JOHNSON, STEVE, Department of Earth Sciences, University of Liverpool, Liverpool, England

High Resolution Sequence Stratigraphy of the Upper Wedge of the Mesaverde Group (Cretaceous), Big Horn Basin, Wyoming

The Clagget Shale and overlying Judith River and Parkman Formations in the Big Horn Basin, Wyoming form a clastic wedge within the upper part of the Mesaverde Group. Sequence stratigraphic analysis reveals a complex history dominated by regionally widespread sequence boundaries at the base of the Judith River and Parkman Formations (Teapot Sandstone). These surfaces define 3rd order sequences within the wedge and provide a framework for consideration of higher order sequence stratigraphic signatures.

The Clagget Formation commences above a maximum flooding surface and records highstand progradation of storm-wave dominated parasequences. Immediately below the first sequence boundary is a sharp-based shoreface sandstone which exhibits growth faults and large scale load-cast deformation. This unit is interpreted as a forced regression, recording a basinward shift within the depositional system that is attributed to a 4th order sequence boundary and may be considered to herald the overlying 3rd order sequence boundary.

The Judith River Formation commences with an incised valley System that includes fluvial deposits, but is dominated by tidally influenced estuarine deposits which accumulated during relative sea level rise. The incised valley is overlain predominantly by coal-bearing coastal plain systems. High order sequences can be recognized in this interval by pronounced, regional scale changes in the character of fluvial systems in the coastal plain and the architecture of their deposits. The Teapot Sandstone is a thick, fluvial-dominated incised valley system the fill of which records, at least in part, higher order fluctuations in relative sea level.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.