Using Infrared Spectroscopy to Assess Paleofluid Flow Characteristics of Clay Gouge of the Moab Fault, UT

The origin, continuity, and mineralogy of fault gouge is crucial for determining the potential degree of fault seal formation. Of particular interest is the role of fluids in gouge formation. These issues have broad applicability in petroleum geology, underground waste disposal, carbon sequestration, and more. The Moab Fault in Utah is a classic example of a sedimentary basin‐bounding normal fault complex in a salt‐related petroleum system with abundant gouge, and spectral signatures of its gouge may be indicative of past fluid flow events and related formation of clay minerals. The Terraspec Halo, a “quantitative reconnaissance” tool and infrared mineral identifier, was applied to exposures of clay gouge at three locations along the 45 km long Moab Fault to assess the nature and origin of the gouge. The method consists of measuring profiles of infrared spectra and scalar values in close spacing across the fault plane and damage zone, extending into the adjacent wall rock. Absorbance peak locations (e.g., Al‐OH absorption feature of clay minerals), help indicate geochemical conditions during mineral formation or alteration. This combination of spectral features can be diagnostic of diagenetic vs. hydrothermal origins for clay minerals. Minerals identified include smectite, magnesium and potassium illite, montmorillonite, hematite, malachite, phengite, and goethite. Carbonate, silica, and copper mineralization in the core and damage zone of two of the sites along the fault suggests significant post‐faulting low‐temperature fluid movement. Spectral indicators of mineral maturity (e.g., Illite Spectral Maturity or ISM) help gauge the temperature of hydrothermal alteration events. Results suggest a distinctly lower ISM in most of the clay gouge compared to surrounding bedrock. This indicates a significant component of low‐temperature hydrothermal alteration and clay formation, and a major contribution of neo‐formed clays to the gouge. Concordant with previous studies utilizing traditional, time and labor‐intensive laboratory analysis, field analysis with the Terraspec Halo produced evidence for both low and high‐temperature fluid migration events. Based on scans transecting wall rock and fault gouge, evidence exists which suggests that sections of the Moab fault acted as a seal in some areas and as a conduit for fluid flow in others. Whether the fault acts as a seal or conduit for fluids at a given site may depend on multiple factors, such as magnitude of displacement, juxtaposed lithologies at a given fault segment, characteristics of gouge material, and complex spatio‐ temporal paleofluid history.