Vertically stacked barrier island systems, Sego Sandstone (Campanian), Northwest Colorado
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The marginal marine deposits of the Sego Sandstone (Campanian) accumulated along a broad 300-500 km wide embayment of the western margin of the Cretaceous Western Interior Seaway. Analysis of outcrops of the Sego Sandstone, emphasizing facies characteristics and relationships, record the vertical aggradation (more than 50 m) of interdeltaic-barrier island systems in the area north of Rangely Dome, Colorado. Though representing the basal deposits of the progradational Mesaverde Group, these vertically stacked barrier island systems reflect a relatively stable shoreline position resulting from the gross balance between basinal subsidence, eustatic sea-level changes, and longshore sediment supply. Seven lithofacies types are recognized within the Sego Sandstone. Those lithofacies reflecting similar inferred depositional processes which occur in close association, were considered genetically related and, for purposes of discussion, are grouped into one of three assemblages designated Lithofacies Assemblage I, II or III. Three lithofacies are included in Assemblage I, an amalgamated, upward-coarsening, sandstone unit. This unit has a predictable vertical lithofacies succession of 1) basal hummocky stratification grading upward into 2) small-scale troughs and 3) planar lamination, and is capped by a coal bed. This succession is interpretated to be a shoreface-through-backshore sequence. The trough cross-stratified, upward-fining sandstones of Assemblage II display pronounced erosional bases and are considered channel-fill deposits. The large- to medium-scale crossbeds have numerous reactivation surfaces, associated clay drapes and bimodal directional trends and are interpreted to be tidal inlet or inlet associated deposits. Faunal mixing further establishes a transition between open marine and lagoonal environments. Lithofacies Assemblage III is dominated by carbonaceous mudstone with subordinate amounts of bioturbated sandstone and ripple-stratified shaly sandstone which is often cut by channels. Collectively, these deposits are interpreted to be low-energy back-barrier accumulations, representing lagoonal/salt marsh, washover and estuarine environments.