Structure of the Presidio Bolson Area, Texas, Interpreted from Gravity Data

To obtain a better understanding of the structure and tectonism of the region, an integrated geophysical-geological study of the Presidio area, Texas, was undertaken using gravity measurements and deep drilling data. New gravity data were combined with existing data to construct simple Bouguer anomaly maps of the Presidio area, and two-dimensional computer modeling of gravity profiles was used to derive earth models. These data outline the major geologic features of the area that are dominated by the effects of Tertiary block faulting and volcanism. The main feature of interest was the Presidio Graben, which is approximately 1.5 km deep near Ruidosa, Texas. One motivation for this study was the collection of a part of the basic scientific data needed to assess the geothermal potential of the area, and the results obtained support the hypothesis that hot springs associated with the Presidio Graben derive their heat from deep circulation along its boundary faults. However, some gravity anomalies observed could be interpreted as indicating the presence of late Tertiary intrusions that could provide heat for the hot springs.