Abstract: Recognition of Middle and Upper Jurassic Depositional Sequences from Palynologic and Sedimentologic Evidence, Western and Central Wyoming

William J. Wade, Robert S. Van Pelt

Two depositional sequences can be distinguished within the Twin Creek Limestone of western Wyoming based on palynologic and sedimentologic criteria. Correlation of these sequences in the Twin Creek trough with coeval sequences on the central Wyoming sheff depends primarily upon sedimentologic interpretations of outcrop sections and cores.

Variations in Twin Creek lithologies and types of organic matter are predictable within a sequence stratigraphic framework. In the Twin Creek trough, lowstand wedges consist of interbedded sandy oolitic grainstones and red argillaceous siltstones; their contained organic matter is primarily inertinite. Overlying transgressive and high stand systems tracts both consist of silty argillaceous wackestones. Transgressive intervals are characterized by increasing dinoflagellate species diversity and amorphous debris, while highstands are characterized by decreasing marine palynoflora and amorphous debris and increasing land plant detritus and inertinite.

The lower and upper sequences in the Twin Creek trough of western Wyoming are correlated with the Gypsum Spring and lower Sundance sequences, respectively, on the central Wyoming shelf. Lowstand wedges in the trough are correlative to sequence boundaries capping each sequence on the shelf. Shelf transgressive and highstand systems tracts consist of evaporites, thin dolomudstones, and silty red and green shales (Gypsum Spring sequence), or green silty calcareous shale with intercalated fine sandstones (lower Sundance sequence).

AAPG Search and Discovery Article #90952©1996 AAPG Rocky Mountain Section Meeting, Billings, Montana