ABSTRACT

The distribution of the Ostracoda from the Shubuta Clay and the Red Bluff Clay of the east-central Gulf Coast region is analyzed in an attempt to evaluate the boundary between the Jacksonian and Vicksburgian Stages.

Of 39 species in the type Shubuta Clay, Clark County, Mississippi, at least ten continue into the overlying Red Bluff Clay. They are: Actinocythereis gibsonensis (Howe and Chambers), Alatacythere ivani Howe, Bairdia hiwanneensis Howe and Lea. Bythocypris gibsonensis Howe and Chambers, Cytherella hannai Howe and Lea, Digmocythere russelli (Howe and Lea). Echinocythereis jacksonensis (Howe and Pyeatt), Krithe hiwanneensis Howe and Lea, Loxoconcha inflata Howe and Howe, and Paracypris media Howe and Howe. Six additional Red Bluff ostracods are direct evolutionary descendants of Shubuta species.

Approximately two-thirds of the Pachuta Marl (Jacksonian) ostracods continue up into the Shubuta Clay, whereas slightly less than 50 percent of the Shubuta ostracods are found in the overlying Red Bluff Clay. At least 70 percent of the Red Bluff ostracods continue unmodified or as direct evolutionary descendants into the overlying Vicksburgian sediments, supporting the view that in the east-central Gulf Coast the Jacksonian-Vicksburgian boundary should be placed at the base of the Red Bluff Formation. This boundary coincides with the first appearance of the following diagnostic ostracods: Actinocythereis dacyi (Howe and Law), Actinocythereis quadrataspinata (Howe and Law), Actinocythereis rosefieldensis (Howe and Law), Actinocythereis thomsoni (Howe and Law), Buntonia n. sp., Hermanites blanpiedi (Howe), Leguminocythereis verrucosus Howe and Law, Occullocythereis kempi (Howe and Law), Puriana n. sp. aff. P. elongorugata (Howe), Quadracythere vicksburgensis (Howe and Law), and Quadracythere weaveri (Howe and Law). These are distinctive Vicksburgian species whose ranges do not extend down into the underlying Shubuta Clay.