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Paleozoic, Mesozoic and Cenozoic rocks totaling about 15,000 feet in thickness are exposed in the Cowboy Pass area in the Confusion Range. The Paleozoic sedimentary rocks are carbonates, with small amounts of sandstone and shale, and the Mesozoic sedimentary rocks are shallow water limestone and shale. Cenozoic deposits are alluvium, lacustrine beds and small amounts of volcanic material. The trend of major folds and faults changes sharply from northerly to northeasterly at Cowboy Pass, and this change is accompanied by minor faulting and folding. Major structures are the result of post Lower Triassic-pre Cenozoic orogeny. During Cenozoic time, high angle normal faults developed, accompanied by local volcanic activity. / text

Clay composition and content profoundly impacts the strength and sealing capacity of a fault zone, reducing frictional resistance to sliding and permeability by as much as 7 orders of magnitude. Previous approaches, including the Shale Gouge Ratio (SGR) and Shale Smear Potential (SSP), have been used to understand and predict the clay content of fault zones. These models are largely limited to mechanical incorporation of detrital clays. This hypothesis stems from field observations of clay gouge and the smearing and associated attenuation of clay-rich shale beds offset by the fault. Recently, diagenesis has been recognized as an additional critical mechanism of clay enrichment In fault zones. My study investigates the relative contributions of both mechanisms of clay enrichment focusing on the implications for fault permeability and strength through structural and elemental mapping of the Moab Fault in Utah. Detailed mapping at Six sites along the Moab Fault in southeast Utah, revealed distinct structural deformation zones as defined by structures and distribution of normally faulted sandstone and shale including: (1) layers of clay-rich gouge separated by slip surfaces that include isolated sandstone breccia; (2) an inner smeared shale adjacent to the gouge showing increasing bed parallel shearing and resulting boudinage closer to the fault, and an outer smear with little shearing but rotation of beds; (3) faulted sandstone hosting deformation bands, slip surfaces, and intersections, joints and veins in locations near relays. Fluid assisted alteration was revealed by a combination of high spatial resolution scan-lines on outcrops element composition and measured sections of measured with a portable X-Ray Fluorescence device. Results to date include: (1) elemental concentrations relative to immobile species (such as Ti) and by structural zone show that Ca, Sr, Rb are preferentially enriched and/or depleted in the fault core, (2) the fault core hosts the greatest alteration; (3) a progressively more extensive and greater density of bed parallel slip surfaces from protolith to gouge where slip surfaces are associated with mixing and disaggregation; (4) stable concentration of elements associated with illite such as K, occurs preferentially in the gouge; (5) localized enrichment and/or depletion reveals solution mass transfer contributed to formation of the fault core and to a lesser extent the damage zones. Elemental mapping clearly demonstrates a compositional evolution of the fault core, and in particular the clay gouge, that cannot be accounted for by mixing of protolithic formations. Thus, observations from elemental mapping show that solution mass transfer influences the formation of clay gouge in the fault zone, in addition to mechanical incorporation of detrital clays from the surrounding protoliths. / Earth and Environmental Science

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The Late Cretaceous foreland basin in central Utah developed adjacent to the Cordilleran fold and thrust belt between Albian and latest Campanian time. Subsidence resulted from the lithospheric 'oad of coeval thrust sheets to the west. Compositional trends of foreland-basin sandstones record unroofing of stratigraphic sequences above ramp-style detachment thrusts until the middle Campanian, when folding above a frontal blind thrust system caused recycling of previously deposited foreland basin detritus. Basement uplifts within the foreland basin terminated subsidence in latest Campanian time. Thrust loading created a westward-thickening basin in which the sedimentary wedge fines eastward. Coarse-grained synorogenic strata along the western edge of the basin are included in the Indianola Group, which consists of a lower marine-dominated sequence and an upper fluvial sequence. The marine sequence correlates with the marine Mancos Shale farther east, while the upper fluvial sequence is equivalent to the Mesaverde Group. Individual lithostratigraphic units are time-transgressive, becoming younger eastward. Eight distinct depositional facies are recognized in the Indianola Group: alluvial fan conglomerate, braided fluvial conglomerate, braided fluvial pebbly sandstone, meanderbelt fluvial sandstone and siltstone, delta distributary sandstone, lagoonal sandstone, siltstone, and mudstone, nearshore marine sandstone, and open marine mudstone and siltstone. The Mesaverde Group was deposited mostly by sandy to pebbly braided and meandering rivers which transported detritus eastward from the thrust belt. Facies in the basin combine to form an offlapping sequence of eastward-fining clastic wedges. Sandstones of the basin are quartzarenites, sublitharenites, and litharenites derived from the sedimentary source terrane of the thrust belt. Detrital carbonate grains are an important fraction of the sedimentary rock fragments that dominate the lithic population of the sandstones. Feldspathic litharenites high in eastern exposures of the Mesaverde Group were derived from an arc terrane lying beyond the thrust belt. Linear petrographic trends shown by triangular QtFL and QpLsLv plots resulted from mixing of detritus from multiple sources. The age of synorogenic deposits and their succession by a Maastrichtian to Paleocene overlap assemblage indicate that foreland basin subsidence and major thrust faulting were continuous from late Albian through late Campanian time in central Utah.