Sedimentologic and Stratigraphic Investigations into the Origin and Fill History of a Tertiary Sinkhole Lake: Gray Fossil Site, TN

In 2004, researchers from East Tennessee State University collected a 39 m split-spoon auger from a concentration of black, laminated, organic-rich, and fossiliferous sediments in Washington County, TN along State Route 75S. Identification of the fossil flora and fauna, coupled with the topography of the local bedrock, indicate a sinkhole (with numerous adjacent depocenters) with ~ 38 m of lacustrine laminated clays. Vertebrate biostratigraphy from surface excavations indicates an age of latest Miocene – earliest Pliocene, whereas pollen from the base of the core suggests a Paleocene age. Analysis of the sediments recovered in the core indicate three main observations: (1) most of the core is characterized by laminations that are defined by grain size (clay-silt couplets), color (varying amounts of carbon and/or ferric iron), and organic content (mature carbon, charcoal), (2) thick organic units are found approximately every 4 cm between 10 m and 20 m depth, and (3) nine sand units are located near the base of the core and make up ~ 0.8 m of the entire core length. Cyclicity is apparent both in the sediment's color pattern, as well as in the magnetic susceptibility record. Through sedimentary analysis of the core, reconstructions of the paleoclimate and paleolandscape can be determined. Textural analysis of the sand units indicates a local source of the coarse clastics, and a paleodrainage from the southeast. Cyclic color patterns (10 m – 20 m depth within the core) suggest climate changes that produced thick organic units overlain by gray-brown clays that then grade into coarser, orange-red silts up-section. The black organics and gray-brown clays indicate reduced conditions, organic preservation, and likely wetter periods when the lake was deep, whereas the orange, coarser units appear to indicate oxidation and shallower lake conditions during dry periods.