Geology of the Glenn Spring Quadrangle, Big Bend National Park

The Glenn Spring area of Big Bend National Park lies in the desert foothills and piedmont upland east of the Chisos Mountains. A recently completed geologic map of the Glenn Spring area, with cross sections and photographs of selected geologic features, illustrates the geologic framework of the area. This map will be available for park visitors to use while viewing the area's spectacular geology. The unique rocks, sediments, and landscape of the Glenn Spring area record a relatively long, complex history of geologic events. Cretaceous marine and continental sedimentation and Paleocene continental sedimentation are recorded by sandstones and mudstones of the Aguja, Javelina, and Black Peaks Formations. Laramide deformation formed northwest-trending folds, including the Mariscal Anticline that extends south of the map area. Oligocene mafic and felsic intrusions and undated intrusions were emplaced, including the prominent Chilicotal Mountain alkali syenite intrusion, Glenn Spring porphyritic microgranite sill, Talley Mountain porphyritic microgranite intrusion, Black Gap microgabbro sill, Chilicotal Mountain microgabbro sill, and numerous dikes. Neogene to Quaternary normal faulting formed north-striking faults. Quaternary episodes of piedmont alluvial aggradation and incision occurred. Quaternary landslides occurred on Chilicotal and Talley mountains that record multiple landslide events. A 15-mi stretch along Juniper Draw from the Chisos Mountains to the Rio Grande was flooded during the Holocene. The flooding was related to Chisos Mountain landslides and subsequent arroyo flooding of diagnostic landslide debris.