QUANTIFYING THE AGE AND VOLUME OF THE SOUTH TEXAS SAND SHEET

Spanning over 4,000 km² and encompassing a significant portion of four counties in southernmost Texas lays the South Texas Sand Sheet (STSS). Despite it being a physiographic boundary, a recharge area for local aquifers, an archive of climate change and a natural corridor for prehistoric human travel, basic knowledge of the STSS is still lacking. Active and stabilized sand dune complexes as well as sand sheet deposits dominate the landscape of the STSS. It overlies and conceals the Pliocene Goliad and the Pleistocene Lissie and Beaumont Formations with a sand thickness that is largely <3 m thick but can reach 12 m in places. Long held ideas contend that sand deposition on the STSS has occurred mostly during the Holocene and probably during the Pleistocene, particularly during the final glacial episode ~18,000 years B.P. but absolute dates to support this contention are lacking. We present the first set of OSL dates from the base of the sand sheet from an ongoing effort to contribute to the understanding of the STSS. These dates largely validate the Holocene to Late Pleistocene time frame (6.4±0.3 and 14.3±0.9 ka), but also hint at the possibility that eolian activity began earlier (39.1±2.4 ka). Using data from over 120 water wells available at the BRACS database, we present a preliminary estimate of the volume of the STSS calculated with RockWorks. With the wells in the western ¼ of the sand sheet still pending to be incorporated in our calculation, the present figure of the volume stands at 15x10⁹ m³. Constraining the volume is important to model recharge rates to local aquifers.