Abstract

Developing interest in Shale Gas plays of the Permian Basin have galvanized research on Carboniferous depositional systems in Texas. Outcrop analogs of the subsurface facies are mostly limited to the small exposures in the Sierra Diablo described by King (1965). Though considerable work has been completed on the shallow carbonate facies of ranges northwest of the Sierra Diablo, and on the more distal facies of the Llano Uplift, strata analogous to quality Shale Gas reservoir found in the Sierra Diablo between these regions remain poorly known. The Carboniferous strata are only exposed along the eastern escarpment, entirely within the Figure 2 Ranch. The Carboniferous portion described in King (1965) was little more than a side note to his significant Permian mapping, comprised of two generalized columns and four fossil localities. Paleontological studies have continued since then, but mostly on material collected in association with King. The research reported here was undertaken to build further on King’s preliminary study, and to develop a framework for more precise stratigraphic and paleontological investigations. Field data includes seven measured sections, corresponding fossil collections, characterization of structural deformation, and mapping of the outcrop area using GPS data and GIS processing.

As a result of this work, the Carboniferous is more completely mapped, and known to include more than 350 meters of section along a 6.5 km outcrop. It represents a largely complete Visean through Moscovian interval, now characterized by ammonoid and conodont biostratigraphy. The overall succession generally records a shift from siliciclastic-dominated deeper marine starved-basin and distal 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 carbonates of the Magdalena Group to the northwest) associated with uplift of the Diablo Platform during the early Moscovian. The improved strati-graphic resolution allows for more precise correlation, facies interpretation, and subsurface modeling.