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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 investigate 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. Geologic 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 may 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 gas 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 technique 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 measurements 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 high, and that Pilgrim Springs may only be the “tip of the iceberg”.

Publication Date

7-17-1981

Keywords

Geothermal resources, Geology, Alaska, Seward Peninsula

Handle

http://hdl.handle.net/11122/15520

Geothermal reconnaissance survey of the central Seward Peninsula, Alaska

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