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Department of Geology and Geophysics, The University of Wyoming, Laramie, WY 82071-3006

Abstract: Stratigraphy, Sedimentology, and Paleontology of the Hell Creek Formation (Maastrichtian) Bowman County, North Dakota

The Hell Creek Formation (Lancian: ~67-65 Ma) on the eastern flank of the Cedar Creek Anticline, southwestern North Dakota, provides an opportunity to study the latest Cretaceous sedimentary history of this area. In this area, the lower 60 m of the Hell Creek Formation (HCF) is comprised of amalgamated, multistorey, channel-form sandbodies, and associated carbonaceous shale. The primary component of these sandbodies is inclined heterolithic stratification (IHS), with lesser amounts of trough cross-stratification, wavy and lenticular bedding, and current ripple stratification. The upper 30 m of the HCF in the study area is characterized by a decrease in the sand/shale ratio (<0.75 vs. >1.20), an increase in pedogenic features and a decrease in the thickness of channel sandstones. These features suggest coastal plain deposition, with increased accommodation creation in response to sea level rise.

Ophiomorpha occurs in estuarine barforms and is associated with fossil logs containing Teredolites borings, suggesting deposition within the brackish water portion of the estuary. Additional ichnogenera preserved within the IHS deposits include Arenicolites, Skolithos, Polykladichnus, and Cylindrichnus. Coastal plain deposits of the upper HCF contain Fe-oxide coated rhizoliths and terrestrial arthropod burrows, occasional fossil logs and in-place stumps. Myledaphus bipartitus, Lissodus selachos, and indeterminate hybodonts are found within estuarine barforms, but are absent from the overlying coastal plain deposits.

Recognition of previously undescribed estuarine and brackish water deposits within the lower part of the exposed HCF in southwestern North Dakota suggests a more complex interplay of sea level and sedimentation than has generally been supposed.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana