Tidal to fluvial transition of the Sego Sandstone, Book Cliffs,
Utah, USA
Marijn van Cappelle1*, Howard D. Johnson1 and Gary J. Hampson1
1Department
of Earth Science and Engineering, Imperial College London, Royal School of
Mines, Prince Consort Road, London SW7 2BP, UK.
*contact: m.van-cappelle12@imperial.ac.uk
Tide-dominated deltas are more poorly understood compared to their fluvial- and wavedominated 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 finingupward 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.