IDEALIZED STRATAL COLUMN FOR A MIXED CARBONATE/CLASTIC SYSTEM; CASPER FORMATION, SOUTHEASTERN WYOMING, USA

In general, a sequence stratigraphic model is a tool to provide a chrono-stratigraphic framework that can be used to correlate sedimentary facies and predict vertical and lateral relationships of lithofacies preserved in clastic shoreline stratal sections (Emery and Myers, 1996). The commonly accepted fundamental principles on which the model is based involve changes in relative sea level and accommodation and the stratigraphy that results from these changes. The original model employs the concepts of sequence boundaries, parasequences and systems tracts to define the subtleties and was conceived from siliciclastic strata. From these data, generalized sequence stratal columns were constructed to illustrate the expected juxtaposed units and their recognizable characteristics.

Subsequent to the original siliciclastic model, there have been a growing number of studies in aeolian strata (e.g., Driese and Dott, 1984), carbonate coastline rocks (e.g., Gianniny and Simo, 1996) and mixed systems (e.g., Miller and Heller, 1994). Although the specific sedimentological signatures and significant surfaces are expressed differently depending on the depositional environments involved (Emery, 1996), these models are all based upon the original concepts and underlying principles of the original. The results of these studies were also used to construct ideal stratal columns to facilitate field recognition of these types of environments.

Although mixed siliciclastic/carbonate systems have been studied, there are a large variety of possible combinations based on the types of depositional environments. For example, one type of combination is an aeolian dune/carbonate platform, a coupling that has not been previously studied in terms of sequence stratigraphic models. The Casper Formation in southeastern Wyoming is a set of strata that was developed from this type of juxtaposition of a dune field and an offshore, epeiric sea carbonate platform. Using drilled core, outcrop and petrographic data, a sequence stratigraphic model has been developed for this type of mixed siliciclastic/carbonate system. From this, an idealized stratal column has been constructed, one that contains multiple lag surfaces, a physical representation of a maximum surface of regression and a series of shoaling upwards carbonate subunits.