A Model for Migrating Shelf-Bar Sandstones in Upper Mancos Shale (Campanian), Northwestern Colorado

Abstract

The Duffy Mountain sandstone was deposited by migrating submarine shelf bars that are now completely encased in marine mudstones of the Mancos Shale (Upper Cretaceous). The shelf bars were deposited at least 10 mi (16 km) offshore and stood approximately 100 to 150 ft (30 to 46 m) above a shallow shelf. The bars were never emergent but were shallow enough for fair-weather waves to form ripples along their crests. These elongate sand bodies trend northeast-southwest, approximately parallel to the shoreline. The sediment was probably derived from reworking of transgressed littoral deposits in southwestern Wyoming. Four subenvironments are recognized: (1) landward flank of the shelf bar, the back bar; (2) the axial crest of central bar; (3) a seaward facing ramp subfacies; and (4) the shelf upon which the shelf bars migrated. Two major processes were active during deposition: (1) fair-weather longshore, southwesterly currents, and (2) storm erosion and westerly, across-the-shelf-bar, sediment transport. During fair weather, sand waves transported sediment along the central bar while the back bar and shelf were being bioturbated. Storms eroded the seaward face of the bar and portions of the central bar, creating hummocky erosion surfaces. Eroded sediment, transported across the bar, was deposited in coarsening-upward spillover lobes in the back bar. After the storm waned, sediments deposited in t e back bar were bioturbated and sand waves were deposited on the eroded central bar. Migration of sand waves onto the eroded face of the bar formed the ramp facies. As the shelf bar migrated, the central bar facies climbed stratigraphically over the back-bar facies. The direction of shelf-bar migration was probably controlled by the interaction of fair-weather and storm-sediment transport.