Two previously undescribed structural basins are outlined by offshore geophysical data across the Shumagin continental shelf in the western Gulf of Alaska. The first basin, herein named Shumagin basin, is between the Semidi and Shumagin Islands and is roughly2equidimensional, with an area of about 3,800 km2. It contains an upper section of presumed Cenozoic sediment as much as 2.5 km thick, characterized by reflection horizons that show divergences in dip, local wedge-outs, and broad folds against the flanks of the basin. This section is in turn underlain by what appear to be complexly deformed sedimentary beds of probable late Cretaceous to early Tertiary age. Shumagin basin is therefore a structural depression in older deformed sedimentary rocks that has been filled by a relatively young deformed sequence
The second basin, herein named Sanak basin, is an elongate fault-bounded basin northeast of Sanak Island with an area of approximately 1,200 km2. The basin contains sedimentary-deposits of presumed Cenozoic age that may be more than 7 km thick and that are broken by probable growth faults indicating deposition contemporaneous with basin subsidence. The northeast and southwest flanks of Sanak basin appear to be underlain by Cretaceous turbidites and early Tertiary intrusive rocks exposed on the nearby outer Shumagin Islands. Anticlinal folding and stratigraphic truncation occurred during basin subsidence.
Available data are insufficient to determine if additional basins are present on the Shumagin shelf or to evaluate with certainty the number and size of potential hydrocarbon bearing structures within Shumagin and Sanak basins. However, the data suggest that prospective stratigraphic and structural features are present.
The Shumagin shelf extends for 500 km along the Alaska Peninsula from the Semidi Islands southwest to Unimak Island at the south end of the peninsula (Fig. 1). As part of the investigations to evaluate the resource potential of the outer continental shelves of the United States, Geophysical Service, Inc., under contract to the U.S. Geological Survey, collected approximately 500 km of gravity, magnetic, and 24- fold common depth point (CDP) seismic reflection data on the Shumagin shelf in 1975. The CDP data were digitally recorded at 24-fold with a 6 second record length using an air gun array as a sound source. Navigati9n was controlled by an integrated satellite doppler sonar system, supplemented by Loran-C. @
Processing was done by Petty-Ray Geophysical Company in Houston, Texas. Gravity data were acquired with a LaCoste-Romberg stable platform gravity meter and magnetic data were collected with a Varian proton magnetometer. The seismic records are available in an open-file report by Bruns and Bayer (1977).
These data outline two structural basins (Fig. 1), herein named Shumagin and Sanak basins, which could contain potential oil and gas resources. Available information is presently insufficient to show if other basins or thick sediment accumulations are present beneath the Shumagin shelf.
