New Mexico Geological Society Annual Spring Meeting — Abstracts

The Western Boundary Fault Zone (WBFZ) is a complex structure that bounds the west side of the Franklin Mountains in El Paso, Texas. The nature and type of faults in the WFBZ are unresolved. Some researchers have suggested the fault zone contains both normal and reverse faults related to Rio Grande rifting and the Laramide orogeny. Part of the difficulty in resolving the type of faulting present is the uncertainty surrounding the age of rocks on the western side of the fault zone. Proterozoic to early Paleozoic rocks are exposed in the Franklin Mountains on the eastern side of the WBFZ; however, rocks on the western side are only locally exposed in a series of small, disconnected outcrops. In this study, we collected samples from these small outcrops and used field observations, fossil assemblages, sandstone petrography, and detrital zircon U-Pb geochronology to determine the age and formation name of the units. Geologic mapping and field observations suggest that the WBFZ consists mainly of small normal faults, and diagnostic fossils suggest the outcrops on the western side of the fault zone may be Cretaceous in age. Some lithologies observed in the WBFZ include the Anapra sandstone, yellow micritic limestone, and dark shale. However, our focus was on collecting the Anapra sandstone for U-Pb geochronologic dating. Ongoing work focuses on comparing sandstone units in the WBFZ to similar sandstone units in the Cristo Rey area, including the mid-Cretaceous Anapra Formation. We are testing whether the composition of sandstones and the spectra of zircon ages between the two exposures are comparable. The results of this study will help resolve the geology of the WBFZ and provide tectonic insights into how the region was affected by contractional and extensional deformation.