31-5 Multi-phase Laramide and Rio Grande Rift Tectonics in Sombrero Peak Ranch, Big Bend Region, Texas - Tascotal Mesa Fault Zone

Session: Best Student Geologic Map Competition (Posters)

Presenting Author:

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Abstract:

Over the last two years, I created a 1:12000-scale  geologic map of the Sombrero Peak Ranch. The Sombrero Peak Ranch is in the northern Big Bend region, in the west Texas borderlands, northeast of Terlingua. My mapping documents fault slip within the eastern Tascotal Mesa fault zone (TMf); first as a Laramide growth fault, which reactivated as a right-handed strike-slip fault. E-W dikes cross-cutting the Tmf indicate N-S extension, possibly occurring 17 Ma or younger.

The Laramide Tornillo Basin contains Late Cretaceous-Early Tertiary shales, sandstones, and conglomerates. NNW trending thrust faults, reverse faults, left-lateral strike-slip faults, and folds characterize the Laramide Orogeny in the Big Bend (Henry et al., 1986). Laramide deformation occurred from 70 -50 Ma (Page et al., 2008), while late Laramide magmatism, forming laccoliths, sills, and dikes, occurred 46-28 Ma (Page et al., 2008).

NNW-trending normal faults and bounding grabens, E-W right-lateral strike-slip faults (including the TMf), and drag folds) characterize the Rio Grande rift in the Big Bend region (Dickerson, 2013; Conley et al., 2023). Rio Grande rift faulting began  24 Ma and continues today;  Rift magmatism formed two calderas and widespread dikes from 31 - 17 (Price et al., 1986).

Over 13 days, I constructed in the field a 1:12000-scale geologic map of Sombrero Peak Ranch. I described and distinguished 3 Cretaceous, 3 Tertiary, and 4 Quaternary map units. I located upper and lower contacts of the Cretaceous Javelina Formation by utilizing criteria from Lehman et al. (2018) and measuring two stratigraphic sections.  I also constructed three cross-sections and used Stereonet 10 (Allmendinger, 2024) to find the fold axis (081 1NE) and axial plane orientations (343 81[JS1]  NE) for an unusual map-scale syncline. I utilized ArcGIS Pro version 3.5 (Esri, 2024) satellite imagery to accurately map complex Quaternary contacts and to create a topographic map with 20-foot contours.

Mapping revealed the following sequence of events: 1) Late Cretaceous-Tertiary units deposited while a NW-trending growth fault formed, 2) NE-trending syncline formed, 3) Laramide laccolith deformed strata into a structural dome, 4) Tmf reactivated as a right-handed strike-slip and drag folds formed, 5) 3 parallel E-W dikes and one sill intruded, 6) Erosion created a pediment surface, 7) Older Quaternary sediments deposited, 8) Recent Quaternary stream sediments and talus deposited.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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