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Abstract: The Cook Mountain Problem: Stratigraphic Reality and Semantic Confusion

Thomas E. Ewing

The Cook Mountain Formation (Fm) illustrates the semantic confusion resulting from extending surface-derived stratigraphic names into the subsurface without a full understanding of basin dynamics.

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 "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, Ceratobulimina eximia 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 horizons. 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 Fm.

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, which unite all co-occurring processes. 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. Sand bodies above the Cook Mountain cycle should be considered Yegua sands.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994

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