Sedimentary Models, Cycles, and Deltas, Upper Cretaceous, Wyoming

Abstract

Sedimentary models developed from modern sedimentation alone are of limited usefulness to the petroleum geologist because of (1) lack of variation resulting from deposition during a consistent trend of sea-level change; (2) poor control in the vertical dimension, particularly in intermediate and deeper water environments; and (3) the lack of direct association with petroleum accumulation. More widely useful models can be derived from certain ancient rock sequences, utilizing principles based on modern sedimentation. Good outcrops, abundant well control, bentonite marker beds, and dominantly stratigraphic oil and gas production make the Upper Cretaceous formations of Wyoming particularly useful in providing models pertinent to both industry and academic needs. The Ericson Sandstone of southwestern Wyoming and its nonmarine and marginal-marine equivalents can be combined with marine elements of the model from eastern Wyoming to produce a complete wide-shelf model consisting of fluvial, paludal, barrier-island, shelf, slope, and basinal facies. Significant differences between the resulting model and previously proposed models are the podlike configuration, the presence of thick slope deposits, and the significant submarine topography at the epicontinental slope. A narrow-shelf model can be derived from the Lewis Shale, Fox Hills Sandstone, and Lance Formation of the Red Desert basin. The complete sedimentary cycle consists of the same units as the complete model. Only regressive sedimentary cycles have been recognized. The maximum marine transgressive shift is reflected in a basinal facies directly overlying the paludal facies of the Parkman in southwestern Powder River basin. The complete model includes a distinct fluvial facies deposited at the mouth of a river and is, by definition, a delta. An incomplete sequence consisting of paludal, barrier-island, and a thin shelf facies is interpreted to be interdeltaic. Several deltas can be delineated on this basis from the Ericson and time-equivalent rocks of Wyoming and adjacent states. When these deltas are related to sequences interpreted as deltaic in older and younger Upper Cretaceous rocks of the area, a shifting pattern of deltaic sedimentation similar to that of the modern delta complex is suggested. If this pattern of shifting loci of deposition is correct, some stratigraphic concepts may require reexamination.