Print this pageDepositional Environments and Ichnofossils of the Sundance Formation from Glendo Reservoir, Eastern Wyoming, Usa
The Sundance Formation is an important hydrocarbon reservoir throughout Wyoming, but there is little consensus as to the origin of the thick sands that comprise the reservoir facies. Over 2.5 km of reasonably well exposed, Upper Jurassic strata (“upper Sundance” and Windy Hill Member-equivalent) crop out along the northeast-facing cliffs northwest of Glendo Dam and on the western shore of Glendo Reservoir in eastern Wyoming. This succession represents offshore tempestites, sandy deltaic foresets, upward-coarsening mouth bars and compound dunes that accumulated during the final regressive phase of the Late Jurassic epeiric sea. Accretionary sets within deltaic foresets, bars, and compound dunes demonstrate an overall west-northwestward progradation of the shoreline. The preserved height of accretionary sets indicates water depths of 2-4 m, in accordance with the shallow slope of the passive, eastern margin of the Late Jurassic foredeep. Tempestites in the offshore were opportunistically colonized by crustaceans and/or polychaetes, resulting in ichnofossil assemblages characteristic of “doomed pioneers.” The silty, fine-grained, hummocky sandstones generated by storm waves contain robust Rhizocorallium, Asteriacites, Palaeophycus, Diplichnites, and Chondrites. Current-aligned coquinas of bivalve, belemnite (Pachyteuthus sp.), and ammonite (Cardioceras sp.) shells are also preserved in these sandstones beds. In some examples, aligned belemnite shells accumulated within burrows as part of the passive fill matrix. Equilibrichnia of moderate-sized bivalves (Siphonichnus) are preserved in ripple and parallel-laminated, fine- to medium-grained sandstones of mouth bar deposits. Some examples show repeated adjustments of the bivalve in response to episodic, rapid accumulation of sand. Dinosaurian footprints are preserved on the surfaces of tidally-influenced mouth bars and represent the activity of 3-4 m long bipeds during bar exposure. Previous interpretations of sandstones in the “upper Sundance” Formation as representing shelfal tidal bars, derived from the west are not supported by architectural, sedimentological, or ichnological data. Instead, sands in the “upper Sundance” Formation at Glendo Reservoir represent deltaic and storm deposited shelfal sands transported from the east and southeast.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009