Evaluation of unconformities in upper jurassic and lower Cretaceous nonmarine deposits, Bighorn Basin, Wyoming and Montana, U.S.A.

Abstract

Although unconformities are common in ancient fluvial sequences, their age significance is often difficult to determine. Where absolute age control is lacking, various physical criteria can be used to evaluate the relative duration and significance of major unconformities. In an area which encompasses over 10 6 km 2 of the west-central United States, a major unconformity is inferred to separate the Morrison Formation of Late Jurassic age from overlying Early Cretaceous conglomeratic sandstones. Previous studies have suggested that the disconformity at this contact may represent as much as 40 Ma. However, physical sedimentologic observations of the Morrison Formation and the overlying Cloverly Formation in the Bighorn Basin of Wyoming and Montana do not support this interpretation. Sandstone composition, clay mineralogy and fluvial architecture show gradational and intraformational changes upsection, rather than an abrupt change at the Morrison/Cloverly contact. In addition, scour surfaces are common within each formation and do not necessarily occur at the formation boundary. The results of this study complement recent absolute and relative age dating which suggests that the Morrison Formation is partially of Early Cretaceous (Neocomian) age, and that the overlying Lower Cretaceous conglomeratic sequence (which includes the lower part of the Cloverly Formation) may be older than Aptian and possibly of Late Neocomian age. The upper part of the Morrison Formation and the lower part of the Cloverly Formation may therefore represent continuous, although episodic, deposition of a fluvial system as it evolved through time.