Geology of a portion of southwestern Brewster County, Trans-Pecos, Texas

The study area is adjacent to the western end of Big Bend National Park, Brewster County, Texas, in a gap between the regions mapped by Maxwell, et al. (1967), McKnight (1968, 1970), and Yates and Thompson (1959). Of the approximately 1082 m of strata that are exposed in the study area, about 1030 are Cretaceous, about 49 are Tertiary, and about 3 are Quaternary. Lowest Cretaceous rocks are massive marine micritic limestones and shales and these comprise, from older to younger, the Santa Elena Limestone, Del Rio Clay, and the Buda Limestone. The superjacent Boquillas Formation is a marine sequence of alternating crystalline limestone, chalk, arid calcareous claystone that grades upward into the Pen Formation, a structureless claystone, also of marine origin. After deposition of these strata, the Cretaceous sea withdrew and the interbedded sandstones and claystones of the Aguja Formation accumulated along the edge of the regressing ocean. The overlying Javelina Formation consists of bentonitic claystones with a few lenticular bodies of sandstone, which were deposited on the newly developing coastal plain. The bentonitic composition reflects an increasing influence of volcanism in the region during the time of deposition and is a precursor to volcanic activity that produced basaltic lavas and volcaniclastic rocks of the Tertiary Chisos Formation. A monzonitic and several gabbroic intrusions of probable Tertiary age are present in Upper Cretaceous strata. Following the episode of volcanism, coarse clastic alluvial and colluvial deposits accumulated until middle Pleistocene time when the Rio Grande established its present course and initiated a period characterized predominantly by erosion. Several terrace levels have been identified in the study area. These are only very broadly correlative with more extensive and better developed terrace surfaces along the Rio Grande to the northwest. Until recently, the area was undergoing planation and developing the younger terrace level, but presently the streams are actively deepening their channels. The region is in the Basin and Range Tectonic Province and northwestward to west-northwestward normal fault and fold trends characterize its structural framework. Broadly, the structure of the study area is dominated in the eastern two-thirds by a homocline of uppermost Cretaceous rocks dipping gently southwestward toward a northwest trending major fault zone that occupies the remaining one-third of the study area. The fault zone raises older Cretaceous rocks to form Mesa de Anguila, a major horst that has been modified by small and discontinuous normal faulting. Folds are of relatively minor importance, most of them being short open flexures with slight structural relief. Normal faulting along regional trends was probably active throughout the period of Tertiary volcanism, but the majority of the block faulting probably postdates the deposition of the volcanic strata. The absence of foreign detritus in the Quaternary deposits, the orientation of the terrace surfaces, and modern drainage patterns indicate that if the Rio Grande ever flowed through the study area, it did so before the development of the present drainage divide.