The fill in Marfa Basin, Texas

The geologic Marfa basin, as indicated by gravity measurements and borehole information, is much larger than the physiographic feature now surrounding Marfa, Texas. Comparisons of gravity measurements and borehole information permit location of areas in Marfa basin where the Tertiary and Quaternary basin fill includes intrusive volcanic rocks. These areas are marked by positive free-air gravity anomalies; areas where the thick fill does not include volcanoes are marked by small negative free-air anomalies. Large negative free-air gravity anomalies are caused partly by the presence of very thick sediments in the Rio Grande River Valley which is thought to occupy a graben in this area.