Paleoecology and Paleogeography of an Extensive Rhodolith Facies from the Lower Oligocene of South Georgia and North Florida

Abstract

An extensive areal occurrence of Lower Oligocene rhodolith limestone has been observed in cores and outcrops extending nearly 290 km along NE-SW trend from south-central Georgia to the Florida Panhandle. Maximum observed thickness of the limestone is 30 m. Such tremendous accumulation of fossil rhodoliths has not been previously described in the literature. The boundaries of the rhodolith facies coincide with the edges of the Suwannee Strait, Cretaceous to Neogene paleobathymetric low, and may be used in subsurface mapping to delimit the edge of the Strait. Most of the rhodoliths are Archaeolithothamnium with mean diameter of 5 cm and most (95%) are spherical (compact) in shape. Rhodolith shape plus their internal laminar growth pattern indicate exposure to high-energy environments, resulting in frequent movement of the algae. The occurrence of Archaeolithothamnium, along with the red alga Lithoporella and dasycladacean alga, suggest moderate to shallow water depth. Rhodoliths overwhelmingly dominated the marine biotic community because the coarse, mobile substrate they produced precluded the development of more diverse faunal assemblage. Species that did live within the rhodolith facies had specific adaptations for survival on such substrate including the ability to encrust (corals and ectoprocts), bore (Lithophaga), live interstitially (Lima), and ease of mobility (scallops, echinoids). INTRODUCTION The purpose of this paper is to show how the interpretation of outcrops and cores containing Oligocene age rhodoliths has aided in determining the position of buried seaway known as the Suwannee Strait. To make this determination, we first described an unusually large areal occurrence of an Oligocene rhodolith facies, and then determined the paleoecological implications of the rhodoliths, associated fauna, and sedimentary facies examined in outcrop and core. Using modern rhodolith deposits as guide, we inferred the position of the Suwannee Strait. The Strait has been described by Applin and Applin (1944) as a channel or trough extending southwestward across Georgia through the Tallahassee area of Florida to the Gulf of Mexico. Although the seaway existed from the Late Cretaceous (Hull, 1962) to Eocene (Chen, 1965; McKinney, 1984), the exact location of the Strait during the Tertiary has been matter of debate. Reports concerning coralline algal nodules, named rhodolites by Bosellini and Ginsburg (1971), can be found in the literature beginning in the late nineteenth century. Early studies (KjelLman, 1883; Foslie, 1894) dealt mostly with the descriptive and taxonomic aspect of these algae that grow as detached, individual, primary nodules. More recent work has been directed towards the ecological relationships of rhodoliths. To this end, several investigators (Logan, et al., 1969; Bosellini and Ginsburg, 1971; Adey and Macintyre, 1973; Focke and Gebelein, 1978; Bosence, 1985) have observed the nodules in Recent environments, thereby providing basic information for paleoenvironmental interpretations. Data from these studies have been used by others (Siesser, 1972; Toomey, 1975; Orszag-Sperber, Poignant, andPoisson, 1977; Pal and Dutta, 1979; Bourrouilh-le-Jan, 1979) to describe environmental conditions of ancient rhodolith-bearing strata. Two significant problems are encountered when attempting paleoenvironmental statements based on the presence of certain algal form. Because classification is based on cell and reproductive structures observed in thin section, identification of the genus can be quite difficult where replacement or dolomitization has taken place. Also, because most of the rhodolith genera have wide, overlapping ranges of ecological tolerances; depth zonation, light penetration, wave energy, and water temperatures cannot be sharply delineated. As result of these difficulties Adey and Macintyre (1973) state that in paleoecological studies many erroneous or