Shallow Geology of North Aleutian Shelf Area, Bering Sea, Alaska: ABSTRACT

In 1981, the geological hazards analysis group of the U.S. Geological Survey's Conservation Division collected 4,009 line-km (2,491 line-mi) of high-resolution seismic reflection data in the south-central Bering Sea. The survey area is bounded on the south by the Alaska Peninsula and Unimak Island and on the north, east, and west by lat. 56°30'N., long. 160°45'W., and long. 165°W., respectively. The U.S. Department of the Interior has tentatively selected this area for inclusion in Outer Continental Shelf Oil and Gas Lease Sale 92 scheduled to be held in 1985. This study was part of the surface and shallow subsurface geological investigation of the sale area. The seismic systems used in this study include a seismic reflection profiler that comprises an array of four 15-in.3 waterguns with both analog and 12-fold processed formats, an 800-joule minisparker, a 3.5-kHz profiler, a narrow-bean fathometer, and a side-scan sonar. A bathymetric map constructed from these data reveals a prominent, 20-m (33 ft) high, gentle scarp which trends obliquely across the survey area. Two fields of 10-m (33 ft) high, undulating features, which might be either waveforms or relic glacial moraines, were detected. Several linear moraine deposits, and several sag depressions related to the presence of near-surface faults were also found in the area. Holocene sediment consists predominantly of silt and fine to coarse-grained sand. The silt component increases toward the western end of the survey area, and gravel is locally present in the eastern end. A Holocene isopach map reveals that sediment distribution is current-controlled. Contemporary current-related features consist of ripple marks, sediment waves, and scour zones. These features generally occur within 60 km (37 mi) of the shore and in water depths of less than 70 m (230 ft). Although current flow generally parallels the shore, side-scan sonographs indicate that the current direction which produced these features is strongly influenced by small and intermediate scale bathymetric features. Faults are present in the southwestern portion of the survey area where they occur in a 30 km (19 mi) wide, east-west trending zone. This zone is an eastward extension of the St. George graben system. Within it, faults trend approximately east-west and sense of movement is exclusively normal. There are also several examples of growth faults. Acoustic anomalies, which may represent gas, are present throughout much of the survey area and occur at two different relatively shallow depths. No relationship appears to exist between acoustic anomalies and faults. End_of_Article - Last_Page 485------------