Abstract

Throughout the Rocky Mountain Region, numerous Upper Cretaceous intervals display the following sequence of deposits from the base upward: (1) marine and/or lagoonal shale; (2) barrier island sandstone; (3) marsh-mudflat deposits; (4) lagoonal-bay deposits; (5) alluvial-coastal plain sediments; and (6) alluvial-coastal plain deposits. These sequences record seaward progradation of coastal plain, transitional, and nearshore marine deposits in response to a large sediment supply and continuous subsidence.

The barrier island sandstone units display a characteristic upward sequence of infra-surfzone, surfzone, and beach deposits which reflect depositional regressions. The vertical succession of sedimentary structures in a barrier island sandstone records building up of the sea floor until a segment emerged and a barrier beach formed.

Sand enters the sea through deltaic distributaries and tidal passes and is transported downcoast by longshore currents and littoral or beach drift to nourish and prograde barrier island sand bodies. The geologic record is strongly biased toward preservation of case histories where large sediment supplies are recorded by seaward progradation of deltas and barrier islands.

Widespread sheets of sandstone and conglomerate, like the Ericson Sandstone of the Rock Springs Uplift and the Castlegate Sandstone of eastern Utah, represent braided piedmont plain deposits of sand and gravel spread beyond the bases of newly uplifted areas under conditions of humid to subhumid climates. These are commonly underlain and overlain by meander belt deposits consisting of broad silty-shale floodplain deposits and narrow channel sandstones and conglomerates. The upward sequence of braided alluvial sheets of sandstone and conglomerate and predominantly shaly meander-belt deposits records progressive headward erosion of tributaries and consequent expansion of drainage basins.