AbstractsEarth & Environmental Science

The Del Rio Formation of the Washita Group of the Comanchean Series in Brewster County, Texas, la a persistent unit in the eastern part the county, but one of varying thickness. On the western front of the Del Norte Mountains and along the Alpine-Terlingua highway the thickness of the clays ranges from 15 feet to 50 feet. Farther south, in the Terlingua district, exposures near the Mariposa mine and California Mountain are from 120 feet to 160 foot thick. Elsewhere in Texas the Del Rio Formation is discontinuous and thin. In Northern and Central Texas, the thickness varies from 25 feet to 125 feet. According to Adkins (1932, p. 388), the formation is absent in much of Trans-Pecos Texas and the Mexican state of Coahuila. In Reeves, Culberson, and Pecos counties it thins northward and disappears, as it does also in the southern Edwards Plateau between Del Rio and San Antonio. The microfauna of the Del Rio Formation is diverse; it contains 92 species of foraminifera belonging to 14 families, and 17 species of ostracodes belonging to 7 families. The foraminifera assemblage is predominantly benthonic and indicates deposition of the Del Rio Formation in an infraneritic bathymetric zone in which the temperature varied from 10^ C» to 25^ C, and salinity was near normal (35<>/oo).