Interpretation of mouth-bar and related lacustrine and fluvial sand bodies from the middle Green River Formation (Eocene), southern Uinta Basin, Utah

David Keighley, Oystein Spinnanger, John Howell, Stephen Flint

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    The Uinta Basin of eastern Utah is an intermontane basin that contains an ~ 2-kmthick succession of mostly carbonate-rich mudrock assigned to the Eocene Green River Formation. In the southwest of the basin, along Nine Mile Canyon and its tributary canyons, the middle member of the Green River Formation contains numerous interbedded sandbodies. Previous researchers have interpreted these sand bodies variably as lacustrine deltaic mouth bars, terminal fluvial distributary bars, and various types of fluvial (delta plain/floodplain/braid plain) bar. Using some modern western U.S. lakes as partial analogues, and taking into account the overall lacustrine basin context of a widely fluctuating, wave-influenced, alkaline-lake shoreline, we again interpret many of the sand bodies to be fluvial in origin. Several sand bodies both truncate and are capped by brown to red-maroon and variegated weak to noncalcareous mudstone with root and desiccation structures, indicating terrestrial deposition well away from the lake shoreline. Others display steep cutbanks from which noncalcareous,
    inclined heterolithic stratification laterally accreted as fluvial side bars. In contrast, utilizing helicopter-based Light Detection and Ranging (LiDAR), we investigated additional sandbodies that may be better examples of deltaic mouth bars. In contrast to the more commonly documented highstand progradational mouthbars of marine and open lake settings, these sand bodies are interpreted to have originated as late-lowstand or transgressive system tract fluvial channels that were then flooded and modified by waves following lake
    transgression. The examples illustrate that any large-scale sandy bed form present in the general vicinity of a closed basin's fluctuating lake shore may be expected to have formed under more than one set of environmental conditions. A revised set of guidelines is therefore presented to aid in the interpretation of lacustrine deltaic mouth bars.
    Original languageEnglish
    Title of host publicationFrom Saline to Freshwater
    Subtitle of host publicationThe Diversity of Western Lakes in Space and Time
    EditorsSW Starratt, MR Rosen
    PublisherGeological Society of America
    Number of pages28
    VolumeSpecial Paper 536
    ISBN (Electronic) 9780813795362
    Publication statusPublished - 2 Apr 2019


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