GEOLOGIC MAPPING AND FRACTURE ANALYSIS OF THE NORTHEASTERN ROSILLOS MOUNTAINS, BREWSTER COUNTY, TEXAS

The long and complex geologic history of the Trans-Pecos region in west Texas is highlighted by Cretaceous and Tertiary tectonism and associated magmatism. Late Cretaceous sedimentary deposits in the Rosillos Mountains were deformed during the Late Cretaceous Laramide orogeny. Subsequent intrusion by an Oligocene quartz syenitic laccolith was followed by Basin and Range extension.

Geologic mapping in the northeastern Rosillos Mountains was completed using a revised stratigraphic column (Lehman, 1985). The improved understanding of the geology of the region allowed for reinterpretation of the geology in the field area. The Upper Cretaceous Boquillos, Pen, and Upper Aguja Formations are identified along with the Rosillos Mountains laccolith. Several contacts previously mapped as faults are reinterpreted as sedimentary contacts. The major structural feature in the field area is the NW-SE-striking segment of the Chalk Draw fault. The fault shows normal displacement with the hanging wall detaching a segment of the Rosillos Mountains laccolith from its main body. A complex zone of faulting, which includes synthetic normal faults, an antithetic normal fault, and small transfer faults, is also present. The Chalk Draw fault appears to terminate near the southern end of the study area.

Regional fracture analyses show wide scatter in the data and no preferred trends are recognizable. However, detailed analyses of fractures at eight locations in the Boquillos, Pen, and Aguja Formations reveals three dominant fracture trends. The dominant fracture attitudes are NW-SE (147°/80°), NE-SW (037°/59°), and WNW-ESE (282°/90°). A fourth, but less prominent, fracture set has lower average dips and is oriented also NW-SE (129°/43°). The origin of the observed fractures can be placed in the established paleo-stress framework for the region. Based on existing data, the maximum stress direction during the Laramide orogeny was oriented NE-SW, which resulted in low-angle NW-SE oriented fractures. During the Basin and Range extension, the maximum stress direction was vertical resulting higher angle structures. The documented flip in the intermediate and minimum stress ~17 Ma is recognized in the NW-SE and NE-SW-oriented fractures in the study area.