GEOLOGIC MAPS OF THE GLENN SPRING AND MARISCAL MOUNTAIN AREAS, BIG BEND NATIONAL PARK

Recent 1:24,000-scale geologic maps of the Glenn Spring and Mariscal Mountain areas, co-authored with the late W. R. Muehlberger, will be presented with cross sections and photographs of selected geologic features. These maps and associated materials illustrate the geologic framework of this area’s spectacular landscape and varied geology, which records Cretaceous through Quaternary sedimentation and deformation. Mariscal Mountain lies astride the border between Texas and Coahuila, Mexico, at the great bend of the Rio Grande. Within the park, the mountain is a ~15-km-long, northwest-striking, asymmetric anticline that is well expressed in Cretaceous carbonate and argillaceous strata (Del Carmen, Sue Peaks, Santa Elena, Del Rio, Buda, and Boquillas Formations), in a thick Eocene gabbro sill that was intruded into the preexisting anticline, and in undated rhyolite sills. Undated rhyolite and thin basalt sills are thrust faulted at the crest of the fold. Neogene(?) to Quaternary solution-collapse features in folded, faulted, and fractured Santa Elena, Buda, and Boquillas strata are as wide as 400 m and formed on both the crest and flanks of Mariscal Mountain. The Mariscal anticline extends northwestward into the Glenn Spring Quadrangle, where late Cretaceous to early Tertiary (Paleocene) deposits (Pen, Aguja, Javelina, and Black Peaks Formations), which record a shift from marine to continental sedimentation, are folded. Most of the Glenn Spring area lies in the desert foothills and piedmont upland east of the Chisos Mountains. Within the Glenn Spring area Oligocene mafic and felsic intrusions and undated intrusions have been emplaced into the sedimentary deposits as sills and dikes. The larger intrusions (Chilicotal Mountain alkali syenite, Glenn Spring porphyritic microgranite, Talley Mountain porphyritic microgranite, and Black Gap microgabbro) form prominent landforms. At Chilicotal and Talley Mountains, thick sill rock caps mudstone and sandstone, and at these mountains, large Quaternary landslides record multiple events. Normal faults, related to regional Neogene to Quaternary extensional deformation, cut two of the Oligocene intrusions in the Glenn Spring area as well as an undated rhyolite sill along the east flank of Mariscal Mountain.