Abstract: Eocene Formation of the Bering Sea Basin Linked to Regional-Scale Tectonism of Alaska--Implications for Energy Gas Resources and the Accumulation of Massive Hydrate Deposits (VAMPs)

David W. Scholl, Andrew J. Stevenson, Patrick E. Hart

The creation of the deep-water Aleutian Basin (Bering Sea) is inextricably linked to the formation of the three co-genetic arcs that structurally frame the basin--the active Aleutian arc subduction zone (SZ), and the fossil submarine Shirshov and Bowers arcs. The origin of these arcs is tied to an early Eocene episode of accelerated tectonism and terrane movement that affected the north Pacific rim from British Columbia westward to Kamchatka. Transpressive tectonism was driven by rapid northward movement of the Kula plate into terrane-clogged SZs of southern Alaska and Kamchatka. A clogged Kamchatka SZ and N-S compression of southern Alaska extruded interior Alaska southwestward along regional strike-slip shear zones toward the Beringian sector of the Pacific rim. Circum-north Pacific continental deformation transmitted shortening stresses to the adjacent Kula plate and formed the offshore family of SZs and arcs that cordoned off the Aleutian Basin from the Pacific Basin.

The massifs of the arcs, and in particular the Neogene opening of between-island passes, influenced basin-floor depositional processes and the preservation of organic matter, exchange of water with the Pacific, productivity of surface waters, and the formation of bottom waters. The basin's sedimentary sequence (4-12 km) is largely terrigenous in character, but dominantly diatomaceous deposits characterize middle Miocene through early Pliocene beds. Younger late Pliocene and Pleistocene deposits are diatom-rich turbidite beds that seismic reflection data reveal host localized massive deposits of methane gas hydrate overlying columns of free interstitial gas. The methane accumulations are recorded in flat-lying beds as velocity structures termed VAMPs, which are detected at a subsurface depth of 400-500 m as anomalous velocity pull-up domes (high velocity hydrate masses) overlying a high-amplitude BSR reflection and velocity push-down depressions below (low velocity gas). VAMPs record sealing of porous beds with hydrate deposits that block vertically migrating thermogenic gases generated from underlying Miocene and older basinal deposits. The volume of hydrated and free gas at a typical VAMP can exceed 0.3 TCF.A conservative estimate of the basin-wide (~400,000 km2) volume of methane associated with VAMPs is 1100-900 TCF.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California