Chemostratigraphy And Paleoenvironmental Significance Of The Pennsylvanian Smithwick Formation, Northern Llano Uplift Region, Texas

Abstract

The Early Pennsylvanian (Bashkirian-Moscovian) Smithwick Formation of Central Texas has been previously described as a marine transgressive, fairly homogeneous and fine-grained dark shale sequence with varying local depositional lithofacies. The deposition of the Smithwick shale through the southern margin of the Fort Worth Basin (FWB) has shown large variability in thickness and deposition style. Lithological heterogeneity of the bounding units throughout the basin is inferred to be the result of a combination of depositional, tectonic and eustatic variations. Regional structural instability from Late Mississippian to Early Pennsylvanian is linked to the reactivation of the Ouachita geosyncline and thrust front, causing deformation of foreland structures, a large influx of sediment from the north and east boundaries, and reported syndepositional faulting (Keller and Cebull, 1973; Trice and Grayson, 1985; Grayson et al., 1990; Pollastro et al., 2003). The current interest in the evaluation and characterization of the Smithwick Formation is based on good core and well-log coverage, revealing its importance as an analogue for studying similar shale sections, and to better understand the structural and depositional constraints of the FWB, and the Pennsylvanian marginal-marine conditions. The purpose of the present study is to acquire and analyze geochemical data from two drill-cores retrieved from Brown and McCulloch counties in Central Texas, and to correlate these drill-cores to previously published studies and well data. The multi-proxy approach can lead us toward a better understanding of the local subsurface geology, geochemical patterns, and environmental processes that occurred during the Early Pennsylvanian in central Texas.