ABSTRACT
The late Holocene sedimentologic framework of the South Texas outer continental shelf (OCS) from Matagorda Bay to the United States-Mexico border was studied as part of the national OCS environmental studies program associated with offshore petroleum lease sales. Late Quaternary sea-level fluctuations have resulted in the development of three genetically distinct sedimentary provinces based on textural-morphologic equilibrium criteria. Seafloor topography and surficial sediment patterns delineate a southern province dominated by relict topography of the Pleistocene and Holocene ancestral Rio Grande delta; the province is composed of a heterogeneous mixture of modern mud and relict muddy sands in textural disequilibrium with the present hydraulic regime. A northern province is dominated by relict topography along the southwestern flank of the ancestral Brazos-Colorado delta; it is composed of palimpsest sandy-mud deposits that have achieved partial equilibrium with present hydraulic conditions, and that appear to be undergoing some net-southward transport. A central interdelta province is characterized by a modern depositional surface; it contains an extensive modern mud blanket that appears to be encroaching southward over relict deposits of the ancestral Rio Grande delta. Surficial-sediment texture suggests a modern sediment-dispersal system characterized by net-offshore and net-southward transport components. Shallow subsurface sediments penetrated by gravity cores indicate that the modern dispersal pattern was established during earlier Holocene time but has been modified during the later stages of the Holocene transgression. Relative to the sand fades, the mud facies has expanded and migrated shoreward with time, thus reflecting a transgressive overlap relation.
