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HomeMy WebLinkAboutGeothermal Reconnaissance Survey of the Central Seward Peninsula AK 1981SWD ool ee & je $<. ial UNIVERSITY OF ALASKA FAIRBANKS, ALASKA UAG R-284 GEOTHERMAL RECONNAISSANCE SURVEY OF THE CENTRAL SEWARD PENINSULA, ALASKA Edited by ts? Eugene Wescott and Donald Turner Report to Division of Geothermal Energy U.S. Department of Energy PROPERTY OF: Alaska Power Authority 334 W. 5th Ave. Anchorage, Alaska 99501 Under Cooperative Agreement DE-FC07-79-ET27034 July 1981 <swd o0N Geophysical Institute Report UAG R-284 GEOTHERMAL RECONNAISSANCE SURVEY OF THE CENTRAL SEWARD PENINSULA, ALASKA Eugene Wescott and Donald L. Turner, Editors Prepared for: The Division of Geothermal Energy of the U.S. Department of Energy Cooperative Agreement DE-FCO7-79-ET 27034 Principal Investigators: Donald L. Turner, Eugene M. Wescott and Juergen Kienle Participating Scientists: Kenneson Dean, Frank Eaton, Robert B. Forbes, Kathryn Sullivan*, Samuel Swanson Research Assistants: Anthony Dunn, Andrew Lockhart, Chester Paris, William Witte July 1981 -ECEIVETS R SFP 7 *NASA Johnson Space Center vectiTiay AUB 7ROena The University of Alaska offers equal educational and employment opportunities. TABLE OF CONTENTS SUMMARN ciclo sais sveists io titers sis atau dss Secures a hee See eS oa) TNTRODUCTION. «2.5 o:c:sc:0ce c's cae Vid S OS Hine eG hOS vale Baas ke ROP GEORGES) 5 Cio cress a's 8.dsd6-t:0b6-d eo 6 ures soa es ea eek CONTINENTAL RIFTING--A NEW TECTONIC MODEL FOR THE CENTRAL SEWARD PENINSULA by Donald L. Turner and Samuel £. Swanson... PTEROUUCTION Gs. . oison cone ss cece cewessciavisteviiccreneses Regional Geology and Geophysics......sccccccccccceeceecs DVETVICW s.o00.c sce ssesiscsevecccsseerieessbecscisins Imuruk Basin--Pilgrim River Valley......ceceececees Imuruk Lake--Kuzitrin River Lowland.....sceeceecece CLES: 1 JSP oe gp aC ce Eee Rhyolite and Basalt Intrusives........cccesccceecce Rift Model........ $.0,.0:0,8. 9.9.0 '9 0.9.00. hnsa-tenceeape-0:0-010re-uiwrenemeveniv'erd Summary and Conclusions......ccccccecccccccccceccecccece Acknowledgements......eseeeeesees REFErENCES....cceccccccncsccscccccccvccccccccccccesacces Pee e errr recess ccceee HELIUM AND MERCURY IN THE CENTRAL SEWARD PENINSULA RIFT SYSTEM, ALASKA by Eugene Wescott.......... Galea bieis'6e Wide aieieieees TREROGUGET ON 325 05:55 ve'e-0 sued Gee's ead o's 5c Siac tigidin'o aie 4/esnears Helium and Mercury Soil Survey Results from the Central DEWAN PENIS. <:5% 4.0 6:b°s's:0 bis e's cco + o'srvlosdonnwel oboe were OGAal S TLS SUwVveys oceanic caress oe oh HED ERTCLAw HUES s. GOnGIUSTONS: 5. is s%iv dees sd Cone iocwes tik chia Soseecs ADPONGIX As ccs cccccncSecceccsccstecovesecveveseivsccaes ROTOR ENCES 65.0550 cin cis one se cdeicdcescsdccecesvcrdvectielelus GRAVITY SURVEY OF THE CENTRAL SEWARD PENINSULA by Andrew EQCRNAGUs 00% dew oc 5.0 detss 0c ool spect cst. osc ice sccisceeltcue nix: INCPOdUCTION. 0... ccecsccesccccccccccsesccsseccceccecece MOthOdS. ccc ccccse Ob 0:6 08056 0.0 8:0 00h 4 0.0560 6 018% Séwovdelwes REGU CSiis'sa st os55 3 eae Wee 0 500.6% 6 eels oes cacace ei yecesnsce PDD OGURA so 5:56: c:oisis 6 «:unce's. 6.5: 0:6 0. 0isis 6 cctscreesa set ews References............0006 eb es ecvoece cesses rversvesusee NNO UN 12 7 20 20 30 33 34 37 37 43 43 54 58 59 TABLE OF CONTENTS (Cont'd) Page DEEP SEISMIC REFRACTION PROFILE IN THE PILGRIM RIVER VALLEY GRABEN by Andrew Lockhart and Juergen Kienle...seeeeee 73 Introduction.......- sistaiae’s sevens okies 6's eeivens esha ee Vesiewes lee Methods and Instrumentation......seereesereess Wessaateees 7k: RESUIES...cececeeee See Reg bclegee oe ce 0) vic gales sies ee 0c0 4° ELD References....eseeee ie vb eleie's aiosiodaee nv ose ee ei ereie sé 80 VLF AND GALVANIC RESISTIVITY SURVEYS IN THE CENTRAL SEWARD PENINSULA by Eugene Wescott....seeeeceeseceerersescces 81 Introduction....ssseee whereas aoe eee Sie ves oateee 81 Measurements. .ceccccccccccccccseessere SRL eee es 83 CONCTUSTONS. ceveccccccccecccecececcessesessresseseeeeeee 100 References....cceecessecees FWP eR TCT TOV ewes eiee-e ¢.0.0:0.0.9.0.8 101 APPLICATION OF RADAR AND INFRARED AIRBORNE REMOTE SENSING TO GEOTHERMAL RESOURCE ASSESSMENT AT PILGRIM SPRINGS, ALASKA by Kenneson G. Dean, Robert B. Forbes, Donald L. Turner, Frank Eaton, Kathryn D. Sullivan...sceseeeeeseeerreces Gish oe 1S IMtrodUCtiOn....cccccecccceccecescerere ‘ve aveceiaeree yee «60-66 103 Physical Setting......-.- Set eee ee TEU 888 iin OS Analysis of Radar Imagery.....+s-++see auc bse § coscics tes 105 Analysis of Thermal Infrared Data....secseeceeees oie tole o'd pe Radiometer Measurements....seeeeers eeisiepowies baie oe Wz Thermal IR Data....+.+eee- os 6 06 bleleteduiee he iaGdease lO Conclusions.....e. Secccccccsesseveses Ties caw sere eeesrs 119 AcknowledgementsS....ssseereceesereres decsccsidiceoovesees 121 REFEFeNCeS...eccececcccecceceecesees eiejeocarecee sites eck te sare 122 Vi SUMMARY The central Seward Peninsula was the subject of a geological, geophysical and geochemical reconnaissance survey during a 30-day period in the summer of 1980. The survey was designed to investi- gate the geothermal energy resource potential of this region of Alaska. Based upon our previous work (Turner and Forbes, 1980) and the 1980 survey, we have proposed a continental rift system model to explain many of the Late Tertiary-to-Quaternary topographic, structural, volcanic and geothermal features of the region. Geo- logic evidence for the model includes normal faults, extensive fields of young alkalic basalts, alignment of volcanic vents, graben valleys and other features consistent with a rift system active from late Miocene time to the present. Rift systems in many parts of the world are known for their abnormal heat flow and significant geothermal potential. Five traverses crossing segments of the proposed rift system were run to look for evidence of structure and geothermal resources not evident from surface manifestations. Gravity, helium and mercury soil concentrations were measured along the traverses. Both helium and mercury soil concentrations have been shown elsewhere to be useful indicators of geothermal resources. We found that mercury soil content varied widely along the traverses and cannot be used to identify areas of interest in the environment of the central Seward Peninsula. Helium in soil gas, however, offers great promise as a geothermal exploration tool. Our surveys found numerous He anomalies that tend to support the rift model. With the exception of two sampling sites, all helium anomalies were found near proposed rift segments. Several areas of significant helium soil os concentration warrant closer study in any further detailed exploration for geothermal resources. Gravity profiles across the proposed rift segments generally show features consistent with a rift system. One traverse, the Noxapaga, has been interpreted by a two-dimensional model, and can be explained by low density sediments filling a valley 1.25 km deep and 32 km wide. Geologic evidence indicates that this valley is a structural feature (graben). Gravity profiling across the Pilgrim River Valley also appears to agree with a graben structure, as supported by geologic evidence. A long-spaced seismic refraction line was run in the Pilgrim River Valley at Pilgrim Springs to determine the depth to crystalline bedrock. Despite some instrumental problems a depth of 425 m was obtained. Previous depth estimates were much shallower (> 200 m; Turner and Forbes, 1980). The revised depth estimate indicates that deeper geothermal reservoirs may be present and that the reservoir potential of the Pilgrim Springs geothermal resource area may be even greater than was previously estimated (Turner and Forbes, 1980). We also carried out deep resistivity and VLF studies in the Pilgrim River Valley to further our understanding of the nature of the geothermal resources at and outside of the hot springs area. Three-dimensional modelling of galvanic resistivity generally agrees with a shallow reservoir as determined by drilling but does not rule out deeper significant reservoirs in the 425 m of valley fill. VLF and galvanic resistivity measurements confirm the existence of low resistivity (presumably hot saline water) under a zone along the Pilgrim River and under a small thaw zone 4 km northeast of Pilgrim Springs. We found that the VLF EM-16R tech- nique agreed well with galvanic resistivity measurements and could be very useful as a regional exploration tool. A National Aeronautics and Space Administration study of remote sensing techniques in the Central Seward Peninsula was also carried out in 1980, centered on Pilgrim Springs. Radar measure- ments proved to be useful in locating linear features under the vegetation which are useful in structural mapping and geothermal resource exploration. Thermal infrared imagery disclosed three warm ground zones in the Pilgrim Springs vicinity under less than ideal conditions. However, the interpretation of infrared imagery appears to be too difficult and expensive to be useful in regional studies of significantly larger areas. We did not discover any new geothermal resource areas in our 1980 work. However, we have established that the central Seward Peninsula may contain a continental rift system with some areas of abnormal helium soil gas concentrations and likely abnormal heat flow, suggesting that the geothermal energy potential of the area is nigh, and that Pilgrim Springs may only be the "tip of the iceberg". REFERENCE Turner, D. L. and R. B. Forbes, (Eds.) 1980, A geological and geo- Physical study of the geothermal energy potential of Pilgrim Springs, Alaska, University of Alaska, Geophysical Institute Report UAG R-271, 165 pp., 1 pl. 3 * INTRODUCTION This is the third of a series of reports on the geothermal energy resources of the Seward Peninsula. Our two previous reports focused on Pilgrim Springs (Figure 1; Turner and Swanson, this report) and gave the results of geological, geophysical, geochem- ical and hydrologic studies, accessible power estimates and recom- mendations for follow-on studies and exploratory drilling targets (Forbes et al., 1979; Turner and Forbes, 1980). During our 1979 investigations at Pilgrim Springs we developed the hypothesis that these hot springs were associated with tensional tectonics and active rifting. We also proposed that the low-lying region extending from the Imuruk Basin through the Kuzitrin valley to the Imuruk lava field (Figure 1; Turner and Swanson, this report) represents an incipient rift through the Seward Peninsula (Turner and Forbes, 1980). In July 1980, we conducted a helicopter-supported geological and geophysical reconnaissance survey of the central Seward Penin- sula, designed to test the rift hypothesis and to provide informa- tion on the regional geothermal energy potential of the area. The results of this work, together with our previous studies have provided evidence for a tectonic model of active rifting extending 250 km across the central Seward Peninsula and offshore into the Bering Sea. This rift model should be useful as a working hypothesis and an exploration model for future, more detailed geothermal studies on the Seward Peninsula. In order to increase scientific yield as well as cost effective- ness, we operated a combined field camp with our NASA- supported project designed to test the effectiveness of remote sensing (syn- thetic aperture radar and thermal infrared) techniques in the exploration and assessment of geothermal energy resources. The Pilgrim Springs area was utilized as a known geothermal target for this study. The results of the remote sensing study are included in this report and integrated with our geological and geophysical work. : REFERENCES Forbes, R. B., E. M. Wescott, D. L. Turner, J. Kienle, T. Osterkamp, D. B. Hawkins, J. T. Kline, S. Swanson, R. D. Reger and W. Harrison, 1979, A geological and geophysical assessment of the geothermal potential of Pilgrim Springs, Alaska, Geophysi- cal Institute, University of Alaska and Alaska Division of Geological and Geophysical Surveys Preliminary Rept., 39 pp-, 1 pl. Turner, D. L. and R. B. Forbes, (Eds.) 1980, A geological and geo- physical study of the geothermal energy potential of Pilgrim Springs, Alaska, University of Alaska, Geophysical Institute Report UAG R-271, 165 pp-, 1 pl. CONTINENTAL RIFTING--A NEW TECTONIC MODEL FOR THE CENTRAL SEWARD PENINSULA by Donald L. Turner and Samuel E. Swanson INTRODUCTION During our 1979 geothermal energy resource investigations at Pilgrim Springs, Alaska, (Figure 1) we developed the hypothesis that these hot springs were associated with tensional tectonics and active rifting. We also proposed that the low-lying region extending from the Imuruk Basin through the Kuzitrin valley to the Imuruk lava field (Figure 1) represents an incipient rift through the Seward Peninsula (Turner and Forbes, 1980). In July 1980, we conducted a helicopter-supported geological and geophysical reconnaissance survey of the central Seward Penin- sula, designed to test the rift hypothesis and to provide informa- tion on the regional geothermal energy potential of the area. The results of this work, together with our previous studies, (Turner and Forbes, 1980) have provided evidence for a tectonic model of active rifting extending 250 km across the entire central Seward Peninsula and offshore into the Bering Sea. The rift model should be useful as a working hypothesis for future geothermal energy resource exploration of the Seward Peninsula. REGIONAL GEOLOGY AND GEOPHYSICS Overview Figure 1 shows the generalized geology of the Seward Peninsula as modified from Hudson (1977). The figure is designed to emphasize 7 Cope Eapenners KOTZEBUE SCALE 1:1,000.000 Figure 1. See Generalized geology of the Seward Peninsula showing distribution of basaltic lava fields (QTb), vents (dots), Quaternary basins (Q) and selected faults. The geology is generalized from Hudson - (1977). Locations of volcanic vents and faults in Imuruk Lake-Koyuk River and Teller areas are generalized from Hopkins (1963) and Hopkins et al. (1974). Faults north of the Kuzitrin River are inferred from topographic lineaments. Kuzitrin Fault mapped by the authors. Figure 8 shows the distribution of basalt flows and vents north of Teller in more detail. the distribution of basaltic lava fields and vents, Quaternary basins, and selected faults believed to be related to the proposed : rift model. As seen in Figure 1, most of the Seward Peninsula is composed of a Precambrian metamorphic basement overlain and overthrust in some areas by Paleozoic carbonates. Alkalic intrusives of Cretaceous age occur along the eastern part of the peninsula. Our 40K-40Ar dating of both metamorphic and igneous intrusive rocks from the Kigluaik Mountains and Hen and Chickens Mountain indicates that this part of the Seward Peninsula basement was subjected to a thermal pulse that was hot enough to cause complete resetting of biotite K-Ar ages. Cooling ages of about 84 m.y. were obtained from all of the dated rocks, indicating that cooling from this thermal pulse occurred in mid-Cretaceous time. It is possible that the thermal pulse may have been related to igneous intrusive activity in the area during Cretaceous time. Ana- lytical data for 40x-40,r age determinations are given in Table 1. Our discussion of the rift model involves geologic events that probably began no earlier than late Tertiary. Pre-late Tertiary rock units are therefore shown in only a generalized way on Figure 1 and will not be discussed in detail in this paper. Imuruk Basin--Pilgrim River Valley As discussed in our previous report (Turner and Forbes, 1980), a considerable body of geologic and geophysical evidence indicates that this low-lying region (Figure 1) represents a graben or half graben structure: OT ANALYTICAL DATA FOR K-Ar AGE DETERMINATIONS TABLE 1 Field kp0 er vad “Oar ral OK Age No. Rock Type Mineral (wt 2) _ (4) (x 1073) (my. + 20) 0179 Pl ‘Schist Biotite 8.260, 8.257 70.2 4.991 83.9 + 2.5 DT79 P2 Gneissic granite Biotite 9.000, 9.010 41.7 §.111 85.9 + 2.6 DT79 P4 Amphibolite Hornblende 1.190, 1.180 72.8 5.051 84.9 + 2.6 DT79 PIIA Pelitic gneiss Biotite 9.313, 9.277 86.1 4.839 81.4 + 2.4 D179 P13 Gneissic granite Biotite 8.480, 8.610 71.5 5.179 87.0 + 2.6 DT79 P14 Granodiorite Biotite 8.480, 8.610 72.4 4.937 83.0 + 2.5 0179 P15 Gneissic granite Biotite 8.950, 8.930 79.9 4.957 83.4 + 25 DT79 P16 Altered Biotite 8.697, 8.717 81.1 5 .003 84.1 + 2.5 granodiorite ‘ DT79 P22c Gneissic granite Biotite 8.953, 8.963 86.2 4.934 83.0 + 2.5 P26 Altered Biotite 8.873, 8.830 82.1 4.911 82.6 + 2.6 granodiorite 8.830 DT79 P36 Orthogneiss Biotite 8.887, 8.877 53.3 5.045 84.8 + 22n Note: rad = radiogenic; o = standard deviation; x = mean; A, + i, = 0.581 x 10-10 yr; 8 Aq = 4.962 x 10710 yr!; 40K Keotal = 1.167 x 1074 mol/mol. (1) (2) The Kigluaik Fault (Figure 1), a major normal fault about 65 km long, marks the northern boundary of the Kigluaik Mountains and separates these mountains from the lowlands of the Imuruk Basin and Pilgrim River valley. Displacement is consistently down to the north. Several ieee moraines are cut by the fault and maximum vertical separations range from 4 to 10 m, probably representing a reasonable measure of total post-Wis- consin displacement along the fault (Hudson and Plafker, 1978). Our mapping along the eastern segment of the fault is consistent with the above observations which came principally from the western and central parts of the fault (Turner et al., 1979). Surficial geologic mapping by Kline et al. (1980) has produced the following evidence that relatively rapid subsidence has been occurring in the Pilgrim River valley: (a) Burial of older stream terraces by modern alluvium. (b) Major changes in the course of the Pilgrim River. (c) The relative absence of thaw lakes and thermokarst fea- tures upstream from the thermally disturbed area around Pilgrim Springs. (d) Grading of old alluvial and outwash fans to a level higher than the present valley floor. (e) Abrupt scarps at the toes of alluvial and outwash fans from the Kigluaik Mountains. (f) A steepening of stream gradient just upstream from the area of maximum apparent subsidence. ll (g) The presence of lacustrine or estaurine clay, silt, and very fine sand beneath 1.5 to 4.5 m of modern floodplain alluvium. (3) Seismic refraction studies in the Pilgrim River valley have shown that crystalline basement of the valley floor is about 400 m beneath Pilgrim Springs (Kienle et al., 1980; Kienle and Lockhart, 1980; Lockhart and Kienle, this report). The total throw of the Kigluaik Fault is therefore at least 400 m. (4) A gravity survey (Kienle and Lockhart, 1980; Lockhart, this report) and a deep resistivity survey (Wescott et al., 1980; Wescott, this report) are consistent with the results of the seismic refraction study, although they cannot in themselves provide precise depths to basement. Although there is no evidence of volcanic activity in the Imuruk Basin-Pilgrim River Valley region (Figure 1), the 1.5 km2 geothermal anomaly at Pilgrim Springs and a second area of thawed ground about 3 km northeast. are indicative of high heat flow in at least part of this region (Turner and Forbes, 1980). Apparently anomalously high lake temperatures near Hen and Chickens Mountain and local reports of an early thaw area near Marys Mountain, both about 3.2 km north of Pilgrim Springs (Figure 1) also suggest anomalously high heat flow in the region. Imuruk Lake--Kuzitrin River Lowland The Imuruk Lake-Kuzitrin River lowland is a 30 km-wide valley extending 100 km northeast from the Precambrian basement high cut by the Kuzitrin river to about 25 km northeast of Imuruk Lake (Figure 1). 12 During our 1980 geologic mapping, we discovered a previously unmapped fault extending northeast about 60 km near the southern border of the lowland. We have named this fault the Kuzitrin Fault (Figure 1). The fault occurs in Precambrian basement rocks along the northern front of the Bendeleben Mountains. Along most of its length, the trace of the fault is marked by distinct scarps and slopebreaks in bedrock ridges of pelitic gneiss. Displacement is consistently down to the north. Scarp height averages about 10 m. In some places where the fault trace crosses ridges, moderately broad notches or swales are developed. Along its southwestern segment, the fault follows the aligned tributary valleys of upper Belt Creek and Lucky Dog Creek, and appears to end at the eastern edge of the Pilgrim River Valley (Figure 1). Glacial moraines are not present along the fault trace, but the fault has produced a scarp in colluvium north of Mt. Bendeleben, suggesting that some displacement has occurred during Quaternary time. To the north of the lowland, a series of strikingly linear, northeast-trending, parallel stream valleys are evident on Landsat imagery and on the topographic maps of the area. Six of these lineaments are shown in Figure 1. Although they have not previously been mapped as faults (Sainsbury, 1974), their strong alignment, their parallelism to the Kigluaik Fault and the fact that they border the Quaternary structural basin of the Imuruk Lake-Kuzitrin River lowland (Hopkins, 1963), suggest that these lineaments may represent en-echelon normal faults bordering a major graben. Our limited time in this area did not permit us to test this hypothesis. 13 The central and eastern parts of the Imuruk Lake-Kuzitrin River lowland are covered by the extensive alkali and tholeiitic basalt flows of the Imuruk Lake lava plateau (Figure 1). Hopkins (1963) mapped these volcanics and distinguished five volcanic units on the basis of degree and character of weathering, the degree of modification of primary surface relief and the presence or absence of a thick cover of windblown silt: the Kugruk Volcanics of estimated late Tertiary or Early Pleistocene age; the Imuruk Volcanics of estimated Early and mid-Pleistocene age; the Gosling Volcanics, believed to be emplaced during and after the interglacial interval between the Nome River (I1linoian) and Salmon Lake (Wisconsin) Glaciations; the Camille lava flow, emplaced near the end of the Salmon Lake Glaciation; and the Lost Jim lava flow, emplaced during the last few thousand years. Our K-Ar dating of three of these units has resulted in some modifications of Hopkins (1963) age estimates, as will be discussed later. Alkali olivine basalt is the most common bulk composition in the Imuruk lava field, but some tholeiitic basalt is also present. The lava flows are relatively thin (3-20 m) compared to their large lateral extent (up to 35 km in length). Hopkins (1963) gives an analysis of a subalkaline olivine basalt and reports typically low silica contents (Si02 = 49.13%) and high alkali values (Nag0 + Ko0 = 4.12%). X-ray fluorescence analysis of eleven samples col lected by the authors indicates that eight are alkali olivine basalts (Si02 = 43.3 to 48.2%, K20 = 0.8 to 2.2%, Nag0 = 2.9 to 4.5%) and three are olivine tholeiites (Si02 = 51 to 52%, K20 = 0.5 to 0.9%, Nag0 = 2.9 to 3.3%). Primary minerals 14 in the alkali basalts include phenocrysts of Olivine, smaller laths of plagioclase (An3¢_5q4) with interstitial augite and spinel. Tholeiitic basalts contain phenocrysts of plagioclase (Angg-54) with finer-grained augite, hypersthene, olivine and spinel. Iddingsite, partially to completely replacing olivine, is the only commonly observed alteration product. Carbonate was noted in the groundmass of a few samples and zeolites have been reported as vesicle-fillings (Hopkins, 1963). The tholeiites show an alteration pattern similar to the alkali basalts. Ultramafic inclusions are found in some of the alkalic basalts (Hopkins, 1963). Preliminary petrographic studies of a suite of inclusions from the Gosling Yolcanics at Virgina Butte (Figure 2) indicates that the ultramafic rocks are predominantly lherzolite and lesser harzburgite (R. B. Forbes, personal communication, 1980). Figure 1 shows the distribution of volcanic vents on the Seward Peninsula from Hopkins (1963). Note the distribution of vents near Imuruk Lake in major, northwest-trending swarms parallel to observed surface faults. Figure 2, modified from Hopkins (1963) shows these relationships in more detail. Note also on Figure 1 the series of vents extending east and southeast along the Koyuk River valley, which also contains extensive basalt flows. The abrupt offset of this valley to the south, also shown on Figure 1, suggests the presence of a major fault, as wil] be discussed later. Hopkins (1963) has shown that faults in the northwest- trending system shown in Figure 2 delineate a broad graben occupied by Imuruk Lake. The faults cut the Imuruk and Gosling flows, but do not cut the Camille or Lost Jim flows, suggesting that the faults were 15 166°00" 16330 6600 EXPLANATION e Volcanic vent IT Fault scarps Linear stream course, probably following trace of a fault 3° “me Gn Isobase on deformed intermediate terrace of Imuruk Lake and on deformed lake bed south of Goodhope River 65°45" 65°45’ 65°30" 650° ess" . e5r15° 164°00° 16330" 163-00" 16720" Figure 2. Volcanic vents, faults, and warped surfaces near the Imuruk Lake area from Hopkins (1963). 16 active until late Wisconsin time but may not have been active during the last 10-15,000 years. Hopkins (1963) also postulated a structural trench in the Koyuk River Valley (Figure 1) because the south side of the valley is bounded by normal faults and because the trend of the valley tran- sects the trend of the limestone and schist units in the adjacent highlands. He observed that the Imuruk Lake graben is aligned with the Koyuk trench and probably represents a continuation of the same structural system. Hopkins also postulated a structural basin on the site of the Kuzitrin flats, as discussed in the following anes (Hopkins, 1963, p. C84): “A structural basin on the site of the Kuzitrin flats is pos- tulated because the late Tertiary erosion surface appears to slope toward the center of the flats from the Bendeleben Mountains on the southeast and from the Seward Peninsula uplands on the west and northwest. The late Tertiary erosion surface dips under the Kougarok gravel in the northwestern and western part of the flats, and the Kougarok gravel appears to dip, in turn, beneath sediments of late Pleistocene age along the Noxapaga and Kuzitrin Rivers. A placermine shaft near Dahl Creek (pl. 2) extended through the Kougarok gravel to within 50 feet of sea level without reaching bedrock (Collier and others, 1908, p. 302). The outline of the Kuzitrin flats transects the strike of the limestone, schist, and gneiss units that underlie adjoining highlands, indicating that the flats probably were not excavated by differential erosion of a weak bedrock unit." 40-40 ar DATING OF BASALTS AND RHYOLITES Basalts Three samples from the basalts north of Teller (Figure 1) and seven samples from the basalts of the Imuruk lava plateau were selected for 40K-40ar dating and sent to Geochron Laboratories 17 for K and Ar analyses. Analytical data are given in Table 2. The basalts meet established criteria for reliable whole-rock dating. The only alteration we detected petrographically was the alteration of olivine to iddingsite in some samples. Two volcanic necks which we mapped as feeders for the basalt flows north of Teller (Figure 1) were dated at 2.7 + 0.2 and 2.5 + 0.3 my. (Hi11 1220) and 2.6 + 0.2 m.y. (Eva Mtn.). Hopkins et al. (1974), reported an age of 2.92 + 0.14 m.y. for one of the flows in this area (age recalculated for new decay constants). Two samples from the Imuruk Formation, the most voluminous of the four basalt formations of the Imuruk lava plateau (Hopkins, 1963), were dated at 2.2 + 0.2 my. Hopkins et al. (1971), reported an age of 5.85 + 0.2 m.y. from a tholeiitic basalt flow from the Lava Camp Mine in the upper Inmachuk River Valley, about 16 miles north of Imuruk Lake (Figure 1). Hopkins (1963) ceovatuues the basalt flows in the Inmachuk River Valley with his Imuruk Formation. The available K-Ar ages thus establish that this formation includes flows ranging in age from about 2.2 to 5.8 my. The age span of 2.5-2.9 m.y. documented for the basalts north of Teller falls within this interval, indicating that basaltic volcanism occurred in both areas at about the same time. This conclusion, and the petrographic and chemical similarities of the lavas in both areas are consistent with the rift model discussed in a subsequent section. Two additional volcanic formations were also dated from the Imuruk lava plateau. Two samples from the Gosling Formation, 18 61 TABLE 2 ANALYTICAL DATA FOR 49-40, AGE DETERMINATIONS OF WHOLE-ROCK BASALT AND RHYOLITE SAMPLES FROM THE SEWARD PENINSULA, ALASKA Oar K rad. pr /40 Ae Age Sample No. (wt. -%) (x 1074 ppm) rad’ total (my. + 10 Basalts North of Teller Hil? 1220 PTB0-50A 1.671,1.665 2.64, 3.70, 2.89 -157,.160, .276 2.7 + 0.2 PT80-50B 1.710,1.719 2.51, 3.23 -144,.254 2.5 + 0.3 Eva Mtn. PT80-51D 1.753 ,1.792 3.30, 2.65, 3.39 -160,.185,.177 2.6 + 0.2 Imuruk Basalts Imuruk Series PTSO-33 -366, .362 0.54, 0.57 -035, .042 2.2 + 0.2 PT80-35A -363, .365 0.48, 0.69, 0.49 -032, .048,.037 2.2 + 0.2 Gosling Series = -971, .965 0.53, 0.58 -036, .058 0.82 + 0.08 PT80-36A - 783.754 0.52, 0.46 044, .036 0.91 + 0.09 Kugruk Series - -985,.991 18.18, 18.30 -545,.577 26.4 + 1.4 PT80-38 1.132,1.135 22.64, 22.03 ~570,.653 28.1 7 1.4 PT80-40 -605, .607 11.81, 12.71 -584,.401 28.8 + 1.8 Basalt Dike Intruding PC Basement in W. Bendeleben Mtns. PS80-14A 2.666 ,2.628 141.1, 140.5 -793,.785 74.9 + 3.0 Rhyolites Intruding PC Basement in W. Bendeleben Mtns. PT80-28 4.062 ,4.069 214.2, 215.8 -880, .899 74.5 + 2.8 PT80-30 4.241,4.216 205.0, 208.6 817, .918 69.0 ¥ 2.6 Analyses by Geochron Laboratories Division, Krueger Enterprises, Inc., Cambridge, Mass. Ages recalculated for new decay constants. which overlies the Imuruk Formation, were dated at 0.82 + 0.8 and 0.91 + .09 m.y., in general agreement with ae (1963) relative age estimates. The Kugruk Formation, however, gave ages much older than these predicted by Hopkins. Three samples from the Kugruk Formation gave ages of 26.4 + 1.4, 28.1 + 1.4 and 28.8 + 1.8 my. (late Oligocene). ‘These lavas appear to be geomorphically much older than the three overlying basalt formations of the Imuruk Lava plateau, yet they occupy a structurally high position in a cliff exposure in the upper Kugruk Valley. We suggest that this structurally high position may be the result of upfaulting of the Kugruk lavas. Because there is no geologic evidence against an Oligocene age for these lavas, we believe that the late Oligocene radiometric ages probably represent a valid age estimate for the time of crystal- lization and cooling of the Kugruk Formation lavas. Rhyolite and Basalt Intrusives Smal] dikes and plugs of vesicular rhyolite and basalt intrude the Precambrian basement in the westernmost Bendeleben Mountains (see Figure 5). Two rhyolites and one basalt were dated. All gave Late Cretaceous ages (Table 2), thus invalidating any possible genetic association with the lavas of the Imuruk lava plateau. RIFT MODEL We propose that a complex continental rift system, shown dia- gramatically in Figure 3 may be responsible for the following late-Tertiary-to-Quaternary structural, tectonic, volcanic and geo- thermal features of the central Seward Peninsula: 20 Te Figure 3. Cope Espenderg KOTZEBUE Port Ciasence Teller SCALE 1:1,000.000 20 Oo Mi =F = s so 100 Km Diagram of proposed rift model for the central Seward Peninsula. The graben structure offshore (PCR) is the Port Clarence Rift (Hopkins et al., 1974) and is shown in more detail on Figures 6-8. Geology generalized from Hudson (1977) and Hopkins et al. (1974). (1) Evidence of north-south extensional tectonics and graben struc- tures in the Imuruk Basin-Pilgrim River valley and Imuruk Lake-Kuzitrin River Lowland basins and in the Koyuk trench. (2) Extensive outpourings of basalt lavas of the Imuruk Lake lava plateau and Koyuk River Valley. (3) Northwest alignment of volcanic vents and normal faults in the Imuruk Lava plateau and Koyuk River Valley. (4) The high level of seismicity in the central Seward Peninsula shown in Figure 4 (Biswas et al., 1980). (5) The presence of the Pilgrim Springs geothermal area and other evidence of anomalously high heat flow in the Pilgrim River valley. The above evidence is consistent with the existence of rift segments B, C, D and E shown in Figure 3. One of the constraints imposed by plate tectonics is that rifts must be part of an inter- connected system. When we examine the regional geology for clues to the geometry of such a system, two major problems arise: At both ends of rift segment B the bedrock geology requires the rift to stop because it runs into Precambrian bedrock highs. We have therefore postulated the existence of two transform faults to connect rift segment B to rift segments C and A. Our 1980 field mapping has confirmed the existence of a north- south-trending bedrock fault connecting proposed rift segments B and C. A geologic map. of the 100 km@ area surrounding the fault is shown in Figure 5. Structural trends and lithologic contacts in Precambrian metamorphic units do not project across this fault. The amount of fault offset cannot be uniquely determined ee, 173° 165° 156° 69° 69° + EPICENTER OF EARTHQUAKE (10 <M, < 45) 66° 66° 62° 173° 165° 156° Figure 4, Epicenters of earthquakes in the Seward Peninsula area located by a ten station local seismic network during 1977 and 1978. The crust and upper mantle structure beneath the area is unknown and a number of earthquake locations have uncertainties of + 50-70 km. The location errors for the rest of the earthquakes are about + 15-20 km (Biswas et al., 1980). 23 EXPLANATION SURFICIAL DEPOSITS Quaternary Colluviat deposits (undifferentiated) INTRUSIVE IGNEOUS ROCKS Cretaceous Vesicular shyolite duresk — Cretaceous ? ha IKy Biothe quartz-monsonlte and granite. massive 10 weakly foliated METAMORPHIC AND INTRUSIVE ROCKS Gneissose granite: wenkly to moderatety toffsted Calc-silicates: dlopside-carbonate-quarty rocks Pelitic gnetss and schist: dominstely Liote paragnelss PRECAMBRIAN Intertayered calcsilicetus and pelitic gneiss and schist Dashed where approximately located; short dashed where indefinite, gradational, or Interred (rom aerlal photogsaphs; dotted where concealed; queried where doubtful —— lee Foult Dashed where approximately located; shurt dashed where indefinite, gredational, or Inferred (rom serlel photographs; dotted where concealed; queried where doubtful ats ‘Sulke end dip of follution Figure 5. Geologic map of the northeastern portion of the Bendeleben A-6 quadrangle showing location of the Labaree Creek Fault, believed to be a transform fault connecting proposed rift segments B and C. Squares = 1 mile. See Figure 3 for location of mapped area. from the available mapping. One of several possible fault solutions requires about 1 km of right-lateral strike-slip offset, consistent with the rift model. We have named this fault the Labaree Creek Fault after the creek which flows along its trace. The surface trace of the fault extends southward about 2.4 km along the deeply-incised valley of Labaree Creek, then crosses a swale in the bedrock ridge immediately to the south and projects into a linear scarp in colluvium on the south slope of the ridge. This scarp is clearly visible on air photos and indicates that the fault has been active during Quaternary time. The fault has been mapped for a distance of 6.4 km. We speculate that it extends for an additional 5-6 km through the alluvium of the Pilgrim River valley to connect with the eastern end of rift segment B. We also speculate that the northern end of the fault connects with the southwestern end of rift segment C, as shown in Figures 3 and 5. The western end of rift segment B appears to terminate at the Precambrian basement high south of Teller (Figures 1 and 2). We must therefore look elsewhere for a geologically permissible place to extend the proposed rift system westward. Marine seismic - reflection work (Grim and McManus, 1970; Hopkins et al., 1974; Johnson and Holmes, 1977) shows a major rift structure trending westward from Port Clarence under the Bering Sea. Figures 6 and 7 show that the structure is present in the deeper subsurface and acoustic basement as well as in the surface of the sea floor (5-15 m scarps). 25 92 Figure 6. \ Wat No eee 162° 140° W 130" steers surface fault —— near surface fault an scarp, height In meters on upthrown side 4% throw of fault inferred extension 7 0 nm 0 0 kilometers ig dU a Surface and near-surface faults in Norton Basin from Johnson and Holmes (1977). Note structures in the area of the Port Clarence Rift (PCR) and Bering Straits Fault (BSF). We propose that these structures represent the offshore continuation of the proposed rift segment A north of Teller (see Figure 3). de Ml aa" N Now Ex \ \ —— doop fault seeere gidge axis a syncline axis Y% throw of fault inferred extension sen nd » 6 wm 0 8 kllomoters rk Figure 7. Deeper subsurface and basement faults and structures in Norton Basin from Johnson and Holmes (1977). Note large graben structure trending east-west in area of PCR (Port Clarence Rift) and BSF (Bering Strait Fault). This graben structure is thought to be an offshore continuation of the proposed rift segment A north of Teller (see Figure 3). We propose that the east-west-trending, 60 km-long valley located about 15 km north of Teller (Figures 1 and 3) is the onshore continuation of this rift system, named the Port Clarence Rift by Hopkins et al. (1974). The valley is filled with Quaternary sedi- ments and contains extensive basalt flows of Quaternary to late Tertiary age (Sainsbury, 1972; Hopkins et al., 1974). Hopkins et al. (1974) have shown that the basalt flows in this area are more extensive than indicated by Sainsbury (1972), and that approximately 30 volcanic vents are present (Figure 8). We made a brief helicopter reconnaissance of the volcanic out- crops on the south side of the area in 1980, but bad weather pre- vented a more thorough study of the extensive volcanic rocks. Two of the vents we visited appear to be eroded volcanic necks with crudely developed columnar jointing. A third vent is less eroded, has a mantle of scoriaceous basalt and feeds a lava flow. All of the basalts sampled are alkali olivine basalts. X-ray fluorescence analysis of seven samples collected by the authors gave the following values: Si0g = 44.7 to 47.7%, K20 = 2.0 to 2.2%, Nag0 = 3.7 to 4.9%. Primary minerals are olivine, augite, plagioclase and spinel. Alteration products include iddingsite around some olivine and carbonate in the groundmass. Inclusions of quartz and ultramafic rocks are found in the basalts. The quartz, probably inherited from the underlying metamorphic rocks, is rimmed by augite and is unstable in the basalts. The ultramafic inclusions are lherzo- lites and xenocrysts of pyroxene. These basalts are chemically and petrographically very similar to the basalts of the Imuruk Basin. 28 ww PORT CLARENCE RIFT ane conibeckee ies ~ \ PORT CLARENCE t BERING SEA - Ze ae Spite ae ee a eee 7 rea z i a | Tom] Lies a — es . . - s PHY! Share| ¥ lee 33 L Gn 62 90m Giasanen Bedrock ) en eee ae te : Conact 133 | na Dashed where soarammately located or interred a § ‘ aL é Ft see 3! I}; (2 bar and bell on dowmnrown side é 3 sciaiaiees ' c | Meamures on eoncanet side 3 | ma i A | ie Gisciet ante non j Cinder cone of eruptive vent } OF Killeston Creek | ee i at snd oe Lager Bees L York Terrace showing ° eaposures of beach grave} Fessw loceiry Figure 8. Cenozoic geologic map of part of the western Seward Penin- sula from Hopkins et al. (1974). Rift segment A extends E-W through the lava field and terminates at a proposed N-S transform fault at the California River. In our tectonic model, this proposed fault connects the western end of rift segment A to the Port Clarence Rift. 29 Having proposed the existence of rift segment A in Figure 3 as a means of getting the proposed rift system out to sea, we make two additional speculations: 1. the eastern end of segment A is connected to the western end of rift segment B by means of a north-south-trending transform fault, as shown in Figure 3, thus. allowing the rift system to circumnavigate the Precambrian bedrock high south of Teller. The proposed transform fault runs down the valley of the Agiapuk River to the Imuruk Basin (areas underlain by Quaternary sediments) and does not appear to violate any of the constraints of the mapped geology of this area (Sainsbury, 1972). 2. The western end of segment A is connected to the Port Clarence Rift by means of a north-south-trending transform fault extending - from the western edge of the basalt field southward along the California River to the Port Clarence Rift (Figures 3 and 8). The possible existence of this transform fault was suggested by David Hopkins (personal communication, 1981), as an alterna- tive to extending rift segment A directly westward out to sea--a hypothesis which would be inconsistent with his Quater- nary geologic mapping in the area (Hopkins et al., 1974). SUMMARY AND CONCLUSIONS We propose the complex continental rift model shown in Figure 3 as an explanation for many of the late Tertiary-to-Quaternary topographic, structural, tectonic, volcanic and geothermal features of the central Seward Peninsula, and for the rift structure offshore in the Bering Sea. 30 Although some features of the model are highly speculative, we believe that the body of geological and geophysical evidence consis- tent with the model is sufficient to support the model as a working hypothesis to be tested by future field work. As one test of the model, five sampling traverses for helium in soil gas were made across rift segments A-D. Nine out of eleven helium anomalies occur near proposed rift segments and suggest abnormally high heat flow in these areas. Fourteen additional helium anomalies were found near Pilgrim Springs, in the center of rift segment B (Wescott, this report). Gravity profiles across rift segments B, C and D also show features consistent with the proposed rift model (Lockhart, this report). The rift model implies that the central Seward Peninsula is spreading apart in a north-south direction. We have not yet been able to do geologic mapping at the eastern end of the proposed rift system. Perhaps the simplest way to complete the necessary geometry to allow the southern half of the peninsula to move south- ward is to postulate a north-south transform fault down the Koyuk River valley, shown as a queried fault in Figure 3. Such a fault would be similar to the short transform fault about 35 km to the west, which we have proposed to account for the abrupt kink in the Koyuk River valley (Figure 3). We have not discussed the extensive basalt fields in two north- south-trending valleys in the northeast corner of the peninsula or the large basaltic field west of Cape Espenberg (Figure 3). Following the rift model, it is perhaps possible that the basalts in the two north-south-trending valleys could represent flows from leaky 31 transform faults parallel to the proposed transforms of the model. If this is so, N-S rifting may also be present in the extensive basalts in the area immediately east of Figure 3. The basaltic field west of Cape Espenberg contains five maars and 3 non-maar vents (D. Hopkins, personal communication, 1981). We believe that the extensive basalts of this area have probably been erupted in a zone of crustal weakness produced by the general N-S extensional tectonics of the Seward Peninsula. The large graben at McCarthy's Marsh bounded by the Bendeleben Fault on the south border of the Bendeleben Mountains is also probably related to N-S extensional tectonics (Hudson and Plafker, 1978). The amount of separation along the proposed rift system is less than the widths of the Quaternary basins which have presumably been enlarged by normal fautting and subsidence of their margins, in addi- tion to rifting. K-Ar dating establishes that volcanism associated with rifting began during late Miocene time. The possible existence of a major rift system extending 250 km across the Seward Peninsula is of major significance for regional geothermal energy resource assessment. The proposed rift segments should, in general, be areas of higher heat flow than their surrounding regions. Our helium survey supports this hypothesis (Wescott, this report). We recommend that the proposed rift system should be considered as an exploration mode} for future, more detailed geothermal studies on the Seward Peninsula. 32 ACKNOWLEDGEMENTS Field assistance with geologic mapping and sampling was provided by Chester Paris and Anthony Dunn. X-ray fluorescence analyses of basalts were kindly provided by the Alaska Division of Geological and Geophysical Surveys, Fairbanks. We thank David Hopkins for making his unpublished data available to us and for providing many helpful suggestions which have improved the Manuscript. John Decker reviewed the manuscript. 33 REFERENCES Biswas, N. N., L. Gedney and J. Agnew, 1980, Seismicity of western Alaska, Bull. Seis. Soc. Amer., v. 70, no. 3, p. 873-883. Grim, M. S. and D. A. McManus, 1970, A shallow seismic profiling survey of the northern Bering Sea, Marine Geology, v. 8, p. 293-320. Hopkins, D. M., 1963, Geology of the Imuruk Lake area, Seward Penin- sula, Alaska, U.S. Geol. Survey Bull. 1141-C, 101 p., 4 pl. Hopkins, D. M., J. V. Matthews, J. A. Wolfe and M. L. Silberman, 1971, A Pliocene Flora and Insect Fauna from the Bering Strait Region, Palaeogeography, Palaeoclimatol., Palaeoecol., v. 9, p. 211-231. Hopkins, D. M., R. W. Roland, R. E. Echols and P. C. Valentine, 1974, An Anvilian (Early Pleistocene) Marine Fauna from Western Seward Peninsula, Alaska, Quaternary Research, v. 4, p. 441-470. Hudson, T., (Compiler) 1977, Geologic Map of Seward Peninsula, Alaska, U.S. Geol. Survey Open-File Rept. 77-796A, Scale 1:1,000,000. Hudson, T. and G. Plafker, 1978, Kigluaik and Bendeleben Faults, Seward Peninsula In: Johnson, K. M., ed., The United States Geo- logical Survey in Alaska--Accomplishments during 1977, U.S. Geol. Survey Circular 722B, p. B47-B50. Johnson, J. L. and M. L. Holmes, 1977, Preliminary Report on Surface and Subsurface Faulting in Norton and Northeastern Chirikov Basins, Alaska, In: Environmental Assessment of the Alaskan Continental Shelf: Hazards and Data Management, NOAA Report XVIII, p. 14-41. 34 Kienle, J., A. Lockhart and J. Peace, 1980, Seismic Refraction Survey of the Pilgrim Springs Geothermal Area, Alaska, In: D. L. Turner and R. B. Forbes, Eds., A Geological and Geophysical Study of the Geothermal Energy Potential of Pilgrim Springs, Alaska, University of Alaska, Geophysical Institute Report UAG R-271, 165 pp, 1 pl., p. 53-72. Kienle, J. and A. Lockhart, 1980, Gravity Survey of the Pilgrim Springs Geothermal Area, Alaska, In: Turner, D. L. and R. B. Forbes, Eds., A Geological and Geophysical Study of the Geo- thermal Energy Potential of Pilgrim Springs, Alaska, University of Alaska, Geophysical Institute Report UAG R-271, 165 pp., 1 pl., p. 73-79. Kline, J., R. Reger, R. McFarlane and T. Williams, 1980, Surficial Geology and Test Drilling at Pilgrim Springs, Alaska, In: D. L. Turner and R. B. Forbes, Eds., A Geological and Geophysical Study of the Geothermal Energy Potential of Pilgrim Springs, Alaska, University of Alaska, Geophysical Institute Report UAG R-271, 165 pp., 1 pl., p. 21-28. Sainsbury, C. L., 1972, Geologic Map of the Teller Quadrangle, Wes- tern Seward Peninsula, Alaska, U.S. Geol. Survey Misc. Invest. Map 1685. Sainsbury, C. L., 1974, Geologic Map of the Bendeleben Quadrangle, Seward Peninsula, Alaska, The Mapmakers, Anchorage. Turner, D. L., S. Swanson, R. B. Forbes, D. Maynard, J. T. Kline, R. Reger and R. McFarlane, 1979, Geologic Map of the Bendeleben A-6 and Eastern Part of the Teller A-1 Quadrangles, Alaska, 35 In: D. L. Turner and R; B. Forbes, Eds., 1980, A Geological and Geophysical Study of the Geothermal Energy Potential of Pilgrim Springs, Alaska, University of Alaska, Geophysical Institute Report UAG R-271, 165 pp., 1 pl. Turner, D. L. and R. B. Forbes, Eds., 1980, A Geological and Geophysi- cal Study of the Geothermal Energy Potential of Pilgrim Springs, Alaska, University of Alaska, Geophysical Institute Report UAG R-271, 165 pp., 1 pl. Wescott, E., R. Sydora, J. Peace and A. Lockhart, 1980, Electrical Resistivity Survey of the Pilgrim Springs Geothermal Area, Alaska, In: Turner, D. L. and R. B. Forbes, Eds., A Geological and Geophysical Study of the Geothermal Energy Potential of Pilgrim Springs, Alaska, University of Alaska, Geophysical Institute Report UAG R-271, 165 pp-, 1 pl-, P- 81-100. 36 HELIUM AND MERCURY IN THE CENTRAL SEWARD PENINSULA RIFT SYSTEM, ALASKA Eugene Wescott INTRODUCTION Turner and Swanson (this report), have proposed "that an inter- connected system of late Tertiary to Quaternary rifts and transform faults extends 250 km across the central Seward Peninsula from Port Clarence to the eastern Koyuk River Valley". The rift model appears to explain many Late Tertiary-to-Quaternary topographic, structural, tectonic, volcanic and geothermal features, and should be useful as an exploration model for geothermal energy resources. Figure 1 shows a diagram of the proposed rift system on a generalized geologic map. The Seward Peninsula covers approximately the same area as the state of West Virginia, but has very few roads. In order to explore for geothermal resources in this large area we conducted a helium and mercury soil sample survey as a rapid exploration technique. Both of these elements have been reported as useful indicators in exploration for geothermal areas. They can be sampled fairly rapidly and are inexpensive to analyze. Helium anomalies have been detected near geothermal sites throughout the world (Bergquist, 1979). Two factors May contribute to helium anomalies in conjunction with geothermal areas: The deep source of most geothermal waters which do not mix with the atmosphere, and the radioactive decay of uranium and thorium in the vicinity of the source waters. Helium is unusual in that its solubility in water increases with temperature above 30°C [Figure 2 37 8e Cape Espenderg KOTZEBUE SCALE 1:1,000.000 4 mi Figure 1. so 100 Km Diagram of preposed rift model for the central Seward Peninsula. The graben structure offshore (PCR) is the Port Clarence Rift (Hopkins et al., 1974). The geology is generalized from Hudson (1977). QTb unit are late Tertiary to Quaternary basaltic lava flows. See Appendix A for geologic units. cm3STP gas/cm3 distilled water Et tt ° 0 20 3 40: 500 6070 Temperature, °C Figure 2. Solubility of noble gases in fresh water (after Mazor, 1972). 39 after Mazor (1972)]. Pressurized hot water will be a very efficient scavenger of helium produced by radioactive decay of uranium and thorium contained in the rocks at depth, and will release helium as it rises towards the surface, cools and de-pressurizes. Since helium is highly mobile it will find faults, minute fractures and paths to rise to the surface. ; We sampled the soil for helium in two ways: The first was to drive a probe 75 cm into the ground and draw off a soil gas sample which was then inserted into a small evacuated steel ampule with a syringe and sealed for later analysis. This method does not work well in wet soil or where the soil is rocky or frozen. In such conditions we used a soil sampling auger to drill a hole 75 cm deep. The soil core at the bottom was then quickly placed in a tin can and sealed with a portable canner. Western Systems, Inc., of Evergreen, Colorado, performed the mass spectrometric helium analyses to a precision of 10 parts per billion. Normal atmospheric He concentration is 5.24 ppm, and any significant soil concen- tration above this represents an anomaly. Mercury content in soils has also been reported as a possible indicator of geothermal resources (Matlick and Buseck, 1975). They confirmed a strong association of Hg with geothermal activity in three of four areas tested (Long Valley, California; Summer Lake and Klamath Falls, Oregon). Mercury deposits often occur in regions containing evidence of hydrothermal activity, such as hot springs (White, 1967). Mercury is highly volatile. Its high vapor pressure makes it extremely mobile, and the elevated temperatures near a geothermal 40 reservoir tend to increase this mobility. The Hg migrates upwards and outwards away from the geothermal reservoir, creating an aureole of enriched Hg in the soi] above a geothermal reservoir. Such aureoles are typically much larger in area than a corresponding helium anomaly. We collected soil samples about 10 cm below the organic layer. The samples were air dried in the shade and sized to -80 mesh using a stainless steel sieve. The -80 portions were stored in airtight glass vials for analysis. The Hg content of the sample was determined by use of a Jerome Instrument Corp., model 301 Gold Film Mercury detector with sensitivity of better than 0.1 ng of Hg. A standard volume of -80 mesh soil (0.25 cc) was placed ina quartz bulb and heated red hot for one minute to volatize all of the Hg, which was collected on a gold foil. Heating of the gold foil in the analysis procedure releases the Hg for analyses as a gas in the standard manner. Calibration is accomplished by inserting a known concentration of Hg vapor with a hypodermic syringe. The background concentration of Hg in soils varies widely from area to area, and must be determined from a large number of samples. It is generally on the order of 10 parts per billion. A question remains as to the application of helium and mercury sampling to Alaska geothermal exploration: How does the presence of permafrost affect the diffusion of He and Hg from source to the soil surface? Further basic resarch on this problem is needed, but we have found He and Hg anomalies in both thawed ground and in thick permafrost areas. Prior to the work on the Seward Peninsula we tried both He and Hg sampling in the vicinity of Chena Hot Springs, Alaska. Figure 3 41 ev TEMPERATURES AT 0.5M AND SOIL He AND Hg CHENA HOT SPRINGS, ALASKA Figure 3. to Falrbonks Q SPRING Map of Chena Hot Springs, Alaska with 0.5 m depth isothermal contours, helium and mercury soil concentrations. shows a map of the Chena Hot Springs area with isothermal contours at a depth of 0.5 m, and the soil concentrations of He and Hg in the area. A high value of 795 ppm He was found near the center of the 40°C isotherm at the west end of the area. In general the mercury values tended to outline the same linear temperature anomaly presumed to be a fault in the underlying granitic basement. HELIUM AND MERCURY SOIL SURVEY RESULTS FROM THE CENTRAL SEWARD PENINSULA Local -Site Surveys In ordér to further assess the usefuTness of He and Hg surveys some limited profiles were made in the approximately 1 km2 thaw ellipse at Pilgrim Springs. Figure 4 shows a map of Pilgrim Springs, the ground temperatures at 4.5 m depth, and the He values. The highest soil concentrations of about 100 ppm He were found near, but not at the highest temperatures at 4.5 m depth (80°C). Figure 5 shows the temperature, He and Hg values along a profile west to east across the hottest temperature anomaly. In general the helium and mercury values are in agreement. Both are anomalously high at station 200W which is suggested as a prime geothermal drilling site. Along a north-south profile on the 0.0 line (Figure 6) the helium values were all close to atmospheric levels, yet a mercury anomaly of 55 ppb was found at a location where the ground tempera- ture was only 20°C. Samples next to the main hot springs pool were low in both He and Hg. These low values are probably due to the high permeability of the sandy soil, and the elevated temperature of this area. The porosity of the soil could allow helium to 43 He ppm 5-6 6—7,5 7.5—10 40-20 >4100 @@©®@ee~oO Figure 4. Map of Pilgrim Springs, Alaska showing the area of thermally disturbed ground temperatures at 4.5 m depth and anomalous He values found. 44 Sb Figure 5. 4.5mT, Hg & He PROFILE ALONG 3005S, PILGRIM GRID i 100 500OW 400W 300W 2OOW 1O0OW O We A Temperature at 4.5 m depth, He and Hg soil. concentrations in a west to east profile A-A' across the Pilgrim Springs thaw ellipse. 9b Figure 6. 4.5m T,Hg AND He PROFILES ALONG N-S BASELINE, PILGRIM GRID N HOT SPRINGS S 200m B i Temperature at 4.5 m depth, He and Hg soil concentrations in a north-south profile B-B' along the N-S baseline at Pilgrim Springs. readily pass through to the atmosphere. Mercury would be easily vaporized by the high temperature and also escape through the porous soil. Some anomalous helium values were found outside the thaw ellipse across the Pilgrim river as shown in Figure 7. Galvanic and EM-16R resistivity measurements show low resistivity layers beneath the surface and suggest the presence of subsurface saline geothermal water in a band along the river shown by dashed lines near station PW80-3 (see companion paper by Wescott, this report). There is a second smaller thaw window in the Pilgrim river val- ley 4 km ENE of Pilgrim Springs (Turner and Forbes, 1980). Dense concentrations of broad leafed-plants, ferns and cottonwoods, which seem to be typical flora of thermally disturbed ground were found there. The ground temperature at 4.5 m was 20°C. The He concentration in the soil was anomalous, 5.52 ppm, and Hg samples were indeterminate, some lower and some higher than normal. No drilling or deeper temperature measurements have been made, but resistivity measurements indicate a low resistivity layer of 2.5 a-m at depth (Wescott, this report). As geological mapping progressed in 1980 the general outlines of a proposed rift system emerged. Five traverse lines were planned to cross the segments of the proposed rift system to measure gravity, VLF, mercury and helium wherever possible. Except for the Nome-Taylor Road, all access to the area was by helicopter or boat. The limited helicopter and field time did not permit stations as closely spaced as might be desirable. Figure 8 shows a map of the Seward Peninsula with the locations of stations on the five traverse lines. Also shown are the locations of 47 HELIUM SOIL CONCENTRATION TERRACE THERMALLY DISTURBED AREA Figure 7. Map of the greater Pilgrim Springs area, showing the locations of significant helium anomalies, inside and beyond the area of thermally disturbed ground. Dots are sample locations where the soil He was near the background level. 48 6p Scale 25 0 25 50 75 100 KM ————S ee Figure 8. Map of Seward Peninsula, Alaska showing 5 traverse lines across segments of the proposed rift system and locations of anomalous helium soil concentrations. Dots indicate locations where soil helium values were less than 5.4 ppm. anomalous He soil concentrations found. With the exception of a smal] anomaly at the north end of the Imuruk Traverse and one at the south end of the Agiapuk traverse, all the helium anomalies lie within the proposed rift segments A, B, C, or D (Figure 1). Helicopter flying range did not allow us to work farther to the east in segment E. Figure 9 shows a geologic cross section, and the He and Hg concentrations along the Imuruk. Traverse. There are two significant helium anomalies two km apart in the lava fields not far from the recent Lost Jim lava flow. The Hg values are also high at these two stations. There is also a minor He anomaly at station 29 just north of the edge of the basalt flow. Figure 10 shows the geological cross section of the Noxapaga traverse as deduced from the geologic maps and computer modelling of the gravity data by Lockhart (this report) with the helium and mercury soil sample values. There are three minor helium anomalies at stations 71 (5.39 ppm), 53 (5.44 ppm) and 52 (5.39 ppm) near the southern fault boundary of the valley. The Hg values are all low on the southern part of the traverse, with a few higher values on the north half. Figure 11 shows He and Hg soil concentrations along the Taylor Road traverse. A small but significant helium anomaly was found at station 16. Two similar significant anomalies were found about 1 km west of station 16 on an island in a lake. The trend is suggestive of a fault alignment in the Precambrian metamorphic complex. Another significant anomaly was found at station 20 nearly in line with the other He anomalies. It is alSo near the center of rift segment C (Figure 1). 50 TS ELEVATION (FEET) GRAVITY Low \ Hg (ppb) so He 40 Hg 30 Ho i 20 z. 10 ona He 4000 —4- er crea ++ ¥ z we * + eee ss —t— yi so BENDELEBEN MTNS. ASSES ae SKeLeTon (EARS LOST Jim CAMILLE 00000 BUTTE BLACK BUTTE 2000 FLOW D g 464542 44 “a3 “a «0 na 38,37 6 6 x BS 32 ns» 2 27:26 «254 23 . DISTANCE N o 1Okm IMURUK TRAVERSE Figure 9. Imuruk traverse, helium and mercury soil concentrations. Two significant helium anomalies are found at stations 43 and 44. The mercury values are also higher than the mean at those sites. The nearby Lost Jim Flow is of very recent age. There is also a minor He anomaly at station 29 and a corresponding Hg anomaly. Vertical exaggeration 6.67x. See Appendix A for geologic units. 2s Low y 40 GRAVITY E £ BENDELEBE . 7 a a - (1 km) —» RMTIS. 4 4s ri) KUZITRIN Mie FAULT MA in AR MEZD) (=1 ken) —o fff NAN HAHN ia Ln a : j coh f We 1 vt Ni iV Ra 63 I DISTANCE j 0 70 km NOXAPAGA TRAVERSE Figure 10. He and Hg soil concentrations along the Noxapaga traverse. The geologic cross section is based upon geological mapping and two-dimensional modelling of gravity data. Vertical exaggeration 6.67x. See Appendix A for geologic units. €s § E ELEVATION (FEET) Hg Hg (ppb) 40 ~e8 20 ° x 5 10 4444 a 44 + oh 4 KIGLUAIK Ucy MINS. t 4 2 \ oN at \ ei \ ae we \ \ PILGRIM \ RIVER 1s COTTONWOOD PaiVen y pr atu t ' US tye SINT at TAT by NUE LT DTA EL TE Y /¥) Zi VY YY, YFG; VGA, 4). CLITA, y WGI. 4 ih, : 8 9 10 W 12 3 14 15 46 v 18 19 a1 22 OISTANCE N oO km TAYLOR ROAD TRAVERSE Figure 11. He and Hg soil concentrations along the Taylor Road traverse. There are large variations in the Hg data. The He anomaly at station 16 is significant, and corresponding values were found 1 km to the west on an island in a small lake. Another significant He anomaly was found at station 20. Vertical exaggeration 6.67x. See Appendix A for geologic units. Figure 12 shows the geological cross section and He and Hg values along the Pilgrim traverse. The traverse is about 15 km west of Pilgrim Springs. There is a small but significant He anomaly near the center of the suspected graben valley. The cor- responding Hg value is low. Figure 13 shows a geologic cross section at the western end of the rift system where the highest helium anomaly on the traverses was located. The corresponding Hg soil concentration is about normal. On the traverse several large Hg anomalies were found, particularly in a small stream valley at station 128. The cause of this anomaly is not evident. At one time during the gold rush mercury was used to amalgamate the fine placer gold in streams. We tested a soil sample in Quartz Creek which was heavily mined and found the Hg content about average. There was no evidence of mining activity in the valley of station 128, or at 131 which was also anomalous. The Hg aureole is expected to be much larger than that around an He source, so the fact that no He anomaly was found at either station does not rule out a geothermal source of the Hg. CONCLUSIONS Our use of He and Hg in the study of the Central Seward Penin- sula was in part research into the usefulness of these elements as geothermal resource indicators in the Alaskan environment. We found that both are probably useful in a hot springs area or known thermal anomaly. However as reconnaissance tools we found that He showed great promise, while Hg was much more difficult to interpret, due to its great variability. If we had made closer spaced measure- 54 Ss 470¢ 4, MT. OSBORN \ ELEVATION (FEET) Figure 12. 100 709 99 108 98 107 97 t OISTANCE J 0 10 km 11200111 101 110 PILGRIM TRAVERSE He and Hg soil concentrations along the Pilgrim traverse. See Appendix A for geologic units. 106 96 = 106 95 94 40 E & = 15 410 Es 0 Vertical exaggeration 6.67x. 9S 471 ppb 140 ppb 2 70 Ho 60 50 40 i 30 EI. z A hana 15 410 5 He 4 35000 A +—+ a ve a 65 o e z 2 2000 = < a 1000 1) 5} ote Cy a CT Ff z WI rere , Gp oh “WM Yeni Ze Vey,; Vel) Sets ae Se Maer rma SMa meth 120 80 123 124 126 126-127 128 129 130 131 8 DISTANCE . 0 10 km AGIAPUK TRAVERSE Figure 13. Agiapuk traverse, helium and mercury soil concentrations. A significant helium anomaly was found at station 123 near basalt flows of 2.6 m.y. age. Two large Hg anomalies were also found without accompanying He anomalies. Vertical exaggeration 6.67x. See Appendix A for geologic map units. ments we might be able to explain this variability, but as it is we can only speculate. Perhaps the presence of permafrost affected our ability to collect samples at a uniform soil horizon. There are probably more varied sources of Hg in the sampled soils than is the case for He. We found that the He anomaly pattern was generally consistent with the proposed rift model. Helium anomalies along our traverses are located at or near proposed rift segments in nearly all cases, suggesting release of He from hot ground water at depth. The implication of higher than average heat flow in these areas is consistent with the rift model. Several significant local He concentrations indicate areas of interest for future geothermal exploration. 57 APPENDIX A Geologica] Map Units Q Tertiary to Quaternary alluvium, valley fill, includes QTu Kougarok gravels and equivalents, till and alluvium. QTb Tertiary to Quaternary basalts of the Kuzitrin Flats and Eva Mtn. Alkalic to tholeiitic in composition with Qb - ultramafic inclusions in the alkali basalts. Ki Cretaceous intrusives, mostly quartz monzonite. Pz Thrust sheets of Paleozoic carbonates and meta-carbonates. Pzc PG Precambrian to lower Paleozoic metasediments. Schists and gneisses of the Nome Group and York Slate. Locally Péms migmatized in the northern Bendeleben Mts. Geologic units generalized from Hudson (1977) and Sainsbury (1972, 1974) with faults on the Imuruk Traverse by Hopkins (1963). 58 REFERENCES Bergquist, L. E., 1979, Helium: An exploration tool for geothermal sites, Geothermal Resources Council Transactions, v. 3, 59-60. Hopkins, D. M., 1963, Geology of the Imuruk Lake area Seward Penin- sula, Alaska, U.S.G.S. Bull. 1141-C. Hopkins, D. M., R. W. Roland, R. E. Echols and P. C. Valentine, 1974, An Envilian (Early Pleistocene) marine fauna from Western Seward Peninsula, Alaska, Quaternary Research, v. 4, 441-470. Hudson, T., (Compiler), 1977, Geologic Map of Seward Peninsula, Alaska, U.S. Geol. Survey Open-File Rept. 77-796A, Scale 1:1,000,000. Matlick, J. S., III, and P. R. Buseck, 1975, Exploration for geo- thermal areas using mercury: a new geochemical technique, In: Proceedings Second United Nations Symposium on the Development and Use of Geothermal Resources, v. 1, 785-792. Mazor, E., 1972, Paleotemperatures and other hydrological parameters deduced from noble gases dissolved in groundwaters; Jordan Rift Valley, Israel, Geochimica et Cosmochimica Acta, v. 36, 1321- 1326. Sainsbury, C. L., 1972, Geologic map of the Teller Quadrangle, Wes- tern Seward Peninsula, Alaska, U.S.G.S. Map 1-685. Sainsbury, C. L., 1974, Geologic map of the Bendeleben Quadrangle, Seward Peninsula, Alaska, The Mapmakers, Anchorage. Wescott, E. M. and D. Turner, 1981, A geological and geophysical study of the Chena Hot Springs geothermal area, Alaska, Uni- versity of Alaska, Geophysical Institute Report UAG R-283. 59 White, D. E., 1967, Mercury and base-metal deposits with associated thermal and mineral waters, In Barnes, H. L., Ed., Geochemistry of hydrothermal ore deposits, New york, Holt, Rinehart, and Winston, 575-631. 60 GRAVITY SURVEY OF THE CENTRAL SEWARD PENINSULA Andrew Lockhart INTRODUCTION In 1979 a gravity survey of the Pilgrim River Valley in the vicinity of Pilgrim Springs was undertaken to aid in defining local bedrock structure and to estimate the depth to basement (Turner and Forbes, 1980). The 1980 gravity program was designed to obtain regional crustal information for the central Seward Penisula in order to aid in understanding the tectonics and assessing the geothermal potential of the region. Gravity anomalies consis- tent with graben and horst block structures were found. METHODS Gravity data were gathered from helicopter traverses across the Kuzitrim flats, the Pilgrim River Valley and the Agiapuk River. A boat traverse down the Kougarok River from Taylor Road to the Kuzitrim River and downstream to the Pilgrim River Valley, and traverses from Nome to Teller along the Teller Road and Salmon Lake to Brakes Bottom along the Taylor Road were also made (Figure 1). In addition, several spot landings were made in the Pilgrim River Valley to extend the 1979 data. In all, 184 data points along 450 km of traverse line were taken with spacings between 1 to 5 km and averaging about 2 km. 4 iocal base station tying the survey to the Alaska gravity base station network (Barnes, 1968) was set in 1979 (Turner and Forbes, 1980). Uncertainties in the tie affect absolute gravity values at Pilgrim Springs less than 0.8 mgal. 61 29 \ SPRINGS 1 \ Figure 1. Map of the Seward Peninsula showing 1980 gravity traverses and represent gravity stations. helium anomalies. Dots A LaCoste-Romberg gravimeter model G-248, was used. Gravimeter drift rates were generally less than 0.1 mgal/hr. The only signi fi- cant exception occurred on the northern half of the Imuruk traverse where a 3 mgal "jump" was noted at the close of a 9 hour flight. Elevation control is the most significant limitation on the gravity data of this survey. Station elevations were determined with one or two Paulin surveying altimeters tied together by record- ings from a base station barograph at Pilgrim Springs. These were checked against elevations from the 1:63,360 scale (50 ft. contour interval) maps of the area. Wherever possible, benchmarks and checked elevations were occupied. The worst elevation error Probably does not exceed 25 feet. This implies an error of 1.5 mgal assuming a standard rock density of 2.67 g/cc. About 50 rock samples collected along traverse lines were analyzed for density. Fifteen samples collected by Swanson and Turner during their geologic mapping were also analyzed. Rocks represented in this collection include the Imuruk basalts, Eva Mt. basalts, Precambrian metamorphics, Paleozoic carbonates and Creta- - ceous intrusives. Imuruk basalt samples varied in density from 2.13 g/cc to 3.00 g/cc depending on porosity and the presence of mafic inclusions. Eva Mtn basalts varied from 2.19 to 2.90 g/cc. Paleozoic carbonates from 2.62 to 2.67 g/cc, and Cretaceous intrusives from 2.51 to 2.56 g/cc. The Precambrian metamorphics varied from 2.51 to 3.09 g/cc, depending on lithology and metamorphic grade. Using seismic refraction data (Lockhart and Kienle, this report) Dale-Drake curves (Grant and West, 1965) give a density of 2.0 + .2 g/cc for the valley fill beneath Pilgrim Springs. 63 ie RESULTS 4, Complete (2.67 g/cc) Bouguer anomalies are shown for the Imuruk, Noxapaga, Pilgrim and Agiapuk traverses in Figures 2 through 5. These traverses cross proposed rift segments at approximately right angles. Note the 30-40 mgal central lows of the Imuruk (Figure 2) and Noxapaga traverses (Figure 3) and the 20-30 mgal low of the Pilgrim traverse (Figure 4). These three traverses were taken across the broad Kuzitrin River Valley and the Pilgrim River Valley. The gravity profiles are consistent with graben development and/or low density valley fill. Figure 3 shows a density structure which would cause the observed anomaly on the Noxapaga traverse. Assuming a density contrast of 0.67 g/cc between Precambrian metasediments and the valley fill, a 1.2 km-deep valley with 5-10° sloping walls may be modeled. If this valley is a tensional feature as proposed (Hopkins 1963), the bounding faults must form a step pattern, rather than a single, large, bounding fault. The Imuruk traverse is problematic, since the 35 mgal low may be due in part to low density basalts, especially on Camille Cone and Hoodoo Hill, and low density intrusives which outcrop immediately to the east of the north and south ends of the traverse and within 20 km east of the center of the traverse end of Imuruk Lake. The Pilgrim traverse (Figure 4) suggests a horst in the center of the graben which may be a continuation of the Hen and Chickens - Mary's Mountain structure (Turner and Forbes, 1980). In contrast to these three traverses, the Agiapuk (Figure 5) and Taylor Road (not shown) traverses are rather flat. The Agiapuk 64 S9 ELEVATION (FEET) 4000. cn BENDELEBEN MTNS. fad sketeTon (EARS LOST Jim CAMILLE HOODOO BUTTE BLACK BUTTE 2000 FLOW mn Pie bg WSS 1000 44 . S e e Ss o RMS SSUGSSES 5 50 49 48 aa 464542 440 434 40 3938.37 36 36 u 33 32 3130 2 28 7 DISTANCE 0 10km Figure 2. IMURUK TRAVERSE Geologic cross section and Bouguer gravity profile along the Imuruk profile. exaggeration 6.67x. See Appendix A for geologic symbols. 2726 «2504 23 N Vertical 99 (-1 km) Figure 3. BENDELEBEN MTNS, eR KUZITRIN Ati i hy YM, i i Hi Ah A Geologic cross section and Bou structure shown in the cross s a 0.67 g/cc density contrast b Vertical exaggeration 6.67x. Na vi N A HAO TA We tn Paty ih A LRA ! a ‘ al iy 4 i | 2 <i Lea eh a nan : Way ia HN q ith Nd Hh 64 DISTANCE 0 10 km NOXAPAGA TRAVERSE guer gravity profile along the Noxapaga traverse. The ection is a two-dimensional model to fit the data, assuming etween the valley fill and the Precambrian metamorphics. See Appendix A for geologic symbols. 49 S mGAL -10 \ MT, OSBORN -20 4000. © 000 = & 2000 ‘ & 1000 7 QP, ies pp q -1000 ts oes GALLI YZ s 113 W12>«111 101 110 = 100 109 99 108 98 107 97 106 96 106 95 84 104 103 102 I DISTANCE } N 0 10 km PILGRIM TRAVERSE Figure 4. Geologic cross section and Bouguer gravity anomaly along the Pilgrim traverse. The gravity high near the center of the valley suggests a horst in the center of the graben. This may be a continuation of the Hen and Chickens - Marys Mountain structure. Vertical exag- geration 6.67x. See Appendix A for geologic symbols. 89 os < 9 E e 10 Lear TT ee - rm 2 -20 4000 G w 3000 = z 2 2000 EVA MTN. < GRANTLEY @ sooo] “AR¥OR Set “Gite — mT ye SITTER. a iy Dy we iy MY seis i, OLED oY i 0 a Zak SAL WEEE FE Wh 120 121 122 80 123 124 125 126 = 127 128 129 130 131 7 DISTANCE " 0 10 km AGIAPUK TRAVERSE Figure 5. Geologic cross section and Bouguer gravity anomaly along the Agiapuk traverse. Vertical exaggeration 6.67x. See Appendix A for geologic symbols. traverse was made across the axis of the proposed rift segment north of Teller and the Taylor Road traverse was made near the proposed Labaree transform fault (Turner and Swanson, this report). Both these traverses were made across bedrock highs; there were no broad sediment-filled valleys crossed. The flat nature of the gravity data along the Taylor Road traverse is consistent with the presence of the bedrock high in this area. The Agiapuk traverse is characterized by relatively uniform gravity data which are inconsistent with the presence of a large prism of low density material. Ramberg and Newman (1978) and Searle (1970) have shown that long wavelength, high magnitude gravity anomalies are present at many large, well-known continental rift systems, such as the Kenya rift valley. The proposed Seward rift system (Turner and Swanson, this report) is clearly a much narrower feature and might be expected to produce a smaller anomaly. The three traverses which show significant gravity lows (Imuruk, Noxapaga and Pilgrim) may be explained by low density valley fill ( sediments and/or low-density basalt) in non-structural basins, low density intrusives at depth, or graben structure with low-density fill. The two-dimensional computer modelling of the Noxapaga Traverse (Figure 3) indicates a depth of low density valley fill of 1.25 km, which would place the valley bottom well below any recent sea level. This suggests that the valley cannot be purely erosional and is more likely a structural feature. Hopkins (1963) has reported geologic evidence showing that the Kuzitrin flats area is indeed a structural basin (see Turner and Swanson, this 69 report). Geologic and seismic refraction studies (Turner and Forbes, 1980; Lockhart and Kienle, this report) indicate that the Pilgrim River Valley is also a structural basin (graben). In summary, the gravity traverses reported here are generally consistent with the geologic and seismic evidence for structural basins at the Kuzitrin Flats and Pilgrim River Valley, and with the rift model proposed by Turner and Swanson (this report). However, the gravity data in themselves do not prove the rift hypothesis because they are subject to several possible model interpretations. 70 APPENDIX A Geological Map Units Q Tertiary to Quaternary alluvium, valley fill, includes QTu Kougarok gravels and equivalents, till and alluvium. QTb Tertiary to Quaternary basalts of the Kuzitrin Flats and Eva Mtn. Alkalic to tholeiitic in composition with Qb ultramafic inclusions in the alkali basalts. Ki Cretaceous intrusives, mostly quartz monzonite. Pz Thrust sheets of Paleozoic carbonates and meta-carbonates. Pzc PG Precambrian to lower Paleozoic metasediments. Schists and gneisses of the Nome Group and York Siate. Locally PGms migmatized in the northern Bendeleben Mts. \ Geologic units generalized from Hudson (1977) and Sainsbury (1972, 1974) with faults on the Imuruk Traverse by Hopkins (1963). 71 REFERENCES Barnes, D. F., 1968, Alaska gravity base station network: U.S. Geol. Survey Open-file report, 304, 8 pp. and tables. Grant, F. S. and G. F. West, 1965, Interpretation theory in applied geophysics: New York, McGraw-Hill. Hopkins, D. M., 1963, Geology of the Imunuk Lake area, Seward Peninsula, Alaska: U.S. Geol. Survey Bull. 1141-C. Ramberg, Ts Be and E. R. Neuman, (Eds.) 1978, Tectonics and geophysics of continental rifts, Volume Two: Dordrecht, Holland, D. Reidel. Searle, R. C. 1970, Evidence from gravity anomalies for thinning of the lithosphere beneath the Rift Valley in Kenya, Geophys. J., Roy. Astron. Soc., Vol. 21, pp. 13-31. Turner, D. L. and Forbes, R. B. (Eds.) 1980, A geological and geophysical study of the geothermal energy potential of Pilgrim Springs, Alaska, University of Alaska, Geophysical Institute Report UAG R-271, 165 pp., 1 pl. 72 DEEP SEISMIC REFRACTION PROFILE IN THE PILGRIM RIVER VALLEY GRABEN Andrew Lockhart and Juergen Kienle INTRODUCTION Our 1979 seismic work at Pilgrim Springs was designed to obtain subsurface information on the stratigraphy and structure of the val- ley fill and to outline the configuration of the crystalline basement (Kienle et al., 1980). We did not reach basement but determined that it was not within 200 m beneath the springs, based on a minimum depth estimate from our longest refraction profile. Other work (geological mapping and gravity) has produced evidence that the Pilgrim River Valley is a graben (Turner and Forbes, 1980). Knowledge of the depth to basement in the Pilgrim River Val- ley is important for assessing the possible existence of geothermal reservoirs in the sediments deeper than the shallower reservoir discovered in the 1979 work. In 1980 we returned to Pilgrim with two seismographs and a radio-telemetered blaster developed at the Geophysical Institute, to increase shot-receiver distance. We shot one 1385 m seismic refraction profile, shown in Figure 1 as J-J'. The profile line was sited to provide basement depth informa- tion in the area of the geothermal drilling targets identified in the previous study of the area (Turner and Forbes, 1980). METHODS AND INSTRUMENTATION Using explosives left from 1979, we loaded six four-inch-dia- meter holes drilled into permafrost at site J (Figure 1). The protective bag in which the explosives were stored had unfortunately torn and many of the nitrocarbonitrate cans were badly corroded 73 Location of Seismic Refraction Profiles TERRACE THERMALLY DISTURBED AREA Figure 1. Map of the Pilgrim Springs area showing the approximate outline of the thermally disturbed ground and the location of 1979 seismic refraction lines and the 1980 long line J-J'. 74 from water damage. Care was taken to use the best appearing cans, but some bad ones had to be used. Two 12-channel Geometrics-Nimbus model ES-1210 seismographs were moved along J-J' to pick up returns from the shots at J. These were linked to the explosions by a radio-telemetered blasting trigger activated at J. The operating principle at the radio blaster entails the transmission of two frequencies which are broken off when the shot is fired. Radios at the seismographs detect the end of the signal and activate the seismographs. Timing errors in the system theoretically should be less than 1 or 2 msec. RESULTS Figure 2 shows the travel time curves for the unreversed 1335 m refraction line. Seven geophone spreads were used to record first arrival times for five shots fired near J (Fig. 1). Array 1 recorded shot 1, arrays 2 and 3 recorded the second shot, arrays 4 and 5 the third, and arrays 6 and 7 recorded the fourth and fifth shots respectively. As one can see from Figure 2, the first arrival times do not lie along a single travel time curve, such as the dashed line through array 3. Evidently the third shot recorded by arrays 4 and 5 was delayed by 150 m sec, the fifth shot recorded by array 7 was delayed 390 m sec, and shots 1 and 4 were apparently delayed by about 30 msec. We suspect the delays were due to the deteriorated condition of the explosives, although the radio-linked trigger may have been at fault. 75 9 700 a ° oO TIME (mSEC) a 2 oO 400 300 200 100 Figure 2. a T a eT TT Tae oat ee alge 1385m Seismic Refraction Profile uf Pilgrim Springs, Alaska er 3920 m/sec __ 5 6 eae weer ner aa Ql et ——-~"5270 m/sec we 3} 771690 m/sec & 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 DISTANCE (m) Travel time curves for the unreversed 1385 m refraction line J-J' run in 1980 using 5 shots at J. The dashed travel time curve was constructed assuming arrivals at array 3 were correct and all others delayed. Groups of arrival times above the travel time curve are thought to be due either to defective explosives, or to the radio-controlled blasting/seismograph start circuitry. Because of the obvious delays in the shots we must make some assumptions to construct a travel time curve to interpret the data. First we assume that the arrival times for array 3 are correct, and have constructed a travel time curve using the slope for each array shifted down to correspond with the data of array 3. The low velocity layer indicated by the dotted line through the origin (Figure 2) is based upon previous refraction profiles. To invert the first arrival times to produce a model of the layering including dipping interfaces, we would require that shots be fired from the other end of the spread at J' (Figure 1). Since this was not done due to time constraints, we can only assume a horizontal layering using the dashed travel time curve. On this assumption we have calculated a layered earth model as shown in Figure 3 for the arrival times to array 6 where the velocity of 5270 m/sec was recorded. This velocity is typical of the crystal- line rocks exposed on either side of the valley, and is assumed to be the basement complex at a depth of 425 m. The 3920 m/sec arrivals from the seventh array may be indicating a dip in the basement to the west, which would agree with gravity gradients at Pilgrim Springs, (Turner and Forbes, 1980). The 425 m depth to crystalline basement has important implica- tions for geothermal energy resources in the Pilgrim River Valley. Resistivity work (Wescott et al., 1980; Wescott, this report) and drilling (Kline et al., 1980) have previously demonstrated the existence of a near surface reservoir about 50 m thick. With 375 meters of sedimentary section between the known reservoir and the crystalline basement complex there is a high probability that 77 82 INTERPRETATION OF UNREVERSED 1385m SEISMIC REFRACTION PROFILE Oe 1400 FT PPCeiT PTLD PERIL CPrrni Pe ee Cea OMe Ee nC ee eee oes al 1690 m/sec 800 m/sec 2090m/sec 100 200 300 DEPTH (m) 400 Figure 3. Generalized cross section calculated from the travel time curve shown in Figure 2, ignoring the low velocity indicated by array 7. other permeable aquifers (geothermal reservoirs) may be present and could be reached by deeper drilling. 79 REFERENCES Kienle, J., A. Lockhart and J. Peace, 1980, Seismic refraction sur- vey of the Pilgrim Springs geothermal area, Alaska, In: Turner, D. L. and R. B. Forbes, Eds., A geological and geo- physical study of the geothermal energy potential of Pilgrim Springs, Alaska, University of Alaska, Geophysical Institute Report UAG R-271, p. 53-72. Kline, J. T., R. D. Reger, R. M. McFarlane and T. Williams, 1980, Surficial geology and test drilling at Pilgrim Springs, Alaska, In: Turner, D. L. and R. B. Forbes, Eds., A geological and geophysical study of the geothermal energy potential of Pilgrim Springs, Alaska, University of Alaska, Geophysical Institute Report UAG R-271, p. 21-28. Turner, D. L. and R. B. Forbes, Eds., 1980, A geological and geo- physical study of the geothermal energy potential of Pilgrim Springs, Alaska, University of Alaska, Geophysical Institute Report UAG R-271. Wescott, E., R. Sydora, J. Peace and A. Lockhart, 1980, Electrical resistivity survey of the Pilgrim Springs geothermal area, In: Turner, D. L. and R. B. Forbes, Eds., A geological and geophysical study of the geothermal energy potential of Pilgrim Springs, Alaska, University of Alaska, Geophysical Institute Report UAG R-271, p. 81-100. 80 VLF AND GALYANIC RESISTIVITY SURVEYS IN THE CENTRAL SEWARD PENINSULA Eugene M. Wescott INTRODUCTION Because of the very large area involved in the 1980 regional reconnaissance for geothermal resources in the central Seward Peninsula region, galvanic resistivity could be used only at a few selected sites. To obtain ground resistivity data along the sam- pling traverses and at other sites we utilized a Geonics EM-16R instrument which measures the VLF signals from one or more of the transmitters used by the Navy to communicate with submarines. We had crystals for two stations in Maine and Maryland (about 6000 km distant) but their signals proved to be too faint for accurate work. The station in Seattle, NLK, operates at 18.6 kHz and is 3100 km distant from Pilgrim Springs. The signal is satisfactory but not outstanding for VLF surveying. The VLF transmitters have a vertical antenna, so the transmitter current is also vertical, creating alternating concentric horizontal magnetic fields radiating outward. When these waves meet conductive zones in the earth there will be induced currents and secondary magnetic fields which may have a vertical magnetic field component along the surface of the earth. The basic EM 16 equipment fs simply a sensitive VLF receiver with means of measuring the vertical field components. Two coils are used in the measurements. The signal from the vertical axis is first minimized by tilting the instrument, giving a measure of the vertical real-component. Next the remaining signal from this 81 coil is balanced out against the signal from a second coil phase shifted by 90°, providing a measure of the quadrature vertical signal. The real and quadrature components measured on profiles perpendicular to the strike of conducting bodies provide data for interpreting the conductivity, width and depth of various shaped bodies. The EM 16R equipment interfaces with the EM 16 to provide quanti- tative information on the resistivity of the ground by use of the magnetotelluric apparent resistivity relationship. The apparent resistivity is derived from the square of the ratio of the electric field to the magnetic field multiplied by the period of the wave and a scale factor. A pair of probes with 10 m spacing are inserted into the ground aligned with the direction to the station to provide the electric field component. Apparent resistivities of 1 to 30,000 ohm-meters can be measured with the EM-16R. The second parameter measured is the phase angle between the electric and magnetic components of the wave. The depth of penetra- tion depends upon the ground resistivity and the frequency of the transmitter. In the YLF range if the ground resistivity is 1000 ohm-m, the depth of penetration is 100 m. If the resistivity is less the depth of penetration is less, etc. Ina uniform earth to beyond the depth of penetration, the phase angle is always 45°. If layers of different resistivity are present the phase angle will differ from 45°. In the case of a two layered earth, if either the resistivity of the upper layer or the sub-stratum, or the thick- ness of the upper layer is known, the use of theoretical curves yields the value of the other two unknown parameters. 82 he resistivity and the phase angle measurements are made by finding a null in an audio tone. The sharpness of the null is dependent upon the received signal strength. Though the signal from NLK in Seattle was sufficient to determine the apparent resis- tivity, the phase angle null was broader and more difficult to measure. Measurements In order to verify the VLF method and measurements we made traverses with the EM-16 and EM-16R over the thaw ellipse at Pilgrim springs where we had previously made careful galvanic resistivity measurements (Wescott et al., 1980). The EM-16 does not show much response to a finite flat plate conductor such as the 50 m thick saline reservoir at Pilgrim Springs except at the boundaries. Thus we did not expect or find an appreciable anomaly with the basic EM-16 measurements. The EM-16R measurements of apparent resistivity were more useful however. On a traverse south along the P-P' line from 600N (Figure 1) at Point P, south to the center of the grid 100 m north of the line T-T' the apparent resistivity varied from about 80 Q-m to less than 10 Q-m as shown in Figure 2. The phase angle was more difficult to measure, but showed consistent results. The phase angle was near 45° measured at stations where the resistivity was low (near 10a-m), indicating that the depth of penetration (10 m) was smaller than the thickness of the low resistivity reservoir. The phase angle generally was greater than 45° north of station 400 N where the apparent resis- tivity was higher. From theoretical curves this indicates that 83 LOCATION OF RESISTIVITY AND EM 16-R SURVEY LINES | I RIVER I 7 (200.747)! pw Ve iM 3 200,32°)] » Oy «| (57,50° &. \ Se 0.52°) ae 5°) PW PW80-13 Pwwe0-9 & con) THERMALLY DISTURBED AREA Figure 1. Map of Pilgrim Springs vicinity showing the locations of galvanic resistivity lines from 1979 and 1980 work and the locations of VLF EM-16R apparent resistivity measurements. 84 100 Figure 2. EM-16R N-S PROFILE, PILGRIM 0.0 E 100N 200N 300N 400N 500N 600N DISTANCE [m] N Apparent resistivity and phase EM-16R measurements along profile P-P'. The phase angle of 45° indicates that the ground appears homogeneous to beyond the depth of penetration (about 10 m). 85 PHASE ANGLE [DEG] there is a low resistivity layer underlying a more resistive surface layer. These results are in very good agreement with the dipole- dipole and Schlumberger resistivity work previously reported by Wescott et al. (1980). We also ran an EM-16R profile (Figure 3) on both sides of the Pilgrim River at station PW80-3 (Figure 1) where there was a small helium anomaly and where there were broad leafed plants typical of thermally disturbed ground, and where we also ran a Schlumberger resistivity depth sounding. The Schlumberger depth profile data and model curve are shown in Figure 4. The fit is not perfect in that the observed data show a steeper decline than the theoretical curve (lower resistivity at depth). The minimum resistivity is suggested to be 3.1 Q-m at a depth of 7.5 m, while the overlying resistances vary from 489 2-m at the dry surface layer to 5021 Q-m at 1.1 m depth, then decreasing in the next few meters. The EM-16R profile shown in Figure 3 crosses the Schlumberger depth profile at about 100N, where the apparent resistivity was 25 Q-m and the phase angle 59°. If we assume a two layer case, with the upper layer of 1000 a-m, then a theoretical curve gives a thickness of 4 m and an underlying resistivity of 10 2-m, in fair agreement with the multi-layered Schlumberger results. Additional comparisons of EM-16R apparent resistivities and Schlumberger depth profiles were made at a thaw window about 4 km NE of Pilgrim Springs where abundant broad leafed plants were found and at another possible thaw window about 3 km NE of Pilgrim Springs. In both cases there was good agreement between the two methods. Figure 5 shows the Schlumberger depth profile data and 86 Pa ( -m) Figure 3. EM-16R PILGRIM PROFILE PW80-3 9 100N 180N PILGRIM DISTANCE [m] RIVER EM-16R profile across the Pilgrim River at station PW80-3 (Figure 1). Helium anomaly and flora suggest the presence of hot water under the northern portion of the profile. 87 PHASE ANGLE ¢ [DEG] 88 RESISTIVITY Figure 4. SCHLUMBERGER PW 80-3 THEORETICAL ——— OBSERVED x x x x Schlumberger depth profile at PW80-3. Very low resistivity material underlies the surface layers as shown by the computer generated fit to the data points. 68 Il RESISTIVITY Figure 5. SCHLUMBERGER fs PW 80-1B THEORETICAL —— OBSERVED xk k xk 4 =3 p, = 863.9 1y=.8 2 = 1419.6 ~~ 13 = 7.6 p3 = 643.9 t4=2.9 p4 = 164.4 ts = 2.1 ps = 47.2 tg= 123 bg = 23.0 ty = 10.6 p) = 12.0 tg= 12.1 Pg = 2.8 BT TET FT EEG FE eee SST T Schlumberger depth profile at small thaw window 4 km NE of Pilgrim Springs. Low resistivity layer suggestive of warm saline water is indicated below the surface layers. model curve at the thaw window 4 km NE of Pilgrim. The EM 16-R readings at the same site were 36 2-m and 68°. By the two layer model curves this is in good agreement with the Schlumberger depth profile. Figure 6 shows the Schlumberger depth profile data and a three layer curve fit using La Compagnie Général de Géophysique (1955) curves at the second possible thaw window 3 km NE of Pilgrim Springs. The data suggest an ice rich frozen layer 3 m thick underneath a thawed surface layer of 0.33 m thickness. Beneath the frozen layer the resistivity is very low, zero in the model curve. The EM 16-R readings at this site were 31 Q-m and 70°. With an assumed two layer case of p] = 30,000 a-m, the EM 16-R values predict a layer 8 m thick underlain by material of resistivity 8 2-m. Helium soil sample values at the first thaw window were significantly higher than normal, and were slightly higher than normal at the second site. One goal of the measurements in the Pilgrim River Valley was to test the hypothesis that hot subsurface saline water rising in the Pilgrim Springs thaw ellipse was migrating northward to the Pilgrim River and then migrating westward below the river bed (Osterkamp et al., 1980). There are several lush cottonwood groves downstream in an almost linear zone on both sides of the meandering river. In several of these groves we found the broad leafed plants which seem to be characteristic of thermally disturbed ground. As shown on Figure 1 we made an EM-16R profile at station PW80-10 ENE of the Pilgrim thaw ellipse and across the Pilgrim River. A helium anomaly of 6.3 ppm was found there, suggesting a source of subsurface hot saline water. The apparent resistivity 90 SOO00 {Q000 RESISTIVITY [a-m] o oO O 400 Figure 6. OA Schlumberger PW80-16 July 29,1980 t,=0.33m P,= 3200 Q-m tp 3m P> = 124,800 a-m P3=0 4.0 10 400 SPACING AB/2 [m] Schlumberger depth profile data and three layer curve at smal] thaw window 3 km NE of Pilgrim Springs. 91 profile is shown in Figure 7. The resistivity is generally higher than in the main Pilgrim area. From 40 to 60 m in from the river the resistivity is about 100 Q-m with a phase angle of 59°. Assuming a surface resistivity of 300 2-m, theoretical curves suggest an underlying resistivity of about 30 2-m, which does not seem to be low enough for a significant hot water or saline layer. Downstream at PW80-3, Figure 1, both a Schlumberger profile, Figure 4 and an EM-16R profile were run (Figure 3). North of the river both methods indicate low resistivities at depth and the presence of warm saline water is likely as there were also broad leafed plants present. South of the river the apparent resistivity is higher. At the next meander bend downstream at station PW80-11, Figure 1, low resistivity at depth is also indicated on the EM-16R profile by low apparent resistivities and high phase angles as shown in Figure 8. Continuing downstream, two measurements were made inland at PW80-12 (Fig. 1) where the resistivities and phase angles were 57 a-m, 50° and 40 a-m, 55°, somewhat higher than upriver. About 200 m farther downstream there is an abrupt change in the character of the vegetation and surface geology caused by a fault which runs by the west side of Pilgrim Springs (Turner et al., 1979). Station 5 on the west side of the transition is in tundra and permafrost. The apparent resistivity was 200 a-m and the phase angle was 74°, If we assume the surface layer of frozen ground has a resis- tivity of 1000 a-m, then it js about 30 m thick and is underlain by a bed of resistivity 30 a-m (see Fig. 1). Further downstream 92 EM-16R PILGRIM PROFILE PW80-10 200 ' é 100 0 10 20 30 40 50 60 70 80 RIVER BANK DISTANCE [m] ENE ——> Figure 7. Apparent resistivity and phase angle EM-16R measurements in the vicinity of station PW80-10. 93 PHASE ANGLE ¢ [DEG] Figure 8. Pa [Q-m) EM-16R PILGRIM PROFILE PW80-11 PHASE ANGLE ¢ [DEG] 100N 160 s DISTANCE [m] N Apparent resistivity and phase angle EM-16R measurements at station PW80-11. / 94 on the south side of the river at station PW80-13 the resistivity was 100 Q-m and the phase angle was 52°. In the next sharp meander bend we ran two Schlumberger depth profiles PW80-7 and PW80-9, shown in Figures 9 and 10 respectively. The computer modelled curve for profile PW80-7 shows high resistivity layers up to 7782 Q-m underlain by lower resistivity. The upper layers are probably frozen ground. The resistivity of the underlying sediments is too high to suggest flow of saline water from a source at Pilgrim. Depth curve PW80-9, Figure 10 is not a very good match between the computer generated fit and the field data. It appears that lateral inhomogeneities in the surface resistivity are present in the data. There appears to be a 2.9 m-thick frozen layer beneath a thawed surface zone underiain by lower resisitvity. The curve indicates a 1.6 m-thick zone of p =12.6 Q-m at a depth of 24.6 m, but due to the poor fit this is uncertain. Downstream in the next meander bend the EM16-R readings at station 7 were 200 2-m and 32°, indicating that it is unlikely that warm saline water is flowing beneath the site. The apparent resistivity continued to increase downstream. At the last site where a lush grove of cottonwoods were found about 3 km beyond station 7 (Fig. 1) we measured 240 a-m and 53°. EM-16 and EM-16R readings were made at some stations about 1 mile apart along the Taylor Road traverse (Wescott, pg. 49, this report) where gravity data and helium and mercury samples were taken. The resistivities were all high, of order 1000 a-m, typical of permafrost perhaps 100 m thick. 95 96 RESISTIVITY Figure 9. SCHLUMBERGER PW 80-7 THEORETICAL —— OBSERVED x x x x Schlumberger resistivity depth profile and computer model curve at PW80-7. of cesiee SAG. 16 0° RESISTIVITY Figure 10. SCHLUMBERGER PW 80-9 THEORETICAL —— OBSERVED x x x x Schlumberger resistivity depth profile and computer model curve at PW80-9. In the 1979 season we made a 100 m dipole-dipole traverse line along P-P' (Fig. 1) Wescott et al. (1980). A two-dimensional finite element computer model of those data indicated a low resis- tivity layer of about 3 a-m, 50 m thick, underlain by higher resistivity material. The model was insensitive to additional layers of low resistivity at greater depths. In 1980 we carried out dipole-dipole measurements east-west across the thaw ellipse, line T-T' on Figure 1, to determine the east-west boundaries and to siniste deeper under the shallow reservoir. We used 100 m dipole spacings separated by n = 10 in some cases. The pseudosection plot of the data is shown in Figure 11. In general features it agrees with the previous N-S pseudosection. The western edge of the thawed ground appears to lie at about 650 m west and the eastern edge at about 350 m east. The western edge coincides with the fault discussed previously. There are some features such as the downward elongation of the 6 2-m contour near 3W, which suggest that detailed three-dimensional modelling might reveal something about the reservoir structure below 50 m depth. We obtained a three-dimensional resistivity modelling program (Dey and Morrison, 1975) and converted it to run on the Geophysical Institute's VAX 11/780 computer. We ran a fairly simple 3-D model using the para- meters determined in the 2-D fit to the N-S profile P-P' and obtained a reasonable general fit (Wescott et al., 1980). Further detailed 3-D modelling should be done in the future if further drilling at Pilgrim Springs is contemplated. To run models of this complexity takes large amounts of personnel and computer time. 98 66 Figure 11. Pseudosection (Figure 1). plot of east-west 100 m dipole-dipole galvanic resistivity along profile line T-T' CONCLUSIONS Resistivity measurements at stations PW80-3 and PW80-12 indi- cate a subsurface conducting layer, which is probably due to circu- lation of warm saline water. From the profiles the reservoir layer is about 150 m wide and trends east-west. The resistivity of the ground underlying the stands of cottonwood vegetation in meander bends further downstream increases on the west side of a Quaternary fault which bounds the west side of the Pilgrim Springs thaw ellipse, suggesting that the offset of sediments by the fault acts as a barrier to the downstream migration of hot saline water. The east-west dipole-dipole traverse determined the boun- daries of the thermally disturbed ground. At the western end the boundary appears to coincide with the fault. At the eastern edge the transition is more gradual, at about 350 m east of the N-S baseline. The EM-16R instrument proved to be a useful tool for rapid reconnaissance resistivity measurements. We did not find any unexplained zones of low resistivity except in the Pilgrim Springs vicinity, but the EM-16R measurements were not carried out on the long helicopter traverses due to lack of space for personnel to make the measurements. Generally the ubiquitous permafrost in the central Seward Peninsula implies that the ground resistivity will be very high except where the ground is thawed. 100 REFERENCES Dey, A. and H. F. Morrison, 1975, Resistivity modelling for arbi- trarilly shaped two-dimensional structures, Engineering Geo- science and Lawrence Berkeley Laboratory, University of Cali- fornia. La Compagnie Générale de Géophysique, Abaques de sondage Electrique, Geophysical Prospecting III, Supplement No. 3, 1955. Osterkamp, T. E., J. P. Gosink, R. B. Forbes, R. G. Gaffi, J. T. Hanscom, M. L. Kane, C. A. Stephens and J. Kline, 1980, A reconnaissance study of the hydrothermal characteristics and accessible power of Pilgrim Springs, Alaska, In: Turner, D. L. and R. B. Forbes, Eds., A geological and geophysical study of the geothermal energy potential of Pilgrim Springs, Alaska, University of Alaska, Geophysical Institute Report UAG R-271, p. 113-156. Turner, D. L. and R. B. Forbes, Eds., 1980, A geological and geo- physical study of the geothermal energy potential of Pilgrim Springs, Alaska, University of Alaska, Geophysical Institute Report UAG R-271, 165 pp., 1 pl. Wescott, E., R. Sydora, J. Peace and A. Lockhart, 1980, Electrical resistivity survey of the Pilgrim Springs Geothermal area, Alaska, In: Turner, D. L. and R. B. Forbes, Eds., A geological and geophysical study of the geothermal energy potential of Pilgrim Springs, Alaska, University of Alaska, Geophysical Institute Report UAG R-271, p. 81-100. 101 APPLICATION OF RADAR AND INFRARED AIRBORNE REMOTE SENSING TO GEOTHERMAL RESOURCE ASSESSMENT AT PILGRIM SPRINGS, ALASKA Kenneson G. Dean, Robert B. Forbes, Donald L. Turner, Frank Eaton, Kathryn D. Sullivan* INTRODUCTION Pilgrim Springs is located approximately 60 miles north of Nome on the Seward Peninsula, Alaska. Extensive geological and geophysi- cal research has been conducted on the springs and the underlying hot water reservoir system to evaluate its potential as a geothermal resource (Turner and Forbes, 1980). Many hot springs are present throughout Alaska. Most of these are relatively inaccessible and only a few have been studied in any detail. If remote sensing techniques could be used to discover and/or evaluate thermal anomalies in Alaskan settings, the assess- ment of the geothermal potential of remote areas could be accelerated. This study evaluates radar and thermal data collected by a high- altitude sensor-platform flown over a known gecthermal target. The sensing platform (a NASA research aircraft, RB57F) contained a synthetic aperture radar (SAR) sensor, and a multispectral scanner. Data were collected over the Pilgrim and Kuzitrin River Valleys, but only the Pilgrim River Valley data were analyzed due to budget constraints. PHYSICAL SETTING Pilgrim Springs is located on the Pilgrim River floodplain (Fig. 1). North and south of the floodplain are terraces adjacent *NASA-Johnson Space Center 103 ~0T Sel, PE aan oO p Extent of Lineament Anal Porn eer. cisrence fleller QV aA : Coan Pe Nome SCALE 1:1,000.000 o 40 80 MI £ = so 100 Km Or! Figure 1. Location diagram and regional geology. Cape Espenderg KOTZEBUE to Hen and Chickens Mountain and the Kigluaik Mountains, respec- tively. The Kigluaik Fault is located along the northern front of the Kigluaik Mountains and is considered to have been active in Holocene time (Hudson and Plafker, 1977). Most of the area is underlain by about 100 m of permafrost (Wescott, et al., 1980) except for the thawed zone (thaw bulb) around Pilgrim Springs. Most of the ground surface in the valley contains standing water either because of a perched water table from permafrost, or a near-surface water table as at Pilgrim Springs. Typical vegetation in the area consists of brush on mountain slopes, tundra on the terraces and brushy wetlands and deciduous forests on the floodplain. Within the thawed zone the forested areas are dominated by poplar with a very broad-leaf understory, which is unusual in arctic environments. Morphologically, the thawed zone includes oxbows, swales, small ponds, wetlands and small cultivated fields. Low-lying areas are usually flooded every spring during break-up. The dense, deciduous vegetation within the thawed zone is distinc- tive on color-infrared photography due to its high reflectance in the near infrared compared to surrounding tundra and brush vegeta- tion. ANALYSIS OF RADAR IMAGERY The x-band (2.4-3.8cm wavelength) SAR data was recorded in the far-range mode, 10 to 20 miles beyond the flight path. Range and azimuth resolution is less than 50 feet. The final SAR product 105 consists of 70mm transparencies for two look directions (south and west) from both like-horizontal (HH) and cross (HV) polarization. The HH imagery from this mission has higher contrast, less grain and presents a wider range of intermediate grey tones as compared to the HV imagery. The HH imagery is more definitive for the delineation and analysis of lineaments and topographic features. However, relative backscatter values on the HH and HV differ for some types of vegetation. Both products provided unique information although the thawed zone could only be seen on the HH image, regard- less of look direction, presumably because the broad-leaf vegetation resulted in stronger radar returns. The primary interest in the radar data was to evaluate its effectiveness in the detection and interpretation of faults, fractures and formational contacts which may influence the upwelling of hot water at the springs. Lineaments were visually interpreted on south- and west-looking HH imagery using monoscopic and stereo- scopic imagery (utilizing both look directions), and on a crude color composite optically formed from south- and west-looking imagery. The latter technique revealed several linears which otherwise would not have been mapped. This technique of forming a color composite from two look directions at right angles to each other shows promise for interpretation of linears. A large number of linears were interpreted from the imagery which initially included erosional and fault scarps, joint ‘rdidy: linear stream channels, layering or bedding in bedrock, and possibly patterned ground associated with permafrost. By comparing inter- preted linears to mapped features in available references (Sainsbury, 106 1972, 1974; Sainsbury, et al, 1969; Hudson, 1977; Beckman and Lathram, 1976; Turner et al., 1979; Kline, 1981) and our own field observations and mapping, we have attempted to eliminate those not related to faults, fractures or meaningful formational contacts (Fig. 2). The density of our mapped linears is much greater than those previously mapped but probably includes some non-fault related features. Persistent and easily identifiable linears are considered to be dominant lineaments, displayed as heavy lines in Figure 2. These features generally trend E-W, and include or are parallel to the Kigluaik Fault and to the structural grain of the Kigluaik Mountains. Inclusion of two dominant N-S trending lineaments (queried dashed lines in fig. 2) is based on sub-paraliel alignment of separate ‘Seaihagh segments. These N-S lineaments are of question- able validity, but are included because they are sub-parallel to many weet aoe linears seen in the same imagery. Shorter and less distinct linears are considered to be sub- ordinate lineaments and are shown as relatively thin lines on Figure 2. These features trend in various directions but a N-S set is dominant. Typically, subordinate lineaments are associated with terrace scarps along the Pilgrim River or trend north from the Kigluaik Mountain front as unusually straight stream channels. Figure 3 illustrates the same linear trends interpreted from the radar imagery without a superimposed topographic base. The linears are oriented in various directions. Prominent linear- sets trend N-S, NE-SW and E-W, with a lesser concentration trending NW-SE. These lineament trends are remarkably similar to the regional 107 \ — Subordinate lineament ——-— — Suspect lineament LI were — Inferred lineament Figure 2. Linear features most likely related to faults or fractures interpreted on the radar imagery. 601 Figure 3. Linear trends interpreted on the radar imagery. Linear Count 1 Degree/Cel | N Linear Lengths 1 Degree/Cel | linear trends described by Metz and Wolf (1980) based on the inter- pretation of enhanced Landsat imagery scenes #£30351-21540 path/row 88/14. The Kigluaik Fault is the dominant lineament previously mapped in this area. Hudson and Plafker (1977) describe the Kigluaik Fault as an en echelon fault system consisting of at least three faults that mark the northern front of the Kigluaik Mountain Range. Holocene displacement is down to the north between 4 m and 10 m and is best developed along central parts. The fault apparently dies out in bedrock at its eastern and western terminations. The region is considered to reflect a regional stress regime that is characterized by N-S extension and may be part of an incipient - continental rift system (Turner and Swanson, this report). Many of the linears mapped from the radar imagery correspond to previously mapped faults (Turner et al., 1979; Kline, 1981). Faults mapped by Turner et al. (1979), are predominantly north-south or east-west trending and are concentrated in the Kigluaik Mountains with a few extending onto or across the Pilgrim River floodplain. Most of these faults are evident on the radar imagery. Faults mapped by Kline (1981, personal comm.) are also, gener- ally, in good agreement with the linears mapped from the radar imagery. Of particular significance are the faults located on the Pilgrim River floodplain, especially those trending NE and WNW. The SAR lineament analysis suggests closely spaced block fault- ing with highest fracture density within 10km of Pilgrim Springs. Dominant fracture patterns trend E-W or NE. It is clear that the E-W linears on the south margin of the Pilgrim River Valley are 110 sub-parallel and related to mapped range front faults along the north front of the Kigluaik Mountains. The NE-trending linears which appear to intersect the dominant E-W linears (faults?), are parallel to a mapped fault, also seen on the imagery. This fault can be traced along a northeasterly trend between Mary's and Hen- and-Chickens Mountains. These linears are also sub-parallel to the Kuzitrin Fault described by Turner et al. (this report) which extends along the north front of the Bendeleben Mountains. The Pilgrim River Valley is filled with at least 425 meters of alluvial, glacial and glaciofluvial deposits (Lockhart and Kienle, this report; Kline, 1981). In some areas, the depth of fill may exceed 425 meters, and it is likely that Tertiary sedimentary rocks may also be present at the base of the section. (Lockhart and Kienle, this report). A critical question is whether: youthful block faulting in the basement rocks could be expressed at the surface through 425 meters or more of relatively unconsolidated sediment. Rigidity induced by permafrost in the overlying sediments would encourage brittle response and breakage rather than conceal- ment by slump and fill. Agreement of radar-mapped linears with Quaternary faults mapped by Turner et al. (1979) and Kline (1981) indicates that many of these linears are fault related despite the thick overburden. ANALYSIS OF THERMAL INFRARED DATA Thermal measurements in this study are based on radiant tempera- tures rather than kinetic temperatures. Radiant temperatures can be measured by instruments that detect electromagnetic radiation 111 in the thermal IR wavelength region, and they are less than accom- panying kinetic temperatures (Sabins, 1978). The measuring instru- ments included a radiometer which was mounted in a helicopter and acquired data from 500 feet above terrain, and a multispectral scanner in the instrument compartment of the RB-57 which acquired thermal IR data along a flight path flown at 60 ,000 feet. Prior to the field work and data acquisition we utilized multi- date seasonal Landsat imagery to target late-freezing or prematurely thawed water bodies and to also target potential areas of anomalous heat flow. Field investigation of targeted areas did not reveal any unusual thermal conditions. The anomalous areas appear to relate to springs or water movement. Radiometer Measyrements Radiant temperatures were measured with a Barnes PRT 5 radio- meter to analyze diurnal variation, and to support the interpreta- tion of high-altitude imagery. Radiometer measurements were recorded on a strip chart. The resulting data were digitized and analyzed by computer. Data were collected from the Pilgrim Springs area as far west as the Imuruk Basin. Measurements of pre-dawn radiant temperatures at 2200 BST (Bering Standard Time) July 13, 1980, revealed three clusters of mean temperatures for surface materials: 5.5°C over mixed snow and bedrock; 10-12.5°C over various vegetation types and surface materials; and 14-16.5°C over water (Figs. 4&5). The standard deviation of measured radiant temperatures for various types of” vegetation and surficial materials exceeded the measured differences between the cover types so that distinctive radiometric signatures 112 €Tl Frequency SURFACE MATERIALS 18 46 414 42 a = mixed snow & rock b = bedrock c = glacial till 10 d = colluvium e@ = swamp deposits f = alluvium WATER 8 g = deltoic deposits / h = rivers 6 1 = Imuruk Basin | = lakes k = Pilgrim Springs pools 3 4 5 GC .-te 8. S40" 44: ) 42: 43: 44°46 46 47 Mean Surface Temp. °C Figure 4. Measured mean radiant temperatures for surface materials in the Pilgrim Springs area, oI 28 26 44 42 a = alpine vegetation b = high brush c = low brush 40 ‘d = riparian wetland ~ e = tundra o c 8 f = wet tundra S g = thawed zone vegetation a h = deciduous forest - 1/6 4 2 4.0 0.5 4 § 6 Ff. > Bi:9H 4. 142 143) 44a te 46 47 Mean Surface Temp. °C Figure 5, Measured mean radiant temperatures for vegetation types in the Pilgrim Springs area. could not be defined. Surfaces within the thawed zone had the highest temperatures of the set and the presence or absence of permafrost had no distinguishable effect on the radiant temperatures. Standard deviations for water temperatures do not overlap. The radiant temperature from the hot bathing pool is 16.5°C compared to subsurface kinetic temperatures which average 81°C (Turner and Forbes, 1980). Radiant temperature measurements were periodically acquired within a 24 hour cycle along a N-S traverse over Pilgrim Springs and the surrounding area on July 12, 1980 at 1300, 1800 and 2240 BST, and on July 13, at 0900 BST. Most surface temperatures peaked between 1300 and 1800 hours, degraded after 1800 and then warmed during the subsequent daylight hours. The temperatures varied as much as 16°C. Unlike other surface temperatures, water temperatures were warmest at 1300, and coolest at 0900, with a 10°C variability. Two thermal lakes were monitored. The warmer of the two lakes produced radiant temperatures of 23.5°C to 13.5°C. The diurnal variations of the lakes generally paralleled the thermal behavior of non-thermally warmed water but had higher mean temperatures. The lake with widely fluctuating temperatures is small and probably shallow. ‘Thermal IR Data The multispectral scanner acquired thermal IR imagery, (10-12 u) on July 18, 1980 at 0830 BST and on July 20, 1980 at 1930 BST. Theoretically the thermal channel of the multispectral scanner has a maximum resolution of 60 feet, and 0.5°C. A pre-dawn flight was planned for 0300, the estimated time of optimum surface cooling, 115 but weather conditions prevented data acquisition at that time. The quality of the resulting imagery is poor due to instrument problems, scattered clouds and possibly some processing difficul- ties. On the daylight image, clouds and/or instrument-related arti- facts obscure Pilgrim Springs but the surrounding terrain is visible. Water bodies are readily distinguishable from the surrounding vegetated terrain. Some thermal definition of various terrains and vegetation types was observed, but was not readily distinguish- able. Based on our radiometer data, radiant temperature measure- ments from daylight thermal imagery appear to be of little use for geothermal studies. Clouds are also present on the evening imagery, although Pil- grim Springs and its immediate vicinity are cloud-free. Comparison of a density-sliced analysis of the evening IR imagery with veget- ation types and inferred composition of surficial materials revealed no correlation. This conclusion is also supported by radiometer measurements. The thermal response of non-geothermally heated wetlands in the area depends upon the amount of exposed water and vegetation cover. In areas where wetlands are adjacent to lakes or.streams, the wetland thermal response is identical to that of the water body. Presumably this is due to heat transfer between the water body and wetland. The thermal response of wetlands not adjacent to lakes or streams is usually identical to the response from surrounding vegetation. 116 On the density-sliced image several areas were detected as warmer than surrounding surfaces. By comparing the sliced image with maps and airphotos, we found that most of these warm areas were related to lakes or streams. Elimination of water-related anomalies revealed several unusually warm areas that appear to be related to the geothermal heat source. These areas are displayed in Figure 6 and the largest areas are labeled A, B and C. Areas A and B lie to the north of the hot springs, along the Pilgrim River. Near-surface river water temperatures (kinetic) increase downstream (to the west in figure 6) from 6.4°C near the center of area B to 7.5°C near the western edge of area A. Ground- water movement, discharge, and electrical resistivity measurements suggest the presence of hot water beneath the river north and west of Pilgrim Springs (Osterkamp et al., 1980; Wescott et al...,_1980; Wescott, this report) especially beneath area A. Radiometer measure- ments do not. support the anomalous thermal responses recorded by the imagery in these two areas. It should be noted, however, that these. measurements were acquired 7 days prior to the acquisition of the imagery. Area C lies along the southern portion of the geothermally thawed zone. The area boundaries do not include the northern part of the thawed zone and actually extend south of this zone, possibly due to image distortions and/or southward migration of warm surface water from the thawed area. This portion of the thawed zone has several hot springs which feed two streams and warm ponds (J. Kline, pers. comm.). The area is a wetland. Radiant temperatures are between 17°C and 23°C compared to less than 16°C in surrounding 117 RADIANT TEMPERATURES >17°C Figure 6. Thermal anomalies in the Pilgrim Springs area interpreted on high altitude thermal imagery obtained on July 13, 1980, 1830 BST. 118 areas. Field investigations recorded the highest subsurface tempera- tures in this region of the thawed zone. Also, electrical conduc- tivity measurements show the presence of hotter and saltier water at depth (Osterkamp et al., 1980) and radiometer measurements indicate high radiant temperatures. Five small thermal anomalies were detected north of area C and between areas A and B (Fig. 6). Three anomalies are generated by the hot bathing pool and the buildings. Surface conditions at the other two anomalies, located near the Pilgrim River, are unknown. The thermal anomaly located approximately 4 km NE of Pilgrim Springs (Turner and Forbes, 1980) was not studied due to cloud cover on the imagery. The thermal lakes and surrounding area are especially suscep- tible to fog during certain atmospheric conditions dependent upon air temperature, humidity and.wind profiles. The fog can cause a cool response on the thermal imagery leading to misinterpretation of the data. During acquisition of thermal imagery, times of excessive fog should be avoided. The relationship of this phenomenon to radiant temperature response over geothermally heated areas needs further study. CONCLUSIONS High altitude radar and thermal imagery can provide useful data on the structural setting and distribution of radiant tempera- tures of geothermal anomalies. Like polarized (HH) radar imagery, with perpendicular look directions, appears to provide the best structural data for lineament analysis. However, more than half 119 of the mapped lineaments are easily detectable on conventional aerial photography. Several suspected lineaments not seen on the photography appear to be related to the Kigluaik Fault while others are oriented parallel to the Kuzitrin Fault. A zone with a radius of about 10 km around Pilgrim Springs appears to have a high density of lineaments. Many of these lineaments represent fractures and/or faults, some of which may provide conduits for the emergence of hot water at that locality. The high-altitude thermal imagery probably is not a usable regional geothermal exploration tool due to confusion between geothermal and non-geothermal responses, but is useful for site- specific evaluations. The amount of detailed data required to eliminate non-geothermal anomalies on the imagery is prohibitive for a regional study, but feasible for a cua lier -stta-apectee project. Generally, if optical analysis techniques are utilized, the surface features are indistinguishable, unless the radiant temperatures differ by about 2°C. Finer temperature gradients could probably be distinguished on the high altitude data if digital analysis techniques were applied. Digital techniques were not evalu- ated because of budget constraints and the poor quality of the data. Within the Pilgrim Springs area, anomalously warm ground or thermal lakes could not be differentiated from other water bodies, based solely on the thermal response. Several anomalously warm areas were detected on the imagery. Of particular interest is an area located along the south edge of the thawed zone where field 120 measurements indicate a high heat flow region. The hot bathing pool and occupied buildings were also detected on the imagery. Our results suggest that high-altitude pre-dawn thermal imagery may be capable of locating relatively large areas of hot ground in site-specific studies of vegetated Alaskan terrain. Gentle lateral gradients are not likely to be detected. ACKNOWLEDGEMENTS Funding was provided by the National Aeronautics and Space Administration, NASA Grant NAG 9-8. Assistance during field investi- gations by Y. Nagashima, was greatly appreciated. We would like to thank J. Kline for his comments on the interpretation of the imagery. 121 REFERENCES Beikman, H. M. and E. H. Lathram, 1976, Geologic Map of Northern Alaska: U.S. Geol. Survey MF 789. Hudson, T., 1977, Geologic Map of the Seward Peninsula, Alaska: U.S. Geol. Surv. Open File Report 77-796A. Hudson, T. and G. 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