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Geology of Northern Alto Relex Area, Roy's Peak Quadrangle, Big Bend National Park, Brewster County, Texas


Alto Relex is a prominent, near vertical fault-line scarp on the westernmost uplifted block of the Sierra Del Carmen Mountains in eastern Big Bend National Park. Recent mapping near Alto Relex in Roy's Peak quadrangle has revealed a variety of structural and intrusive features. Multiple tectonic events have affected the Big Bend region. Laramide compression and possibly transpression created folding, thrust faults, and strike-slip faults. Tertiary igneous intrusions, such as the nearby McKinney Hills laccolith, displaced bedding and may have injected magmatic fluids into preexisting fault planes. Eventually, Basin and Range extension overprinted earlier structures. The general geology of the study area has been featured on previous maps but lacks detail. Field mapping indicates that Cretaceous strata have been warped into short- and long-wavelength folds and broken by a variety of faults, including normal, thrust, bedding plane and strike-slip. Some areas have slickenlines and chatter marks with different orientations within a short distance, suggesting a complex structural history. One of the main units in the area, the Boquillas Formation, is age equivalent to the oil producing Eagle Ford Shale of South Texas. Analysis of thin sections will provide more information about the diagenetic history and depositional setting of the Boquillas, Buda and Santa Elena Formations in the study area. Clay cake modeling will provide a better understanding of the different stress regimes created by various tectonic influences. This clay model research is mainly geared towards understanding structural features created by igneous intrusions.