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Why Hydrate-linked Velocity-amplitude Anomaly Structures are Common in the Bering Sea Basin: A Hypothesis

By
David W. Scholl
David W. Scholl
U.S. Geological Survey, Menlo Park, California, U.S.A.
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Ginger A. Barth
Ginger A. Barth
U.S. Geological Survey, Menlo Park, California, U.S.A.
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Jonathan R. Childs
Jonathan R. Childs
U.S. Geological Survey, Menlo Park, California, U.S.A.
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Published:
January 01, 2009

Abstract

The thick sedimentary sequence (2-12 km [6500-39,400 ft]) underlying the abyssal floors (3-4 km [9800-13,100 ft]) of the Bering Sea Basin is shal-lowly (<360 m [<1181 ft]) underlain by large (>2 km [>6500 ft] in diameter, ~200 m [~656 ft] thick) deposits of concentrated methane hydrate. Mound-shaped bodies of hydrate are displayed on seismic reflection records as velocity-amplitude anomaly (VAMP) structures imaged as velocity pull-ups overlying pushdowns. The VAMPs are numerous (hundreds to thousands) and occur across an area of approximately 250,000 km2 (96,525 mi2).

The abundance of VAMP structures is conjectured to be a consequence of high rates of basinwide planktonic productivity; of preservation of organic matter; biosiliceous sedimentation; of silica diagenesis; and of high heat flow; and deposition of a thick (700-1000 m [2296-3281 ft]), upper section of perhaps latest Miocene but mostly glacial-age (early Pliocene and Quaternary) turbidite beds and diatom ooze. Stacking of this upper Cenozoic sequence of water-rich beds heated underlying diatomaceous deposits of Miocene and older age and enhanced the generation of thermogenic methane and the diagenetic conversion of the opal A of porous diatom beds to the denser and contractionally fractured opal-cristobalite tridymite phase of porcellaneous shale. Silica transformation expelled large volumes of interstitial and silica-bound water that, with methane, ascended through the shale via chimneys of fracture pathways to enter the porous (~60%) upper Cenozoic section of diatom ooze and turbidite beds. Ascending methane entered the hydrate stability field at approximately 360 m (1180 ft), above which concentrated deposits of methane hydrate formed as either pore-filling accumulations or more massive lenses.

The deposition of high-velocity methane hydrate above a multitude of chimney structures transporting low-velocity, gas-charged fluids toward the sea floor is posited to account for the widespread recording of VAMP structures in the Bering Sea Basin.

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Contents

AAPG Memoir

Natural Gas Hydrates—Energy Resource Potential and Associated Geologic Hazards

T. Collett
T. Collett
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A. Johnson
A. Johnson
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C. Knapp
C. Knapp
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R. Boswell
R. Boswell
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American Association of Petroleum Geologists
Volume
89
ISBN electronic:
9781629810270
Publication date:
January 01, 2009

GeoRef

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