ABSTRACT

The Choctawhatchee (late Miocene) deposits exposed at Jackson Bluff, on the Ochlockonee River, are composed of two fossiliferous units separated by a slight erosional disconformity.

Comparison of fossil molluscan and foraminiferal assemblages with extant communities in the Gulf of Mexico, western Atlantic and Caribbean indicates that the Choctawhatchee sediments were deposited in an open marine, near-shore shallow to intermediate shelf zone, at depths of less than 21 fathoms.

The lower part of the lower unit ("Ecphora facies") is transgressive over the nonmarine Hawthorne (medial Miocene?), and the deposits representing maximum water depth for the section lie a few feet above the base of this unit. The upper part of the lower unit was deposited under shoaling conditions. The overlying unit ("Cancellaria facies") is transgressive, but was deposited at a depth of less than 8 fathoms.

The terms "Ecphora facies" and "Cancellaria facies" as applied to this section, are rejected.

Comparison of the Jackson Bluff Choctawhatchee deposits with those at Alum Bluff, Liberty County, Florida, indicates that the lower unit at Jackson Bluff is contemporaneous with Units 2 and 3 ("Ecphora shell bed") and Unit 4 (Aluminous clay) at Alum Bluff, and that the upper unit at Jackson Bluff is contemporaneous with the upper Choctawhatchee sand (Unit 5) at Alum Bluff.