Syndepositional gravity tectonics in the northwestern Gulf of Mexico basin have obscured the geometry of Tertiary shelf-slope stratification and thereby inhibited the recognition of relict shelf edges. However, examination of the modern shelf margin, formed primarily by deltaic deposition during the late Pleistocene lowstand of sea level, can lead to alternative criteria for recognizing Tertiary shelf margins. Late Pleistocene shelf-margin deltas, in contrast to inner-shelf deltas, are characterized by rapid subsidence and growth faulting, thick progradational cycles, and steep clinoform stratification. High subsidence rates result from deep-seated gravity sliding of the continental slope, which creates a strongly extensional regime along the shelf margin. Many downdip Te tiary formations are similarly characterized by large growth faults with high expansion ratios in deltaic sequences; hydraulic isolation of shallow-water sandstones by large fault offsets leads to overpressuring. These structurally-complex downdip trends, typically with geopressured gas reservoirs, represent the shelf-margin megafacies.

Mapping of these shelf-margin trends provides a concise summary of the Cenozoic depositional and structural history of the basin. Major influxes of sand to the shelf margin correspond to episodes of rapid progradation and are interpreted as large shelf-margin deltas. Pre-Pleistocene shelf-margin deltas do not appear to be synchronous across the basin, and therefore are probably a function of sediment supply rather than sea-level fluctuations. The three largest such Tertiary delta complexes can be correlated with major tectonic episodes in likely source areas in western North America: (1) the late Paleocene (lower Wilcox) Rockdale delta system in east Texas coincides with the major pulse of Laramide uplift in the southern Rockies; (2) the mid-Oligocene (Frio) Norias delta system in sou h Texas coincides with extensive ash-flow volcanism in the Sierra Madre Oriental; and (3) the Neogene ancestral Mississippi delta system in Louisiana coincides with reactivation of the southern Rockies and regional uplift.

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