BIOLOGICALLY FORMED SILICA SPHERES FROM THE PINECREST SHELL BEDS OF THE UPPER TAMIAMI FORMATION, FLORIDA

The Pliocene Pinecrest beds of southwest Florida are composed of a diverse and abundant molluscan fauna that forms the bulk of these fossiliferous units. Carbonate mud and coarse-grained siliciclastic sands comprise the matrix between the large shell clasts of the beds. A recent investigation into the preservation of benthic foraminifera assemblages has revealed small silica spheres found at the base of four of the ten depositional beds that encompass the Pinecrest at the SMR Quarry where this study was undertaken. These spheres, found both inside and outside of articulated specimens of Mercenaria campechiensis, are relatively spherical, ~200 µm diameter, and are of uniform shape. Most are translucent and consist of amorphous silica; however, a few are white to gray in color within their interiors. All spheres consistently show a structure of concentric layering that is readily observed in thin section. Some spheres are conjoined with smaller spheres, whereas all spheres have an identical ovoid ‘scar' located on their exterior. Other silica spheres found documented in previous studies have been attributed to meteorite debris, fly ash, phytoliths, or opal precipitation. The silica spheres found in this study, however, do not adhere to the criteria for any of these traditional sources. They are not of extraterrestrial origin due to their consistent size, shape, and mineral composition. They cannot be fly ash because of their size and presence within articulated Mercenaria campechiensis valves which have been closed since the Pliocene; and only exposed, partially, for a few days prior to collection. Their shape and structure are uncharacteristic of phytoliths. If the spheres are inorganic opal precipitate there would have to be an extremely high level of silica saturation and flow in the ground water through the largely carbonate-rich sediments of the Florida Platform to induce precipitation. The uniform nature of the spheres, in size, shape, and composition suggests a biological origin. The ovoid patch on all of the spheres would be difficult to form if they were produced by abiotic means, whereas they could readily represent an attachment scar to an organism that controlled the formation of the concentric internal layers.