Abstract

The combination of a semi-enclosed sea with a broad shelf, high sediment influx and a wave-dominated energy regime make the northwest Gulf of Mexico unique in North America for studying the processes that have controlled sand occurrence on the continental shelf during the Quaternary. Synthesis of Late Pleistocene/Holocene deposits based on 34 000 km of high resolution seismic reflection profiles plus extensive gravity coring, vibracoring, and grab sampling show that patterns of sand deposition were controlled by glacioeustatic changes in sea level, paleogeomorphology, and salt tectonism. During low stands of sea level, river courses were incised into the continental shelf and fluvial sands were deposited in alluvial systems comparable in size to the modern Mississippi River. Width of main courses reached 40 km and that of major channels, 8 km. Thickness of sand fill in the ancient channels was as much as 46 m, and grain sizes range from gravelly to silty sand. Deltaic sands accumulated in sediment depocentres along the shelf margin. The five, large, shelf margin deltas of the Late Wisconsinan low stand have been identified. Sequences of distinct high-angle clinoforms (foresets) yielding oblique tangential reflection patterns characterize these deltas. Reworked sands mark both sea level oscillations at the shelf margin and stages in transgression across the shelf. These relict sands represent several environments of deposition: sheets and barriers capping former delta fronts; shoreline sands such as Sabine Bank; and deposits such as Trinity Shoal, which represent reworking of an early lobe of the modern Mississippi delta. Position of streams were determined off Louisiana by active diapirie domes and off Texas by regional southward dip. Continued movement of the diapiric structures caused channel adjustments locally and major diversions of stream courses regionally. Interaction of sediment loading and diapirism at the shelf margin, and beyond, created interdiapiric basins and dams below the shelf margin deltas. These became catchments for sand on the upper continental slope. Modern sand deposition is confined amost entirely to the inner shelf. Thin and irregular, but widespread, Holocene sands found off Texas as far as mid-shelf may be hurricane deposits.