Temperature variations and their relation to groundwater flow, South Texas, Gulf Coast Basin

Isothermal surfaces and temperature gradients confirm the presence of anomalously high geothermal gradients along the Wilcox growth fault zone in South Texas. This anomaly has been attributed to the advection of heat via upwelling basinal fluids. However, there is also evidence that the same processes are occurring along the Vicksburg/Frio growth fault zones. Although a previous study showed a general increase in the temperature gradient to the southwest within South Texas, this study demonstrates that the trend is discontinuous. Shallow data suggest that advecting fluids are escaping from the compactional regime and perturbing the temperature field within the overlying meteoric regime. This study enlarges the data base of Bodner (1985) by including temperature data of less than 200°F within the original study area and the entire range of temperature data to the south. Isothermal surfaces produced at 50°F intervals from 100°F to 400°F indicate that beginning with the 150°F isotherm, a perturbation occurs at approximately 3500 feet to 4500 feet (1070 to 1370 m) along the Wilcox growth fault trend -- well above the top of the geopressured, compactional regime. This perturbation suggests that advecting fluids are escaping from the compactional regime to the meteoric regime above. In addition, the isothermal surfaces show that the perturbation along the Wilcox growth fault trend becomes more prominent with depth. However, the isothermal surfaces do not show a perturbation along the Vicksburg/Frio growth fault trends. Temperature gradients calculated within numerous subregions of the study area confirm the presence of temperature gradients as high as 3.4°F/100 ft (62.2°C/km) within the Wilcox growth fault zone. In addition, a contour map of temperature gradients within the compactional regime reveals the presence of higher temperature gradients (> 2.5°F/100 ft or 45.8°C/km) within the Vicksburg/Frio growth fault zones. This is the only thermal evidence that processes occurring in the Wilcox growth fault zone may also be occurring in the Vicksburg/Frio growth fault zones. Finally, a contour map of temperature gradients within the meteoric regime indicates that the highest gradients (> 2.0°F/100 ft or 36.6°C/km) occur roughly along the Wilcox growth fault zone. As with the 150°F isotherm, this suggests that advecting fluids are escaping from the compactional regime and perturbing the temperature field in the overlying meteoric regime.