--> --> Mancobrara Stratigraphy Unraveled: A tale of Mancos to Niobrara Formation stratigraphic continuity across the Coniacian/Santonian Western Interior Seaway

AAPG Rocky Mountain Section Meeting

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Mancobrara Stratigraphy Unraveled: A tale of Mancos to Niobrara Formation stratigraphic continuity across the Coniacian/Santonian Western Interior Seaway

Abstract

It has long been known that laterally time-equivalent sediments deposited in uppermost Turonian through Campanian time in the Western Interior Seaway vary in composition from west to east. Presently, this once continuous stratigraphic interval has been segmented into basins by Laramide tectonism and geographic areas by governmental entities resulting in many different names being given to the same stratigraphic interval. Complications arise when correlating from one basin to another. There is a desire by many geologists to push the eastern, carbonate dominated, prolifically oil and gas productive, Niobrara Formation name west into the realm of the clastic dominated Blue Gate Member of the Mancos Shale. Work on Western Interior biostratigraphy and radiometric dating confirms that at least part of the Mancos shale is time equivalent with the Niobrara Formation with no gap in deposition along this west to east transect. In the absence of a clear-cut distinction regarding which formation name to use due to the various lateral lithologies, the lure of a blended name such as “Mancobrara” is strong. This study is a synthesis of the work of many previous authors and is a clarification of the Niobrara to Mancos transition across the Douglas Creek Arch. It presents a western-side-of-the seaway look at what occurred while the Niobrara Formation was being deposited to the east.