STRATIGRAPHY AND PALEONTOLOGY OF LATE CRETACEOUS TO PALEOCENE STRATA ON THE WESTERN ROSILLOS MOUNTAIN RANCH, BREWSTER COUNTY, TEXAS

The Rosillos Mountain Ranch (RMR) is adjacent to the north-central border of Big Bend National Park (BBNP) in West Texas. There are exceptional exposures of Cretaceous- to Paleogene-aged fossil-bearing terrigenous rocks in the western-most area of the RMR that are also found in BBNP, including the Aguja, Javelina and Black Peaks Formations. Previous geological maps of the region, including the 2007 USGS geologic map of BBNP, which includes the RMR, relied heavily on aerial photos and older maps for contact boundaries in remote areas. Unfortunately, many of these boundaries are incorrect. One purpose of this study is to correct these lithologic contacts in the southwest of the RMR, with detailed field mapping, including using plentiful high resolution in-the-field georeferenced images. Additionally, detailed lithostratigraphic descriptions are used to further establish the distinctiveness of the formations, as there is some confusion in the current literature. A detailed stratigraphic section of the area, including high resolution georeferenced photo documentation along the transect, illustrate the lithostratigraphic relationships of the three formations. Another goal of the project is to augment the current fossil collections of the area. Fossils collected from the field area are currently being prepared, including a small turtle, hadrosaur material including a distal tibia and juvenile ilium, and an Alamosaurus ulna.