Stratigraphic Overview of Upper Cretaceous (Early Campanian-Late Maestrichtian) Montana Group, Powder River Basin, Wyoming: Implications for Complex Interplay Between Eustatic Sea Level Fluctuations, Sedimentation Rates, and Intraforeland Basin Subsidence

Edmund R. Gustason, Paul E. Devine, Jon McClurg, Charles J. Rappold

Isopach maps of chronostratigraphic and lithostratigraphic units from the Late Cretaceous (early Campanian-late Maestrichtian) Montana Group of the Powder River basin, Wyoming, reveal a complex interplay between eustatic sea level fluctuations, sedimentation rates, and intraforeland basin subsidence rates. The Montana Group is characterized by numerous asymmetrical, coarsening- and thickening-upward, progradational deltaic, strand-plain, and/or shallow-marine deposits that thin eastward and merge into thick offshore-marine and pelagic deposits of the Pierre Shale. From oldest to youngest these are the Gammon, Shannon, and Sussex Members of the Steele Shale, the Parkman and Teapot Sandstone Members of the Mesaverde Formation, the Teckla Sandstone Member of the Lewis Shale, and the Fox Hills Sandstone. Formation tops and bentonite beds from approximately 30,000 well logs were correlated throughout the Powder River basin and adjacent areas.

The asymmetrical, coarsening- and thickening-upward character of these Upper Cretaceous clastic wedges indicates that regressions, which reflect eustatic sea level lowering, are marked by a combination of relatively high sedimentation rates and foreland basin subsidence, whereas transgressions are marked by relatively low sedimentation rates and movement on basement faults within the foreland basin. The "shelf sand ridges" of the Gammon, Shannon, and Sussex Members of the Steele Shale may have formed during the early phases of transgression following periods of sea level lowstands.

Isopach maps of chronostratigraphic intervals reveal that subsidence rates were as much as seven times greater in the southern and southwestern parts of the Powder River basin. The area of greatest subsidence was in the vicinity of the present-day Hanna basin, south-southwest of the Powder River basin. However, most shoreline trends and shelf-to-basin slope breaks at the time of peak regression are oriented north-south or are perpendicular to isopach contours, indicating sedimentation rates in the southern part of the Powder River basin kept pace with subsidence rates in this intraforeland basin. Variations in shoreline trends through time reflect fluctuations in the balance between sedimentation rates and intraforeland basin subsidence rates.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.