CARBONIFEROUS STRATIGRAPHY OF THE SIERRA DIABLO, CULBERSON COUNTY, WEST TEXAS

Recent interest in developing Shale Gas plays in the Permian Basin make it increasingly important to understand the broader Carboniferous depositional systems of Texas. Outcrop analogs of the subsurface strata are mostly limited to the small exposures described by King (1965) in the Sierra Diablo. While considerable work has concentrated on the shallow carbonate facies of ranges northwest of the Sierra Diablo, and on the more distal carbonate and shale facies of the Llano Uplift, strata in the Sierra Diablo between these regions remain poorly known. King's Carboniferous sections are little more than a side note to his Permian work, comprising two meager columns and four fossil localities. Paleontological studies have continued since then, but mostly on material collected during King's work. This research was undertaken to build further on the preliminary work of King, and to develop a framework for more precise stratigraphic and paleontological investigations.

Carboniferous strata of the Sierra Diablo are exposed along the eastern escarpment, entirely within the Figure 2 Ranch. Field data include numerous measured sections, fossil collections, characterization of structural deformation, and mapping of the outcrop area using GPS data and GIS processing. The Carboniferous as now mapped contains more than 350 meters of section along a 6.5 km outcrop, and represents a largely complete Visean through Moscovian interval. Overall, these strata record a pronounced shift from siliciclastic-dominated deeper marine starved-basin and shelf facies (similar to the Barnett Formation, Marble Falls Group, and Smithwick Formation of Central Texas), to cyclic carbonate platform facies (more similar to the shallow shelf carbonates of the Magdalena Group) through the Bashkirian-Moscovian boundary interval. Paired with the paleontological data (particularly ammonoid, conodont, and fusulinid data), the improved stratigraphic resolution allows for more precise correlation across the Permian Basin, and further to the western ranges and the Llano Uplift.