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Abstract

Scholl, D. W., G. A. Barth, and J. R. Childs, 2009, Why hydrate-linked velocity-amplitude anomaly structures are common in the Bering Sea Basin: A hypothesis, in T. Collett, A. Johnson, C. Knapp, and R. Boswell, eds., Natural gas hydrates—Energy resource potential and associated geologic hazards: AAPG Memoir 89, p. 308323.

DOI:10.1306/13201108M893346

Copyright copy2009 by The American Association of Petroleum Geologists.

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

David W. Scholl,1 Ginger A. Barth,2 Jonathan R. Childs3

1U.S. Geological Survey, Menlo Park, California, U.S.A.
2U.S. Geological Survey, Menlo Park, California, U.S.A.
3U.S. Geological Survey, Menlo Park, California, U.S.A.

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 shallowly (lt360 m [lt1181 ft]) underlain by large (gt2 km [gt6500 ft] in diameter, sim200 m [sim656 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 push-downs. 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 (sim60%) 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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