60-35 Fluvial Facies and Paleoenvironmental Transitions in the Big Bend Region

Session: 2YC and 4YCU Geoscience Student Research Poster Showcase

Presenting Author:

Authors:

Abstract:

The Sombrero Peak Ranch, in the Big Bend region of West Texas and bordering Big Bend National Park, offers exposures of Upper Cretaceous to Lower Tertiary formations that allow for detailed paleoenvironmental and structural analysis. These units were deformed by transpressional folding and faulting during the Laramide orogeny and transtensional faulting associated with the Rio Grande Rift (Page et al., 2008). This study combines facies analysis with mapping structures to reconstruct depositional environments and evaluate tectonic influences. Two measured stratigraphic sections reveal 10 facies; mudstone, sandstone, and cross-bedded sandstone are most common, grouped into 4 facies associations: point bar sequences, floodplain deposits, crevasse splays, and mid-channel bars. Facies associations, organized into 2 facies successions, fit meandering and braided fluvial depositional models (Nichols, 2009). 

Facies, facies models, and formation thicknesses change across a mapped fault within the Tascotal Mesa fault zone of Dickerson (2013), suggesting deposition occurred during Laramide reverse faulting. My current preferred working hypothesis attributes these variations to uplift and erosion producing unconformities on the upthrown side of the fault during active displacement.

Several hypotheses explaining variations in thickness and facies in the Sombrero Peak Ranch are being tested: (1) syndepositional folding led to thickened and deeper facies near the syncline axis; (2) growth faulting on a Laramide normal fault produced thickening on the downthrown side; and (3) an unconformity on the upthrown side of a Laramide thrust fault produced thinning (Lehman et al., 2018, 2022). Units under investigation include the deltaic Aguja Formation, consisting of shales, mudstones, and sandstones, the fluvial Javelina Formation containing mudstones and conglomeratic sandstones, and the lacustrine-fluvial Black Peaks Formation consisting of mudstones and sparse sandstones (Lehman et al., 2018). 

Methods I am applying include: 1:12,000-scale geologic mapping, using Google Earth images to locate exposures, and thin-section petrography of collected samples to assess mineralogy and texture. Facies analysis follows the approach of Nichols (2009) and Van Yperen et al. (2019). Successions are interpreted using Walther’s Law. Stratigraphic sections, measured with a Jacob’s staff, include descriptions of rock type, color, bed thickness, fossil content, grain size, sedimentary structures, and bioturbation. Sombrero Peak measured sections are correlated with published sections on both sides of the syndepositional fault in Paint Gap and Grapevine Hills (Lehman et al., 2018, 2024). 

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

doi: 10.1130/abs/2025AM-8562

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