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In the Rock Springs uplift of Wyoming, the late Cretaceous Almond formation consists of a lower alluvial unit and an upper transitional and marine unit which exhibits cyclic deposits. Typical cycles display the following sequence of units from the base upward: (1) marine and/or lagoonal shale, (2) barrier island sandstone unit, (3) marsh or mud flat deposits, and (4) lagoonal or bay deposits. These rhythms reflect the lateral shifting of three contemporaneously existing depositional entities: (1) marine environment--in which surfzone and infra-surfzone sands, and offshore muds accumulated; (2) barrier island environment--consisting of foreshore beach, backshore beach, and fringing marsh or mud flat deposits; (3) lagoonal environment--in which predominantly fine-grained se iment, carbonaceous material, and oyster reefs accumulated.
The barrier island sandstone units display a characteristic sequence of sedimentary structures that reveals their origin. This succession of primary features records the building-up of the sea floor until it emerged and a beach was formed.
Evidence indicates that seaward growth of barrier islands was accompanied by an expansion of the lagoons which resulted in a progressive flooding of the landward margins of the barriers. Thus positions previously occupied by a barrier island were successively blanketed with lagoonal deposits. As the distance between a seaward-advancing barrier island and the mainland increased, a threshold limit was approached beyond which the amount of sand supplied to the seaward face was insufficient to permit further seaward growth. The operation of negative processes (subsidence, compaction, erosion, and perhaps an independent rise in sea-level) soon brought about the submergence or "drowning" of an abandoned barrier.
The effects of a rapid transgression were mimicked as lagoons merged with the open sea and marine conditions were rapidly extended to the edge of the mainland. Emergence of a new chain of barrier islands near the mainland shore initiated another cycle. Thus rhythmic seaward migration, progressive isolation, cessation of growth, and submergence of barrier islands simulated the effects of slow regressions and rapid transgressions and produced the cyclic deposits.
Similar ancient barrier island units are present in other Cretaceous formations both in the Rock Springs uplift and in numerous other localities.
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