MAPPING OF THE UPPER CRETACEOUS BOQUILLAS FORMATION IN BIG BEND NATIONAL PARK, TEXAS

Geologic maps are presented for parts of the Solis, San Vicente, Rio Grande Village, Roys Peak, McKinney Springs, Dagger Flat, Bone Spring, Bone Spring NE, and Persimmon Gap quadrangles in Big Bend National Park, Texas. The maps show primarily those areas underlain by the uppermost Buda Fm, Ernst Mbr of the Boquillas Fm, and the lower part of the San Vicente Mbr of the Boquillas Fm. Mapping of these units in the listed areas show significant differences from the published maps by Maxwell and others (1967) and Moustafa (1988). In several areas rocks previously mapped as Santa Elena, Del Rio, and Buda formations proved instead to be Buda Fm and the lower Ernst Mbr of the Boquillas Fm. In other areas, rocks previously mapped as Del Rio Fm are actually a thin veneer of brown gravel overlying the Buda Fm. In some cases, rocks mapped as Del Rio Fm are in fact the lowermost ±20m of the Ernst Mbr. Examination of aerial photography reveals how these inconsistencies could have occurred. A number of structures (especially faults) identified by Moustafa (1988) are not present and certain faults not identified by previous workers are identified in the present study. Most faulting appears to represent an extensional tectonic setting. Major faults in mapped areas trend N20°W to N45°W and are sub-vertical. Splay or tear faults branch off of the major faults and the amount of displacement on the splay faults decreases with distance along the splay away from the major faults. Secondary folds are subhorizontal and subparallel to some major faults suggesting dip-slip displacement. Along some other major faults, secondary gently plunging folds at ±45° to fault strike suggest strike-slip displacement. The mapping in these nine quadrangles is intended as provisional base mapping for 1:24,000 geologic quadrangle maps and to serve as support for the USGS 1:100,000 geologic map of Big Bend National Park and the National Park Service.