GEOLOGIC MAPPING OF CRETACEOUS AND TERTIARY ROCKS ON THE PITCOCK ROSILLOS MOUNTAIN RANCH, BREWTSER COUNTY, TEXAS

Upper Cretaceous and Paleogene strata are exposed on the Pitcock Rosillos Mountain Ranch, bordered on three sides by Big Bend National Park (BBNP), Brewster Co., Texas. The Pen, Aguja and Javelina Formations of Late Cretaceous age, record the terminal Cretaceous regression of the Western Interior Seaway and contain abundant vertebrate remains. The Tertiary Black Peaks, Hannold Hill and Canoe Formations record deposition in a floodplain dominated by fluvial activity. Maxwell et al. (1967) produced a detailed geologic map of BBNP and adjacent areas, which extends onto the Pitcock ranch and includes these units.

Our objective was to produce a geologic map and cross sections of the sedimentary strata on and around the Rosillos laccolith, on the Pitcock ranch, through field mapping and remote sensing data. We worked on mapped and unmapped areas of the ranch and tested the validity of the Maxwell et al. map where it falls on the ranch. Thomas Lehman and students from Texas Tech University have remapped areas of the park south of the ranch, updating and clarifying the original Maxwell et al. map.

Numerous differences were found between the Maxwell et al. map and our interpretations. A normal fault north of the Canoe Valley syncline was not observed while in the field. Instead, continuous deposition of Javelina and Black Peaks Formations had occurred with the K/T Boundary located immediately to the east of our field area. The K/T Boundary is also located inside the southern border of the ranch. An anticline of Cretaceous sediments southwest of the Canoe's south-bounding fault has also been identified in our map but is absent in the map by Maxwell et al. In Cottonwood Wash Canyon, a gentle syncline of Cretaceous units is present and easily visible, with its fold hinge perpendicular to the canyon walls. Maxwell et al. mapped large exposures of Hannold Hill Formation in this western region that we are unable to locate. The syncline extends toward the west and plunges steeply to the southwest of our field area. Other slight differences between the two maps are have been found, while many features coincide. The geologic map we produced will be of great importance because it includes previously unmapped areas and revisions to a previously constructed map of the area.