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Over 20,000 km of high-resolution seismic data (40-2000 Hz) and hundreds of platform boring descriptions and samples have been analyzed in the northern Gulf of Mexico (offshore Texas, Louisiana, Alabama, and Florida) to reconstruct the depositional environments during the most recent lowstand in sea level. The lowstand units are interpreted to lie between the oxygen isotope Stage 2 sequence boundary (approximately 18,000 yr BP) and the oxygen isotope Stage 2 to 1 transgressive surface. Our study demonstrates a large amount of lateral variability in Lowstand depositional environments and corresponding deposits in the northern Gulf of Mexico. The deposits include sand-dominated and mud-dominated prograding complexes, wedges, and thin units characterized by periods of erosion and nondeposition. Sediments vary in thickness from 75 meter thick delta lobes offshore east and south Texas to thin mud drapes of negligible thickness off the central Texas shelf. The variability in the lowstand depositional environments appears to be controlled by sediment supply, shelf gradient and width, and tectonics. Understanding the contributions of different combinations of controlling factors in the various Lowstand depositional environments along the northern Gulf of Mexico, allows the development of depositional models to predict lowstand reservoir character and distribution in ancient strata.

Undoubtedly, bypass of fluvial sediments through canyon-indented slopes is an important contributor to lowstand depositional systems. However, due to complex interactions with other mechanisms (sediment supply, antecedent topography, salt tectonics, etc.) the question of how the process of sediment bypass of the shelf margin-delta system is initiated has not been resolved. In this seismic-stratigraphic study of near-surface stratigraphy of the Alabama/west Florida shelf (an area with low subsidence, no faulting and no salt tectonics) the isolated effect of fluvial response to relative-sea-level fall is investigated. In the study area, shelf-margin-delta depositional units are primarily composed of discrete prograding wedges. Based on available age constraints, we infer that these depositional cycles are associated with failing limbs of 100 ka glacioeustatic relative-sea-level cycles of the Pleistocene. Seismic-stratigraphic analysis, contour mapping and paleogeographic reconstruction of shelf-margin-delta environments indicate that slope-canyon connections between the shelf margin delta and upper slope did not develop in spite of frequent and widespread subaerial exposure of the shelf. Slope canyons may be lacking in the study area due to the absence of salt tectonics and growth faulting. Another possibility is that the sandy nature of surface sediments offshore Alabama/west Florida inhibits organized drainage from the subaerially-exposed continental shelf during relatively short Pleistocene lowstands. Nonetheless, based on the absence of shelf-to-slope connections in the study area, we propose that, acting alone, lowstand fluvial incision of the shelf edge is not an important initiator of slope canyons and fluvial bypass to to the basin.

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