CORRELATING THE COOK MOUNTAIN: A SYNTHETIC APPROACH TOWARDS A REGIONAL STRATIGRAPHIC FRAMEWORK, MIDDLE EOCENE (LUTETIAN – BARTONIAN) OF MISSISSIPPI AND ALABAMA

The middle Eocene (Lutetian – Bartonian) Cook Mountain Fm. of Mississippi was deposited on a shallow carbonate shelf during a regional sea level highstand and in association with the Middle Eocene Climate Optimum. The formation is useful for subsurface mapping due to its widespread extent and disconformable lower contact with the Kosciusko Fm., readily observable on wireline logs where carbonates rest sharply atop a shale marker. In outcrop, this surface is evident where the sandy wackestones of the Archusa Mbr. overlie the Dobys Bluff Tongue of the Kosciusko. Less apparent is the expression of this surface and adjacent strata at classic middle Eocene sections in Alabama, where equivalent sediments are mapped within an undifferentiated Lisbon Fm.

While typically correlated with carbonaceous sands and clays within the “middle” Lisbon, sporomorph taxa from the Dobys Bluff Tongue suggest an equivalence with the basal Cubitostrea sellaeformis Zone, or “upper” Lisbon, within the Nuxpollenites crockettensis Assemblage Zone previously recognized in the Crockett Fm. of Texas. At Little Stave Creek (Clarke County, AL) these beds are contained within calcareous nannofossil zone NP16, while the “middle” Lisbon rests firmly within zone NP15 based on the occurrence of Chiasmolithus gigas. Subsurface cross-sections demonstrate the lateral persistence of Cook Mountain carbonates, while terrigenous sediments within the Kosciusko condense into a relatively thin sand and marl unit exposed in the AL outcrop belt. Here, field relations indicate that the basal “upper” Lisbon is capped by a silty clay bed and associated firmground burrowed with irregular Thalassinoides and sparse Gyrolithes. We suggest the equivalence of this surface with the base of the Cook Mountain in MS, which is itself expressed as a firmground bored with Gastrochaenolites.

From a genetic viewpoint, the “upper” Lisbon and Dobys – Cook Mountain interval records a widespread transgression that occurred after the close of the Sparta depositional episode, with the base of the Cook Mountain interpreted herein to represent a maximum flooding surface. This revision is conceptually similar and biostratigraphically consistent with the modern treatment of the Crockett Formation in Texas and will aid in the regional correlation of this classic middle Eocene section.