AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

Tidal to Fluvial Transition of the Sego Sandstone, Book Cliffs, Utah, USA


Tide-dominated deltas are more poorly understood compared to their fluvial- and wave-dominated counterparts, partly because fluvial-tidal interaction results in more complex depositional processes and less systematic facies trends compared to those that characterize fluvial- and wave-dominated deltas. Hence, there is still a need to develop more widely applicable facies models of such systems. In this study, we document the vertical and lateral architecture of offshore, delta front and delta plain deposits in the proximal, tide-dominated deltaic strata of the Sego Sandstone in the vicinity of Green River, Utah, USA. Here, the net-regressive Sego Sandstone is underlain by marine mudstones of the Mancos Shale (Buck Tongue) and overlain by coal-bearing delta plain deposits of the Neslen Formation. The Sego Sandstone is subdivided into the Lower Sego Sandstone, containing four coarsening-upward regressive successions bounded by flooding surfaces, which is separated from the Upper Sego Sandstone by a prominent marine mudstone (Anchor Mine Tongue). The eastern (paleoseaward) part of the study area is represented by a typical coarsening-upward regressive succession, comprising offshore mudstones that grade upward into lower shoreface deposits overlain by laterally extensive, inter-channel tidal bar deposits across a low-relief erosion surface. Tidal bar deposits contain cross-bedded sandstones with mud drapes and mud clasts, and heterolithic lenticular and wavy bedding. The bar deposits are locally overlain by lithologically similar channel-fill deposits with several meters of erosional relief at their bases. Towards the west (paleolandward), inter-channel tidal bar deposits pass over distances of 5-10 km into fining-upward tidal flat successions that comprise, from base to top, sand-, mixed- and mud-flat deposits. The marine mudstone that separates the Lower and Upper Sego Sandstone passes westward into lower shoreface deposits. Tidal flat deposits in the Upper Sego Sandstone are traced further west (paleolandward) into delta plain deposits of the Neslen Formation, which comprise coal, carbonaceous shale, mudstones, and inclined heterolithic strata. This study documents in detail the facies characteristics and stratigraphic architecture of a part of the Sego Sandstone that records the transition between delta front and delta plain deposits within an overall tide-dominated delta depositional setting.