Distribution and significance of coarse biogenic and clastic deposits on the Texas inner shelf.

Sediments of the Texas inner shelf are generally fine grained, coarse clasts (>0.5mm) are uncommon (<1%) over much of the area. Higher concentrations of coarse material, however, occur in discreet 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 or 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.