--> --> A New Stratigraphy for the Pierre Shale at the Cedar Creek Anticline, Montana

AAPG Rocky Mountain Section Meeting

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A New Stratigraphy for the Pierre Shale at the Cedar Creek Anticline, Montana

Abstract

The Cedar Creek Anticline (CCA) is an asymmetrical, plunging, northwest-trending fold at the southwest edge of the Williston Basin. It stretches from the northwest corner of South Dakota through the southwest corner of North Dakota to the Yellowstone River in east central Montana. It is complex and difficult to understand, attributed to several major geological events beginning in the Devonian and culminating in the early to mid- Paleogene. Most of the stratigraphic research at the anticline has been focused on subsurface, petroleum-bearing layers. However, there is also a fascinating surface exposure of Cretaceous Pierre Shale (and layers above) that has attracted research as far back as the 1850s with F. Hayden and others. Although the basic stratigraphy of the Pierre at Red Bird, Wyoming, the CCA, and elsewhere was worked out more than 50 years ago by James Gill, Bill Cobban, Gale Bishop, and others prior to them, many questions and uncertainties have remained due to extensive slumping of the Pierre, variation from site to site, and the paucity of index fossils at key points. We, with early mentorship from Bill Cobban, including in the field, have continued studying the Pierre at the CCA for over 30 years and at more than 120 sites, focused on the northwest fifth of the CCA. Accumulated and recent findings of a sufficient number of critical specimens have permitted us to refine the stratigraphy, including a precise placement of the Campanian-Maastrichtian boundary. The new findings also allow us to more accurately describe the marine paleoenvironments associated with the new stratigraphy and better place this region into the larger context of the Cretaceous Western Interior Seaway. We also comment on the present-day geomorphology of the CCA and how it might have attained its modern form.