THE EFFECTS OF WEATHERING ON OUTCROP-BASED FRACTURE CHARACTERIZATION: A CASE STUDY FROM THE STILLWELL ANTICLINE, WEST TEXAS

We performed a study of limestone beds that define the Stillwell anticline, west Texas, to investigate the effects of weathering on outcrop-based fracture studies. The anticline is an asymmetric, NE-vergent fold with a shallow, SW-dipping backlimb, a subhorizontal middle limb, and a steep, NE-dipping forelimb. A previous outcrop-based fracture intensity study, which focused on fracture characteristics in a single limestone bed across the anticline, revealed statistically significant differences in fracture characteristics at each structural position. However, this earlier study suggested that fracture orientations could not be linked to regional tectonic stresses, and the intensity of fractures observed at the outcrop was significantly greater than the intensity observed with optical petrography. When combined with petrographic study that revealed a significant weathering rind on rocks at the outcrop, it became apparent that weathering processes, which include in-situ brecciation resulting from extensive dissolution, shrinkage, and precipitation processes, could be responsible for at least some of the fractures observed in outcrop.

In this study, we investigate the effects of weathering on fracture analysis by documenting fracture characteristics from fresh surfaces (exposed using a large sledge) at the same structural positions and locations used in the earlier study. These characteristics include fracture orientation, length, aperture, morphology, and fill type. Based on these data, we present a comparison of the two data sets (weathered surface vs. fresh surface), which reveals the relative importance of weathering upon outcrop-based fracture network characterization.