Date of this Version
A sedimentological, stratigraphical and palynological study was conducted on the upper Cretaceous (Campanian) Mesaverde Group over 27.5 km of a north-south-orientated outcrop belt in the northern Bighorn Basin, Wyoming. During the Campanian, the study area lay in the western part of the Cretaceous Western Interior Seaway. The study aims to provide a stratigraphic and paleoenvironmental analysis of the Mesaverde Group and to integrate findings with data from outcrop belts to the south (southern Bighorn Basin), north (Montana) and east (Powder River Basin).
The Mesaverde Group (371 - 515 m thick) comprises a succession of mainly coarsening-upward stratal cycles, with some anomalous, sharply-based sandstone bodies that are out of context with respect to underlying facies. The trace fossil assemblage is typically of low diversity with only a few ichnotaxa recurring throughout the succession. The palynofacies analysis shows predominantly low abundance of palynomorphs and dinoflagellates indicating a high energy environment. There is, however, a wide range in types and abundance of phytoclasts present. Paleocurrent data indicate a dominantly southeastward direction of sediment dispersal with bi-modal sediment dispersal patterns within individual facies.
This study indicates a similar lithological assemblage to previous studies of the Mesaverde Group; however the interpretation of depositional environment from this study points towards a dominantly deltaic depositional environment, something that only a subset of previous studies has done. The depositional environment changes vertically throughout the section from distal to proximal, with facies associations ranging from basinal to coastal plain. The palynofacies analysis performed in this study is consistent with the lithofacies analysis but also reveals some flooding events that are not revealed by the lithology. Finally, four sequence stratigraphic cycles are interpreted throughout the Mesaverde Group identified by incised lithosome bases and major dislocation of facies. Overall this study provides a basis for future study on the Mesaverde Group and allows for a more detailed paleoenvironmental evaluation of the unit.
Advisor: Christopher R. Fielding