Integrated Geochronology of a Classic Pliocene Fossil Site in Florida: Linking Marine and Terrestrial Biochronologies

Abstract

One of the most spectacular fossil molluscan faunas known to paleontologists is exposed in a large shell pit near Sarasota, Florida, where on the order of 1,000 well-preserved, shallow marine species are found. Sandwiched between extensive shell beds is a unit containing a rich vertebrate fauna (ca. 100 species) with a distinctive mammalian component indicative of the late Blancan Land Mammal Age (2.5-2.0 Ma). In contrast, the surrounding shell beds have traditionally been assigned an older (early to mid-Pliocene) age based upon biocorrelation of the benthic invertebrate (molluscan) fauna. Precise dating is difficult, however, as preservation of planktonic micro- and nannofossils was not favored in the nearshore paleoenviron-mental setting. In a multi-faceted approach, we integrated invertebrate biochronology (ostracodes), vertebrate biochronology (Land Mammal Age), paleomagnetism (a unique application in such coarse shell beds), and strontium isotope chronostratigraphy with existing age data from the literature in an attempt to resolve this chronologic dichotomy and link the marine and terrestrial records. All sites yielded reversed magnetic polarities. Based upon the vertebrate biochronology, as well as ostracode correlations and the $$^{87}Sr/^{86}Sr$$ ratios from each unit, we place the principal bone-bearing unit (Unit 4) and overlying beds within the lower portion of the Matuyama Chron (ca. 2.5-2.0 Ma). The underlying units (Units 5-10) are probably older, with a depositional age between 2.5 and 3.5 Ma. The lowermost unit (Unit 11) may be as old as early Pliocene.