Depositional Dynamics of Almond Formation, Rock Springs Uplift, Wyoming: ABSTRACT

Abstract

Abstract In the Rock Springs uplift of Wyoming the Upper Cretaceous Almond Formation consists of a lower alluvial unit and an upper transitional and marine unit which displays cyclic deposits. Upper Almond cycles display the following sequence of deposits from the base upward: (1) marine and/or lagoonal shale, (2) barrier-island sandstone, (3) marsh or mudflat deposits, and (4) lagoonal-bay deposits. These rhythms reflect the lateral shifting of three contemporaneously existing depositional entities: (1) the marine environment in which surfzone and infra-surfzone sands, and offshore muds accumulated; (2) the barrier-island environment consisting of foreshore beach, backshore beach, and fringing marsh or mudflat deposits; (3) the lagoonal-bay environment in which predominantly fine-grained sediment, carbonaceous shale, 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 barrier beach was formed. Evidence indicates that seaward growth of barrier islands was accompanied by expansion of 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 volume of sand supplied to the seaward face was insufficient to permit further seaward growth. The operation of negative processes (subsidence, compaction, erosion, and possibly an independent rise in sea level) soon brought about the submergence or drowning of an abandoned barrier island. 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. The mechanism outlined above would produce a lateral and vertical juxtaposition of potential reservoir sand bodies and organic-rich muds. It seems likely that natural stratigraphic traps would be created after consolidation of the muds.