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The Cook Mountain Problem: Stratigraphic Reality and Semantic Confusion
Thomas E. Ewing
Historical inconsistency as to what constitutes the Cook Mountain Formation illustrates the semantic confusion resulting from extending surface-derived stratigraphic names into the subsurface without a full understanding of basin architecture.
At the surface, the Cook Mountain Formation consists of fossiliferous marine shale, glaucony and marl, and marginal-marine sandstone and shale between the nonmarine Sparta Formation sandstones below and the nonmarine Yegua Formation sandstones and lignitic shales above. Fossils are abundant, including the benthic foraminifer Ceratobulimina eximia.
As subsurface exploration began, the first occurrence of Ceratobulimina eximia ("Cerat") was used as the top of the marine "Cook Mountain Shale" below the Yegua section. Downdip, the overlying Yegua was found to become a sequence of marine shales and marginal-marine sandstones, the lower part of which yielded "Cerat." Because of this, the lower sandstones were called "Cook Mountain" in many fields. At the Yegua shelf margin, "Cerat" is absent. Different exploration teams have used their own definitions for "Cook Mountain," leading to substantial confusion.
Regional sections show that the entire outcropping Cook Mountain Formation is correlative downdip with one cycle of marine shale, silt, and marl, which lies well below any "Yegua" or "Cook Mountain" sandstone beds. All "Yegua" and "Cook Mountain" sandstones correlate updip to nonmarine Yegua Formation. However, by lithostratigraphic criteria, all of these sandstones could be included in the Cook Mountain Formation.
In general, formal lithostratigraphic units are not useful in understanding the subsurface geology of clastic shelf-margin sequences. Subsurface exploration relies on identification of sequences that record contemporaneous depositional processes and resultant facies. Usually, one identifies regional flooding surfaces to approximate time lines. In this light, I propose using the term Cook Mountain in the subsurface for (1) the Top Sparta flooding surface and (2) the outcrop-correlative cycle. Sandstone bodies above the Cook Mountain cycle should be considered Yegua sandstones.
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