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A 1.5 km long natural cliff outcrop of nearly horizontal Eagle Ford Formation in south Texas exposes northwest and southeast dipping normal faults with displacements of 0.01–7 m cutting mudrock, chalk, limestone, and volcanic ash. These faults provide analogs for both natural and hydraulically-induced deformation in the productive Eagle Ford Formation – a major unconventional oil and gas reservoir in south Texas, U.S.A. – and other mechanically layered hydrocarbon reservoirs. Fault dips are steep to vertical through chalk and limestone beds, and moderate through mudrock and clay-rich ash, resulting in refracted fault profiles. Steeply dipping fault segments contain rhombohedral calcite veins that cross the fault zone obliquely, parallel to shear segments in mudrock. The vertical dimensions of the calcite veins correspond to the thickness of offset competent beds with which they are contiguous, and the slip parallel dimension is proportional to fault displacement. Failure surface characteristics, including mixed tensile and shear segments, indicate hybrid failure in chalk and limestone, whereas shear failure predominates in mudrock and ash beds – these changes in failure mode contribute to variation in fault dip. Slip on the shear segments caused dilation of the steeper hybrid segments. Tabular sheets of calcite grew by repeated fault slip, dilation, and cementation. Fluid inclusion and stable isotope geochemistry analyses of fault zone cements indicate episodic reactivation at 1.4–4.2 km depths. The results of these analyses document a dramatic bed-scale lithologic control on fault zone architecture that is directly relevant to the development of porosity and permeability anisotropy along faults. [ABSTRACT FROM AUTHOR]

Structures in Cretaceous strata in the Devils River Uplift of west Texas include folds, thrust faults, and bedding-parallel veins indicative of contractional deformation in a thrust faulting stress regime. In this study we analyze a series of small-wavelength (meters to tens of meters) folds in roadcut exposures northwest of Del Rio, Texas. The folds have WNW-ESE dominant trends, gently plunging axes, and are gentle to tight with upright to inclined axial surfaces of variable vergence. These structures are interpreted as detachment folds, with detachments within volcanic ash or mudrock beds. Observed thickening in anticline cores was by ductile flow of volcanic ash or mudrock, small-scale folding, and thrust or reverse faulting. Bed-perpendicular calcite veins are common and in some cases are offset by flexural slip on fold limbs, indicating that folding occurred after brittle failure and extension vein formation in lithified rock. Although rare, bed-parallel calcite veins are also present in the exposure and indicative of a thrust faulting stress regime. Occasional normal and reverse or thrust faults are also present in the exposures. Fluid inclusion analysis of vein cements from extension fractures and faults yielded homogenization temperatures of 71–101 °C, and estimated trapping depths of 2.2–3.5 km prior to and coeval with folding. We interpret these folds as having formed during NNE-SSW directed Laramide shortening (∼80–40 Ma) in a thrust-faulting stress regime with >2 km of overburden, rather than as either penecontemporaneous with deposition or after exhumation by caliche formation, as previously interpreted. • Analyses of small-wavelength folds in Devils River Uplift, west Texas. • Folds are typically detached above volcanic ash or mudrock beds. • Calcite veins offset by flexural-slip on fold limbs – bed-parallel veins rare. • Vein fluid inclusions trapped at 71–101 °C, and depths of 2.2–3.5 km. • Folds record NNE-SSW directed Laramide shortening. [ABSTRACT FROM AUTHOR]

Field observations and structural analysis of a 50-m-wavelength gentle anticline document bed-parallel fibrous gypsum veins ("beef") and calcite veins associated with contractional folding in the Boquillas Formation, east of Langtry, west Texas. Gypsum beef veins with bed-perpendicular fibers accommodated vertical extension of 13% in the anticline, diminishing to 0% in adjacent synclines. Bed-parallel calcite veins accommodated 0–4% vertical extension across the structure, with no clear correspondence of vein distribution to structural position. Homogenization temperatures from two-phase fluid inclusions in calcite veins indicate trapping depths of 2.0–2.7 km. Source rock analysis from unoxidized organic-rich mudrock at the site shows that the rocks never reached hydrocarbon maturation conditions, therefore vein formation cannot be explained by in situ hydrocarbon expulsion. Bed-parallel veins with vertical or bed-perpendicular fibers document a thrust faulting stress regime during contractional folding near the frontal limit of Laramide deformation. Gypsum vein formation may be related to meteoric water interacting with pyrite or H 2 S to form sulfuric acid, dissolution of carbonate, and deposition of gypsum as an auxiliary mineral. Switching between calcite and gypsum mineralization suggests opening and closing of fault and fracture controlled fluid pathways during deformation. • Gentle anticline in mudrock contains calcite veins and fibrous gypsum beef. • Gypsum beef accommodated 13% vertical extension in anticline, 0% in synclines. • Beef formed during amplification of Laramide detachment fold. • Beef not associated with in situ hydrocarbon expulsion. • Switching between calcite and gypsum mineralization records changing fluid sources. [ABSTRACT FROM AUTHOR]