Geochronology of the Trans-Pecos Texas volcanic field
— John Andrew Wilson

The occurrence of interbedded fossiliferous volcaniclastic sediments and potassium-argon datable rocks makes the Trans-Pecos volcanic field doubly valuable for geochronology. The superposition of vertebrate fossil faunas and volcanic flow rocks has furnished a correlation of the classical biochronological scale and the radiochronological scale. The span of time when both scales interdigitate stretches from about 45 to 20 million years before present or from middle Eocene to Miocene in the Big Bend area of Texas. Older and younger vertebrate faunas are known in West Texas but they predate or postdate the volcanic activity.

The Trans-Pecos volcanic field lies to the east, southeast, and south of Van Horn, Texas (fig. 1). It is the easternmost and southernmost Tertiary volcanic field in the continental United States. It covers an area of approximately 16,190 sq. km. in Texas, and the volcanic rocks extend southward into Mexico. For the last thirty years the area has been intermittently studied by geologists interested in extracting oil from beneath the volcanics, by those searching for minerals, radioactive or other, and lately by those testing for a source of geothermal energy. So far, none of their efforts have turned up commercial quantities of any of these. Over the same span of time, a probably greater number of students of geology have studied parts of the area with more academic goals in mind. Much of the work of the latter group is scattered in theses and dissertations at several universities or in shorter articles on a single quadrangle or on the results of a particular research project. The Geology of Big Bend National Park, Texas, by Maxwell and others (1967) is a comprehensive study but it covers only the Park. The Guidebook to the Geology of the Big Bend Area by Maxwell and Dietrich (1965) described the areal geology south of U.S. 90 but did not include the Vieja area or the northern Davis Mountains. The papers presented at a symposium on the Cenozoic Geology of the Trans-Pecos Volcanic Field in Texas (Walton and Henry, eds., 1979), were devoted to a wide variety of studies: geochemistry, geophysics, radiometric dating, volcaniclastic sediments, and sedimentary diagenesis. These studies are continuing and, one hopes, will eventually be incorporated in a synthesis of the volcanic field as a whole. This contribution is intended as an update for my particular field of study, which deals with the biostratigraphy of the volcaniclastic sediments and their local and regional correlation.

Moderately large collections of vertebrate fossils have been recovered from volcaniclastic sediments in six separate areas (fig. 2) on the periphery of the Trans-Pecos volcanic field. The collec-tions have come from sediments that span the period from middle Pleistocene to early Miocene (Table 1). The rocks from the lower part of the section, middle Paleocene to middle Eocene, are entirely sedimentary and the only volcanic elements included are from a remote source. Local volcanism within the area began slightly earlier in the southern part of the field. The oldest vertebrate fauna associated with locally derived volcaniclastic rocks is early late Eocene, whereas to the north in the Vieja area the earliest fauna is late late Eocene. In the northeastern Davis Mountains and also in the northwestern part of the field in the Van Horn Mountains, the oldest tuffs contain early Oligocene vertebrate fossils. No vertebrate fauna younger than early Oligocene is known at present that is overlain by volcanic flow rock. Those fossils that have been found in the uppermost part of the section that might be late Oligocene are not identifiable, but that part of the section has not been thoroughly searched. An early Miocene fauna has been found in grabens along the Rio Grande near the village of Castolon in Big Bend National Park. A similar fauna, as yet unstudied, was found by Margaret Stevens of Lamar University, up the Rio Grande from Lajitas in rocks mapped by McKnight (1970) as the uppermost part of the Rawls Formation. The early Miocene rocks are intruded by dikes but not overlain by flow rock. The beginning and last phases of local volcanism in the Trans-Pecos volcanic field can be precisely dated with vertebrate faunas and the peak of volcanic activity was in the Oligocene.