Background
Formation of the eastwardly advancing Rocky Mountain Cordillera in southwest Texas and neighboring New Mexico culminated with the Laramide orogeny. The timing of the Laramide orogeny has been broadly interpreted to be late Cretaceous through Eocene (80 to 35 Ma) based on synorogenic sedimentary deposits and radiometric dating of rocks uplifted/emplaced during the orogeny. Vast sedimentary deposits in front of the large-scale Laramide structures were subtly deformed during the Laramide orogeny, producing faults and fractures that strongly influence fluid movement and accumulations in oil and gas basins and aquifers. The Balcones fault system also transects this region, with faulting historically estimated to have begun in the Oligocene (34-23 Ma) and been primarily active through the Miocene (23-5 Ma).
SwRI geoscientists have for over two decades conducted numerous studies near and beyond the Laramide front, with particular focus on fault and fracture systems that control or influence groundwater movement, oil and gas migration and trapping, and as analogs for other hydrocarbon reservoirs around the world. This work has produced an inventory of more than 400 calcite veins with rich associated datasets for kinematic history, relative timing of deformation, and fluid inclusion and stable isotope data. High-precision uranium-lead (U-Pb) dating of calcite has the potential to revolutionize our understanding of fault and fracture ages and place absolute time constraints on deformation stages critical to understanding regional structural and tectonic development, and associated oil and gas reservoirs, aquifers, and disposal or storage of CO2 and hydrogen.
Approach
The objective of this basic research project is to leverage the extensive SwRI archive of samples and associated structural data and fluid inclusion analyses – using high-precision U-Pb dating of calcite – to determine ages of faults and fractures, and place age constraints on the Laramide deformation cycle and earlier or later tectonism (e.g., Balcones fault system). The initial phase of the project focused on screening our sample archive and selecting an initial suite of samples that represent key stratigraphic units and deformation stages. Laboratory analyses, including initial uranium content screening, petrographic characterization, and in-situ Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb calcite geochronology were conducted by SwRI and University of Texas at Austin (UT) personnel at the UTChron Laboratory in Austin, Texas. Leveraging learnings from initial screening analyses, we prepared additional samples and performed LA-ICP-MS U-Pb analysis focusing on high-impact samples that represent key deformation episodes from host-rock strata that are most amenable (i.e., containing sufficient uranium concentrations) for U-Pb dating. The project culminates with synthesis, integration, and reporting, including reconciling new absolute ages with other timing information across the study area.
Accomplishments
LA-ICP-MS U-Pb dating was performed on syn-kinematic calcite veins from normal faults, strike-slip faults, thrust faults, subvertical opening-mode veins, and subhorizontal opening-mode veins (‘beef’) in Paleozoic (mainly Permian) and Cretaceous strata. These analyses yielded 187 new ages that document sequential pre-, syn-, and post-Laramide deformation across southeast New Mexico, and west and central Texas. These new data constrain Devonian veins in Ordovician deep-water strata, and several fault and vein ages from Wolfcampian or older strata indicative of Ancestral Rockies or Ouachita deformation. Sevier-age (~150-100 Ma) thrust-faulting regime deformation (thrust faults, ‘beef’) is recorded by samples from southeast New Mexico and west Texas, where this early stage of Rocky Mountain Cordilleran deformation had not previously been identified. Laramide-age (~80-35 Ma) thrust-faulting regime deformation (thrust faults, ‘beef’) and strike-slip-faults are recorded in southeast New Mexico, west Texas, and south-central Texas (Devils River uplift vicinity). Laramide-age normal faults and subvertical opening-mode veins reflecting normal- or strike-slip-faulting regime deformation indicate NE-directed maximum horizontal stress at and beyond the Laramide macrostructural front in west and south-central Texas. This Laramide-associated normal faulting includes new 39-58 Ma ages from the Balcones fault zone in San Antonio that are now documented in a new paper on these results published in Journal of Structural Geology (Ferrill et al., 2025). Post-Laramide ages (~35-10 Ma) from thrust-faulting-regime mesostructures were determined in the Guadalupe Mountains foothills, Glass Mountains, and Devils River uplift vicinity. These ages may reflect persistent thrust-faulting-regime deformation during post-Laramide erosional overburden removal. Post-Laramide ages (35-0 Ma) of normal-faulting and possible strike-slip regime deformation record a regional pattern of NW-SE directed extension consistent with residual Laramide foreland stress conditions in south-central Texas. Post-Laramide ages also show a regional pattern of SW-NE directed extension consistent with post-Laramide over-relaxation, and Basin and Range tectonism across southeast New Mexico and far west Texas.
Published Technical Papers
Ferrill, D.A., Cawood, A.J., Smart, K.J., Lehrmann, D.J., Evans, M.A., Stockli, L.D., Stockli, D.F., 2025. Fault zone deformation and fracture intensity in chalk-dominated carbonates. Journal of Structural Geology, https://doi.org/10.1016/j.jsg.2025.105469.
Technical Presentations
Ferrill, D.A., Cawood, A.J., Smart, K.J., Stockli, L.D., Stockli, D.F., Evans, M.A., 2025 (in press). New constraints on timing and stress regimes of pre-, syn-, and post-Laramide mesostructures in New Mexico and Texas. GSA Connects 2025 – Geological Society of America Annual Meeting, San Antonio, Texas.
Patents: None.
Resulting Project Work: None.