MINERALOGICAL AND TEXTURAL CHARACTERIZATION OF MIXED-SOURCE EOLIAN SANDS ON THE SOUTH TEXAS SAND SHEET

Between the Rio Grande and the Nueces River is the South Texas Sand Sheet (STSS), a patch of sand ~7,000 km2 stretching ~100 km inland from the shoreline. The STSS consists largely of sand sheet deposits, and active and relict SE-NW sand dunes. It’s geology, arid climate, and diverse biotic communities make the STSS a unique physiographic unit, and one of the least understood geologic features of the Gulf Coastal Plain Province. One important issue that has remained unexplored is the origin and composition of the sand. Color differences on the sand sheet and the active dunes, observed along three main highways that cut across it (US 77, US 281, and TX 1017), range from white to brown to deep red, and suggest mineralogical differences that reflect distinctive sources. Two potential sources contributing sand to the STSS are proposed: 1- a coastal source from the beach/barrier island, and 2- a deflation (wind erosion) source from the underlying unit, the Goliad Formation.

To better understand the geologic history of the STSS, a comparative study of the textural and mineralogical composition of surface sediment from across the STSS is underway. Two reference samples, one from the coastal dunes at South Padre Island, “the coastal source”, and one from an outcrop of the Goliad Formation near San Isidro, “the deflation” source are being examined texturally and mineralogically. Analyses of ten other samples, spanning the length of the sand sheet in a SE to NW direction, will follow. The mineralogy characterization is based on detailed microscope grain counting using optical properties of minerals at 50X magnification. Standard granulometric analyses are employed. For all samples the fraction of sediments corresponding to the mode, (typically the 3 phi) is being characterized with portable X-ray fluorescence to further differentiate the sediments.