Rocks recently dredged along the Bering Sea continental slope include fossiliferous, gray-green arkosic sandstone of Late Jurassic age. The sandstone was recovered from acoustic basement at water depths ranging from 1,500 to 2,800 m. These arkosic rocks were sampled at nine sites along a segment of the Beringian margin that extends about 550 km northwest of the Pribilof Islands toward eastern Siberia. Preliminary lithologic and petrographic examination of the feldspathic sandstone and lesser amounts of siltstone indicates that these rocks are equivalent to units in the Naknek Formation,

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which is exposed in southern Alaska and along the Alaska Peninsula. The megafossil Buchia rugosa was found in quantity in one dredge haul. This Late Jurassic pelecypod implies that the rocks were deposited in a neritic or shallow-water environment. These Jurassic strata are overlain unconformably by diatomaceous mudstone or sandstone as old as late Eocene or early Oligocene.

Geophysical work indicates that the Jurassic rocks were recovered from an acoustic basement complex that can be traced northwestward from near the western tip of the Alaska Peninsula to Siberia, a distance of nearly 1,250 km. The Mesozoic basement complex consists structurally of a series of interconnected ridges that underlie the outer shelf and crop out along the adjacent continental slope. Previous theories on the tectonic evolution of the Bering Sea implied that the continental margin should be underlain either by (1) deformed Mesozoic trench or slope deposits that were structurally accreted to the margin by oblique convergence between the Kula(?) and North American plates or (2) by disrupted fragments of Mesozoic slope beds deposited along a transform or strike-slip boundary that separated the two plates. However, rocks dredged from the margin now indicate that a belt of shallow-water Upper Jurassic sandstone underlies the Beringian margin between southwestern Alaska and eastern Siberia. This belt, which structurally may include younger rocks, subsided in early Tertiary time to form the existing Beringian margin. Collapse along the margin was more than 3 or 4 km; in some areas beneath the outer shelf, the Mesozoic framework may have subsided more than 10 km.

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