Occurrence of Plio-Pleistocene phosphatized macro-invertebrates from the upper west Florida slope, eastern Gulf of Mexico

Abstract

Numerous phosphatized internal molds of an articulate brachiopod, along with fossils from four other phyla (Cnidaria, Mollusca, Annelida, and Echinodermata), were collected from several dredge sites in the Gulf of Mexico. The samples were collected during two cruises aboard the RV Suncoaster in December 1989 and May 1993, approximately 250 km west-southwest of Tampa, Florida. These cruises were not designed to collect fossils, rather they were aimed at studying the origin of phosphorite nodules and hardgrounds developing in response to marginal upwelling of the Gulf of Mexico Loop Current along the western margin of Florida. The invertebrate fossils were collected as part of the phosphorite nodule dredge hauls and are the focus of this study. Mineralogical analysis of these internal molds, including a brachiopod and the largest of the three echinoid specimens, indicates they are composed of francolite, low-magnesium (Mg) calcite, aragonite, and variable amounts of quartz, which is consistent with nonferruginous nodules recovered from the same areas. Petrographic examination also identified glaucony and iron-oxyhydroxides (FeOOH) within the phosphatic molds. The timing of phosphogenesis associated with the fossils and nonferruginous nodules has been identified as occurring after a late Miocene (Tortonian) sea level lowstand, most likely during Plio-Pleistocene sea level highstands that favored marginal upwelling over the west Florida slope. We interpret the paleoecology of the fauna to represent a continental shelf environment of shallower depth than where the fossits were collected (511-520 m) along the west Florida slope. Though all the taxa have ranges of depth from which they have been recorded, the best synthesis using all the fossils indicates an environment of less than several hundred meters depth. It is likely that some transportation of the organisms occurred after death. Also, significant exposure effects (such as encrusting organisms and borings) are present on the fossils. Therefore, some mixing of the shallower water and deeper water fauna possibly occurred prior to their ultimate inclusion within the limestone outcrops on the west Florida slope.