STRIKE-SLIP FAULT INDICATORS IN EASTERN ERNST TINAJA CANYON, BIG BEND NATIONAL PARK, TEXAS

Detailed geologic mapping (1:12,000) has revealed numerous structural features previously unmapped by past researcher at Ernst Tinaja, a narrow canyon in eastern Big Bend National Park with locally folded and faulted Cretaceous strata. The Sierra del Carmen Mountains rise to the east, with uplifted, thrusted Laramide monoclines overprinted by Basin and Range extensional normal faulting. West of Ernst Tinaja lies the Tornillo graben (another B&R feature) and the Chisos Mountains which are a result from Tertiary volcanics. A few miles north are Paleozoic units that were folded and thrusted during the Ouachita orogeny. Thus Ernst Tinaja sits at the nexus of several major structural and volcanic trends in North America.

Intense folds are common in the thin-bedded Boquillas Formation (Eagle Ford equivalent) near the mega-pothole called Ernst Tinaja. The Boquillas overlies thick, unfolded but faulted Buda Limestone. Some clayey Boquillas layers thicken slightly in fold hinges, but folds are otherwise mostly parallel, flexural-slip, open to tight, possibly drag folds. Orientation of mesoscale thrusts in the Boquillas match flexural-slip directions. Several steep breccia zones cutting the canyon may be a combination of B&R extension and strike-slip faulting.

 Going upstream (east), the canyon makes an abrupt 90⁰ turn to the north. Numerous outcrops of horizontal slickensides indicate primarily dextral and lesser sinistral strike-slip movement in this location. Vertical slickenlines were identified near a bend in the strike-slip canyon where the fault planes strike changes from northwest to a northeast and back to the northwest after the bend. Chattermarks indicating normal faulting suggest a negative flower structure based on the movement of the fault and the geometry of the bend. This location can give insight into the working of strike-slip faults and the geomechanics of releasing/ restricting bends. A clay cake model is currently in progress to model the structures in this area to better understand the stresses which affected this area.