FRACTURES AND VEINS ALONG THE BLAINE ESCARPMENT, OKLAHOMA: EVAPORITE PRECIPITATION UNDER CONTEMPORARY TECTONIC STRESS

We analyze the fracturing and vein-filling processes within the Flowerpot Shale Formation along the Blaine Escarpment, north-west Oklahoma. The study included detailed mapping of fractures and veins, and an analysis of the fracturing mechanisms, e.g., tectonic stresses, dissolution subsidence, or diagenetic-related origins. In the field, the fractures in this shale unit are typically identifiable as polymorphs of gypsum veins. Fracture zones were measured within six main exposures that are distributed along 115 km of the escarpment, while considering the spatial distance to shallow bodies of subsurface salt deposits previously mapped by Vosburg (1963). We measured fracture density, spacing, vein-fill, bed-boundedness, lithology, kinematic indicators, and proximity to surface salt precipitation. The dominant vertical fracture sets trend ENE-WSW throughout the six mapped exposures. The exposures that are located close to areas of heavy surface salt precipitation display chaotic bed inclinations, and thick bed-bounded satin spar gypsum veins with sinusoidal fibers which indicate layer-parallel shear. The red shale beds throughout the escarpment are separated by thin green/grey shale layers that are typically gypsum-rich. These layers bound and arrest many vertical fractures, and act as discontinuity surfaces along which bedding-parallel gypsum veins could form. The chaotic bed inclinations, bed-bounded vertical veins within bedding plane fractures exhibiting shear indicators, and proximal distance to heavy surface salt deposits suggests dissolution subsidence-related failure. We also consider the widely accepted concept that fracture orientations reflect the prevailing stresses during the fracturing period and accordingly, we envision two possible mechanisms for preferred vein orientation related to fracture initiation resulting from salt body dissolution: (1) Similarities in trend between the axis of the current maximum horizontal tectonic stress (e.g., Kolawole et al., 2019), and measured fracture/vein sets imply stress regime-driven fracture propagation; and (2) The orientations of the fractures/veins that closely align with preexisting subsurface, reactivated faults which controlled the exposed structures.