MAPPING AND SECTION ASSESMENT AT THE DALQUEST DESERT RESEARCH STATION, BIG BEND, TEXAS

The Dalquest Desert Research Station (DDRS) is located in the southeastern corner of the Bandera Mesa South 7.5’ Quadrangle. The mesa and canyon topography of the DDRS contains a view of the late Eocene-early Oligocene strata in the central Big Bend. The strata are transected by the Tascotal Mesa Fault (TMF), an E-W trending fault that links the region’s more dominant Basin and Range fault systems. The area has not been mapped at high resolution and lacks a modern integrated assessment. We conducted high-resolution geologic and structural mapping and section measurement.

The mapped section includes the Devils Graveyard Fm. (P_Edg), the Terneros Creek Rhyolite (P_Etc), a localized intrusive dome, the Solitario Conglomerate (P_Esc), the Mitchell Mesa Rhyolite (P_Emm), the Tascotal Fm. (P_Et), and the Rawls Fm. (P_Er). An alluvial pediment covers much of the canyon top, the canyon floors contain a dissected boulder conglomerate and minor alluvium. The P_Edg unconformably overlies the Cretaceous, with lower P_Edg (168.6 m of variegated muds and tuffs), middle P_Edg (104 m of interbedded largely buff and red colored tuffs, muds, channel sands, and conglomerate), and the P_Edg, Bandera Mesa member (88.4 m of buff-to-white muds, tuffs, sands, and breccias). This is conformably capped by an incomplete section of the P_Emm (45 m of orange welded tuff with lesser brecciated material), faulted out against the P_Et. This unit is marked by lacustrine limestone benches with interbedded breccia and muds (67.5 m, but thickness varies laterally). It is conformably overlain by the interbedded tuffs and basalt flows of P_Er (incomplete section, 170 m to mesa top). The P_Esc is confined to the southeastern portion of the map area, underlying the P_Emm.

Near-vertical, oblique-slip faulting dominates the area. The southern wall of the TMF is down-dropped by as much as 300 m, where the P_Et accommodates substantial strain through ductile deformation. A 320°-striking fault provides a smaller southwestern downdrop through the central mapped area. The area is marked by smaller imbrications from these faults and associated deformation.

This project confirms and integrates prior work on regional processes. The high-definition map also suggests that that the TMF produced subequal amounts of vertical and horizontal movement, which may constrain current work on this feature.