ABSTRACT

An N45E shoreline trend and five major shelf facies were interpreted from surface and subsurface data in the upper Mancos (Campanian) shelf sands in northwestern Colorado. The central bar, up to 20 m thick, coarsens upward from fine to medium-grained sandstone containing large-scale landward-dipping accretion surfaces. Smaller cross-beds have reactivation surfaces and indicate two transport directions (west and southwest). Hummocky surfaces are also present. The ramp unconformably overlies the seaward edge of the central bar. Large-scale seaward-dipping ramp units have cross-strata indicating only westward transport. Up to 15 m of bioturbated back bar occurs landward and below the central bar. Overall, the back bar coarsens upward but contains (1) fining-upward sequences(< 2 m thick); (2) coarsening-upward sequences (< 2 m thick); and (3) bioturbated isolated sand waves. Paleocurrents were to the west. Shelf bioturbated clayey siltstones gradationally underlie the back bar and overlie the reworked bar top.

These facies represent large-scale migrating sand ridges with axes parallel to the shoreline. The dominant processes were: (1) major storms that eroded the seaward edge and crest of the sand ridges and transported sediment landward (westward paleocurrents). Back-bar deposits include storm washovers and graded beds. Channels, hummocky cross-strata, and hummocky erosion surfaces were formed on the bar crest. (2) Between storms, contour currents (southwesterly) transported sand onto and along the bar crest. The ramp facies represents sand waves climbing onto the eroded seaward face of the bar.

The sand-ridge migrated over the back-bar deposits causing that facies to thicken landward.

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