ABSTRACT

Evidence for the Pliocene age of units, previously considered Miocene, in limey deposits of the eastern Florida Panhandle first emerged in the 1970's. A wide gap still exists in the generally recognized distribution area of Pliocene marginal marine and shelf deposits between these calcareous units and the Mississippi delta area to the west. Drill sample studies aimed at narrowing this gap revealed several siliciclastic lithosomes of Pliocene age within a dominantly continental, highly brackish sequence between 125-460 ft, east and south of Mobile Bay. Planktonic forams in these fine-grained sediments included Globigerina riveroae (upper part of planktonic zone N. 18 and Zone N. 19), G. nepenthes , Pulleniatina praecursor and Globorotalia dutertrei. Pliocene ostracodes Loxoconcha edentonensis, Malzella devexa, Puriana mesocostalis and the typical Jackson Bluff bivalve Nuculana trochilia were also present.

Several nannoplankton taxa (Discoaster asymmetricus, Reticulofenestra pseudoumbilica and Sphenolithus neoabies), exclusively or nearly exclusively of Pliocene age (Bolli and Saunders, 1984) occurred in the shelf units but most taxa ranged into the Miocene or (in the case of Helicosphaera sellii) Pleistocene. The locally abundant Cretaceous forms and the Late Miocene Discoaster quinqueramus indicate reworking from older deposits. The marine Pliocene and correlative brackish deposits are designated as the new Perdido Key Formation.

A paralic, siliciclastic coastal Mississippi sequence above the Rangia johnsoni-microjohnsoni faunizone (widely regarded as coextensive with the Pascagoula Formation or Member) has been named by Brown et al. (1944) the Pliocene Graham Ferry presence of Cyrtopleura costata (in mold form) is confirmed. A Hemphillian (U. Miocene - L. Pliocene) vertebrate fauna in N. Mobile County has not yet proven the Pliocene age of the enclosing deposits either.

For continental and brackish sedimentary sequence of poorly defined age and stratigraphic position in the three-state area the term "undifferentiated Neogene clastics" has been recommended, coupled with designation of the directly underlying firmly defined horizon (e.g., "undifferentiated post-Amos Neogene clastics"), to replace a number of questionable formation names.