LATE EOCENE ZOOGEOGRAPHY OF THE EASTERN GULF COAST REGION

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Abstract
Calcareous upper Eocene (Jacksonian) sediments in southeastern Alabama, south-western Georgia, and Florida contrast markedly with stratigraphically equivalent terrigenous deposits from central Alabama westward. The enclosed fossils, chiefly marine invertebrates, permit subdivision of the Jacksonian Stage into stratigraphic units (zones) and geographic units (biofacies). The present study is concerned primarily with the abundance and distribution of invertebrate fossils, particularly cheilostome bryozoans, in the four major biofacies of the eastern Gulf Coast Jacksonian. Biofacies 1, containing abundant mollusks and smaller foraminifers in a matrix of terrigenous detritus, is characterized by bryozoans having free, discoidal (lunulitiform) zoaria. Biofacies 2 is composed almost entirely of skeletal elements of larger foraminifers and cheilostomes exhibiting erect, branching (eschariform), and encrusting (membraniporiform) zoaria. Biofacies 3, including a fauna dominated by lagenid and buliminid foraminifers, is devoid of bryozoans. Biofacies 4, incorporating abundant echinoids and foraminifers, contains a bryozoan fauna dominated by species having erect, jointed (cellariiform) zoaria. The boundaries between the major biotopes in which the biofacies accumulated were not stationary during the Jacksonian. In middle Jacksonian time biofacies 1 and 2 adjoined in central Alabama, 2 and 3 in eastern panhandle Florida, and 3 and 4 in northern peninsular Florida. Categorizing individual cheilostome faunules as associations has made possible reconstruction of the four major biotopes in harmony with known ecological requirements and tolerances of living cheilostomes. On this basis it is concluded that biofacies 1 and 2 accumulated on the continental shelf, biofacies 3 in a channel (Suwannee Strait), and biofacies 4 on a submarine plateau (Ocala Bank). The cheilostome faunas of shelf and bank were quite distinctive at the beginning of the Jacksonian. A native Gulf Coast fauna inhabited the shelf, and a fauna having many elements in common with western Europe dominated the bank. During Jacksonian time the faunas became increasingly similar through extinction of endemic species and interchange of longer-lived species as the strait became a lessj effective barrier to migration. In seeming contradiction to the widely held opinion that faunal migration may be geologically instantaneous where there is no major barrier, radial dispersal of some species within the confines of the Ocala Bank involved at least the amount of time necessary for the accumulation of a biostratigraphic zone. The Ocala Bank was a submarine feature comparable to the present Great Bahama Bank and was separated from the North American continental shelf proper by the Suwannee Strait, the Eocene analogue of the present Straits of Florida. During the greater part of Jacksonian time, the water covering the Ocala Bank was probably 50–100 feet deeper than the water over the present Bahamian platforms, so the Ocala fauna shows greater similarity to that of the submarine banks of the Gulf of Naples. Two genera and 18 species of cheilostomes are newly described. An additional 18 species are discussed or figured. The superfamily Microporacea is emended, and the superfamily name Scrupocellariacea is proposed to replace the “division” Cellularina.

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