The structure and origin of the large submarine canyons of the Bering Sea

Abstract

Three exceptionally large and long submarine canyons — Bering, Pribilof, and Zhemchug — incise the continental slope underlying the southeastern Bering Sea. Bering Canyon, the world's longest known slope valley, is approximately 400 km long and has a volume of 4,300 km 3. The volume of Pribilof Canyon is 1,300 km 3 and that of Zhemchug is 8,500 km 3; Zhemchug Canyon may well be the world's largest slope valley; most other large submarine canyons have volumes less than 500 km 3. Pribilof and Zhemchug canyons are further distinguished by the headward bifurcation of their slope axes to form elongated trough-shaped basins behind the regionally projected position of the shelf edge. These troughs are superimposed over structural depressions formed by down-faulted basement rocks of Mesozoic and older ages. Prior to canyon cutting these depressions were filled with as much as 2,600 m of shallow-water diatomaceous, tuffaceous, and detrital sediments largely of Tertiary age. Deposition of these sediments took place concurrently with general margin subsidence of at least 2,000 m. The data and conclusions presented in this paper stress that the location, trend, and shape of the enormous submarine canyons cutting the Bering margin are structurally determined. However, axial cutting and headward erosion within the relatively unconsolidated Tertiary strata and the older, lithified basement rock is thought to have been caused by basinward-sliding masses of sediment; these unstable sediment bodies accumulated on the upper continental slope and outer shelf, probably near the mouths of major Alaskan rivers. Bering Canyon was periodically cut and filled by axial sedimentation during Late Tertiary and Quaternary time. Pribilof and Zhemchug canyons, however, are thought to have been excavated entirely during the Pleistocene. It is presumed that, during one or more periods of glacially lowered sea level, the Kuskokwim and Yukon rivers emptied into or near the heads of Pribilof and Zhemchug canyons. The enormous size and unusual shape of Zhemchug Canyon resulted from the breaching of the seaward wall of an outer-shelf basement depression and the subsequent removal of nearly 4,500 km 3 of Tertiary deposits filling it.