Geology of the Northern Alto Relex Area, Big Bend National Park, Brewster County, Texas

Detailed, 1:12,000-scale geologic field mapping was conducted around Alto Relex in the northern Sierra Del Carmen Mountains in Big Bend National Park. Alto Relex is a prominent, near vertical, ~1000 foot-high fault-line scarp. Cretaceous rocks in the area were folded and faulted by Laramide compression, intruded by Tertiary sills and overprinted by high-angle normal faults from Basin and Range extension. During the Laramide Orogeny and Basin and Range extension, the underlying basement shear zone called, the Texas Lineament, was reactivated creating a transpressional and transtensional regimes. The general geology of this area was outlined by Moustafa (1988) working primarily from aerial data and was partly mapped by Cooper et al. (2011). This study provides more detail in the structurally complex area around Alto Relex. Cretaceous rock units exposed in the study area include the Santa Elena Limestone, Del Rio Clay, Buda Limestone, Boquillas Formation and the Pen Formation. In some areas the thin-bedded, flaggy Boquillas contains small folds that have an average strike of N30°W. Minor thrust faults were found in multiple rock units striking approximately N13°W. A large sinistral strike-slip fault on the east side of Alto Relex was traced for over 3,000 feet, striking N40°W. Distinctive ii reddish-brown knobs of fault breccia on the west side of Alto Relex appear to have been silicified or hydrothermally altered. This breccia probably marks the location of a large basin-bounding normal fault. Both map-scale and outcrop scale folding is present. One map-scale monoclinal feature has an orientation similar to the Laramide compression direction and could be a Laramide fault-propagation fold overlying a reverse fault, or a fault-propagation fold that was produced over a Basin and Range normal fault. Some fault planes in the study area have slickensides and chatter marks with different orientations within a short distance, suggesting a complex structural history