Thrust v. stylolite

Abstract

Mesostructures in subhorizontal thinly bedded calcareous mudstones of the Permian Bell Canyon Formation in the western Delaware Basin (Guadalupe Mountains foothills, west Texas) include vertical tectonic stylolites kinematically linked with low-angle thrust faults. Schmidt rebound is high (38–70) for all but a few beds, indicating competent rock. Competent beds exhibit contrasting behavior ranging from (i) no visible tectonic stylolites and numerous small-displacement (<1 cm) thrust faults, to (ii) tectonic stylolites with rare thrust faults. Thrust fault intensity correlates positively with total carbonate, and negatively with clay, clay + quartz, quartz + feldspar, and total organic carbon (TOC). Tectonic stylolites are abundant in beds with 3.5–6.5% clay. Stylolite intensity correlates positively with TOC. Lowest-rebound beds exhibit ductile flowage or internal folding rather than stylolite formation, and tend to terminate small-displacement thrust faults in adjacent beds. Tectonic stylolites and thrust faults are consistent with formation in a thrust faulting stress regime during NE-directed Laramide shortening. Fluid inclusion analyses of vein calcite from thrust faults indicate maximum burial depths of 3.3–4.2 km at the time of deformation. Deformation behaviors are highly sensitive to composition, with coeval thrust faulting in clay-poor beds versus stylolite formation in slightly more clay-rich beds.