GEOLOGY OF THE ALTO RELEX AREA, SIERRA DEL CARMEN, BIG BEND NATIONAL PARK, BREWSTER COUNTY, TEXAS

Detailed mapping around Alto Relex in the western Sierra Del Carmen Mountains in Big Bend reveals Cretaceous rocks that have been folded, sheared and thrusted by Laramide compression. Then overprinted by high-angle normal faults from Basin and Range extension and cut by Tertiary sills probably associated with the nearby McKinney Hills intrusion. Alto Relex is a prominent, near vertical, ~300 m-high fault line scarp. The general geology of this area was delineated by Moustafa (1988) and was partly mapped by Cooper et al. (2011); this study focuses on the Alto Relex area in more detail. Cretaceous units in the area include the Santa Elena Limestone, Del Rio Clay, Buda Limestone and Boquillas Formations. In some areas the thin-bedded, flaggy Boquillas contains folds that have an average axial plane of ~N30W, 86° SW, low plunge axes, an average inter-limb angle of ~95° and wavelengths of ~2 m. Minor thrust faults were found in the upper Santa Elena striking ~N18W, 42° SW on average. A large sinistral strike-slip fault on the east side of Alto Relex has been traced for over 700 meters, striking N40W; it may be related to a similar feature found in upper Ernst Canyon a few miles to the south. Distinctive reddish-brown knobs of fault breccia on the east side of Alto Relex appear to have been silicified, possibly by magma fluids. Some locations in the study area have slickensides and chatter marks with different orientations within a short distance, suggesting a complex structural history.