Fault and fracture response to Neogene extension in the western Delaware Basin, Texas

We performed an integrated investigation of normal faults in the Brushy Canyon Formation near the eastern edge of Cenozoic Basin and Range/Rio Grande Rift deformation to understand the origin, timing, and fluid history of normal faulting in the western Delaware Basin. Two roadcut exposures in the southern Guadalupe Mountains along the eastern edge of the Salt Flat Graben reveal (i) steep normal faults in subhorizontal layers, and (ii) tilted layering and antithetic normal faults in a hanging-wall rollover of the asymmetrical graben. Normal fault orientations match the orientations of abundant opening-mode fractures in well-cemented turbidite sandstone beds. In horizontal layers, opening-mode fractures and faults are near-vertical, and where bedding is tilted 20-30°, fractures and faults remain bed-perpendicular. We interpret that progressive rotation of vertical opening-mode fractures during layer tilting increased slip tendency driving shear displacement. Faults with 1-50 cm displacements in sandstone abruptly terminate in claystone beds (∼60-95 wt % clay) due to ductile flowage, compartmentalizing deformation. Minor normal displacement occurred along weak laminae in sandstone beds, offsetting bed-perpendicular calcite-filled opening-mode fractures. U-Pb dating of calcite from normal faults and opening-mode fractures yield ages of 20.5-4.0 Ma, consistent with regional timing for Basin and Range extension. Oil inclusion fluorescence in fault and opening-mode-fracture hosted calcite veins indicate ∼29 to 32 API gravity oil. Very low δ 13 C and δ 18 O stable isotope values are consistent with hydrocarbon oxidation. These results document progressive structural evolution and fluid movement in the Delaware Basin, providing context for hydrocarbon migration and fault reactivation by hydraulic fracturing, hydrocarbon production, and wastewater injection.