Don't mess with Texas: a geologic field trip to the Trans-Pecos

By Matthew Morriss
Whitman College, Walla Walla, Washington

In order to better to understand the rocks, structures, and geologic history of southwestern Texas, Whitman College students spent half of a semester studying the geology of this part of the Basin and Range Province. Through this in-class research, students knew what to expect in West Texas; however, the time spent in the field was essential for a better understanding of the area’s geology. The Proterozoic rocks in the region are remnants of island arcs and marine sediments which accreted to the southern margin of Laurentia during the Grenville Orogeny.  Fossiliferous Cretaceous limestones represent the warm waters of the Western Interior Seaway. The Eocene-Oligocene (47-32 Ma) volcanics of the Trans-Pecos Magmatic Region are a unique assemblage of volcanics.  In this remote, sparsely populated corner of Texas, extrusive rocks are spread over three distinct regions: the Davis Mountains, the Chisos Mountains (in Big Bend National Park), and the Bofecillos Mountains.  The older volcanics are associated with the rapidly subducting Farallon plate, producing rhyolitic and trachytic lava flows and pyroclastics.  The initiation of continental rifting in the late Oligocene resulted in continental rift-style basalts.  In addition to geologic pursuits, the course included a naturalist perspective, as the Trans-Pecos is renowned for birding and ecological refugia.