REDBEDS OF THE UPPER ENTRADA SANDSTONE, CENTRAL UTAH: FACIES ANALYSIS AND REGIONAL IMPLICATIONS OF INTERFINGERED SABKHA AND FLUVIAL TERMINAL SPLAY SEDIMENTS

The western extent of the Entrada Sandstone has been described as laterally continuous “earthy” red beds, and categorized as sub- to supra-tidal marine influenced sediments. Recent workers have reexamined the sedimentary facies hosted by the Entrada Sandstone, and findings suggest purely terrestrial depositional environments. The purpose of this study was to collect detailed outcrop measurements of these and other facies present in the upper Entrada Sandstone and to place them in context within a regional sedimentary system. Sedimentary features and bedforms have been well-preserved, and stacking patterns of sedimentary facies allow precise identification of depositional settings. These ranged from eolian, to fluvial subcritical, critical, supercritical, and waning flow, to desiccation and paleosol development. These settings were interpreted as belonging to sabkha, overbank splay/paleosol, distal terminal splay, and hyper-distal terminal splay environments. Ancient analogs include the Blomidon, Skagerrak, and Ormskirk Formations, which have been described as dryland fluvial systems that terminated onto saline mudflats. Modern analogs were found in fluvial terminal splays in ephemeral Lakes Eyre and Frome, Australia. The sedimentary system of the upper Entrada of the San Rafael Swell is interpreted as an interfingering fluvial terminal splay and inland sabkha system. These are marked by a wide array of sedimentary structures. During arid conditions, the only source of water was evaporative pumping of a high water table. During rare occasions when surface water flowed through the system, extreme flooding events produced the highest-criticality flow conditions yet described in geological literature. Supercritical bedforms observed include antidunes, chutes and pools, and cyclic steps. The succession of these facies reveals allogenic and autogenic processes that were active at the time of deposition, including episodes of tectonic uplift and fluvial system avulsions.