Distribution and Significance of Coarse Biogenic and Clastic Deposits on Texas Inner Shelf: ABSTRACT

Abstract

Sediments of the Texas inner shelf are 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 which 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 mollusks including Ragina sp. and Crassostrea virginica. Rounded sandstone 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 mollusks, and (3) rock fragments show similar trends outlining ancestral Ri Grande, Brazos-Colorado, and Trinity deltas. A patchy, arcuate trend between Pass Cavallo and Aransas Pass is enigmatic. Criteria used to determine relative ages 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 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. However, no single explanation adequately accounts for areal variations in coarse material. Reworking of delta-plain and estuarine deposits during and following sea-level rise is common to all areas which at present are also sites of insignificant coarse-sediment influx. Sabine-Bolivar trends are interpreted as transgressive lags derived from erosion of a late Pleistocene Trinity delta previously dissected by the Sabine River during Wisconsin glaciation. In contrast, Brazos-Colorado and Rio Grande tre ds are interpreted as possibly compound strandline features associated with subsidence, erosion, and retreat of Holocene deltas. Upwelling of nutrient-rich shelf waters and freshwater inflow also may have increased productivity of shelf benthos near the Rio Grande delta. End_of_Article - Last_Page 1608------------