Atka basin is a major depression in the Aleutian Terrace, south of the Andreanof Islands in the Aleutian chain (174°W). Sediments flooring the basin were recovered at Site 186 of the Deep Sea Drilling Project on the southern edge of the basin. Most of the sedimentary fill is diatomaceous silty clay. Interbedded in the silty clay are numerous layers of turbidite sand and vitric volcanic ash. The sand layers are mostly volcanic-rock fragments and volcanic glass, although some feldspar and quartzose debris, including chert, is present. Most of the sediment in the basin must have originated by erosion and volcanic eruptions in the adjacent central Aleutian arc.

Neogene sedimentation rates at Site 186 exceeded 180 m/m.y. and in the early Pliocene may have been more than 300 m/m.y. These high rates were caused by extensive shelf erosion perhaps associated with extensional rifting and volcanism in the late Miocene and early Pliocene.

The sediments of Atka basin are structurally and temporally part of the "late series" of the Aleutian Ridge and are probably representative of the vast volumes of late Tertiary debris that has accumulated on the ridge and in adjacent parts of the Bering Sea. The reactive nature of the turbidite sediments in Atka basin suggests that other basins along the Aleutian Ridge may not be particularly favorable locations for hydrocarbon accumulation.