THE GEOLOGY OF THE CHISOS MOUNTAINS

The Chisos Mountains in Big Bend National Park are underpinned by a thick buildup of Eocene and Oligocene tuffaceous sedimentary rocks, lava flows, and ignimbrites that accumulated above widespread Late Cretaceous and Paleocene marine and marginal-marine sedimentary strata. The layered rocks are intruded by dikes, sills, stocks, and laccoliths of several eruptive and intrusive centers. Deposition and emplacement took place during four distinct episodes; 1) the formation of the Cretaceous and Paleocene substrate, 2) the influx of tuffaceous and sedimentary strata, 3) a period of eruptions and intrusions, and 4) localized deposition during denudation. There were periods of regional compressional tectonism in the Paleocene and early Eocene (Laramide) and extensional tectonism primarily in the Miocene (Basin and Range) that variously affected the Chisos-Mountain strata. The Cretaceous and Paleocene was a time of clastic sedimentation in shallow seas or lowland marshes and rivers. By early Eocene (prior to 46-47 Ma) sandstone, tuffaceous sandstone, and basalt/andestite flows of the lower Chisos Formation began to accumulate in low areas associated with actively forming synclines. Folding had ended by 46-47 Ma when the lava flows of the Alamo Creek Basalt erupted in the area, but the older Chisos beds are locally folded. Tuffaceous sandstone continued to accumulate in the Chisos Formation until the initial eruptions associated with the Pine Canyon Caldera first covered the region about 33 Ma. The eruptive and intrusive period spanned several million years in the Chisos Mountains beginning with the Pine Canyon eruptions and ending 28-29 Ma with the formation of the Sierra Quemada center. Widespread trachyandesite flows and a small mafic volcanic center at Dominguez Mountain also formed during this episode. Basins formed and filled with sediment on either side of the Chisos Mountains during Basin-and-Range extension, but extensive erosion has removed most of the fill. The denudational period that resulted from the introduction of the Rio Grande into the region is associated with extensive and complex surficial deposits.