--> Abstract: Shelf-Edge Deltas, Linked Downslope Deposits, and Gas Hydrates: High and Low Sea Level Responses, Northern Gulf of Mexico, by Harry H. Roberts, Richard H. Fillon, and Larry M. Cathles; #90039 (2005)

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Shelf-Edge Deltas, Linked Downslope Deposits, and Gas Hydrates: High and Low Sea Level Responses, Northern Gulf of Mexico

Harry H. Roberts1, Richard H. Fillon2, and Larry M. Cathles3
1 Coastal Studies Institute, Louisiana State University, Baton Rouge, LA
2 Earth Studies Group, New Orleans, LA
3 Department of Geological Sciences, Cornell University, Ithaca,

Stacked and laterally offset clinoform wedges that represent deltas deposited during periods of Pleistocene falling-to-low sea level characterize the shelf-edge of the northern and northwestern Gulf of Mexico. During the latest 100 ka glacioeustatic cycle the Mobile River prograded across the shelf, building a delta at the shelf-edge shortly before the glacial maximum. This well studied shelf-edge delta provides a model in which thick, sand-rich clinoform sets occur at or near the shelf-slope transition. Density underflows and delta front failures produce prodelta aprons composed of thinly bedded heterolithic deposits that grade downslope into subtle channel-levee systems and slope fans. At sea level highstands, the turbidite toe sets of these deltas fall within the gas hydrate stability zone. Direct observations indicate that periodic thermal loading of the seabed by warm Loop Current intrusions and eddies causes gas hydrate decomposition and gas release. Capillary sealing in turbidites prevents vertical gas migration, but encourages up-dip lateral gas migration and charging of the self-edge clinoforms. Seismic profiles confirm that clinoform sets are gas-charged and leaking gas to the water column. Under falling-to-lowstand conditions, a decrease in hydrostatic pressure and the impact of warm surface layer water causes progressive gas hydrate decomposition deep within the upper hydrate stability zone. Resulting gas expulsion and a decrease in sediment strength can cause massive shelf edge failures and canyon formation. At the glacial maximum, shelf-edge deltas commonly occupy the evacuation scars and density underflows are funneled downslope to nourish submarine fans.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005