Polyphase Laramide Structures and Possible Folded Tertiary(?) Sills at Dagger mountain, Big Bend National Park, Texas

Abstract

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.