INFLUENCE OF THE YOAKUM CANYON ON THE SANDSTONE MORPHOLOGY IN THE CARRIZO SAND

The Texas Water Development Board recently completed an aquifer characterization of the Carrizo-Wilcox Aquifer in the Gonzales County, Texas area as part of the Guadalupe-Blanco River Authority’s Mid-Basin Water Supply Project. This study included lithologic interpretation using both geophysical well logs and driller’s descriptions from well reports. The study area contained a portion of the Yoakum Canyon, one of the largest submarine canyon features in the Wilcox Group, which spans over 100 km from Bastrop County toward the Gulf of Mexico, has a width of over 16 km, and a total thickness of over 1,066 meters. The development of the large-scale erosional canyons prevalent in the Wilcox Group is complex and was likely related to the subduction of the Pacific Plate creating uplift along the paleo-Gulf of Mexico shoreline interacting with several regressive-transgressive sequences. The Yoakum Canyon allowed for sediment bypass of the shelf margin remained open throughout the deposition of the Wilcox Group. Sediments within the infilled Yoakum Canyon are characterized by predominately clay and shale with some isolated sandstone beds and were likely deposited during a marine transgression. The Carrizo Sand unconformably overlies the Wilcox Group and contains distinct thick, sharp-based, high-resistivity sand units. The Carrizo Sand is interpreted to have been deposited in a tidally-influenced deltaic depositional environment. The basal sandstones of the Carrizo Sand within the study area are crossbedded and are overlain by massive, high-porosity sandstones which can be over 150 meters thick. The Carrizo Sand is distinctly thicker over the Yoakum Canyon and the sandstones within this area have a different character as seen in geophysical well logs. The Carrizo Sand overlaying the Yoakum Canyon has more interbedded clay and sandstone bodies are thinner and more vertically isolated. On average, the resistivity of these sandstone units is also lower than the massive sands found outside the Yoakum Canyon footprint. This study shows that the presence of the Yoakum Canyon created more accommodation space and potential sediment bypass to the Gulf. The change in character of these sandstone bodies has implications for hydrogeologic studies in the Carrizo Sand and groundwater supply projects in the study area.