Neogene Turbidite Sedimentation in Komandorskiy Basin, Western Bering Sea

Abstract

Sediments underlying the abyssal plain in Komandorskiy basin were recovered at Deep Sea Drilling Project Site 191 in the east-central part of the basin (168.1°E). Most sedimentary fill is diatomaceous silty clay. Interbedded in the silty clay are layers of turbidite sand and volcanic ash. The sand layers range in composition from volcanic lithic sand to chert-rich sand containing as much as 40% quartzose debris. The sand probably originates on the continental shelf in the westernmost Bering Sea, and is deposited by turbidity currents crossing the abyssal plain. Chert-rich sand may originate on the shelf between Mys Olyutorskiy and Ostrov Karaginskiy, and volcanic lithic sand may originate farther south toward Mys Kamchatskiy. Below 500 m, sediments of the abyssal plain are altered extensively. The silty clay is compacted to mudstone, and sand layers are graywacke. Porosity in the sand layers is destroyed completely, and the deepest sand layer has textures that approach those of semischist. The graywacke layers contain considerable quantities of volcanic and sedimentary lithic debris, and may have originated on the continental shelf in the Komandorskiy Islands. Neogene accumulation rates in Komandorskiy basin were about 60 m/m.y.-1 in the late Miocene, increasing to 300 m/m.y.-1 in late Pliocene and early Pleistocene time. These high rates were maintained by extensive continental-shelf erosion during late Cenozoic sea-level transits, and by exceedingly high diatom productivity in the overlying waters of the Bering Sea. A slight decrease in accumulation rates in the late Pleistocene may be the result of a decline in diatom productivity. Unlithified Neogene deposits of Komandorskiy basin are distal representatives of contemporaneous deposits in continental-margin basins and have some characteristics of potential reservoir rocks. Their more proximal counterparts at shallower-water depths deserve increased attention as possible hydrocarbon sources.