne,
1956; Grantz and Jones, 1960) and is overlain unconformably
by the Eocene Wishbone Formation. The Wishbone consists of
2,950 ft (900 m) of thick, massive conglomerates and sand-
stones containing clasts derived from the Talkeetna Mountains
to the north (figs. 49 and 50). The formation at the east end
of the Matanuska coalfield is unconformably overlain by flat-
lying Tertiary basalt. Gabbro sills and dikes and other Tertiary
volcanic rocks also intrude the coal-bearing Chickaloon For-
mation and increase the coal rank along the intrusive contact.
The Wishbone Formation is equivalent to the coal-bearing
West Foreland Formation in the south-southwest part of the
Cook Inlet Basin. The West Foreland Formation consists of
abundant conglomerates and sandstones and minor siltstones,
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Figure 41. Tectonic and volcanic settings of the Cook Inlet Basin. Modified from McGowen and others in Swenson (1997).
Southern Alaska-Cook Inlet Coal Province 43
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others in Swenson (1997).
44 Alaska Coal Geology, Resources, and Coalbed Methane Potential
mudstones, and coal beds (Wahrhaftig and others, 1994).
Houston (1994) reported a few coal beds as thick as 1 ft (30
cm) in the 4,100-ft-thick (1,259 m) West Foreland Formation
in the Cape Douglas-Katmai National Park area west of the
Shelikof Strait and southeast of the Cook Inlet (see fig. 40).
Flores and Stricker (1993a, 1993d) described and inter-
preted the depositional environments of the Chickaloon and
Wishbone Formations. Stratigraphic variations of the Chick-
aloon sandstones, mudstones, and coal beds in the Wishbone
Hill coal district are shown in figures 51, 52, 53, 54, 55, and
56. Sandstones are erosional based and range from lenticular
(fig. 47) to tabular shape (fig. 48). Lenticular-shaped sand-
stones were deposited in fluvial channels and the tabular-
shaped sandstones were deposited in crevasse splays. Coal
beds (fig. 46, a photograph of the upper part of the Chickaloon
Formation in the Wishbone Hill coal district) of the Chick-
Cook Inlet Depositional Systems
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Figure 43. Depositional model of the Kenai Group in the Cook Inlet Basin. Modified from McGowen and others in Swenson (1997).
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Figure 44. Generalized chronostratigraphic column of
the coal-bearing Kenai Group and related rock units
in the Southern Alaska-Cook Inlet coal province.
aloon Formation were deposited in topogenous or low-lying
mires associated with low-gradient bedload meandering (fig.
57) and anastomosed (fig. 57) fluvial systems. The low-lying
mires formed on abandoned belts of meandering streams dur-
ing lateral aggradation influenced by autocyclic processes.
In contrast, low-lying mires related to anastomosed streams
developed during vertical aggradation controlled by basin sub-
sidence. Growth faulting promoted prolonged peat accumula-
tion in mires on upthrown blocks and caused stream capture
on downthrown blocks. The Wishbone Formation was depos-
ited in alluvial fans and braided stream deposits that were shed
from the Talkeetna Mountains (see fig. 57; Flores and Stricker,
1993d). The West Foreland Formation, an equivalent of the
Wishbone Formation, was interpreted by Houston (1994) as
being deposited in braided streams and associated flood plains.
The sediments were derived mainly from the Alaska-Aleutian
volcanic arc terrane. The West Foreland coal beds were depos-
ited in abandoned braid belts and flood plains.
Upper Tertiary Kenai Group
The upper Tertiary rocks in the Southern Alaska-Cook
Inlet coal province include the Kenai Group consisting, from
bottom to top, of the Oligocene Hemlock Conglomerate,
Oligocene to middle Miocene Tyonek, upper Miocene Beluga,
and upper Miocene to Pliocene Sterling Formations (Flores
and Stricker, 1993b, 1993c; Flores, Stricker, and Bader, 1997;
Flores, Stricker, and Stiles, 1997; Flores and others, 1999).
The Kenai Group is more than 25,000 ft (7,620 m) thick. All
these formations are coal bearing, with the Tyonek and Beluga
Formations containing numerous thick, minable coal beds.
In the offshore and onshore Cook Inlet Basin (fig. 58) the
Hemlock, Tyonek, Beluga, and Sterling Formations vary in
thickness and lithostratigraphy (figs. 59, 60, and 61). Gener-
ally, the formations, especially the Tyonek and Beluga, thicken
toward the central part of the basin. The Tyonek Formation is
generally sandstone dominated toward the western part of the
basin, the Beluga Formation is generally sandstone dominated
toward the eastern part, and the Sterling Formation appears to
be sandy in the central and eastern parts.
Southern Alaska-Cook Inlet Coal Province 45
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Figure 45. A generalized
stratigraphic column
of the Chickaloon and
Wishbone Formations in
the Matanuska coalfield.
Minable coal zones
occur in the uppermost
part of the Chickaloon
Formation. Modified
from Flores and Stricker
(1993a).
46 Alaska Coal Geology, Resources, and Coalbed Methane Potential
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Figure 46. Photograph of coal beds of the Chickaloon Formation in the Wishbone Hill coal district. For scale,
the Jonesville coal zone is 20 feet (6.1 meters) thick.
Figure 47. Photograph of the lenticular fluvial-channel sandstone (20 feet or 6.1 meters thick) and associ-
ated rocks of the Chickaloon Formation in the Wishbone Hill coal district.
Southern Alaska-Cook Inlet Coal Province 47
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Figure 48. Photograph of the tabular crevasse splay sandstone and associated flood-plain deposits of the
Chickaloon Formation in the Wishbone Hill coal district. Hammer in lower left is 1 foot (0.3 meter) long for
scale.
Figure 49. Vertical and lateral lithofacies variations of the Wishbone Formation in the Wishbone
Hill coal district.
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EXPLANATION
48 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Figure 50. Photograph of the braided-stream-deposited conglomerates and sandstones in the Wish-
bone Formation in the Wishbone Hill coal district. Hammer is 1 foot (0.3 meter) long for scale.
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EXPLANATION
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Covered interval
Normal fault
Covered
Covered
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Palmer
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EXPLANATION
CANADACANADA
UNITED
STATES
ALASKA
Area of map CANADAFigure 51. Stratigraphic cross section of the lower part of the Chickaloon Formation in the Wishbone Hill coal
district. Adopted from Flores and Stricker (1993d). See inset map for location of cross section.
Southern Alaska-Cook Inlet Coal Province 49
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Figure 52. Photograph of the lower part of the Chickaloon Formation showing the Midway coal zone
and adjoining fluvial-channel sandstones in the Wishbone Hill coal district. Jacob staff on the sand-
stone (see right side) is 5 feet (1.5 meters) for scale.
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Sandstone Channel and crevassesplay
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EXPLANATION
�Covered interval
Tree trunks
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M ooseCreekEskaCreekGraniteCreekEXPLANATION
CANADACANADA
UNITED
STATES
ALASKA
Area of map CANADAFigure 53. Stratigraphic cross section of the middle part of the Chickaloon Formation in the Wishbone Hill coal
district. Adopted from Flores and Stricker (1993b). See inset map for location of cross section.
50 Alaska Coal Geology, Resources, and Coalbed Methane Potential
���������������������������������Figure 54. Photograph
of the upper part of the
Chickaloon Formation
showing the Premier
coal zone (50 feet or
15.2 meters thick),
Jonesville coal zone (30
feet or 9.1 meters thick)
and associated fine-
grained sediments in
the Wishbone Hill coal
district.
CANADACANADA
UNITED
STATES
ALASKA
Area of map CANADAConglomerate
Sandstone
Mudstone,siltstone,
and silty sandstone
Coal,carbonaceous shale,
and coal and carbonaceous shale
}
}
}Mire
Channel and channel lag
Overbank and
channel plug
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Trough crossbeds
Point bar surface
Rippled laminations
Foreset ������������������Rock Interpretation
EXPLANATION
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M ooseCreekEskaCreekGraniteCreek�����������������������������
���Matanuska Formation
���Wishbone Formation���Chickaloon Formation
���Tsadaka Formation
EXPLANATION
Figure 55.
Stratigraphic
cross section of
the upper part of
the Chickaloon
Formation in
the Wishbone
Hill coal dis-
trict. Adopted
from Flores and
Stricker (1993a).
See inset map for
location of cross
section.
Southern Alaska-Cook Inlet Coal Province 51
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������Normal fault;U=up,D=down
Mires
Sandbars
Alluvial fans
U D
Direction of lateral movement along fault
EXPLANATION
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Figure 56. Photograph of the upper part of the Chickaloon Formation showing the Jonesville coal zone
overlain by fluvial-channel sandstones (>50 feet or >15.2 meters thick) of the Wishbone Formation in
the Wishbone Hill coal district.
Figure 57. Paleogeographic map showing depositional environments of the Chickaloon
Formation in the Matanuska coalfield. Modified from Flores and Stricker (1993a).
52 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Hemlock Conglomerate
The Hemlock Conglomerate is unconformable, grada-
tional, and interfingering with the West Foreland Formation
(fig. 42). It consists mainly of pebble to boulder conglomer-
ates containing quartz, chert, metamorphic, volcanic, and
plutonic rock fragments. Minor conglomeratic sandstones are
interbedded, which are arkosic in composition with sparse
heavy minerals, predominantly epidote and garnet (Calder-
wood and Fackler, 1972; Magoon and Egbert, 1986). How-
ever, the formation contains a few thin coal beds and many
siltstone beds and is the main producing horizon for oil in
the offshore Cook Inlet (Magoon and Anders, 1990). Detrital
rocks are interpreted as deltaic and lacustrine deposits, and
apparently the sediments were derived from the north. Most
coals formed in interdistributary low-lying mires. Together
with the Bell Island Sandstone and the Tsadaka Formation,
temporal equivalents at the east end of the Cook Inlet Basin,
the Hemlock Conglomerate forms a variable sheet deposit 655
ft (200 m) thick, with a maximum thickness of about 2,772 ft
(845 m). The formation is Oligocene in age (Wolfe and Tanai,
1980; Magoon and Egbert, 1986).
In the Cape Douglas-Katmai National Park area west of
the Shelikof Strait (fig. 40) and southeast of the Cook Inlet,
Houston (1994) described the 2,772-ft-thick (845 m) Hemlock
Conglomerate as consisting of conglomerates and sandstones
Figure 58. Map showing lines
of stratigraphic cross sec-
tions (see figs. 59–61) of the
Kenai Group in the offshore
and onshore Cook Inlet Basin.
Map also shows areas of cross
sections (see figs. 72–73, 75,
85–86, 90–91) of the Kenai
Group in the Chuitna area,
Capps Glacier area, along the
west coast of Kenai Peninsula,
and along the north coast of
Kachemak Bay.
Southern Alaska-Cook Inlet Coal Province 53
���������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������EXPLANATIONSterlingFormationBelugaFormationTyonekFormationHemlockConglomeratePre-HemlockConglomerateFigure 59. Offshore north-south cross section (A–A’) of the Kenai Group along the axis of the Cook Inlet Basin. Modified from Alaska Geological Society (1969a). See figure 58 for location of cross section.
54 Alaska Coal Geology, Resources, and Coalbed Methane Potential
deposited in meandering streams (figs. 62 and 63). Minor
siltstones and mudstones were interpreted to be deposited
in overbank and flood-plain environments (figs. 64 and 65).
Sparse coal and carbonaceous shale beds, which vary from
2 inches to 2.5 ft (5 cm to 0.75 m) thick, were interpreted to
have accumulated in mires developed on abandoned flood
plains and meander belts. These streams derived sediments
from the Alaska-Aleutian volcanic arc terrane.
Hite (1976) interpreted the sandstones of the Hemlock
Conglomerate distributed along the central part of the Cook
Inlet Basin (figs. 59 and 60) as being deposited in a marine-
influenced environment. Based on size analysis and verti-
cal variability mapping, 18 percent of the analyzed samples
showed evidence of tidal transport by bidirectional currents.
Twenty-five percent of the analyzed samples indicate trans-
port by turbidity or density suspension currents. Hite (1976)
interpreted the coal beds to have accumulated in coastal mires
and mapped them as two bands (fig. 66) parallel to the basin
margins. Also, Hite (1976) suggested that the paleogeographic
setting of the basin during deposition of the Hemlock Con-
glomerate was very similar to the modern Cook Inlet, which
is composed of coastal plains influenced by tidal estuaries,
flats, and marshes. Although the present report agrees with this
scenario, the elongate shape and coastal-parallel (northeast-
southwest orientation) nature of the Hemlock sandstones in the
central part of the Cook Inlet Basin, which were interpreted
by Hite (1976) as tidal channel and turbidity deposits, are here
reinterpreted as tidal sand-flat deposits. These tidal sand flats
were probably derived by reworking of deltaic sediments of
the streams that deposited the Hemlock Formation in the Cape
Douglas-Katmai National Park area southeast of the basin.
Tyonek Formation
The Tyonek Formation (Wolfe and Tanai, 1980) consists
of a sequence of sandstones, siltstones, mudstones, carbo-
naceous shales, and coal beds as much as 7,640 ft (2,330 m)
thick (fig. 67; Calderwood and Fackler, 1972). Sandstones, the
most common rock type, are basally erosional, crossbedded,
thick, and vertically stacked (fig. 68). Individual coal beds
are as much as 33 ft (10 m) thick. A sandstone:mudstone ratio
map of the formation by Hartman and others (1971) shows
more than 50 percent sandstone in an area along the west side
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Sterling Formation
Beluga Formation
Tyonek Formation
Hemlock Conglomerate
Pre-Hemlock
EXPLANATION
Figure 60. Offshore (west) to onshore (east) cross section (B–B’) of the Kenai Group across the Cook Inlet Basin. Modified
from Alaska Geological Society (1969b). See figure 58 for location of cross section.
Southern Alaska-Cook Inlet Coal Province 55
�������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������SterlingFormationBelugaFormationTyonekFormationHemlockConglomeratePre-HemlockPost-SterlingEXPLANATIONFigure 61. Onshore north-south cross section (C–C’) of the Kenai Group along the western part of the Kenai Peninsula or eastern margin of the Cook Inlet Basin. Modified from Alaska Geological Society (1969c). See figure 58 for location of cross section.CC'
56 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Figure 62. Photograph of conglomerates in the Hemlock Conglomerate in the Katmai National Park.
Jacob’s Staff is 5 feet (1.5 meters) long for scale. Photograph courtesy of Frank Ethridge.
Figure 63. Photograph of sandstones in the Hemlock Conglomerate in the Katmai National Park. Man is 5.5
feet (1.7 meters) tall for scale. Photograph courtesy of Frank Ethridge.
Southern Alaska-Cook Inlet Coal Province 57
Figure 64. Photograph of thin coal and carbonaceous shale beds in the Hemlock Conglomerate in the Kat-
mai National Park. Man is 6 feet (1.8 meters) tall for scale. Photograph courtesy of Frank Ethridge.
Figure 65. Photograph of braided stream deposits (conglomeratic lower part) in the Hemlock Conglomer-
ate. The upper part contains fluvial-channel sandstones and carbonaceous siltstones. Jacob’s staff in the
lower part of the photograph is 5 feet (1.5 meters) long for scale. Photograph courtesy of Frank Ethridge.
58 Alaska Coal Geology, Resources, and Coalbed Methane Potential
of the Cook Inlet Basin and in the extreme northeastern part
of the basin. The sites with higher sandstone content were
interpreted as loci of sediment input. An extensive mudstone
band containing as little as 10 percent sandstone extends
south-southwestward along the western part of the Kenai Pen-
insula. Sandstone content increases southeastward of this band
of maximum mudstone content. This site of extensive belt of
mud represents the distal point of sandstone input.
Hite (1976) presented an isolith sand map (net sandstone
thickness isopach map) of the Tyonek in the Cook Inlet Basin
(see fig. 69) that shows the high concentration of sandstone
along the western part of the basin and decreasing toward the
eastern part. In addition, on the west side, the contour lines
define a lobate concentration of sandstone, which flares as
elongate bodies to the northeast and southeast of the basin. We
interpret these sandstone concentrations as alluvial-fan delta
and tidal-flat deposits (fig. 70).
The alluvial-fan delta and tidal-flat lithofacies of the
Tyonek Formation were studied by Flores and others (1994)
and Flores, Stricker, and Stiles (1997) whose descriptions of
the vertical and lateral variations of these facies in the Chuitna
River drainage basin, about 10 miles (16 km) northeast of
Tyonek, are depicted graphically in figures 71, 72, and 73.
The tidal sand flat and intertidal lithofacies as described by
Flores and others (1999) 2.6 mi (4.2 km) northwest of Wasilla
are shown in figure 74. Sediments of the tidal sand flats may
have been sourced from the alluvial-fan delta sediments and
reworked by tidal currents. Streams from adjacent areas to the
north and east of the Cook Inlet Basin may also have contrib-
uted minor amounts of sediment.
The net total thickness of coal beds penetrated in wells in
the Tyonek Formation increases toward the northwestern part
of Cook Inlet Basin (see fig. 66; Hite, 1976; Wahrhaftig and
others, 1994). The beds are concentrated along the northwest
margin of the basin, from Kalgin Island northeastward along
the west shore of the inlet to the Susitna River. In that area,
the coal isopach map displays lobe shape with fingerlike
extensions oriented to the east and southeast (see fig. 66).
Lobe-shaped coal concentrations, which also correspond to
the sites of lobate concentrations of sandstones, are interpreted
as having been deposited in mires associated with alluvial-fan
deltas. At those sites, abandoned alluvial-ridge braid belts of
the fan deltas served as raised platforms where mires devel-
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Kenai
Tyonek
Anchorage
K nikA rm
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CookInletContour Interval =200 Feet
UNITED
STATES
ALASKA
Area of map CANADA
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EXPLANATION
Isopach Line
Figure 66. Net coal thickness isopach map of the
Tyonek Formation in the Cook Inlet Basin. Modified from
Hite (1976) and Wahrhaftig and others (1994).
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Figure 67. Photograph of coal beds and interbedded
fluvial-channel sandstones and mudstones in the Tyonek
Formation in the Chuitna River drainage basin. The
sandstone in the upper part of the outcrop is 75 feet (22.8
meters) thick for scale.
Figure 68. Photograph of fluvial-channel sandstones and
Chuitna coal bed in the Tyonek Formation in the Chuitna
River drainage basin. The sandstone in the lower part of
the outcrop is 50 feet (15.2 meters) thick for scale.
oped as much as 28 ft (8.5 m) of minable coal (see figs. 72 and
73) as described at the Diamond Chuitna coal-mine lease area
by Flores and others (1994) and Flores, Stricker, and Stiles
(1997). However, the coal beds are associated with intertidal
sediments indicating development in supratidal mires (figs.
75, 76, and 77). The fingerlike pattern of the net coal thickness
isopach map (see fig. 66) indicates tidal influence much like
the mires along the coast in west-central Sumatra, Indonesia
(Flores and Moore, in press).
The total coal isopach map (fig. 66) shows thinning to
the northeast, southeast, and southwest toward the zone of
minimum sandstone content (Hartman and others, 1971; Hite,
1976). The southwest-northeast orientation of the net coal
thickness isopach, a trend that is perpendicular to the south-
east-oriented deltaic wedges, indicates accumulation of coal
in low-lying tidal sand flat and supratidal mires. Tidal influ-
ence in the Tyonek Formation in the Barabara Point, south-
west of Kachemak Bay (see fig. 40) in the eastern Cook Inlet
was described by Stricker and Flores (1996). However, the
tidal deposits overlie a sequence of conglomerate, sandstone,
siltstone, and mudstone, with coals a few inches to 2 ft (few
centimeters to 0.6 m) thick (fig. 77). Mudstones and siltstones
are the most abundant lithologies, which are interpreted to
be derived from the Chugach metamorphic rock complex.
Stricker and Flores (1996) and Myers and others (1998)
interpreted this sequence as an alluvial fan drained mainly by
anastomosed streams.
The thick Capps Glacier coal bed (figs. 78, 79, and 80)
and coal beds along the Beluga and Chuitna Rivers (Barnes,
1966; Adkison and others, 1975; Myers and others, 1998) are
in the Tyonek Formation (Magoon and others, 1976). Within a
single coalfield, correlation of the coal beds from well to well
has proved difficult for distances of more than a mile (few
kilometers), which indicates considerable lenticularity of the
coal seams and the intervening sedimentary rocks. The rapid
changes in the lateral and vertical stratigraphy of the coal beds
and intervening rocks are shown in figures 81 and 82. In both
cross sections, the Capps Glacier coal bed is traceable for a
mile or two; however, the coal bed splits and merges laterally
as influenced by the thinning, pinching out, and thickening of
the intervening fluvial channel sandstones. The pattern of the
Southern Alaska-Cook Inlet Coal Province 59
EXPLANATION
Isopach line
S
usitna
Riv
e
r
CookInletTurnagain
Arm
S u s i t n a
F l a t s
Harriet
Point
West Foreland
East Foreland
K a c h e m a k BayKalgin
Island
Contour interval 100 feet
Anchorage
Kenai KnikA rm
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UNITED
STATES
ALASKA
Area of map
CANADA
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Figure 69. Net sandstone thickness isopach map of the
Tyonek Formation in the Cook Inlet Basin. Modified from
Hite (1976).
Tidal flats and sandbars
Alluvial fan conglomerates
Tidal swamps
Mudstone and siltstone
Tidal channels
EXPLANATIONAncestralCookInlet��������������������������������������
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�������������������������������Figure 70. Paleogeographic map
(block diagram) showing depositional
environments of the Tyonek Forma-
tion in the Cook Inlet Basin. Modified
from Hite (1976).
60 Alaska Coal Geology, Resources, and Coalbed Methane Potential
coal bed wrapping around the sandstones is enhanced by dif-
ferential compaction. That is, the sandstones are less compact-
ible than adjacent mudstones. In outcrop, however, individual
coal beds have been traced for as much as 6.2 mi (10 km)
(Barnes, 1966; Ramsey, 1981).
Nearly flat-lying outliers of the Tyonek Formation along
the southeast shore of Kachemak Bay near Seldovia Bay and
at Port Graham (see fig. 40) rest unconformably on metamor-
phic rocks of Triassic and Jurassic age and appear in part to
fill steep-sided valleys and in part to be downfaulted (Stone,
1906; Martin and others, 1915; Magoon and others, 1976).
The occurrence on the northeast side of the entrance to Port
Graham was the site of the plant fossils on which Oswald Heer
in 1869 (Wahrhaftig and others, 1994) established the “Arctic
Miocene” flora of Alaska (see Stone, 1906), and the locality
on which the name “Kenai Formation” (now Kenai Group)
was established by Dall (1896). Portlock first reported coal
there in 1786 (see Stone, 1906, p. 54). Coal (chiefly lignite)
was mined at this site by the Russians from 1855 to 1867, but
it could not be produced at a profit, and operations ceased
when Alaska was sold to the United States in 1867 (Stone,
1906).
Beluga Formation
The Beluga Formation (fig. 42) is as much as 4,900 ft
(1,500 m) thick, is composed of interbedded conglomerates,
sandstones, siltstones, mudstones, carbonaceous shales, and
coal beds (see fig. 44). The sandstones are the most abundant
rock type and coal beds are the least common. The sandstones
are erosional based, as much as 50 ft (15 m) thick, cross-
bedded, and vertically stacked (figs. 83 and 84; Flores and
Stricker, 1993b). Stratigraphic variations of the sandstones and
coal beds are shown in figures 85 and 86. These lithic units are
drab-gray in color. In the outcrop, the color of the formation
is used to distinguish it from the overlying buff to light-brown
Sterling Formation (Barnes and Cobb, 1959; Wolfe and others,
1966; Merritt and others, 1987). Abundant heavy minerals
(Kirschner and Lyon, 1973) and metamorphic rock fragments
in the locally pebbly sandstones led Hayes and others (1976)
to interpret the Beluga Formation to be derived mainly from
the Kenai and Chugach Mountains. Paleocurrent analysis of
crossbeds of the sandstones by Rawlinson (1984) and Kremer
and Stadnicky (1985) indicate a westerly transport direction of
its sediments. The Beluga Formation is well exposed in beach
bluffs along the northwest side of Kachemak Bay and the
south-southwest end of the Kenai Peninsula between Homer
and Anchor River (Magoon and others, 1976; Merritt and oth-
ers, 1987). There it contains numerous coal beds with indi-
vidual beds as thick as 6.6 ft (2 m) (Barnes and Cobb, 1959).
The formation was dated as middle and late Miocene (Wolfe
and Tanai, 1980).
Environments of deposition of the Beluga Formation are
interpreted to be braided and meandering streams and alluvial
fans (fig. 87; Hayes and others, 1976; Hite, 1976; Rawlinson,
1984; Merritt, 1986). Recent studies by Flores and Stricker
(1992, 1993b) indicate deposition in an alluvial plain drained
by alternating braided streams and crevasse splay-anasto-
mosed streams. Through time the braided streams evolved
into the crevasse splay-anastomosed streams, which, in turn,
evolved into the braided streams. Regional and basin subsid-
ence and autocyclic avulsion processes caused these alterna-
tions through time. Flores and Stricker (1993b) suggested that
the coal beds accumulated in mires on abandoned braid belts
and anastomosed stream belts. However, the coal beds formed
in mires on braid-belt deposits were thicker and more exten-
sive than coal formed in mires developed on crevasse-splay-
anastomosed deposits.
Hayes and others (1976) suggested that the Beluga For-
mation was deposited by meandering streams along the length
of the western part of the Cook Inlet Basin. These stream
deposits, which are the chronostratigraphically equivalent of
the Lignite Creek Formation (Central Alaska-Nenana coal
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Figure 71. Generalized stratigraphic column of minable
coal beds in the Tyonek Formation in the Chuitna River
drainage basin and adjoining areas. Modified from Flores
and others (1994) and Flores, Stricker, and Bader (1997).
Southern Alaska-Cook Inlet Coal Province 61
Conglomerate
Sandstone
Siltstone and mudstone
Coal and carbonaceous shale
Clinker (thermally alteredoverburdenbyin-placeburningofcoal)
EXPLANATION
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Coal and carbonaceous shale
Conglomeratic sandstone
Mudstone,siltstone,and sandstone
Mudstone and siltstone
Fluvial sandstone
EXPLANATION
Figure 73. Stratigraphic cross section of the coal beds, fluvial-channel sandstones and intertidal deposits in the
Diamond Chuitna lease area east of the Chuitna River drainage basin. See figure 58 for location of the cross section.
Adopted by Flores, Stricker, and Stiles (1997).
Figure 72. Three-dimensional cross sections (fence diagram) of the Chuitna coal bed and interbedded
erosional-based sandstones deposited by braided streams of the Tyonek Formation in the Chuitna River
drainage basin. Adopted from Flores and others (1994). See figure 58 for location of the cross section.
62 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Figure 74. A, Stratigraphic lithofacies sequence in the Tyonek Formation showing tidal sandstone flats facies near
Wasilla. Adopted from Flores and others (1999). B, Explanations of sedimentary structures for figures 74A and 77.
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EXPLANATION
Very fine to fine-grained sandstone (vf -fss)
Medium-grained sandstone (med ss)
Coarse-grained sandstone
Mudstone and siltstone
Coal and carbonaceous shale
Lenticular beds
Flaserlike beds
U-shaped burrows
Root marks
Thalassinoides-like burrows
Synaeresis cracks
Southern Alaska-Cook Inlet Coal Province 63
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Coal and carbonaceous shale
Sandstone
Mudstone,siltstone,and sandstone
Mudstone and siltstone
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EXPLANATION
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Figure 75. Stratigraphic
cross section of the coal
beds and fluvial-channel
sandstones in the Dia-
mond Chuitna lease area
east of the Chuitna River
drainage area. Adopted
from Flores, Stricker, and
Bader (1997). See figure
58 for location of the
cross section.
Figure 76. Verti-
cal lithofacies
and associated
geophysical logs
of minable coal
beds (Reds 1, 2,
and 3, and Blue)
and interbedded
fluvial-chan-
nel sandstones,
and flood plain
claystones and
siltstones in the
Diamond Chuitna
lease area east of
the Chuitna River
drainage area.
Adopted from
Flores, Stricker,
and Stiles (1997).
64 Alaska Coal Geology, Resources, and Coalbed Methane Potential
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Coal
EXPLANATION
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EXPLANATION
Figure 78. Photograph of the Capps Glacier coal bed (50
feet or 15.2 meters thick) and overlying fluvial-channel
sandstones in the Capps Glacier area.
Figure 79. Photograph of the coal beds and interbedded
flood-plain/crevasse splay deposits in the Capps Glacier
area. Man for scale is about 5.9 feet (1.8 meters) tall.
Figure 77. Vertical lithofacies of coal beds (Reds 1, 2, and 3)
and interbedded tidal and intertidal sandstones, siltstones,
and mudstones in the Diamond Chuitna lease area east of the
Chuitna River drainage area. Adopted from Flores, Stricker,
and Stiles (1997). See figure 74B for explanation of symbols for
sedimentary structures.
Figure 80. Structural
cross section (north-
south) of the Capps
Glacier coal bed and
associated rocks of the
Tyonek Formation in the
Capps Glacier area. See
figure 58 for location of
the cross section.
province; Wolfe and Tanai, 1980), have their watershed in
the central Alaska interior. The meandering streams were fed
by transverse alluvial fans that were better developed along
the eastern part of the basin than along the western part. The
ancestral Chugach-Kenai uplift was uplifted more than the
ancestral Aleutian-Alaska Arc Range (see fig. 87).
Sterling Formation
The Sterling Formation is as much as 10,990 ft (3,350 m)
thick and consists of sandstones, conglomeratic sandstones,
siltstones, mudstones, carbonaceous shales, and coal beds (see
fig. 43; Kirschner and Lyon, 1973; Hayes and others, 1976;
Hite, 1976); Hartman and others, 1971; Calderwood and Fack-
ler, 1972). The sandstones are as much as 200 ft (61 m) thick,
fining upward, basally scoured, and multistory (figs. 88 and
89); their vertical and lateral variations are exhibited in figures
90 and 91. Tonsteins or volcanic ash units are commonly
interbedded with the coal beds. Hornblende and volcanogenic
hypersthene are abundant in the sandstones. These heavy min-
erals indicate that the Sterling Formation was probably derived
from the ancestral Aleutian-Alaska Arc Range to the west
(see fig. 58). Coal beds are generally no more than 3 ft (1 m)
thick, but a few are as thick as 8 ft (2.5 m) (fig. 92; Barnes and
Cobb, 1959; Calderwood and Fackler, 1972). Coal is lignitic
throughout much of the formation but is high-volatile subbi-
Southern Alaska-Cook Inlet Coal Province 65
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EXPLANATION
BARABARA POINT
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Sandstone
Claystone
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EXPLANATION������������������������������������������������������
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Figure 81. Stratigraphic cross section (northeast-southwest) of the rocks of the Tyonek Formation at Barabara Point showing len-
ticular conglomerates, sandstone, and coal beds. The sandstone and conglomerate in the southwestern part of the cross section
represent paleovalley deposits incised into basement rocks. See figure 58 for location of the cross section.
Figure 82. Stratigraphic (structural) cross section of the Capps Glacier coal bed and associated rocks of the Tyonek
Formation in the Capps Glacier area. See figure 58 for location of the cross section.
66 Alaska Coal Geology, Resources, and Coalbed Methane Potential
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Figure 83. Photograph of the fluvial-channel sandstones (average 60 feet or 18.3 meters thick), flood plain mudstone
and siltstones, and coal beds of the Beluga Formation along the coastal bluffs in west Homer, Kenai Peninsula.
Figure 84. Photograph of a coal bed (3.5 feet or 1.1 meters thick) and crevasse splay deposits of the Beluga For-
mation along the coastal bluffs west of Homer, Kenai Peninsula.
Southern Alaska-Cook Inlet Coal Province 67
Sandstone
Mudstone and siltstone
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Coal and carbonaceous shale
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EXPLANATION
Sandstone (fluvial channel)
Sandstone (interfluve crevasse splay)
Siltstone and mudstone (interfluve)
Coal and carbonaceous shale (mire)
Measured stratigraphic section
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Figure 85. Stratigraphic cross section of the Beluga Formation showing thick coal beds (for example, Cooper coal bed),
fluvial-channel sandstones, and flood-plain mudstone and siltstone along the coastal bluffs west of Homer, Kenai Peninsula.
Adopted from Flores and Stricker (1993b). See figure 58 for location of the cross section.
Figure 86. Stratigraphic cross section of the Beluga Formation showing interbedded thin to thick coal beds (for
example, Cooper coal bed), fluvial-channel sandstones, and flood-plain deposits along the coastal bluffs west of
Homer, Kenai Peninsula. Adopted from Flores and Stricker (1993b). See figure 58 for location of the cross section.
68 Alaska Coal Geology, Resources, and Coalbed Methane Potential
tuminous near the base. The Sterling Formation was dated as
latest Miocene and Pliocene age by Wolfe and Tanai (1980).
Hite (1976) and Hayes and others (1976) interpreted the
Sterling Formation as being deposited by meandering and
braided streams (fig. 93), with the former mainly draining the
basin axis and the latter draining the margins of the basin. The
braided streams built alluvial fans that were better developed
along the western part of the basin than along the eastern part.
Flores and Stricker (1993c) interpreted the Sterling Forma-
tion as being deposited in low-sinuosity (braided) streams that
evolved into high-sinuosity (meandering) streams. A close
facies association exists between thick coal beds and deposits
of the low-sinuosity streams. Mires in high-sinuosity streams
were commonly choked by overbank and flood-plain
sediments.
Coalfields:
Matanuska Coalfield
The Matanuska coalfield is the most important Paleocene
coalfield in Alaska because it contains high-rank minable coal
beds. This coalfield occupies a graben along the extent of the
Matanuska Valley, between the Talkeetna Mountains on the
north and the Chugach Mountains on the south (fig. 94). Coal
beds of the Chickaloon Formation are distributed in an area
about 62 mi (100 km) long, from Moose Creek on the west to
Anthracite Ridge on the east (Capps, 1927).
Coal districts in the Matanuska coalfield were divided
into leases under the Federal Coal Leasing Act of 1915. The
Wishbone Hill coal district (about 15 mi2 or 38 km2 in area) is
on the north side of the coalfield between Moose and Granite
Creeks. More than 20 coal beds, with thicknesses exceed-
ing 3 ft (0.9 m), are known in the Wishbone Hill coal district
(Belowich, 1994). There, individual coal beds are as much as
23 ft (7 m) thick, but average 8 ft (2.4 m) thick. Mining began
in 1917 at the west end of the district. The Federal Govern-
ment operated the Eska mine in 1917 and started a second coal
mine, the Chickaloon, on the Chickaloon River. At one time or
another nine mines operated in the Wishbone Hill coal district
between 1917 and 1970, and three or four coal mines operated
in the Chickaloon-Castle Mountain coal district during the
same period of time. The latter district was about 12 mi2 (31
km2) in the area around the old mining camps in the Chick-
aloon River Valley. Annual coal production in both districts
averaged about 50,000 short tons (45,360 metric tons) from
1917 to 1940, 160,000 short tons (145,000 metric tons) from
1940 to 1951, and about 240,000 short tons (217,700 metric
tons) from 1952 to 1970. A total of 3 × 106 short tons (2.7 mil-
lion metric tons) was produced from open pit mines and the
rest from underground mines. Total coal production was about
7.7 × 106 short tons (7 × 106 metric tons) between 1915 and
1970, after which production of oil in the State eliminated the
market for coal (Merritt and Belowich, 1984).
Coal beds within the Chickaloon Formation vary in
thickness considerably or pinch out altogether within short
distances as shown in figures 51, 53, and 55. Correlation of
exposure of the Premier coal zone and associated coal beds
Sandstone
Conglomerate
Mudstone and siltstone
Coal and carbonaceous shale
Mires
Normal fault
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Figure 87. Paleogeo-
graphic map (block
diagram) showing
depositional environ-
ments of the Beluga
Formation in the Cook
Inlet Basin. Modified
from Hayes and oth-
ers (1976).
Southern Alaska-Cook Inlet Coal Province 69
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Figure 88. Photograph of fluvial-channel sandstones and thin coal of the Sterling Formation along the coastal bluffs
in the Clam Gulch area, Kenai Peninsula. Men are 6 feet (1.8 meters) tall for scale. See figure 58 for location of the
cross section.
Figure 89. Photograph of fluvial-channel sandstones overlying thin (3 feet [0.9 meter]) to thick (12 feet [3.6 meters])
coal beds of the Sterling Formation along the coastal bluffs between the Clam Gulch and Ninilchik, Kenai Peninsula.
70 Alaska Coal Geology, Resources, and Coalbed Methane Potential
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Conglomeratic sandstone (Channel)
Sandstone (Overbank and crevasse splay)
Planar crossbeds
Megaforesets Root marks
Ripple laminations
Convolute laminations
Bentonitic (Smectitic -volcanic ash)
Trough crossbeds
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EXPLANATION
Siltstone }(Overbank,flood-plain,and channel plug)Mudstone
Figure 90. Stratigraphic cross sections showing variations in fluvial-channel architecture in the upper part of the Sterling For-
mation in the Clam Gulch area, Kenai Peninsula: A, Lower part of Clamgulchian type section; B, Middle part of Clamgulchian
type section; C, Upper part of Clamgulchian type section. See figure 58 for location of the cross section.
Southern Alaska-Cook Inlet Coal Province 71
����������������������������������Tickmarkslocatepositionofstratigraphicsections.������������������SandstoneSiltstoneandmudstoneCoalandcarbonaceousshaleEXPLANATIONFigure 91.
72 Alaska Coal Geology, Resources, and Coalbed Methane Potential
was performed by Flores and Stricker (1993a) in the Wishbone
Hill district where minable coal beds split and merge over
short distances laterally as shown in figure 95. In this coal dis-
trict, four groups of minable coal beds, one to six beds in each
group, are separated by 49–295 ft (15–90 m) of interburden
rock in a section 1,180–1,510 ft (360–460 m) thick. Com-
bined, 12 minable beds totaled about 49 ft (15 m) in thickness.
The thickest coal bed is about 10 ft (3.3 m) thick (Barnes and
Payne, 1956; Barnes and Sokol, 1959).
Six to 10 coal beds were penetrated by drilling in the
Chickaloon coal district, most less than 3 ft (1 m) thick, but
one bed is more than 14 ft (4.3 m) thick. The beds are lenticu-
lar and vary in thickness within 197–295 ft (60–90 m) later-
ally, making correlations, reserve calculations, and prospecting
across transverse faults difficult.
The Anthracite Ridge coal district covers about 30 mi2
(77 km2) at the eastern end of the coalfield (fig. 94). The
number of coal beds in this district is uncertain owing to poor
exposures and complex structure. A few beds in the coal dis-
trict are as thick as 3.9–6.5 ft (1.2–2.0 m) and one reaches
39 ft (12 m); the coal beds are exceptionally lenticular.
The intensity of deformation and abundance of igneous
dikes and sills in the Chickaloon Formation increase eastward
along the Matanuska coalfield. A few small dikes occur in the
Wishbone Hill coal district, and thick sills are abundant in the
Anthracite Ridge coal district. Heating induced by the igneous
intrusions may be the main reason for the increase in coal rank
������
Figure 92. Photograph of thin to thick coal beds in the lower
part of the Sterling Formation. Hammer on left of photograph
between lower and middle coal beds is 1 foot (0.3 meter) long
for scale.
Conglomerate
Sandstone
Mudstone and siltstone
Coal and carbonaceous shale Fault
Mire
EXPLANATION�������������������������������������������
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Figure 93. Block diagram showing depositional environments of the Sterling Formation in the Cook Inlet
Basin. Modified from Hayes and others (1976).
Southern Alaska-Cook Inlet Coal Province 73
GraniteCreekIntrusive rocks (Tertiary)
Wishbone Formation (Eocene)
Chickaloon Formation (Eocene and Paleocene)
Matanuska Formation (Cretaceous)
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Figure 94. Map showing the geology and coal districts in the Matanuska coalfield.
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Sandstone Channel and crevasse
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and sandstone
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EXPLANATION
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Rocks Interpretation
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EXPLANATION
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Figure 95. Cross section of the Premier coal zone of the Chickaloon Formation in the Wishbone Hill coal district.
Modified from Flores and Stricker (1993d). See figure 96 for location of cross section.
CANADACANADA
UNITED
STATES
ALASKA
Area of map CANADACANADACANADA
UNITED
STATES
ALASKA
Area of map CANADA
74 Alaska Coal Geology, Resources, and Coalbed Methane Potential
from subbituminous to anthracite eastward in the coalfield.
However, Barnes (1962) suggested that heat generated by
tectonic activity was more important than that from igneous
intrusions. Merritt (1985) described the natural coking of coal
adjacent to an intrusive diabase sill in which the contact tem-
perature reached 1,020˚F (550˚C). The coal bed along the con-
tact was locally coked and raised to semianthracite, but about
165 ft (50 m) away from the contact, the coal was ranked
high-volatile bituminous A. The coal rank in the Anthracite
Ridge coal district also changes abruptly from low-volatile
bituminous to semianthracite or anthracite within about 196 ft
(60 m) toward an intrusion (see fig. 40; Waring, 1936).
Structures in the Matanuska coalfield are typically com-
plex. The doubly plunging Wishbone Hill syncline, a relatively
simple structure, has beds that dip 20˚–40˚ on either flank;
the structure is cut by two sets of transverse faults (fig. 96).
Structural complications on its northwest flank make the coal
beds in some structural blocks difficult to mine and preclude
meaningful estimation of reserves (Barnes and Payne, 1956).
With the possible exception of the Castle Mountain district,
structural complexities increase eastward. In the Chickaloon
district, beds dip as much as 90˚; in the Chickaloon mine, coal
beds are overturned (Chapin, 1920) and faulted. Large areas
of the Chickaloon Formation are covered by a thick mantle
of glacial till and crop out only along stream bluffs (Capps,
1927). Anthracite occurrences on the south flank of Anthracite
Ridge are bordered on the north by a high-angle fault of large
displacement and are in tightly folded and locally overturned
synclines cut by many faults.
Susitna-Beluga Coalfield
The Susitna-Beluga coalfield is situated in the Susitna
Lowland (see fig. 40) north of the Cook Inlet between the
Talkeetna Mountains on the east and the Alaska Range on the
north and west. Glacial and alluvial deposits mainly cover the
Susitna Lowland. Coal beds are in the Kenai Group. The rocks
are exposed in isolated areas but mainly along the banks and
tributaries of the Susitna, Yentna, Beluga, and Chuitna Rivers.
These coal-bearing rocks underlie an area of at least 3,440
mi2 (8,910 km2). Barnes (1967a) studied these exposures and
concluded that the potentially minable coal beds are located
in a 400-mi2 (1,036-km2) area at the southwestern end of the
coalfield in the drainage basins of the Chuitna and Beluga
Rivers. There, the coal beds range from lignite to subbitumi-
nous and range from a few inches (few centimeters) to more
than 50 ft (15 m) thick. Barnes (1967a) has traced a few of
the thick (30–50 ft or 9.1–15 m) coal beds for distances of
more than 7 mi (11 km) along the course of the Chuitna River.
Flores and others (1994) and Flores, Stricker, and Stiles (1997)
have described the lateral variations of these coal beds, which
are shown in figures 73, 74, and 75. Myers and others (1998)
traced one 50-ft (15-m) coal for 4 mi (6.4 km) in the Capps
Glacier area (see fig. 40). Other thick (10–25 ft or 3–7.6 m)
coal beds are exposed along the Beluga, Skwentna, and Yentna
Rivers (see fig. 40). Along the southeast margin of the Alaska
Range, the Kenai Group rocks lie in downfaulted or down-
warped basins (Barnes, 1966; Magoon and others, 1976; Reed
and Nelson, 1980).
Tc
F i g .9 5
Matanuska River
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Tsadaka Formation (Oligocene)
Syncline
Wishbone Formation (Eocene)
Chickaloon Formation (Eocene and Paleocene)
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Strike-slip fault (U,up;D,down)
Direction of plunge
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EXPLANATION
Figure 96. Geologic map of the Wishbone Hill coal district showing doubly plunging syncline disrupted by normal
faults. Modified from Flores and Stricker (1993a).
CANADACANADA
UNITED
STATES
ALASKA
Area of map CANADA
Reed and Nelson (1980) divided the Tyonek Formation
in the Susitna-Beluga coalfield into two members. The basal
member consists of 40 percent conglomerate, 20 percent
sandstone, and 40 percent siltstone, claystone, and coal; the
latter are in beds as much as 56 ft (17 m) thick. The overlying
member consists predominantly of sandstones about 558 ft
(170 m) thick, composed of repetitive cycles 23–75 ft
(7–23 m) thick and grading from coarse-grained, pebbly sand-
stone at the base to silt and clay with coal or bony coal at the
top. The Tyonek is overlain by the Sterling Formation,
consisting of an orange to light gray, massive pebble to
boulder conglomerate, as much as 2,525 ft (770 m) thick.
Barnes (1966) reported two negative gravity anomalies
beneath the Susitna Lowland —one between Johnson Creek
and Yenlo Mountain and north of the Skwentna River, and the
other between Yenlo Mountain and the Susitna River, centered
at the confluence of the Kahiltna and Yentna Rivers
(see fig. 40). Barnes interpreted both anomalies as thick fill
of the Kenai Group that may contain potential for large
deposits of coal.
Broad Pass Coalfield
The Broad Pass coalfield underlies a narrow trough
extending northeastward from south of the divide of the
Alaska Range, on the headwaters of the Chulitna River (see
fig. 40), to the north end of the Cook Inlet-Susitna Lowland
(Wahrhaftig, 1965; Barnes, 1967a). The coalfield is about 5 mi
(8 km) wide and is bordered by mountains that rise abruptly to
elevations of about 3,300–8,200 ft (1,000–2,500 m). Although
Mesozoic and older metamorphic and igneous rocks are
mainly exposed in the coalfield, several small areas of coal-
bearing rocks of the Kenai Group occur. Only two of these
areas are known to contain coal resources: Costello Creek and
Broad Pass Station on the Alaska Railroad (see figs. 1 and 40).
Only a 7-mi2 (18-km2) area was mapped with coal-bearing
rocks in the Costello Creek and a 1.5-mi2 (3.8-km2) area near
the Broad Pass Station. A detailed U.S. Bureau of Mines-U.S.
Geological Survey investigation in the Costello Creek area
disclosed a lower unit of interbedded sandstone, mudstone,
and coal beds, 0–85 ft (0–26 m) thick, overlain by an upper,
predominantly sandstone unit, as much as 490 ft (150 m)
thick, lacking coal beds (Wahrhaftig, 1944).
The coal beds at the Broad Pass Station, 8–10 mi
(13–16 km) east of the Costello Creek area, are interbedded
with white to orange sandstones and gravelly conglomerates
(Hopkins, 1951). These coal beds are correlated to the Sterling
Formation of the Susitna-Beluga coalfield. Coal has been
reported south of these areas along the Chulitna River. The
coalfield contains orange to yellow gravels exposed in railroad
cuts and streambanks, which resemble the Nenana Gravel
in the Central Alaska-Nenana coal province and the Sterling
Formation of the Susitna Lowland.
Kenai Coalfield
The Kenai coalfield lies on the lowland between the
Kenai Mountains on the east and the Cook Inlet on the west,
in the western part of Kenai Peninsula (see fig. 40). The
coalfield contains the thick, coal-bearing Beluga and Sterling
Formations of the Kenai Group and is divided into two coal
districts: the northern Kenai and southern Homer coal districts
(Barnes, 1967a). The northern Kenai coal district includes
mainly outcrops of the Sterling Formation, and the coal beds
are exposed mainly along the coastal bluff from north of Clam
Gulch to south of Ninilchik (see figs. 58 and 97; Merritt and
others, 1987). The coal beds are mainly thin in the upper part
and thicker in the lower part of the formation. The Homer
coal district contains outcrops of both the Beluga and Sterling
Formations, which are mainly exposed along the coastal bluffs
from north of Anchor Point to Homer and along the north
shore of Kachemak Bay (see figs. 58 and 98) on the southern
end of the Kenai Peninsula. The coal-bearing rocks are com-
pletely concealed by as much as several hundred feet of glacial
and alluvial deposits, particularly in the northern Kenai coal
district. However, where the Sterling coal beds are exposed
along the coastal bluffs, they are as thick as 12 ft (3.8 m) and
are laterally continuous for more than 1.75 mi (3 km) (Flores
and Stricker, 1992).
The Homer coal district (Barnes and Cobb, 1959) is
about 1,200 mi2 (3,110 km2) in area and includes as much as
5,000 ft (1,525 m) of the Beluga and Sterling Formations.
These formations contain at least 30 coal beds ranging
individually from 3 to 7 ft (0.9 to 2.1 m) in thickness (Barnes,
1967a). Flores and Stricker (1993b) reported that Beluga coal
beds range from a few inches (few centimeters) to 8.2 ft
(2.5 m) thick and average 3.2 ft (1 m) (fig. 99). Thin coal beds,
a few inches to 1 ft (a few centimeters to 30 cm) are traceable
laterally from a few tens to hundreds of feet. Thicker coal
beds, greater than 2 ft (>0.6 m), are traceable laterally as much
as a few miles. The thickness-to-length ratio of coal beds
indicates they vary from lenticular (1:9) to elongate
(1:1,000–3,000). Stratigraphic variations of the coal beds in
the Homer coal district are shown in figures 100 and 101. Coal
beds of the Beluga Formation are thick and laterally
continuous where they are interbedded with thick and
extensive sandstones, which were deposited by meandering
streams (see fig. 100). Beluga Formation coal beds are thin
and discontinuous where interbedded with thin and lenticular
sandstones, in which case anastomosed streams (see fig. 101)
deposited the sandstones.
Coal was mined intermittently since 1888 along the
north shore of Kachemak Bay by the Alaska Coal Company
at Millerʼs Landing northwest of Homer (Barnes, 1967a). In
1891, the U.S. Navy mined 50 short tons (45 metric tons) from
four localities on Kachemak Bay. In 1894, the North Pacific
Mining and Transportation Company began development in
Eastland Canyon (about 1 mi northeast of Kachemak Bay). At
least 650 short tons (590 metric tons) of coal was produced
from this underground mine and shipped to San Francisco.
Southern Alaska-Cook Inlet Coal Province 75
76 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Underground mines were also opened from 1894 to 1897 west
of McNeil Canyon.
Coal mining shifted to the west of Homer along the beach
bluffs of the Cook Inlet from 1899 to 1951. The Cook Inlet
Coal Fields Company developed the Cooper coal bed from
five mine shafts in the beach bluff on Bidarki Creek, about
a mile (1.6 km) west of Homer. The 1899–1902 total coal
production from these mines was only a few hundred short
tons. In 1915, Bluff Point (see fig. 58) underground mine was
opened near Bidarki Creek and produced about 1,400 short
tons (1,270 metric tons). Barnes (1967a) reported production
from this mine to be about 1,200 short tons (1,090 metric tons)
in 1921, 2,700 short tons (2,450 metric tons) in 1922, and 700
short tons (635 metric tons) in 1923. No production records
were found for 1924 to 1945. In 1946, the Bluff Point mine
was taken over by Homer Coal Corporation, which blocked
out reserves of stripping coal. No reported production was
recorded from this operation, which operated until 1951. Total
production in the Homer coalfield is at least a few thousand
tons.
Coal Resource Assessment in the
Southern Alaska-Cook Inlet Coal
Province
The coal resource assessments of various workers in the
Southern Alaska-Cook Inlet coal province vary in magnitude
and coal resource categories, which resulted in different esti-
mates. We reconstructed these diverse coal resource estimates
following guidelines of the coal-resource classification system
of Wood and others (1983). This new reporting system of
the coal resources of the Southern Alaska-Cook Inlet coal
province modified from previous estimates is summarized in
table 1. Following is a historical account of the variable coal
resource assessments in the coal province.
Figure 97. Photograph of a 4-foot-thick (1.2 meters) coal
bed interbedded with fluvial-channel sandstones and flood
plain mudstones and siltstones in the Sterling Formation in
the Clam Gulch area.
Sandstone
Mudstone-siltstone
Coal and carbonaceous shale
Ferruginous sandstone
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Root mark
EXPLANATION ��������
Figure 98. Stratigraphic cross section showing interbedded
coal beds, fluvial-channel sandstones, and flood-plain mud-
stones and siltstones in the lowermost part of the Sterling
Formation along the north shore of Kachemak Bay east and
west of McNeil Canyon. See figure 58 for location of the
cross section.��������������������������������������
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EXPLANATION
Conglomerate
Sandstone
Mudstone and siltstone
Coal and carbonaceous shale
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Figure 99. Photograph of a coal bed with tonstein partings
and related rocks of the Beluga Formation along the beach
bluffs on the northern shore of the Kachemak Bay. Mat-
tock is 2 feet (0.6 meter) long for scale.
Figure 100. Stratigraphic cross section showing interbed-
ded coal beds, fluvial-channel sandstones, and flood-plain
mudstones and siltstones in the uppermost part of the
Beluga Formation west of McNeil Canyon. See figure 58 for
location of the cross section.
��������
Coal Resource Assessment in the Southern Alaska-Cook Inlet Coal Province 77
��������
���������
Tick marks locate position of measured stratigraphic sections
������
���������
Conglomerate
Sandstone
Mudstone and siltstone
Coal and carbonaceous shale
Figure 101. Stratigraphic cross section showing interbedded coal beds, fluvial-channel sandstones, and flood plain
mudstones and siltstones in the uppermost part of the Beluga Formation at the mouth of Fritz Creek. See figure 58 for
location of the cross section.
78 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Barnes (1967a) estimated identified coal resources in the
Southern Alaska-Cook Inlet coal province to be about 2,910
million short tons (2,640 million metric tons). Later, McGee
and Emmel (written commun., 1979) estimated the identified
coal resources to be about 34,320 million short tons (31,130
million metric tons).
Matanuska Coalfield
Estimates of coal resources produced by various work-
ers for the Matanuska coalfield were reported by Merritt and
Belowich (1984) as being as high as 200 × 106 short tons
(181 × 106 metric tons) for measured to inferred coal
resources and as high as 2.4 × 109 short tons (2.2 × 109 metric
tons) for hypothetical coal resources. The most reliable coal-
resource estimates are from Barnes (1967a), who reported
137 × 106 short tons (125 × 106 metric tons) of combined
measured, indicated, and inferred coal resources, and from
Merritt and Belowich (1984) who reported 24 × 109 short tons
(22 × 109 metric tons) of hypothetical coal resources (table 6).
In the Wishbone Hill coal district, Barnes (1967a)
reported total coal resources of 112 million short tons
(101 million metric tons), based on apparent rank of
bituminous coal with thicknesses greater than 14 inches
(35 cm), and between 0 and 2,000 ft (0–610 m) of overburden.
Total resources were divided into 6.6 million short tons
(6.0 million metric tons) measured coal resources, 51.7 million
short tons (47 million metric tons) indicated coal resources,
and 53.7 million short tons (49 million metric tons) inferred
coal resources.
In the Chickaloon-Castle Mountain coal district, Barnes
(1967a), reported total coal resources of 25 million short tons
(23 million metric tons), based on apparent rank of
bituminous coal, with thicknesses greater than 14 inches (35
cm), and between 0 and 2,000 ft (0–610 m) of overburden.
Total resources were divided into 0.0 measured coal resources,
0.7 million short tons (0.6 million metric tons) indicated coal
resources, and 24.3 million short tons (22 million metric tons)
inferred coal resources.
In the Anthracite Ridge coal district, the only identified
minable bed of anthracite, 4.2 –6.6 ft (1.3–2.0 m) thick, under-
lies an area of no more than 2.5 acres (1 hectare) and totals no
more than 22,000 short tons (20,000 metric tons)
(Waring, 1936; Merritt and Belowich, 1984). One other
reported anthracite occurrence (Merritt and Belowich, 1984),
too thin to be mined, is on a large active landslide (Detterman
and others, 1976).
Susitna-Beluga Coalfield
Barnes (1967a) reported identified coal resources of the
Susitna-Beluga coalfield as 2.4 billion short tons
(2.2 billion metric tons) (table 6). Total resources were
estimated for subbituminous coal beds greater than 2.5 ft
(0.76 m), with overburden to 1,000 ft (0–305 m) in the drain-
age basins of the Yentna, Skwentna, Beluga, and Chuitna
Rivers, the Capps Glacier coal district, and an area southwest
of Tyonek. Indicated coal resources are 56 million short tons
(51 million metric tons) in the Yentna River Basin, 123 million
short tons (116 million metric tons) in the Skwentna River
Basin, 260 million short tons (236 million metric tons) in the
Beluga River Basin, 1.54 billion short tons (1.4 billion metric
tons) in the Chuitna River Basin, 406 million short tons
(368 billion metric tons) in the Capps Glacier district, and
9.4 million short tons (8.5 million metric tons) southwest of
Tyonek.
Wahrhaftig and others (1994), based on Barnesʼ 1966
report, calculated indicated coal resources of (1) 4.5 × 106
short tons (4.1 × 106 metric tons) in beds less than 6.5 ft (2 m)
thick in the Peters Hills; (2) about 44 × 106 short tons
(40 × 106 metric tons) of coal mainly in beds more than
10 ft (3 m) thick in the Fairview Mountain area; (3)
20 × 106 short tons (18 × 106 metric tons) of coal mainly in
beds more than 6.5 ft (2 m) thick in the Johnson Creek area;
and (4) 110 × 106 short tons (100 × 106 metric tons) of coal in
the downfaulted half graben along Canyon Creek area. A
drilling program by Mobil Oil Corporation resulted in
estimates of 500 × 106 short tons (450 × 106 metric tons) of
coal within 250 ft (76 m) of the surface in beds 10 to 50 ft
(3 to 15 m) thick, in two leased areas totaling 23,000 acres
(9,300 hectares). One area includes the Canyon Creek drain-
age basin and the other extends from the Skwentna River
northward across Johnson Creek (Blumer, 1981).
Table 6. Estimates of coal resources for the Tertiary Kenai Group in the Matanuska, Susitna, Broad Pass, and Kenai coalfields in the Southern Alaska-Cook Inlet coal province.[Resource estimates are in millions of short tons (multiply by 0.907 to obtain metric tons)]
Coal field Source Classification
CoalResource estimate
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Broad Pass Coalfield
The hypothetical and identified coal resources of the
Broad Pass coalfield reported by Wahrhaftig and others (1994)
were 13.3 × 106 short tons (12 × 103 metric tons) of coal com-
bined for two beds with a maximum thickness of 10 ft
(3 m). According to Barnes (1967a), 64 × 103short tons
(58 × 103 metric tons) of coal was mined from 1940 to 1954,
and the rest was unminable (table 6).
Hopkins (1951) estimated that at least 13 x 106 short
tons (12.2 × 106 metric tons) of coal exist beneath the area of
known exposures of Tertiary rocks at Broad Pass Station, but
only 300 × 103 short tons (270 × 103 metric tons) of lignite
with an ash yield of 8–25 percent was actually measured.
Barnes (1967a) reported total coal resources of the Broad
Pass coalfield as 64 million short tons (58 million metric tons).
Based on coal-bed thickness of 2.5 ft (0.75 m) for subbitu-
minous rank and overburden of 0–1,000 ft (0–305 m), Broad
Pass Station area contains as much as 0.3 million short tons
(0.27 million metric tons) indicated coal resources, 63.3
million short tons (57 million metric tons) inferred coal
resources, or a total coal resource of 63.6 million short tons
(57.4 million metric tons). Costello Creek area contains 0.3
million short tons (0.27 million metric tons) indicated
coal resources.
Kenai Coalfield
Coal resources of the Kenai coalfield are mainly con-
centrated in the Homer coal district. Barnes and Cobb (1959)
calculated indicated coal resources of 400 × 106 short tons
(360 × 106 metric tons) for coal beds greater than 2 ft
(0.6 m) thick, of which 50 × 106 short tons (45 × 106
metric tons) are in beds more than 5 ft (1.5 m) thick. These
coal beds are covered by <990 ft (300 m) of overburden.
Barnes and Cobbʼs investigation indicated that all except the
extreme northern and northeastern parts of the Homer coal
district, about 750-mi2 (1,940-km2) area, is underlain by coal
beds greater than 2 ft (0.6 m) thick. Thus, the potential coal
resources of the Homer coal district may be several billion
short tons. Barnes (1967a) reported the total coal resources
of the Homer coal district to be more than 318 million short
tons (290 million metric tons). McGee and Emmel (written
commun., 1979) reported Kenai coalfield onshore resources as
0.2 million short tons (0.18 million metric tons) measured coal
resources, 318 million short tons (290 million metric tons)
identified coal resources, and 34 billion short tons (31 billion
metric tons) hypothetical resources (table 6).
Based on geophysical logs from drill holes throughout
the Cook Inlet Basin, McGee and OʼConnor (1975)
calculated a hypothetical resource of 1.2 × 1012 short tons
(1.1 × 1012 metric tons) of coal of apparent lignite rank to a
depth of 10,000 ft (3,048 m) and 110 × 109 short tons
(100 × 109 metric tons) to a depth of 2,000 ft (610 m). McGee
and Emmel (written commun., 1979) reported about 34.32
billion short tons (31 billion metric tons) of hypothetical
coal resources in the offshore Cook Inlet Basin. Affolter and
Stricker (1987b) estimated a hypothetical resource of
0.8 × 1012 short tons (0.7 × 1012 metric tons) of the Kenai
Group coal to lie beneath the waters of Cook Inlet.
Coal Quality
Coal in the Southern Alaska-Cook Inlet coal province
ranges from lignite to anthracite (table 7). The lignite with
heat values of 5,410–8,020 Btu/lb (3,010–4,460 kcal/kg) and
subbituminous coal with heat values of 8,060–9,520 Btu/lb
(4,480–5,290 kcal/kg) are mainly in the Broad Pass, Susitna-
Beluga, and Kenai coalfields (Barnes, 1967a). Bituminous
coal with heat values of 10,390–14,380 Btu/lb (5,770–7,990
kcal/kg) and semianthracite with heat values of 10,720–13,420
Btu/lb (5,960–7,460 kcal/kg) coal are mainly in the Matanuska
coalfield (Barnes, 1967a).
In the Matanuska coalfield, the ash yield varies from 2.4
to 21.7 percent, sulfur from 0.2 to 1.0 percent, and moisture
content from 1.1 to 20.3 percent (as-received basis; Barnes,
1967a). Run-of-the-mine coal quality in the Wishbone Hill
coal district varies from 9 to 29 percent ash, 0.3 to 0.4 percent
sulfur, and 4.0 to 6.0 percent moisture content (as-received
basis; Germer, 1986; Belowich, 1994).
In the Susitna-Beluga coalfield, coal varies from 2.1 to
30.5 percent ash yield, 0.1 to 0.3 percent sulfur, and 11.3 to
33.1 percent moisture contents (as-received basis; Barnes,
1967a). In the Chuitna River drainage basin, Affolter and
Stricker (1994) reported ash yield that varies from 4.83 to
38.67 percent, sulfur content from 0.08 to 0.18 percent, and
moisture content from 19.19 to 26.15 percent. In the Beluga
River drainage basin, the ash yield varies from 3.59 to 29.87,
sulfur content from 0.08 to 0.32, and moisture content from
16.78 to 7.49 percent. In the Capps Glacier district the coal
beds contain ash yield of 9.3–40.3 percent, sulfur 0.12–0.33,
and moisture 4.8–26.0 percent (as-received basis; Affolter and
Stricker, 1986). Flores and others (1994) suggested that the
higher sulfur content in coal beds in the Chuitna River drain-
age basin was influenced by tidal incursions into mires.
In the Broad Pass coalfield, the ash yield varies from 6.0
to 21.2 percent, sulfur from 0.2 to 0.6 percent, and moisture
content from 8.7 to 35.8 percent (as-received basis; Barnes,
1967a).
In the Kenai coalfield, with emphasis on the coal beds
in the Homer coal district, ash yield varies from 3.8 to 22.0
percent, sulfur content from 0.1 to 0.4 percent, and moisture
content from 16.5 to 30.4 percent (Barnes and Cobb, 1959;
Barnes, 1967a). Affolter and Stricker (1994) reported ash yield
ranging from 4.80 to 26.90 percent, sulfur content from 0.20 to
Coal Resource Assessment in the Southern Alaska-Cook Inlet Coal Province 79
80 Alaska Coal Geology, Resources, and Coalbed Methane Potential
1.30 percent, and 11.0 to 26.50 percent moisture content from
the Kenai coalfield.
Coal Petrology
The coal petrology of the Tyonek coal beds in the Chuitna
River drainage area was investigated by Rao and Smith
(1986). Vitrinite (or huminite) is the most abundant maceral
and varies from about 66 to 92 percent. Minor liptinite varies
from 4 to 18 percent and inertinite from 0 to 9 percent. The
woody or huminite maceral is composed mainly of cypress
trees (Rao and Smith, 1986). However, oak, beech, hickory,
elm, walnut, alder, and birch trees are represented in the peat-
forming mires. The huminite maceral is either unevenly dis-
tributed vertically throughout the coal beds or it increases in
the upper and lower parts of the coal beds. Liptinite macerals
in some coal beds increases in the upper part of the coal beds.
Inertinite appears to be less preferentially distributed verti-
cally in the coal beds than the huminite and liptinite macerals.
However, local peak occurrences of inertinite indicate genera-
tion of fusinite or charcoal that is formed by forest fires during
dry periods. High occurrence of liptinite suggests differential
decomposition of the more resistant exinite from vegetal
matter. The high concentration of huminite in the lower part
of coal beds indicates that the mires were initially vegetated
by abundant trees, which evolved into less woody vegetation
through time. The high concentration of huminite in the upper
part indicates that the mire supported more woody vegetation
through time.
Coalbed Methane Potential
The coal resources of Alaska (about 5,526 billion short
tons; see table 1) contain significant potential economic
coalbed methane resources. Methane derived from coal, which
has migrated and is stored in interbedded sandstone reservoirs
in the Cook Inlet Basin, is presently being developed. Coal-
bed gas or methane-rich gas is stored (adsorbed) in the coal
along fractures, cleats, and pores and (or) within (absorbed)
the molecular structure of the coal. Gas is stored in the coal
by molecular attraction on the surfaces of the structures of the
coal. Methane is a by-product of fermentation during deposi-
tion and coalification during burial of peat. The ability of the
coal to store gas is a function of rank or grade of coalification
(for example, lignite, subbituminous, bituminous) and tem-
perature and pressure. Generally, more methane is stored in
higher rank coal and at high pressure whereas higher tempera-
ture decreases storage capacity. Methane generated in higher
rank coal (for example, bituminous) is thermogenic in origin,
and methane produced in lower rank coal (lignite and subbitu-
minous) is biogenic in origin. Biogenic gas is generated during
bacterial activity by methanogens or anaerobes that produced
methane as a by-product of their metabolism. In most cases
methanogens do this by reducing carbon dioxide with hydro-
gen to produce methane. Biogenic gas generated from lignites
in Alaska was determined from a 1994 U.S. Geological Survey
test well in the Yukon Basin (Flats), where the coal beds are
more than 21 ft (6.4 m) thick.
A major by-product of development of coalbed meth-
ane, especially for subbituminous coal, is coproduced water.
Volumes of water produced in major methane-producing
Table 7.Range (minimum and maximum values) of quality parameters for Tertiary coal
deposits in the Matanuska, Broad Pass, Susitna, and Kenai coalfields in the Southern
Alaska-Cook Inlet coal province.
[All analyses except Calorific value (Btu) are in percent. Values reported on an as-
received basis. Modified from Merritt, 1984]
Area Moisture Volatile
matter
Fixed
carbon
Ash
yield
Total
sulfur
CalorificvalueBtu per pound
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basins in the conterminous United States vary significantly
between bituminous and subbituminous coal. The volume of
coproduced water from bituminous coal ranges in average
from 48 to 240 barrels (7,632 to 38,160 liters) of water per day
per well and from the subbituminous coal the average is about
440 barrels (91,600 liters) (Flores, 2000). Hence, the water:
gas ratio for the bituminous coal ranges from 0.029 to 0.51
barrel per thousand cubic feet (16.3 to 286 liters per 100 m3)
and from the subbituminous coal is 2.88 barrels per thousand
cubic feet (21,360 liters per 100 m3) (Flores, 2000). In order to
produce the methane from the coal, the reservoir needs to be
dewatered, which results in the depressurization of the reser-
voir. This water can be disposed of either on the surface, into
ponds or existing drainages, or reinjected below the surface.
Regulations, quality, and amount of the coproduced water
influence the choice of a disposal system. Coproduced water
from subbituminous coal of the Tertiary Fort Union Forma-
tion being developed in the Powder River Basin of Wyoming
is freshwater. It contains concentrations of dissolved solids
mainly of bicarbonate, and trace elements and pH values that
are generally below and within recommended drinking-water
standards (Flores, 2000). Thus gas operators in that basin are
permitted to dispose of the coproduced water on the surface;
however, the large volume of water being disposed of is affect-
ing the environment (for example, biota, ephemeral drainages,
ground-water supply). Water-disposal problems may influence
potential development in Alaska where the permafrost (for
example, Northern Alaska-Slope coal province) is thick (Fer-
rians, 1965), and freezing temperatures at the surface for much
of the year may curtail surficial disposal by ponding or along
preexisting drainages. The quality of water such as concen-
tration of total dissolved solids and location of coalbed gas
production where recharge areas are juxtaposed to brackish-
marine bodies of water (sea, ocean, bay) may prevent surface
disposal or reinjection, which may contaminate ground-water
supply.
Smith (1995) reported that Alaskaʼs in-place coalbed
methane resources might be as much as 1,000 trillion cubic
feet (tcf) (28 trillion cubic meters [tcm]) based on estimates of
the gas content of as much as 245 standard cubic feet per ton
(scf/t) for the coals. The high coalbed-methane resource esti-
mate of Smith (1995) utilized 200 scf/t for both the subbitumi-
nous and bituminous coals in the Northern Alaska-North Slope
coal province and 152 scf/t for the subbituminous coal in the
offshore area in the Southern Alaska-Cook Inlet coal province.
Our investigations of the subbituminous coals in the Powder
River coals indicate gas content ranging from 0 to 99 scf/t,
averaging 25 scf/t (Stricker and others, 2001). If the Powder
River Basin coalbed-methane content is applied for the Alaska
subbituminous coal, Smithʼs estimate will be reduced to about
one-half the volume.
Northern Alaska-Slope Coal Province
The voluminous lignite, subbituminous, and bituminous
coal of the Northern Alaska-Slope coal province indicates
a high potential for large biogenic and thermogenic gas
resources. The abundance of bituminous coal with 1,910 bil-
lion short tons (1,732 billion metric tons) and subbituminous
coal with 1,960 billion short tons (1,778 trillion metric tons)
in the Northern Alaska-Slope coal province (fig. 102) indi-
cates a high potential for thermogenic and biogenic methane
resources. Outcrop and surface-projected mean vitrinite
reflectance values in the Northern Alaska-Slope coal province
range from 0.31 to 1.71 percent, which corresponds to lignite
to low volatile bituminous coal ranks (figs. 103 and 104; table
8a and 8b). Coal rank generally increases southward toward
the Brooks Range where the vitrinite reflectance values
exceed 1.71 percent (see table 8a and 8b). Thus, the vitrinite
reflectance values suggest a range of coal maturation in which
coalbed methane, both biogenic and thermogenic, may be gen-
erated from subbituminous and bituminous coals, respectively.
Tyler and others (2000) and Clough and others (2000)
evaluated the potential coalbed methane for the rural com-
munities in the Northern Alaska-Slope coal province. They
suggested that based on depth, coal thickness, and depositional
systems, primary coalbed methane targets and potential explo-
ration fairways occur mainly in the Cretaceous Nanushuk
Group. These workers identified the area between the east-
ern boundary of the National Petroleum Reserve of Alaska
(NPRA) to Chukchi Sea (see figs. 8 and 10) as containing the
highest coalbed methane potential because of the thickest net
coal, which is >300 ft (91 m) (see fig. 8). Potential methane
development in this area may be from most of the coal beds
that lie at an average depth of 2,000 ft (610 m).
Drilling depths for coalbed methane are recommended
below the permafrost zone, which is as much as 2,000 ft deep
(610 m) (Ferrians, 1965), to 6,000 ft (1,830 m). McKee and
others (1986) suggested that permeability is very low below
the 6,000-ft (1,825-m) threshold. The Meade Test Well No. 1
and Kaolak Test Well No. 1 have related gas shows with coal
beds as much as 30 ft (9.1 m) thick as well as interbedded
sandstones at depths between 1,240 and 2,200 ft (378 and
670 m) (Collins, 1959). Here, Barnes (1967a) reported that
there are as many as 60 coal beds with a net coal thickness of
350 ft (107 m) within a 4,600-ft (1,400-m) interval. Methane
gas shows associated with coal beds in the Nanushuk Group
and Corwin Formation were recorded at depths between the
surface to about 1,420 ft (430 m) by Husky Oil NPR, Opera-
tions, Inc. (1982–83). The presence of high gas content in sub-
bituminous coal beds in the Nanushuk and Colville Groups in
the NPRA was also reported by Claypool and Magoon (1988).
These investigators also noted that the shallow, immature
nature of the coal beds make for an unfavorable thermogenic
gas source. However, similar subbituminous coal beds of the
Fort Union Formation in the Powder River Basin of Wyoming
are currently producing coalbed gas from depths of 250–
1,500 ft (76–460 m).
Kenai Coalfield 81
82 Alaska Coal Geology, Resources, and Coalbed Methane Potential
The vitrinite reflectance values of the Cretaceous coal-
bearing rocks (Corwin-Chandler, Grandstand, Torok Forma-
tions, the Pebble shale unit, and underlying Jurassic-Devonian
rocks) in the Northern Alaska-Slope coal province are shown
in figures 105 and 106. The vitrinite reflectance values down
to 6,000 ft (1,830 m) range from about 0.30 to 0.66 percent,
which corresponds mainly to lignite to subbituminous coal
through subordinate high-volatile bituminous C coal (Stach
and others, 1982). Vitrinite reflectance values are superim-
posed on the cross section of the Nanushuk Group and under-
lying rocks (fig. 107). Here, the vitrinite reflectance values
of the coal-bearing Corwin-Chandler Formations range from
<0.5 to >0.7 percent in the western part (updip) of the North-
ern Alaska-Slope coal province and from <0.5 to <0.6 percent
in the eastern part (downdip). This indicates that the coal beds
may have generated mixed biogenic and thermogenic methane
in the western part of the coal province and mainly biogenic
methane in the eastern part. The extent of Nanushuk coal
beds and the high-potential coalbed methane resources in the
western part of the Northern Alaska-Slope coal province is
shown in figures 108 and 109. In figure 108, the depths to the
vitrinite reflectance value of 0.6 percent are superimposed on
the base of Nanushuk Group and the net coal thickness of the
Nanushuk Group. When the vitrinite reflectance contours are
merged with the extent of the Nanushuk coal and where the
Nanushuk coal beds have a net thickness of >400 ft (192 m),
the area of highest coalbed methane potential is in the south-
western part of the Northern Alaska-Slope coal province.
Callahan (1979) suggested that the North Slope gas
is biogenic and generated by a microbial activity. Carbon
isotopic analyses of near-surface (0–4,920 ft, 0–1,500 m) gas-
hydrate- and coal-bearing units by Collett (1993) yielded car-
bon isotopic values averaging about –49 permil. This indicates
that the methane in near-surface strata is from mixed biogenic
and thermogenic origin. However, based on vitrinite reflec-
tance values (0.30–0.66), the gas-hydrate and coal-bearing
rocks probably were not subjected to high temperatures; thus,
the thermogenic gas may have migrated from greater depths.
Tyler and others (2000) suggested that in addition to tar-
geting coal beds for conventional methane exploration, strati-
graphic and structural traps should be explored for coalbed
methane potential. Conventional play for thermogenic gas in
the coal that migrated updip and was trapped below the perma-
frost was also recommended for exploration. The permafrost
zone serves as a seal for trapping migrating gas. Stratigraphic
traps were suggested by Tyler and others (2000) where
coal beds pinch out updip behind progradational shoreline
sequences (for example, delta-front, barrier-shoreface sand-
stones) in the Nanushuk Group. Structural traps may be found
in fault-cored anticlines (for example, Meade and Wainwright
arches) (see fig. 8; Tyler and others, 2000).
Central Alaska-Nenana Coal Province
The coalbed methane potential for the Central Alaska-
Nenana coal province is not as high as the Northern Alaska-
Slope coal province. The coal beds in this coal province are
mainly subbituminous, range from 50 to 66 ft (15 to 20 m) in
thickness, and occur to depths of 3,000 ft (910 m). In addi-
tion, the Healy Creek Formation is sealed by thick mudstones
of the overlying Sanctuary Formation. Exploration targets for
potential coalbed methane are along the axes of large synclinal
basins such as the Healy Creek and Lignite Creek Basins. In
these basins, most of the coal resources in the Healy Creek
and Suntrana Formations are thick (as much as 65 ft or 20 m
thick) and found from 1,000-to 3,000-ft (305 to 914 m) depths
(Wahrhaftig and others, 1994).
Although the rank of the Healy Creek, Suntrana, and
Lignite Creek coals is mainly subbituminous, Affolter and
Stricker (1994) reported heating (calorific) values ranging
from 6,130 to 9,210 Btu/lb (3,410 to 5,120 kcal/kg), which
correspond to lignite to subbituminous coal. Outcrop and
surface-projected vitrinite values of the coal-bearing Usibelli
Group in the Central Alaska-Nenana coal province range
from 0.21 to 0.48 percent, which corresponds to lignite to
subbituminous C coal ranks (fig. 104). Coal ranks generally
increase south-southeast toward the Alaska Range, indicating
that methane generated in these mainly subbituminous coal
deposits is biogenic. The rank and quality (low ash and sulfur)
of the Healy Creek, Suntrana, and Lignite Creek coals beds are
very similar to the subbituminous coal beds of the Fort Union
Formation in the Powder River Basin of Wyoming, which are
producing economic biogenic methane at an average of
25 scf/t. In that basin, coalbed methane is produced as close
as 1–2 miles (1.6–3.2 km) from coal strip mines (Stricker and
others, 2001). However, the strip mining has liberated gas by
pressure reduction. Because the Fort Union coal beds have
high water saturation, depressurization from dewatering dur-
ing strip mining releases and subsequently causes migration of
gas by desorption and diffusion through the microstructures in
the coal. Thus, success in developing the coalbed methane for
the Healy Creek and Suntrana coal beds should probably be
focused on areas removed from old underground coal mines
and current strip mines.
Southern Alaska-Cook Inlet Coal Province
The coalbed methane potential for the Southern Alaska-
Cook Inlet coal province is high. This resource potential varies
from the Kenai, Broad Pass, and Beluga coalfields, which
contain lignite and subbituminous coal, to the Matanuska coal-
field, which contains bituminous and semianthracite coals.
Magoon and Anders (1990) reported that the gas pro-
duced from the Kenai Group in the Cook Inlet is biogenic.
Gas is mainly derived from the Tyonek and Beluga Forma-
tions. This gas is produced primarily from gas-driven–sand-
stone reservoirs (table 9) in the Tyonek, Beluga, and Sterling
Coalbed Methane Potential 83
BITUMINOUS
COAL
Barrow
B R O O K S R A N G E
SUBBITUMINOUS
COAL
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CANADAUNITEDSTATES
ALASKA
Area of map
UNITED STATESALASKA
CANADA
ARCTI C
O C E A N
Beaufort
Sea
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Figure 103. Distribution of surface vitrinite reflectance values at sea level in the Northern Alaska-Slope coal
province.
Figure 102. Map of the Northern Alaska-Slope coal province showing distribution of bituminous and sub-
bituminous coals. Modified from Sable and Stricker (1987).
84 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Formations (Brimberry and others, 1997; Flores and others,
1998). Figure 110 shows the distribution of gas and oil fields
in the Cook Inlet Basin producing mainly from the Tyonek,
Beluga, and Sterling sandstones. Figures 111 and 112 show the
vertical distribution and occurrence of gas in the Sterling and
Beluga sandstone reservoirs; gas accumulations in associated
coal beds in the Kenai field are shown in figure 113 (Brim-
berry and others, 1997; Flores and others, 1998). Since 1958,
when gas production from the Cook Inlet was first recorded
by the Alaska Department of Natural Resources, Division of
Oil and Gas (1997), the total (gross) production from these
sandstone reservoirs was about 7,993 billion cubic feet. This
gas is thought to be derived from the Tyonek, Beluga, and
Sterling coal beds (Kelly, 1968). Coal mines in the Matanuska
coalfield have emitted methane from the Chikaloon coal beds,
which has caused several mine explosions in 1937 and 1957
(Barnes and Payne, 1956; Smith, 1995).
Thirteen out of 18 coal beds in the Tyonek Formation
in the upper Cook Inlet Basin (northwest of Wasilla) were
determined to contain coalbed methane by Smith (1995). Gas
content ranges from 63 ft3 per short ton (1.97 scm3/gm) at stan-
dard temperature and pressure (STP) for coal beds at a shallow
depth of 500 ft (152 m) to 245 ft3 per short ton (7.6 scm3/gm)
at STP for coal beds at a depth of 1,200 ft (366 m). Vitrinite
reflectance values range from 0.47 to 0.58 percent and gener-
ally increase with depth. The carbon isotope composition of
the coalbed gases range from –49.3 to –43.3 permil δ13C with
slightly heavier isotope values at depth (Smith, 1995). In
general, biogenic methane is isotopically light with methane
δ13C values ranging from –55 to –90 permil (Rice and Clay-
pool, 1981; Rice, 1993). However, biogenic methane can be as
heavy as –40 permil, which can be produced by reduction of
isotopically heavy carbon dioxide (Jenden and Kaplan, 1986).
Thus, the gas from the Tyonek coal beds may be slightly bio-
genic but mostly thermogenic. Chemical composition is 98–99
percent methane with minor amounts of carbon dioxide and
nitrogen (see table 9; Flores and others, 1998).
Attempts to develop Tyonek coal beds by energy com-
panies (Union and Ocean Energy) in the Wasilla area were
affected by coproduced water problems. Large amounts of
ground water were encountered, which posed production
problems in separating the coalbed methane from the copro-
duced water as well as water-disposal problems by reinjection.
Similar problems were met by gas operators in developing the
coal beds of the Fort Union Formation in the Powder River
Basin of Wyoming. However, the gas operators in that area
are permitted to dispose of coproduced water at the surface.
Other targets for coalbed methane development in the Upper ARCTICO C E A N
Beaufort Sea
0.1 -0.5
0.51 -0.75
0.76 -0.99
1.0 -1.97
1.98 -4.75
EXPLANATION
(Vitrinite reflectance)
Anchorage
Juneau
Barrow
ALASKA
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Figure 104. Map showing surface vitrinite reflectance values in the Northern Alaska-Slope, Central
Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
Coalbed Methane Potential 85
Table 8a. Vitrinite reflectance values of coals across the surface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces. [Modified from Johnsson and others, 1992]
Record
number Quadrangle scale of 1:250,000 North
latitude
Westlongitude Formation Mean
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86 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Table 8a. Vitrinite reflectance values of coals across the surface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record
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Mean
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Coalbed Methane Potential 87
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Table 8a. Vitrinite reflectance values of coals across the surface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record
number
Quadrangle scale of 1:250,000 Northlatitude
Westlongitude Formation Mean
Ro (%)
88 Alaska Coal Geology, Resources, and Coalbed Methane Potential
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Table 8a. Vitrinite reflectance values of coals across the surface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record
number
Quadrangle scale of 1:250,000 Northlatitude
Westlongitude Formation Mean
Ro (%)
Coalbed Methane Potential 89
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Table 8a. Vitrinite reflectance values of coals across the surface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record
number
Quadrangle scale of 1:250,000
Northlatitude
Westlongitude Formation Mean
Ro (%)
90 Alaska Coal Geology, Resources, and Coalbed Methane Potential
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Table 8a. Vitrinite reflectance values of coals across the surface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record
number
Quadrangle scale of 1:250,000 Northlatitude
Westlongitude Formation Mean
Ro (%)
Coalbed Methane Potential 91
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Table 8a. Vitrinite reflectance values of coals across the surface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record
number
Quadrangle scale of 1:250,000 Northlatitude
Westlongitude Formation Mean
Ro (%)
92 Alaska Coal Geology, Resources, and Coalbed Methane Potential
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Table 8a. Vitrinite reflectance values of coals across the surface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record
number
Quadrangle scale of 1:250,000 Northlatitude
Westlongitude Formation Mean
Ro (%)
Coalbed Methane Potential 93
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Table 8a. Vitrinite reflectance values of coals across the surface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record
number
Quadrangle scale of 1:250,000
Northlatitude
Westlongitude Formation Mean
Ro (%)
94 Alaska Coal Geology, Resources, and Coalbed Methane Potential
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Table 8a. Vitrinite reflectance values of coals across the surface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record
number
Quadrangle scale of 1:250,000 Northlatitude
Westlongitude Formation Mean
Ro (%)
Coalbed Methane Potential 95
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Table 8a. Vitrinite reflectance values of coals across the surface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record
number
Quadrangle scale of 1:250,000 Northlatitude
Westlongitude Formation Mean
Ro (%)
96 Alaska Coal Geology, Resources, and Coalbed Methane Potential
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Table 8a. Vitrinite reflectance values of coals across the surface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record
number
Quadrangle scale of 1:250,000 Northlatitude
Westlongitude Formation Mean
Ro (%)
Coalbed Methane Potential 97
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Table 8a. Vitrinite reflectance values of coals across the surface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record
number
Quadrangle scale of 1:250,000
Northlatitude
Westlongitude Formation Mean
Ro (%)
98 Alaska Coal Geology, Resources, and Coalbed Methane Potential
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Table 8a. Vitrinite reflectance values of coals across the surface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record
number
Quadrangle scale of 1:250,000 Northlatitude
Westlongitude Formation Mean
Ro (%)
Coalbed Methane Potential 99
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Table 8a. Vitrinite reflectance values of coals across the surface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record
number
Quadrangle scale of 1:250,000 Northlatitude
Westlongitude Formation Mean
Ro (%)
100 Alaska Coal Geology, Resources, and Coalbed Methane Potential
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Table 8a. Vitrinite reflectance values of coals across the surface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record
number
Quadrangle scale of 1:250,000 Northlatitude
Westlongitude Formation Mean
Ro (%)
Coalbed Methane Potential 101
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Table 8a. Vitrinite reflectance values of coals across the surface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record
number
Quadrangle scale of 1:250,000
Northlatitude
Westlongitude Formation Mean
Ro (%)
102 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Table 8b. Vitrinite reflectance values of coals across the subsurface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces. [Modified from Johnsson and others, 1992]
Record
number
Quadrangle scale of 1:250,000 Well name North latitude West longitude MeanRo (%)
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Coalbed Methane Potential 103
Table 8b. Vitrinite reflectance values of coals across the subsurface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record number
Quadrangle scale of 1:250,000
Well name North
latitude
West longitude
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104 Alaska Coal Geology, Resources, and Coalbed Methane Potential
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Table 8b. Vitrinite reflectance values of coals across the subsurface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record number
Quadrangle scale of 1:250,000 Well name North
latitude
West longitude MeanRo (%)
Coalbed Methane Potential 105
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Table 8b. Vitrinite reflectance values of coals across the subsurface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record number
Quadrangle scale of 1:250,000
Well name North
latitude
West longitude MeanRo (%)
106 Alaska Coal Geology, Resources, and Coalbed Methane Potential
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Meade Quadrangle, National Petroleum Reserve in
Alaska. Modified from Magoon and Bird (1988).
Figure 106. Vitrinite reflectance values for the Tunalik No.
1 well, National Petroleum Reserve in Alaska. Modified
from Magoon and Bird (1988).
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Table 8b. Vitrinite reflectance values of coals across the subsurface in the Northern Alaska-Slope, Central Alaska-Nenana, and Southern Alaska-Cook Inlet coal provinces.
— Continued
Record number
Quadrangle scale of 1:250,000
Well name North
latitude
West longitude MeanRo (%)
Coalbed Methane Potential 107
Cook Inlet are in the Tyonek area where the coal beds in the
Tyonek are as much as 50 ft (15 m) thick occurring at shallow
depths of less than 2,000 ft (610 m). Also, the infrastructure
(for example, pipeline) of existing petroleum development is
readily available in the area.
A basinwide variation in thermal maturity of the Cook
Inlet Basin was determined by Johnsson and others (1993)
from vitrinite reflectance values of 30 offshore and onshore
wells (fig. 114). Reflectance values range from 0.24 to 0.95
percent with high values (0.6–0.8 percent) occurring at
increasing depths in the northeastern and southwestern parts of
the Cook Inlet Basin (see fig. 114). This indicates that tec-
tonic deformation and volcanism along the Aleutian volcanic
arc influenced the high reflectance values in the northeastern
part of the basin, particularly in the Matanuska Valley. The
localized upgrading of the thermal maturity in the Matanuska
Valley, based on reflectance values of the Tyonek coal beds, is
shown in figure 115 (Smith, 1995). Here, the Tyonek coal beds
are mainly subbituminous, but the older coal beds in the east-
ern part of the valley range from bituminous to semianthracite.
The central part of the basin maintains reflectance values from
0.4 to 0.6 percent, which indicate burial influence by thick
sedimentary rock sequences of 12,000–13,000 ft (3,660–
3,960 m) (see figs. 58, 59, and 60) along the basin center.
Thermal maturity measured from vitrinite reflectance data is
relatively low. Shi-Ming (1996) suggested, in a study of clay
mineral diagenesis, that temperatures never exceeded 167°F
(75°C) (fig. 116). Rapid rate of subsidence and sedimentation
of the Cook Inlet Basin probably controlled the low thermal
maturity of the Tyonek, Beluga, and Sterling coal beds. Gener-
alized vitrinite reflectance lines are superimposed on the cross
sections in figures 117 and 118.
Coal beds identified in lithologic logs of the Tyonek and
Beluga Formations in the Edna Mae Walker No. 1 well in
Kenai Peninsula (see fig. 116) are directly associated with the
high gas shows indicated on the mud logs (fig. 116). These
coal beds contain as much as 2.5 percent by volume of coalbed
methane marked by high gas kicks (see fig. 116). However,
based on the downhole hot wire total gas results (see high gas
kicks in fig. 116), the coal beds in the upper part of the Tyonek
Formation contain by far the most coalbed methane resources.
Coal beds in the lower part of the Beluga Formation consist of
moderate amounts of coalbed methane resources. Coal beds
of the Sterling Formation contain very low coalbed methane
concentrations. The difference in the coalbed methane content
between the Beluga and Sterling coals may be related to the
variation in their rank, beds in the Sterling Formation being
mainly lignite and those in the Tyonek and Beluga beds being
mainly subbituminous (Barnes and Cobb, 1959). Vitrinite
reflectance values of the Sterling Formation coal beds range
from 0.32 to 0.44 percent, the Beluga coal beds from 0.42 to
0.58 percent, and the Tyonek Formation coal beds from 0.45 to
0.66 percent; all values increase with depth (fig. 117). These
vitrinite reflectance values are closely similar to the subbitu-
minous Paleocene Fort Union coal beds (0.31–0.49 percent)
in the Powder River Basin of Wyoming and Montana, which
have an average gas content of 25 scf/t. Also, the gas content
(based on the hot wire total gas and methane in mud logs;
see fig. 116) of the Powder River Basin coal beds appears to
increase with depth, from below 6,000 ft (1,830 m) to more
than 13,000 ft (3,960 m). However, producibility of gas from
coal-bed reservoirs at these depths may be negligible due
to low permeability below 6,000 ft (1,830 m) (McKee and
others, 1986). Thus, by comparison, coal beds of the upper
Tyonek and lower Beluga Formations contain the best coalbed
methane potential in the Kenai Peninsula, especially reser-
voirs less than 6,000 ft (1,830 m) deep. The upper part of the
Beluga Formation is mainly exposed along the beach bluffs in
the southern Kenai Peninsula. Thus, the targeted coal beds of
the lower Beluga and upper Tyonek occur in subcrop and at
shallower depths than in the Edna Mae Walker well along the
south coast of the Kenai Peninsula.
The hypothetical coal resources of the coal-bearing Kenai
Group in the Cook Inlet Basin were estimated to be as much
as 1.55 trillion short tons (1.45 trillion metric tons) (see
table 1). As much as 1.5 trillion short tons (1.36 trillion short
tons) of these coal resources is offshore (see table 1). Based
on the gas contents of the Tyonek coal beds in the upper Cook
Inlet by Smith (1995), which range from 63–245 scf/t (1.97–
7.6 scm3/gm) at STP, the in-place methane resources in that
part of the basin may be high. However, based on the sub-
bituminous and lignite ranks and the similarity of the vitrinite
reflectance values of the Tyonek, Beluga, and Sterling coal
beds in the central and southern parts of the basin to those of
the Fort Union coal beds in the Powder River Basin, these
coals may provide a lower-end estimate of the gas content in
the Cook Inlet Basin.
Summary
Nearly all the coal resources calculated for Alaska are
in Cretaceous and Tertiary rocks distributed in three major
coal provinces. The Cretaceous coal resources, generally
of bituminous and lignite rank, are mainly in the Northern
Alaska-Slope coal province with 3,200 billion short tons
(2,902 billion metric tons) of hypothetical resources. A minor
amount of Tertiary coal resources are in the Northern Alaska-
Slope coal province with 670 billion short tons (608 billion
short tons) of hypothetical resources. Most of the Tertiary
coal resources, mainly lignite to subbituminous with minor
bituminous and semianthracite, are in the Central Alaska-
Nenana and Southern Alaska-Cook Inlet coal provinces with
more than 1,600 billion short tons (1,451 billion metric tons)
of combined measured, indicated, inferred, and hypothetical
resources. These three coal provinces contain about 87 percent
of the total coal resources and represent most of the minable
coal beds of Alaska. Combined coal resources (measured,
indicated, inferred, and hypothetical resources) in the North-
ern Alaska-Slope, Central Alaska-Nenana, and Southern
Alaska-Cook Inlet coal provinces are about 5,526 billion short
108 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Pebble shale unitPre-pebble shale45612345SP R111098654321SP RSP R123456SP R1234SP R12312345621 SP RSP R345SP R67SP R123Dominantly nonmarine faciesEXPLANATIONDominantly shallow marinesandstone and shaleBottomsets of westerly derived Torok deposited onbottomset turbidite facies of southerly derivedTorok and Fortress Mountain Formations at some horizon belowForesets projected fromseismic line 68E-77in syncline to southBottomsets (basinal)Base of pebble shale unit fromwell and seismic data andadjusted for tectonic thickening of Torok FormationPre-pebble shale unitFormationKukpowrukFormationCorwin Formation1,700-ft Torok removedto account for tectonicthickeningTopsets (shelf)Arbitary location of nomenclature changeGrandstand FormationNanushuk Group NinulukFormationSeabee Formation , Colville Group Chandler FormationNanushuk GroupGR TT1234567891011SP R13456Foresets(slope)????6.7°6°??WESTTunalikNo. 1KaolakNo. 143 km107 km MeadeNo. 162 kmOumalikNo. 118kmEastOumalikNo. 157 kmTitalukNo. 140 kmWolf CreekNo. 346 kmUmiatNo. 1UmiatNo. 2McCullochCol. U. 2GubikNo. 110km9km18km35 kmEASTB.P. ItkillikNo. 1Torok Pebble shale unitTorok-SeabeeFormations, undivided550 ft of fault repetition removed760 ft of fault repetition removed Torok Formation2°1.8°GR = Gamma raySP = Spontaneous potentialR = Resistivity.70.61.57.62.56.54.57360 (.57)488429 (.37)460(.43).60.57.52.60.65.64.700.50.60.52.63.62.75.62.60.7.60.66.57.49.45.55.66.75.69.63.56.58.71.63.55.56.59.64.52.53.60.49.52.65.47.52.58.62.58.60.58.63.60.73.61.53.58.53.51.51.49.73.82.86.450.70Foreset (slope)Figure 107. Stratigraphic cross section of the Nanushuk Group with superimposed vitrinite reflectance values. See figure 8 for location of cross section.
Summary 109
Figure 108. Coalbed methane potential in the Nanushuk Group coals based on the thickness and vitrinite
reflectance of the nonmarine part of the group in the Northern Alaska-Slope coal province. Adopted
from Smith (1995).
Figure 109. Distribution of surface vitrinite reflectance (Ro) values in the Northern Alaska-Slope coal province.
110 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Figure 110. Map of the Cook Inlet Basin showing distribution of oil and gas fields offshore and
onshore. CBM #1 is the well studied by Smith (1995).
Table 9. Properties of sandstone reservoirs and associated gas in the Sterling
and Beluga Formations. Modified from Brimberry and others (1997).
Reservoir data of Kenai field
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tons (5,012 billion metric tons). Of this total, 13.5 billion short
tons (13.2 billion metric tons) are identified coal resources
mainly from the Central Alaska-Nenana and Southern Alaska-
Cook Inlet coal provinces. Thus, only a small fraction of the
total coal resources of Alaska is known, and a large amount is
undiscovered.
Coal mining has been intermittently attempted in the
Central Alaskan-Nenana and Southern Alaska-Cook Inlet coal
provinces. A dozen or more underground and strip mines in
these two coal provinces have produced over 40 million short
tons (36 million metric tons). Thus, only a small fraction of the
identified resources has been produced of the more than 13.5
billion short tons (billion metric tons) that are estimated to
occur in these coal provinces. Alaskan coal resources have low
sulfur content (averaging 0.2–0.4 percent) compared to the
coal in the conterminous United States. This low-sulfur coal
is within or below the minimum value mandated by the 1990
Clean Air Act amendments. The extremely large identified
coal resources are located near existing infrastructure, which
should aid in their development, transportation, and market-
ing. The short distance of these resources to countries in the
western Pacific would appear to make them more marketable
there than in the conterminous United States.
An untapped resource is coalbed methane. With more
than 5,500 billion short tons (5,012 billion metric tons) of
combined coal resources of Alaska coal, the in-place gas
resource is an exceedingly large volume. A large part of the
measured, indicated, inferred, and hypothetical coal resources,
about 5,482 billion short tons (4,972 billion metric tons), is in
the Northern Alaska-Slope and Southern Alaska-Cook Inlet
coal provinces where in-place and planned infrastructure
(pipelines, highways, and so on) can assist in the transporta-
tion and marketability of coalbed gas. The shallow depths to
a large portion of the methane-bearing coal beds in onshore
areas make the gas more accessible for future development.
Summary 111
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Figure 111. Facies profile of the lower part of the Sterling Formation and accompanying downhole logs showing horizons of gas
accumulation. The Sterling facies include fluvial-channel sandstones and flood-plain mudstones and siltstones.
112 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Acknowledgments
The authors acknowledge Heather Mitchell, Dean Han-
cock, Jennifer Goldsmith, and Steve Dunn for their assistance
in generation of the digital illustrations for this paper. The
data for this study were collected from 1988 to 2002 with the
help of numerous geologists, too many to mention here, from
the U.S. Geological Survey, Alaska Department of Natural
Resources Division of Oil and Gas and Division of Geological
and Geophysical Surveys, Alaska Geologic Materials Center,
and Bureau of Land Management. Funding for part of this
study from the Division of Oil and Gas is very much appreci-
ated. Finally, we thank the Usibelli Coal Mine Inc., Beluga,
Diamond Alaska, and Placer Dome coal companies, ARCO
Alaska Inc. and International ARCO, and Marathon Oil Com-
pany, Anchorage, Alaska and Littleton, Colorado, for sharing
information and permission to describe their core data.
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Figure 112. Facies profile of the upper part of the Beluga Formation and accompanying downhole logs showing horizons of
gas-perforated intervals.
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CANADA
UNITED STATES
ALASKA
Area of map CANADA
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�������������������������������Figure 113. Location map of the Kenai gas field in the Kenai
Peninsula. Gas accumulations in the Beluga and Sterling
Formations occur on a doubly-plunging anticline. KTU 43-
6X is the well described in figures 110 and 111.
Acknowledgments 113
Anchorage
Seward
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0.3 -0.4
0.4 -0.5
0.5 -0.6
0.6 -0.7
0.7 -0.8
Cook Inlet Basin
Subsurface perspective
Mean Ro (%)
EXPLANATION
Oil well
Fault
Castle MountainFaultBruinBayFault
Border RangeFault
CANADA
UNITEDSTATES
ALASKA
Area of map CANADAFigure 114. Basinwide
and vertical variations of
vitrinite reflectance (Ro)
values in the Cook Inlet
Basin. Modified from
Johnsson and others
(1993).
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Palmer
Depth to 0.6 vitrinite reflectance
in thousands of feet (km)
Tyonek Formation isopach
in thousands of feet (km)
Tyonek Formation
Chickaloon Formation
EXPLANATION
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in thousands of feet (kilometers)
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Tyonek Formation
Prospective area
Figure 115. Coalbed
methane prospect area
and depths to vitrinite
reflectance values of 0.6
percent superimposed
on the thickness isopach
of the Tyonek Forma-
tion south of the Castle
Mountain fault in the
northeastern part of the
Cook Inlet. Adopted from
Smith (1995).
CANADACANADA
UNITED
STATES
ALASKA
Area of map CANADA
114 Alaska Coal Geology, Resources, and Coalbed Methane Potential
Figure 116. Downhole geophysical logs, hot wire total gas and methane contents,
vitrinite reflectance values, and illite diagenetic values in the Edna Mae Walker drill
hole. SP=spontaneous potential; mV=millivolt; SN=sonic; Ro=vitrinite reflectance;
%=percent. Ipf=illite peak profile at 10 angstroms; Ic= illite crystallinity. Modified
from Shi-Ming (1996).
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Acknowledgments 1150.40.31,0002,0003,0004,0005,0006,0007,0008,0009,00010,00011,00012,000SP(mV)-10025SN(ohmm)SP(mV)SN(ohmm)SP(mV)SN(ohmm)SP(mV)SN(ohmm)SP(mV)SN(ohmm)SP(mV)SN(ohmm)SP(mV)SN(ohmm)SP(mV)SN(ohmm)SP(mV)SN(ohmm)-25100OLD MANS BAY No 11,0002,0003,0004,0005,0006,0007,0008,0009,00010,00011,00012,00013,00014000-1250-25150KALGIN ISLAND1,0002,0003,0004,0005,0006,0007,0008,0009,00010,00011,00012,00013,00014,000-10025-25300REDOUBT SHOALS No 11,0002,0003,0004,0005,0006,0007,0008,0009,00010,00011,000-10025-25200KUSTATAN No 11,0002,0003,0004,0005,0006,0007,0008,0009,00010,000-15025-25300GRAYLING No 1-A10002000300040005000600070008000900010000-1750-25225McARTHUR POINT No 1100020003000400050006000700080009000100001100012000-1250-25150TYONEK STATE No 1100020003000400050006000700080009000100001100012000-15025-25175TOWER No 1?????????10002000300040005000600070008000900010000-12500200TRADING BAY No 1A SOUTHA' NORTHEXPLANATION0.60.50.60.50.4Sterling FormationBeluga FormationTyonek Formation Hemlock ConglomeratePre-Hemlock FormationFigure 117. Stratigraphic cross section of the Kenai Group in the offshore Cook Inlet Basin with superimposed vitrinite reflectance values. See figure 58 for location.
116 Alaska Coal Geology, Resources, and Coalbed Methane Potential
(ft)Depth(ft)Depth(ft)(ft)1,0002,0003,0004,0005,0006,0007,0008,0009,00010,00011,00012,000SP(mV)-100 25SN(ohmm)SP (mV)SN(ohmm)SP (mV)SN (ohmm)SP (mV)SN(ohmm)SP (mV)SN (ohmm)-25 100OLD MANS BAY No 11,0002,0003,0004,0005,0006,0007,0008,0009,00010,00011,00012,00013,00014,000-100 75-25 225NINILCHIK No 1-125 25-25 27501,0002,0003,0004,0005,0006,0007,0008,0009,00010,00011,00012,00013,00014,000DEEP CREEK No 12001,0002,0003,0004,0005,0006,0007,0008,0009,00010,00011,00012,00013,00014,00015,00016,000-125750EDNA MAE WALKER No 1-25 300-150 001,0002,0003,0004,0005,0006,000ANCHOR RIVER No 1SECTION B TO B' (WEST TO EAST SOUTH PART OF STUDY AREA)B (WEST)B' (EAST)DepthSterling FormationBeluga FormationTyonek FormationHemlock Formation Pre-Hemlock 0.60.50.40.30.30.40.50.6Depth(ft)DepthEXPLANATIONFigure 118. Stratigraphic cross section of the Kenai Group in the onshore Cook Inlet Basin with superimposed vitrinite reflectance values. See figure 58 for location.
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Fairbanks
Anchorage
JUNEAU
NORTH SLOPE BOROUGH
NORTHWEST ARCTIC BOROUGH
LAKE & PENINSULA
BOROUGH
KODIAK ISLAND
BOROUGH
KENAI PENINSULA
BOROUGH
MATANUSKA-SUSITNA
BOROUGH
CITY & BOROUGH OF YAKUTAT
DENALI BOROUGH
ALEUTIANS EAST BOROUGH
FAIRBANKS NORTH STAR
BOROUGH
CITY & BOROUGH
OF SITKA
HAINES BOROUGH
CITY & BOROUGH
OF JUNEAU
MUNICIPALITY OF ANCHORAGE
KETCHIKAN GATEWAY
BOROUGH
ALEUTIANS EAST BOROUGH
BRISTOL BAY
BOROUGH
180˚170˚160˚150˚140˚130˚120˚110˚160˚170˚
50˚
60˚
EXPLANATION
Coal rank
Anthracite
Bituminous
Lignite
Subbituminous
Ownership
U.S. Forest Service
U.S. Fish and Wildlife
Wild and Scenic Rivers
Bureau of Land Management
Military land
Native-owned land
State-owned land
State-and Native-owned lands
Private land
Borough boundary
0 190 380 570 76095
MILES02805608401,120140
KILOMETERSARCTIC OCEANGulf of Alaska
B
ea
ufort Sea
BRISTOL BAY
KOTZEBUE SO
U
N
D
KUSKOKWIM BAY
NORTON SOUND
A L E U T I A N I S L A N D S River
RiverRiver
Riv
e
r
RiverRiverKobuk
YukonYukonColville
NoadakRiver
River
Kuskokwim River
Koyukuk
Tanana
SusitnaPoint Hope
Barrow
Harrison
B
a
y
Cook InletY akutat B ayPrince William
Sound
Port Graham
Figure 2. Map showing the coal rank and land ownership of Alaska
STRATIFIED SEDIMENTARY SEQUENCE
Mainly marine. Includes some volcanic rocks. In part metamorphosed
HOLOCENE DEPOSITS—Alluvial, glacial, lake, estuarine, swamp, landslide, flood-
plain, and beach deposits
PLEISTOCENE DEPOSITS—Alluvial, glacial, dune sand, loess, and reworked sand and
silt deposits
QUATERNARY DEPOSITS—Alluvial, glacial, lake, eolian, beach, and volcanic depos-
its. Includes the marine Bootlegger Cove Clay
MIOCENE ROCKS—Sandstone, siltstone, conglomerate, ar---gillite, graywacke, and basal-
tic rocks. Includes the Bear Lake Formation on the Alaska Peninsula, the Narrow Cape For-
mation (Oligocene or Miocene) on Kodiak and Sitkinak Islands, and the Chuniksak Forma-
tion (Miocene?) on Attu Island
OLIGOCENE ROCKS-—Volcanic conglomerate, sandstone, volcanic breccia, shale, and
siltstone. As shown, includes the Meshik Formation and Stepovak Formation on the Alaska
Peninsula and the Sitkinak Formation on Sitkalidak, Sitkinak, and Chirikof Islands
TERTIARY ROCKS—Sedimentary rocks concealed beneath Quaternary cover on Point
Hope and volcanogenic sedimentary rocks and flows, dikes, and sills on the Alaska Peninsula
and Umnak Island
CRETACEOUS ROCKS—Volcanic graywacke, mudstone, and sandstone with some
coal-bearing rocks in the Yukon-Koyukuk province; graywacke and shale of the Kuskokwim
Group in the Kuskokwim Mountains; and shelf deposits of sandstone, siltstone, shale, and
limestone of (1) the Kennicott, Moonshine Creek, Schulze, Chititu, and MacColl Ridge For-
mations in the southern Wrangell Mountains; (2) the Matanuska Formation in the Matanuska
Valley; and (3) the Kaguyak Formation on the Alaska Peninsula
UPPER CRETACEOUS ROCKS—Shale, sandstone, and conglomerate of the Ninuluk
Formation of the Nanushuk Group and the Seabee and Schrader Bluff Formations of the Col-
ville Group in the Arctic 0151 Coastal Plain and Foothills; nonmarine and marine clastic
rocks, siltstone, and shale of the Chignik and Hoodoo Formations on the Alaska Peninsula;
graded beds of sandstone and slate of the Kodiak Formation on Kodiak and Afgonak Islands;
sandstone and mudstone of the Shumagin Formation on Shumagin and Sanak Islands
LOWER CRETACEOUS ROCKS—Graywacke, sandstone, shale, siltstone, and con-
glomerate of part of the Tiglukpuk Formation of former usage, and the Okpikruak, Fortress
Mountain, Torok, and Kukpowruk Formations in the western Arctic Foothills; the Kongakut
Formation, Bathtub Graywacke, and Tuktu and Grandstand Formations in the eastern Brooks
Range and Arctic Foothills; unnamed graywacke, argillite, conglomerate, and minor lime-
stone southeast of the mouth of the Kuskokwim River; interlayered submarine and subaerial
andesitic fragmental volcanic rocks, flows, tuffs, and volcanic clastic rocks of the Chisana
Formation north of the Wrangell Mountains; and unnamed graywacke, argillite, and minor
andesite on Etolin Island
UPPER TERTIARY ROCKS—Sandstone, siltstone, shale, mudstone, and conglomer-
ate of Miocene and Pliocene age. Includes upper part of the Sagavanirktok Formation on the
Arctic Coastal Plain, and the Yakataga Formation in the Gulf of Alaska area
MIDDLE TERTIARY ROCKS—Siltstone, sandstone, organic shale, and, locally, vol-
canic rocks. Includes the Poul Creek, Katalla, and Topsy Formations ranging from Oligocene
to Miocene age in Gulf of Alaska area
LOWER TERTIARY ROCKS—Interbedded sedimentary, volcanogenic, and volcanic
rocks of Paleocene, Eocene, and Oligocene age on the Alaska Peninsula and Aleutian Islands
and intensely deformed marine and continental clastic rocks of Paleocene and Eocene age in
the Gulf of Alaska area. Includes the Tolstoi and Belkofski Formations of Burk (1965) in the
Alaska Peninsula; the Ghost Rocks Formation on Kodiak Island; the Amchitka and Banjo
Point Formations on Amchitka Island; the Gunners Cove Formation on Rat Island; the Kru-
gloi Formation on Agattu Island; and the Kulthieth, Kushtaka, and Tokun Formations and
clastic rocks of the Orca Group in the Gulf of Alaska area
CRETACEOUS AND JURASSIC ROCKS—Argillite, shale, graywacke, quartzite, con-
glomerate, lava, tuff, and agglomerate almost barren of fossils; probably includes rocks ranging
in age from Early Jurassic to Late Cretaceous. In places moderately to highly metamorphosed
(amphibolite facies)
CRETACEOUS AND UPPER JURASSIC ROCKS—Graywacke, slate, argillite,
minor conglomerate, volcanic detritus, and interbedded mafic volcanic rocks. Includes the
Valdez Group and part of the Yakutat Groups and Sitka Graywacke. Mildly metamorphosed,
locally to greenschist
LOWER CRETACEOUS AND UPPER JURASSIC ROCKS—Shallow and
deep-water clastic deposits (Oxfordian to Barremian) north of the Wrangell Mountains; in-
cludes sandstone, arkose, siltstone, and limestone of the Staniukovich Formation (Burk,
1965) and the Herendeen Limestone on the Alaska Peninsula; and slate, graywacke, and con-
glomerate of the Seymour Canal Formation on Admiralty and Kupreanof Islands
LOWER CRETACEOUS AND UPPER JURASSIC(?) ROCKS—Melange of
flysch, greenstone, limestone, chert, granodiorite, glauco----phane-bearing greenschist, and lay-
ered gabbro and serpentinite. Melange consists of Upper Jurassic(?) and Lower Cretaceous
pelitic matrix enclosing blocks several kilometers in dimension of Permian to Lower Jurassic
rocks. Includes the Uyak Formation, McHugh Complex, melange within the Yakutat Group,
and the Waterfall Greenstone and Khaz Formation of the Kelp Bay Group
JURASSIC ROCKS—Shale, siltstone, and sandstone. Includes the Kingak Shale along the
northern front of the Brooks Range, the Glenn Shale (which includes rocks of Triassic and
Cretaceous age) in the east-central part of the State, the Nizina Mountain Formation and Kot-
sina Conglomerate along the southern Wrangell Mountains, and unnamed slaty detrital rocks
on Gravina and Annette Islands
UPPER JURASSIC ROCKS—Sandstone, siltstone, shale, and conglomerate on the
Alaska Peninsula, Cook Inlet area, and southern flank of the Talkeetna Mountains. Includes
the Chinitna and Naknek Formations
MIDDLE JURASSIC ROCKS—Argillite, graywacke, and conglomerate southeast of
the Kuskokwim River and sandstone, shale, siltstone, and conglomerate on the Alaska Penin-
sula and Cook Inlet area where it includes the Kialagvik and Shelik Formations and Tuxedni
Group
LOWER JURASSIC ROCKS—Sandstone and argillite interbedded with volcanic
flows and pyroclastic rocks of the Talkeetna Formation in the Cook Inlet area and southern
Talkeetna Mountains
JURASSIC AND (OR) TRIASSIC ROCKS—Chert and argillite north of the Porcu-
pine River; limestone with minor dolomite, shale, and chert of the Chitistone Limestone, Ni-
zina Limestone, McCarthy Formation, and Lubbe Creek Formation along the southern Wran-
gell Mountains; and hornfels and phyllite of the Hazelton(?) Group in southeast Alaska
TRIASSIC ROCKS—Shale, chert, and limestone of the Shublik Formation and quartzitic sand-
stone of the Karen CreekóSandstone on the north flank of the Brooks Range
UPPER TRIASSIC ROCKS—Limestone, shale, and chert of the Kamishak Formation
in the Cook Inlet area; a shelf facies of limestone, tuff, tuffaceous conglomerate and breccias
at the southern tip of the Kenai Peninsula (west of the Border Range's fault) and equivalent
rocks on Shuyak, Afognak, and Kodiak Islands; a deep-water flysch and melange facies of
chert, pillow basalt, and associated graywacke, argillite, and minor ultramafic rocks (east of
the Border Ranges fault) on the southern Kenai Peninsula; and chert, limestone, sandstone,
and greenstone of the (1) Whitestripe Marble and Pinnacle Peak Phyllite (both Triassic?) on
Chichagof and Baranof Islands, (2) Hyd Group on Admiralty Island and Keku Straits area,
and (3) Nehenta and Chapin Peak Formations on Gravina Island
TRIASSIC AND PERMIAN ROCKS—Sandstone, siltstone, and shale of the Sadlero-
chit Group on the north flank of the Brooks Range; mafic volcanic rocks, red beds, lime-
stone, andcalcareous argillite in the Chulitna River area; argillite, limestone, siltstone, con-
glomerate, and abundant gabbroic sills in the east-central Alaska Range where it includes the
upper part of the Mankomen Group; and schist, graywacke, slate, conglomerate, phyllite, and
andesite flows and tuffs on Admiralty Island where it includes the Barlow Cove Formation
JURASSIC, TRIASSIC, AND PERMIAN ROCKS—Shale, siltstone, chert, and gray-
wacke in the Brooks Range. Includes upper part of the Nuka Formation and the Siksikpuk
and Shublik Formations
MESOZOIC AND PALEOZOIC ROCKS—Sandstone, shale, chert, dolomite, and
conglomerate, in a discordant rock sequence of unknown provenance that includes rocks of
Mississippian, Triassic, Jurassic, and Cretaceous age in the western Brooks Range (includes
Nuka Formation); Lower Jurassic, Pennsylvanian, and Permian rocks, in part covered by Ter-
tiary sedimentary rocks and intruded by granitic rocks of Tertiary age, in north-central Chu-
gach Mountains; and slate, quartzite, schist, and phyllite with interlayered beds of marble,
layered gneiss, and amphibolite of Ordovician to Jurassic or Cretaceous age along the west
flank of the Coast Mountains
PERMIAN ROCKS—Chert, shale, and siltstone of the Siksikpuk and Echooka Formations
in the central Arctic Foothills and volcanic argillite and graywacke with local chert, pillow
flows, limestone, and dolomite of the Cannery, Pybus, and Halleck Formations on Admiralty,
Kuiu, and Kupreanof Islands
PERMIAN AND PENNSYLVANIAN ROCKS—Basaltic to andesitic lavas and deriv-
ative volcaniclastic rocks, tuffs, minor gabbro, and local shallow-water sedimentary rocks
metamorphosed to greenschist facies and, locally, amphibolite facies of unnamed phyllite,
slate, schist, greenschist, amphibolite, gneiss, and migmatite in St. Elias Mountains
PENNSYLVANIAN ROCKS—Siltstone, sandstone, and limestone of the Klawak Forma-
tion and Ladrones Limestone on Prince of Wales Island
PENNSYLVANIAN AND MISSISSIPPIAN ROCKS—Limestone, conglomerate,
shale, dolomite, and chert of the Kekiktuk Conglomerate and Kayak Shale (both of Missis-
sippian age) of the Endicott Group and the Alapah and Wahoo Limestones of the Lisburne
Group
MISSISSIPPIAN ROCKS—Conglomerate, shale, limestone with subordinate chert, and
dolomite of the Kekiktuk Conglomerate and Kayak Shale of the Endicott Group and the Utu-
kok Formation and Wachsmuth and Alapah Limestones of the Lisburne Group on the north-
ern flank of the Brooks Range. Limestone, dolomite, and interbedded chert of the Iyoukeen
Formation on Chichagof Island and Peratrovich Formation on Prince of Wales Island
JURASSIC TO MISSISSIPPIAN ROCKS—Unnamed slate and quartzite northwest of
Porcupine River and the Lisburne and Sadlerochit Groups and Kingak Shale at northeast
front of Brooks Range
TRIASSIC TO DEVONIAN ROCKS—Radiolarian chert, slate, and argillite
[These geologic unit descriptions are modified from Beikman (1980, Geologic map
of Alaska: U.S. Geological Survey Special Map, scale 1:2,500,000, 2 sheets).
Several rock units were combined to simplify and reduce the size of the digital files]
PALEOZOIC ROCKS—Limestone, marble, dolomite, and chert on Seward Peninsula and
St. Lawrence Island; limestone, slate, and conglomerate in central Alaska Range; argillite
and graywacke slightly metamorphosed west of Chulitna River; flysch, conglomerate, lime-
stone, and pillow basalt southwest of Mount McKinley; marble, in places containing tremo-
lite, in Wrangell Mountains where it includes parts of a Devonian section designated the
Kaskawulsh Group in the Yukon Territory (Canada); and sedimentary, metasedimentary, and
metavolcanic rocks in southeastern Alaska
UPPER PALEOZOIC ROCKS—Argillite, chert, shale, limestone, and siltstone.
Greenstone, limestone, shale, clastic sedimentary rocks, schist, gneiss, and undifferentiated
metamorphic rocks east of Juneau
MISSISSIPPIAN AND (OR) DEVONIAN ROCKS—Sandstone, graywacke, quartz-
ite, and conglomerate. Includes the Noatak Sandstone in western Brooks Range and the Ke-
kiktuk and Kanayut Conglomerates in eastern Brooks Range
DEVONIAN ROCKS—Phyllite, hornfels, graywacke, and sandstone on the Seward Penin-
sula; pyroclastic rocks and ash flows interbedded with sedimentary rocks metamorphosed to
schist and gneiss on north-central flank of Alaska Range; limestone east of Kuskokwim Bay;
clastic rocks and limestone of the Kennel Creek Limestone (which may also include Silurian
rocks) and Cedar Cove Formation on Chichagof Island; schist, phyllite, marble, and amphib-
olite of the Retreat Group and Gambler Bay Formation on Admiralty and Kupreanof Islands
and equivalent rocks to the north and south; and limestone, shale, graywacke, conglomerate,
and basaltic rocks of the St. Joseph Island Volcanics (Devonian?), Wadleigh Limestone, and
Port Refugio Formation on Prince of Wales Island
UPPER DEVONIAN ROCKS—Shale, sandstone, chert, conglomerate, and quartzite in
eastern and central Brooks Range and limestone and dolomite in western Brooks Range. In-
cludes the Hunt Fork Shale, Kanayut Conglomerate, Kugururok Formation, and Eli Lime-
stone (Middle and Upper Devonian)
DEVONIAN AND SILURIAN ROCKS—Limestone, dolomite, marble, and shale of
the Katakturuk Dolomite and Skajit Limestone in Brooks Range and the Karheen Formation
in Prince of Wales Island
SILURIAN ROCKS—Graywacke, shale, siltstone, limestone, sandstone, and argillite. In-
cludes siltstone, mudstone, limestone, conglomerate, sandstone, graywacke, minor red beds,
and volcanic rocks of the Rendu Formation and Willoughby Limestone in Glacier Bay area;
the Point Augusta Formation on Chichagof Island; the Bay of Pillars Formation on Admiral-
ty, Kuiu, and Prince of Wales Islands; and the Kuiu Limestones and Heceta Limestone on
Prince of Wales Island
ORDOVICIAN ROCKS—Limestone and shale on Seward Peninsula; argillite, chert and
limestone of the Hood Bay Formation on Admiralty Island
SILURIAN AND ORDOVICIAN ROCKS—Graywacke, conglomerate, shale, silt-
stone, tuff, lava, and local limestone of the Descon Formation on Prince of Wales Island
CAMBRIAN ROCKS—Siltstone, sandstone, and phyllite
ORDOVICIAN, CAMBRIAN, AND PRECAMBRIAN ROCKS—Phyllite, sand-
stone, siltstone, limestone, chert, and quartzite
PALEOZOIC AND (OR) PRECAMBRIAN ROCKS—Sandstone, limestone, shale,
chert, phyllite, argillite, and quartzite of the Neruokpuk Formation in the northeast Brooks
Range; quartz mica schist, mafic greenschist, calcareous schist, chloritic schist, phyllite, and
quartzite along south flank of Brooks Range and southwest through Kokrine-Hodzana High-
lands; schist and quartzite of the Birch Creek Schist of former usage in Yukon-Tanana High-
lands; highly metamorphosed clastic rocks including the Keevy Peak Formation in north
flank of Brooks Range; and volcanogenic greenschist with interstratified marble in Prince of
Wales, Long, and Dall Islands, where it includes the Wales Group and possibly the Descon
Formation
LOWER PALEOZOIC ROCKS—Rocks of Cambrian through Devonian age, in pla-
ces metamorphosed to greenschist and amphibolite facies. Sedimentary rocks include lime-
stone, dolomite, argillite, chert, and graywacke; metasedimentary rocks include schist,
quartzite, slate, greenstone, carbonate rocks, and phyllite. Includes the Holitna Group in Kus-
kokwim Mountains; the Tonzona Group along Kuskokwim River; rocks formerly included in
the Birch Creek Schist in Yukon-Tanana Upland; and unmetamorphosed rocks of the Funnel
Creek, Adams, Hillard, Road River, McCann Hill, and Hillard Formations and Puppets For-
mation on Gravina and Annette Islands
LATE PROTEROZOIC ROCKS—Siltite, phyllite, graywacke, quartz schist, and graph-
itic schist of slate of the York region on Seward Peninsula; schist, gneiss, and small amounts
of amphibolite and marble east of Kuskokwim Bay; quartz wacke, semischist, phyllite, and
argillite of the Neruokpuk Formation in northeastern Brooks Range; phyllite, slate, and silt-
stone east of Fort Yukon; and limestone, dolomite, sandstone, shale, and basalt of the Tindir
Group north of Tintina fault
YOUNGER LATE PROTEROZOIC ROCKS—Schistose, argillaceous, dolomitic
limestone and tactite on Seward Peninsula
OLDER LATE PROTEROZOIC ROCKS—Schist, gneiss, and migmatic and meta-
morphic rocks, including rocks equivalent to slate of the York region, in the Kigluaik and
Bendeleben Mountains on the Seward Peninsula
Tm
To
TpPLIOCENE ROCKS—Sandstone, siltstone, and conglomerate. Includes the Tachilni For-
mation on the Alaska Peninsula and the Tugidak Formation on Tugidak and Chirikof Islands
Qh
Qp
K
KJ
J
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DS
S
O
SO
TERTIARY CONTINENTAL DEPOSITS—Sandstone, siltstone, claystone, shale, conglomer-
ate, and coal beds. Include the Sagavanirktok Formation in the Arctic Coastal Plain; the Gakona Forma-
tion in east-central Alaska Range; the Healy Creek, Suntrana, Sanctuary, Lignite Creek, and Grubstake
Formations and Nenana Gravel and related unnamed rocks in west-central Alaska Range; the Chicka-
loon, Wishbone, and Tsadaka Formations in the Matanuska Valley; and the Kenai Group (Hemlock,
Tyonek, Beluga, and Sterling Formations) in Cook Inlet area. It also includes the Kootznahoo Forma-
tion on Admiralty, Kuiu, Kupreanof and Zarembo Islands; the Frederika Formation in Wrangell Moun-
tains; and the Cantwell Formation in central Alaska Range. Rocks range in age from Paleocene through
Pliocene
TERTIARY AND CRETACEOUS CONTINENTAL DEPOSITS—Conglomerate, brec-
cia, sandstone, arkose, mudstone, shale, tuffaceous rocks, and lignite beds. Includes the Arkose
Ridge Formation (Cretaceous?) in Matanuska Valley
CRETACEOUS CONTINENTAL DEPOSITS—Sandstone and conglomerate, siltstone,
claystone, shale, coal, coaly shale, ironstone, and bentonite beds. Includes the Corwin Formation
(Lower and Upper Cretaceous) of Nanusuk Group and Killik Tongue of Chandler Formation of
Nanusuk Group, the Niakogon Tongue of Chandler Formation of the Nanushuk Group, and the
Prince Creek Formation of Colville Group on the Arctic Coastal Plain and in the Yukon-Koykuk
Basin, and margins of the basin
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UPPER AND (OR) MIDDLE DEVONIAN ROCKS—Conglomerate, graywacke,
phyllite, shale, sandstone, siltstone, and limestone. Includes the Nanook Limestone in Shu-
blik Mountains
sSa
DESCRIPTION OF MAP UNITS
CONTINENTAL DEPOSITS
SCALE 1: 3,800,000
Anchorage
Fairbanks
180˚ 171˚ 162˚ 153˚ 144˚ 135˚ 72˚
171˚ 162˚ 153˚ 144˚ 135˚
72˚
63˚
63˚
54˚54˚
Ice
Water
Fault—Dotted where concealed
Coastline
Contact
180˚
EXPLANATION
UNDIFFERENTIATED METAMORPHIC, IGNEOUS, ULTRAMAFIC, AND
VOLCANIC ROCKS
METAMORPHIC AND IGNEOUS ROCKS—Small masses of metamorphosed
sedimentary, volcanic, and igneous rocks largely of pre-Cretaceous age scattered throughout
the Aleutian Range batholith and amphibolite facies schist along north side of Matanuska
Valley. Includes intercalated blueschist, quartz mica schist, greenschist with subordinate am-
phibolite, and marble; metachert at southern tip of Kenai Peninsula and on Afognak Island;
and metasedimentary, metaplutonic, and metavolcanic rocks near Anchorage and along south
side of Matanuska Valley. Also includes hornfels, schist, amphibolite, minor marble, and un-
divided metamorphic rocks north of Icy Strait in southeastern Alaska; gneiss, schist, phyllite,
and undifferentiated metasedimentary and metaigneous rocks in the Yukon-Tanana Upland;
and metasedimentary and metaigneous rocks, including schist and gneiss of many different
compositions, primarily of the greenschist and amphibolite facies, in the Yukon-Tanana Up-
land (formerly included in the Birch Creek Schist). Rocks range in age from Mesozoic
through Paleozoic
ULTRAMAFIC AND IGNEOUS ROCKS—Granite to granodiorite, gabbro, and
syenite to diorite. Rocks range in age from Cenozoic through Precambrian
VOLCANIC ROCKS—Trachyte to andesite, basalt, and rhyolite to dacite. Rocks range
in age from Cenozoic to Paleozoic
Kc
TKc
Tc
CENOZOIC AND LATE PROTEROZOIC ROCKS
Figure 3. Map showing the geology and structure of Alaska. Modified from S.J. Moll, Scott Bie, Devon Peterson,
D.C. Pray, F.H. Willson, J.W. Schmidt, J.R. Riehle, and T.P. Miller (unpublished data, 1997, U.S. Geological Survey,
Reston, Virginia). After Beikman (1980).
KALTAG FAULTSHAW CREEKFAULTTINTINA FAULT
STRAND
DENALI
FAULT
FAREWELL
FAULT
MC KINLEY
TOTSOHU
N
D
A FA
U
LT
STRAND
IDITAROD-NIXON FORK FAULT
ANIAK-THOMPSON CREEKCHIROSKEYFAULTMULCHATNA FAULTCASTLE MOUNTAINFAULT
BRUIN BAYCONTACT FAULT
BORDER RANGES FAULTEAGLE RIVERFAULTS
T
R
A
I
CLA
RE
N
CFAULT
F
A
U
L
TTOGIAK- TIKCFAULTHIKHOLTINAFAULTFAULTHINES CREEK
F
AI
RWE
A
T
H
E
R F
A
U
L
T
C
A
T
HAM
STRAI
T
FAULTPERIL STRAIT LT
0
ARCTIC
OCEAN
GULF OF
ALASKA
MILES
One inch = approximately 60 miles
150
CANADA
300
0KILOMETERS
One centimeter = approximately 97 kilometers
241483
Port Graham
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S e w a r d P e n i n s u l a
N u i a t o H i l l sA L A S K A R A N G EK U S K O W I M M O U N T A I N SChugach Mountains
B R O O K S RANGE
CENOZOIC
MESOZOIC
PALEOZOIC
PRECAMBRIAN
QUATERNARY
TERTIARY
CRETACEOUS
JURASSIC
TRIASSIC
PERMIAN
PENNSYLVANIAN
MISSISSIPPIAN
DEVONIAN
SILURIAN
ORDOVICIAN
CAMBRIAN
PRECAMBRIAN
CENOZOIC AND LATE
PROTEROZOIC
ROCKS
Kc
Kc
Kc
Kc
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Tm
Tm
To
To
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KJ
KJ
KJ
KJ
KJ
KJ
KJ
KJ
KJ
KJ
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JPJP
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hM
hM
M
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MJM
JM
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MD
MD
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DS
DS
DS
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SO
SO
Qh
Qh
Qh
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ALEUTIAN
TRENC
H
Bristol BayKotzebue Sound
Beaufort Sea
Kuskokwim Bay
Norton Sound
TESHEKPUK
LAKE
Harrison
B
a
y
Point Barrow
COLVILLERIVERIcy Cape
Cape Lisburne
Point Hope
Bering Strait
St. Lawrence Island
Norton
B
a
y
KOBUK
RIVER
KOYUKUK
RIVER
YUKONRIVERYUKONRIV
E
R
PORCUPINE RIVER.TANANA
RIVER
Cook InletILIAMNA LAKE
Kodiak
Island
Yakutat Bay
Lituya BayProjection: Albers Equal Area
Central meridian: 153˚ W.
St. Matthew
Island
Nunivak Island
St. Paul
Island
St. George
Island
Umnak Island
Unalaska Island
Amlia Island
Atka Island
Adak
Island
Tanaga
Island
Kanaga
Island
BERING SEA
Unimak IslandA L E U T I A N I S L A N D S
KUSKOKWIMRIVERSUSITNARIVER
Prince
William
Sound
Montague IslandUnga
Island
Seguam
Island
Trinity
IslandsShelik of StraitA L E X A N D E R A R C H I P E L A G O Cross SounDixon Entrance
Prince
of Wales
Island
Baranof
Island
Admiral
Island
Tp
Tp
TKc
TKc
TKc
TKc
ee
e
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U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY Digital Data Series-77
Jd
Juneau
This map shows generalized geology in Alaska and therefore a number of features
that are listed in the map unit descriptions are not indicated on the map due to space
limitations. Also the data that are on this CD-Rom do not duplicate this graphic,
only the generalized geology and faults. The other data that are represented on this
graphic (rivers, coastline, and cities) are available from the Department of
Natural Resources of Alaska on the web at http://www.asgdc.state.ak.us/
homehtml/pubaccess.html.
CORRELATION OF MAP UNITS
CONTINENTAL DEPOSITS
QhQ
P
S
OSO
MD
JM
JP
KJ
D
M
h
g
PPh
hM
Qp
Tp
Tm
To
J
K
T
d
e
Jd
dP
dD
DS
cg
fg
Ope
gpe
sSa
STRATIFIED SEDIMENTARY SEQUENCE
Mainly marine. Includes some volcanic rocks. In part metamorphosed
Kc
TKcTc
Port Graham
River
RiverRiver
Riv
e
r
RiverRiverKobukYukonYukonColvilleNoadakRiver
River
Kuskokwim R i ver
Koyukuk
Tanana
Susitna1000 200 MILES50
0 100 200 300 KILOMETERS
Physiographic Map of Alaska
IceARCTIC OCEANGulf of Alaska
B
ea
ufort Sea
ALASKACANADAARCTIC COASTAL PLAIN
ARCTIC FOOTHILLS RANGE
Y U K O N F L A T S
C E N T R A L & E A S T E R N B R O O K S R A N G E
INTERIO R O R IN T E R M O U N T A IN P L A T E A U
SOUTHERN CORDILLERA
NORTHERN
CORDILLERA
135˚144˚153˚162˚171˚72˚
63˚
54˚
Anchorage
JUNEAU
Fairbanks
Hope
Point Barrow
B R IS T O L B A Y
KOTZEBUE SO
U
N
D
K U S K O K W I M B A Y
N O R T O N S O U N D
Harrison
B
a
y
Cook InletA L E U T I A N I S L A N D S D IX O N E N T R A N C E
Y akutat B ayPrince William
Sound
Figure 4. Map showing the physiographic regions of Alaska. Modified from Plafker and Berg (1994).