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ABSTRACT: Was the Aleutian Basin More Enclosed, Its Deeps Oxygen Deficient, and the Geothermal Gradient Steepened in the Middle Tertiary? Implications for the Petroleum Potential of the Bering Sea

David W. Scholl, Andrew Stevenson

Circulation of north Pacific waters through the Aleutian Basin of the deep Bering Sea (3500-4000 m) is presently vigorous and affects all depths. Energetic currents enter and exit the Bering Sea via deepwater (1000-4000 m) passes in the Aleutian Ridge. But geologic evidence suggests that in the middle Tertiary (or earlier), the basin was possibly stagnant or poorly ventilated.

A sequence of undeformed sedimentary deposits typically more than 4 km thick underlies the south-central bathymetric deep of the basin. The basinal sequence includes hundreds of velocity-amplitude anomalies (VAMPs), which are deep-water bright spots associated with underlying velocity pull-downs. The VAMPs are thought to record the pooling of thermogenic gas at subbottom depths of 800 to 1000 m. Because VAMPs are largely confined to the basin's bathymetric low, subsurface source beds that supplied the gases must be located here. Favorable preservation of organic matter over the basin deep may reflect a former oxygen-poor bottom-water environment. Tectonic reconstructions indicate that prior to the middle Tertiary a more laterally continuous Aleutian Ridge restricted circulation with t e north Pacific, a circumstance that would have favored incomplete deep basin ventilation.

New information concerning the paleoceanographic history of the Bering Sea, the prospect that organic-rich source beds have been deposited, and the likelihood that an episode of early Tertiary backarc spreading enhanced the basin's geothermal gradient, combine to improve the petroleum potential of the Aleutian basin.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990