32-4 Deformation Mechanisms in the Edwards Formation: Contraction, Extension, and Dissolution

Session: Latest Research Advances in Structural Geology and Tectonics (Posters)

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Abstract:

US Highway I-10 roadcuts between Kerrville and Sonora Texas contain poorly understood exposures of intense localized deformation in the Edwards Formation, including normal and thrust faults, breccia pipes and horizons, and folds. The Edwards Formation hosts the Edwards aquifer, which supports over two million people, and these surface structures may reflect those within the aquifer. Calichification and gypsum dissolution occur throughout the Edwards plateau and have been hypothesized to cause this deformation, but these processes do not comprehensively explain the relatively large (1-4 m) scale folding and faulting.

The thrust faults and folds observed in the field do not share a consistent vergence direction. Some folds drape into breccia pipes, interpreted to be due to collapse of underlying strata. Contrarily, intact folds have also been observed cross-cutting the pipes, implying either the prior existence of unrelated folding or dissolution and collapse of pre-existing drape folds. Many folds show no interaction with collapse features, suggesting they do not share the same deformation mechanism. Tectonics likely do not account for all thrusts and folds due to the distance from the nearest plate boundary and inconsistent vergence direction. It is possible, however, that some folds and thrusts do originate from Laramide age tectonic forces, while others originate from soft sediment deformation or karst processes.

To determine the deformation mechanisms and the magnitude of contraction and extension present, this study uses cross-section based structural analysis and structure-from-motion photogrammetry to model and restore select roadcuts. Calcite will be sampled within fault planes, folds, and collapse breccias. To determine the timing of deformation events, samples will be analyzed using U-Pb dating, and fluid inclusion thermometry will be used to determine the temperature and depth of formation.

This study will identify the number of deformation events and constrain their timing. Regardless of their individual origin, the faults and folds interact with gypsum dissolution, brecciation, and karstification. Extensive karstification and brecciation may lower terrain stability and the feasibility of man-made structures in the study area. The risk of modern collapse can be assessed based on the depth of brecciation; shallow collapse may pose a risk to development. Understanding this interaction will therefore advance knowledge of regional tectonics and improve hydrologic models of the Edwards aquifer.

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

doi: 10.1130/abs/2025AM-9254

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