ABSTRACT

Sediments of the Texas inner shelf are generally fine grained; coarse clasts (> 0.5 mm) are uncommon (<1%) over much of the area. Higher concentrations of coarse material, however, occur in discrete areas that apparently represent positions of former deltas. Coarsest constituents are predominantly whole shells and shell fragments with subordinate amounts of lithic clasts. The calcareous skeletal debris represents a mixture of extant shelf fauna and relict brackish-water molluscs including Rangia spp. and Crassostrea virginica. Rounded sandstone, limestone, and mudstone clasts up to 7 cm long and caliche nodules are common in some areas. Maps showing 1) coarse fraction percent, 2) distribution of brackish-water molluscs, and 3) rock fragments show similar trends outlining ancestral Rio Grande, Brazos-Colorado, and Trinity deltas; a patchy, arcuate trend between Pass Cavallo and Aransas Pass is enigmatic. Criteria used to determine post-depositional history and possible sources of shell debris for each of the four trends are degree of abrasion, fragmentation, etching, boring and discoloration.

Possible explanations for concentration of coarse material include high biological productivity, low rates of terrigenous clastic sedimentation, selective deposition by modern shelf processes, and reworking of locally shelly relict deposits exposed on the seafloor during the Holocene transgression. Of these possibilities, no single explanation adequately accounts for areal variations in coarse material. Reworking of delta-plain and estuarine deposits during and after sea-level rise is characteristic of areas that are now receiving insignificant amounts of coarse-sediment. The Sabine-Bolivar trend is interpreted as a transgressive lag derived from erosion of a late Pleistocene Trinity delta. In contrast, Brazos-Colorado and Rio Grande trends are interpreted as compound strandline features associated with subsidence, erosion, and retreat of Holocene deltas.