Holocene depositional history of Lavaca Bay, central Texas Gulf coast




Byrne, J. R.

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Sediments in Lavaca Bay represent a texturally fining upward marine transgressive sequence. The entire sediment sequence is 120 feet (36.4 m) thick at the mouth of Lavaca Bay. Gravel and sand from the Lavaca-Navidad fluvial-deltaic complex comprise the oldest Holocene sediments in the bay. Successively younger sediments are represented by up to 80 feet (24.2 m) of bay-estuarine mud. The bay-estuarine sediments reflect three major mud sources. These are: (1) suspended load transported by the Colorado-Brazos fluvial complex, which was discharged into marine and estuarine bodies over 50 miles (83 km) to the northeast and carried southwest by longshore currents, (2) erosion of Pleistocene deltaic sediments exposed along the bay margins, and (3) suspended load transported by the Lavaca-Navidad fluvial complex, discharging into the head of Lavaca Bay. Comparison of the volumes of mud presently filling this Pleistocene valley, with calculated volumes of mud that have been supplied from these potential sources has resulted in a rough argillaceous sediment budget for this coastal estuary. Sufficient mud had been made available from the Lavaca-Navidad fluvial complex and the bluffs exposed along the bay margins to completely fill Lavaca Bay since Recent stillstand of the Gulf of Mexico. However, the sediment accumulation rate in Lavaca Bay is slow due to the transport of significant quantities of sediment out of Lavaca Bay by waves and surface currents. In addition, lithologic characteristics and facies relationships of the respective muds indicate that discharge of suspended load by the Colorado-Brazos fluvial complex had been an important source as well. Progradation of the Lavaca-Navidad fluvial-deltaic complex since Recent stillstand has resulted in a regressive sediment sequence which overlies transgressive sediments at the head of Lavaca Bay. The clay mineral suite in sediments in Lavaca Bay reflects clay minerals in the source area. No changes in the clay minerals were detected as a result of increasing salinity, sediment depth and time. X-ray diffraction data indicate significant potassium fixation by montmorillonite after treatment with 1N potassium chloride. Sediment facies are the main control on trace element distribution in Lavaca Bay. In general, high trace element concentrations are correlative with clay sediment content and, to a lesser degree, organic carbon content. Data suggest that both "detrital" and "nondetrital" trace elements contribute to trace element accumulations in Lavaca Bay