The Mancos Shale is a geologic formation dominated by marine mudrocks below storm wavebase (Kellogg 1977), deposited in the Western Interior Seaway during the Late Cretaceous geologic period. Within this formation are intervals that consist of very-fine to medium-grained sandstones displaying both sheet-like and channel-form geometries, with sedimentary structures that appear wave influenced. The area of interest is Hatch Mesa, an outcrop along the Book Cliffs in eastern Utah. Previous studies have interpreted the depositional setting of the sandstone intervals to be prodelta, shelf and basin foreslope environments (Kellogg, 1977; Swift et al., 1987; Chan et al., 1991; Taylor and Lovell, 1995; Cole et al., 1997). Since then, Pattison et. al (2007) has interpreted these sequences to be channelized and lobate wave-influenced turbidity current deposits formed in prodelta to outer-shelf environments, with the identification of turbidites and wave-influenced deposits. Recent analysis of density-current hydrodynamics suggests that sedimentary structures similar to those in the Mancos sandstones can be generated by bedforms associated with both sub- and supercritical sediment gravity flows in the absence of wave influence (Yokokawa et al., 2010; Cartigny et al., 2014; Fedele et al. 2016). The structures observed in flumes experimentally have a similar architecture to the bedforms Pattison et al. (2007) described. Based on observations in the field, supplemented by well log interpretations, it has been determined that what has previously been interpreted as the product of storm wave-influenced deposition may have been deposited by super-critical density currents in the absence of wave influence. The implication of this study is that the sandstone intervals were deposited in deeper water than previously thought. Further study will include additional field-based documentation in order to produce a more comprehensive picture of the mechanics of the bedforms within the sandstone intervals of the Mancos.
Author: Emma Palko
Faculty Advisor: Brian Currie, Department of Geology and Environmental Earth Science
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