Description and Analysis of the Thin-bedded Facies and Deposition Mechanisms of the Mancos Shale, Uinta Basin

Offshore fine-grained sediment transport and deposition in epicontinental seas have been debated over a long period. Hyperpycnites, sediment-gravity flows, turbidites, tidal influences and tempestites are theoretical mechanisms for mud transport toward the basin. A core from the Mancos Shale in Utah is an example of an offshore deposited formation and preserves various sedimentary structures and ichnofacies. With millimeter scale analysis of thin-bedded facies, a measured section was constructed along with facies classification and a bioturbation index log. Approximately 240 beds and 28 bed sets are measured. Frequency of events is approximately 1 per 1.25 years, assuming sedimentation rate is 1 mm/yr. Grain sizes vary from clay to very fine sand. Mudstones are interpreted to be deposited by bottom offshore currents where sandstones are mainly influenced by storm-modified waves. Thus, this core is interpreted to be deposited in a wave or storm-dominated middle shelf, mixed with offshore bottom current-influence and high hemipelagic content. Dominant ichnofacies is Cruziana. An average bioturbation index of 3-4, reveals a moderate sedimentation rate and an aerobic environment.