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Abstract

The morphology of platform margins as depicted on high-resolution seismic data and the development of outer shelf, eutrophic ecologic assemblages can be used as keys in understanding the evolution of carbonate platforms. The Baltimore Canyon platform, offshore United States East Coast, and the Liuhua platform, Pearl River Mouth Basin, People's Republic of China, exhibited the effects of environmental collapse prior to their extinction by drowning.

Reprocessed seismic data, lithologic data, and biostratigraphic data from the Baltimore Canyon area show that environmental deterioration of the platform immediately preceded or was coincident with deltaic progradation. This implies that slope-front fill seaward of the platform is probably coeval with platform deposits, and that a previously identified carbonate sequence boundary may actually be an older drowning sequence. A seismic sequence boundary should be placed at the top of the youngest drowning sequence.

Late-growth reefs appear to be discontinuous along both the Baltimore Canyon and Liuhua platform margins. The proximity of prograding deltas appears to be the main control on the location of late-growth reefs on both platforms, though tectonic subsidence (local faulting) may govern their distribution along the southern part of the Liuhua platform margin.

The horizontal-planar onlap of basinal shales onto the Liuhua platform margin could be misinterpreted as always representative of an unconformable contact with the platform sequence. In reality, local differences in highstand off-bank transport of platform and platform margin sediments may have produced a progradational fore-slope (a wedge of fore-slope debris) in some areas, and only horizontal-planar onlap in others.

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