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The mid-Cretaceous (Albian-Cenomanian) boundary in the central United States traditionally is placed at a major unconformity that records a withdrawal of marine waters from the Western Interior Seaway. Although this unconformity can be located with reliability in many areas of western North America and the Gulf Coast, its placement along the eastern margin of the interior seaway is more problematical. Largely as a matter of convenience, unconstrained by biostratigraphic data, the base of Cenomanian strata has been placed at the base of the sandstones of the Dakota Formation in its type area of eastern Nebraska and western Iowa, where they overlie Paleozoic and Precambrian rocks. Palynological evidence from Dakota sections in the type area now suggests that the mid-Cretaceous unconformity may instead be traceable to a horizon within the formation, separating the upper Woodbury Member from the lower Nishnabotna Member. If this interpretation proves to be correct, the Nishnabotna is chronostratigraphically and depositionally equivalent to Albian units such as the Newcastle and Muddy Sandstones of Wyoming and the subsurface “J” Sandstone of the Denver Basin. Given the historical development of the use of the term “Dakota,” this suggested interpretation also calls into question the advisability of continuing to include the Nishnabotna Member within the Dakota Formation.

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