STRATIGRAPHY AND DEPOSITIONAL ENVIRONMENT OF THE MESA VERDE FORMATION IN THE NORTHERN BIGHORN BASIN OF WYOMING

A sedimentological, stratigraphic and palynological study was conducted on the upper Cretaceous (Campanian) Mesa Verde Formation 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 Mesa Verde Formation and to integrate findings with data from outcrop belts to the south (southern Bighorn Basin), north (Montana) and east (Powder River Basin).

The Mesa Verde Formation (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; where abundances are high, there is a high dominance of an individual palynomorph. There is, however, a wide range in types and abundance of phytoclasts present. Paleocurrent indicated a dominantly southeastward direction of sediment dispersal.

Throughout much of the unit, coarsening-upward cycles are interpreted as the product of deltas that prograded southeastward into a shallow sea. In the upper part of the unit, shorter coarsening-upward cycles may indicate progradation of coastal systems into lagoons or bays. Sharply-based sandstones are interpreted as the product of incised tidal channels and coastal fluvial distributary channels, and are interpreted to overlie sequence boundaries. The internal stratigraphy of the Mesa Verde group is comparable to that of adjacent areas, with the Eagle Member, the Claggett Tongue of the Cody Shale and the Judith River Group culminating in the Teapot Sandstone Member all present. A series of major flooding surfaces form excellent correlation horizons. Some sandstones within the Eagle Member thin and pinch out towards the south. The Claggett Member also varies in thickness considerably over the study area.