DAGGER MOUNTAIN, BIG BEND NATIONAL PARK, TEXAS, CONTAINS THREE PHASES OF LARAMIDE THROUGH BASIN AND RANGE FOLDS

Dagger Mountain is a 5 km-long, doubly-plunging, southwest-vergent anticline adjacent to a doubly-plunging syncline. Mapping at 1:12,000 scale of Dagger Mountain (DM) in Big Bend National Park reveals details about three phases of Laramide and Basin and Range structures. DM is located in northern Sierra del Carmen, near the eastern margins of the Cordilleran orogen and Basin and Range province. Mapping and descriptive structural analysis complement previous mapping at 1:12,000 – 1:75,000 scales (Poth, 1979; Moustafa, 1988; Cooper and others, 2011; Turner and others, 2011; Maxwell and others, 1967). Poth and Moustafa were graduate students advised by Bill Muehlberger. DM exposes Cretaceous Santa Elena Limestone, Del Rio Clay, Buda Limestone, and Boquillas Formation. At least two phaneritic mafic, feldpathoid-rich sills intrude the Boquillas Formation. A felsic phase of a similar, possibly correlative sill south of DM is dated at 32.47 ± 0.41 Ma ( ⁴⁰Ar/³⁹ Ar on groundmass; Morgan and Shanks, 2008). Sills thicken southward from 6.4 m and 12.8 m to 15.9 m and 33.4 m. One well-exposed sill can be traced from one map-scale fold limb, through the hinge, and into the other limb. The DM anticline is a first-phase (D1) fold. D1 map- and outcrop-scale folds contain subvertical axial planes striking ~342, fold axes at ~162 02, and interlimb angles that average 108° and range from 34° - 166°. Second-phase (D2) folds produced NNW and SSE plunges of the DM anticline. D2 map- and outcrop-scale folds display subvertical axial planes striking ~038, fold axes at ~038 2, and interlimb angles that average 124°,and range from 41°- 166°. Third-phase (D3), high-angle faults striking ~348 and ~320 cross-cut first-phase folds and Tertiary sills. D3 drag folds adjacent to faults contain axial planes striking ~348 – 354 and dipping 82 NE, fold axes at 153 01, and interlimb angles that average 148°, and range from 120° - 162°. DM structures are significant because: a) few polyphase fold localities have been documented in the Big Bend region, b) the west-verging DM anticline and other D1 folds show fault-propagation fold characteristics, c) the sill that appears folded in a map-scale D1 fold suggests that Laramide deformation here is post-32 Ma or that the sill intruded both limbs and hinge of an existing fold.