Author Keywords

Dagger Mountain, Big Bend National Park, Laramide orogeny, Basin and Range, Sierra del Carmen, folded sills, Santa Elena Limestone, Del Rio Clay, Buda Limestone, Boquillas Formation


Dagger Mountain, in Sierra del Carmen within Big Bend National Park, Texas, is a 5 km-long, doubly-plunging, southwest-vergent anticline adjacent to a doubly plunging syncline. Dagger Mountain lies near the eastern margins of the Cordilleran orogen and the Basin and Range province. Mapping at 1:12,000 scale reveals details about three phases of Laramide and Basin and Range structures. 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). Four distinctive formations of Cretaceous age crop out on Dagger Mountain: Santa Elena Limestone, Del Rio Clay, Buda Limestone, and Boquillas Formation. At least two phaneritic, mafic, feldpathoid-rich sills intrude the Boquillas Formation. A similar, possibly correlative sill south of Dagger Mountain is dated at 32.47 ± 0.41 Ma ( 40Ar/39 Ar on groundmass; Morgan and Shanks, 2008). One well-exposed Dagger Mountain sill can be traced from one map-scale fold limb, through the hinge, and into the other limb. The Dagger Mountain anticline is a first-phase (D1) fold. D1 map- and outcrop-scale folds contain subvertical NNW-striking axial planes and subhorizontal fold axes. Second-phase (D2) folds produced NNW and SSE plunges of the DM anticline. D2 map- and outcrop-scale folds display subvertical NE-striking axial planes and subhorizontal fold axes. Third-phase (D3) high-angle faults strike NNW and NW and cross-cut D1 folds and Tertiary sills. Drag during D3 faulting produced D3 folds. Dagger Mountain structures are significant because: a) few polyphase folds have been documented in the Big Bend region, b) the west-verging Dagger Mountain anticline and other D1 folds show fault-propagation fold characteristics, c) the apparently folded Tertiary(?) sill suggests that Laramide deformation at Dagger Mountain is post-32 Ma and unusually recent. Alternatively, the sill could be as old as Cretaceous, or the sill could have intruded both limbs and hinge of an existing fold.