SUMMARY OF RECENT STUDIES IN THE ROSILLOS MOUNTAINS, BREWSTER CO., TEXAS

The Rosillos Mountains are the geomorphic expression of an almost completely unroofed Tertiary laccolith emplaced in mainly Cretaceous strata. The northern portion of the range lies within the boundaries of BIg Bend National Park, but the the southern portion is on an enclave of private land within the park. Over the last ten years, graduate students and faculty at Texas Christian University have conducted a series of geological investigations on the laccolith and surrounding sedimentary strata exposed on the privately-owned Rosillos Mountain Ranch. Remote sensing studies have been used to define and map alluvial fans ringing the laccolith and to detect previously unmapped remnants of roof rock. The distribution of springs has been related to fracture patterns in the laccolith. Preliminary geomorphic, sedimentologic and stratigraphic studies of alluvial fans on the southern flank of the Rosillos Mountains have provided new information on the unroofing history of the laccolith and geomorphic evolution of Tornillo Creek. Most of the roof rock of the laccolith was removed before the onset of alluvial fan deposition. Incised streams flowing across a pediment developed on the surrounding Cretaceous strata contain gravels derived from roof rocks in immediate contact with the laccolith. The fans contain little in the way of debris derived from the roof rocks, but instead consist almost exclusively of material eroded from the laccolith. Terrace deposits along Tornollo Creek contain gravels derived from the western side of the park mixed with debris from the laccolith.