A SEDIMENTOLOGICAL CASE FOR ALLIGATOR GASTROLITHS AT THE GRAY FOSSIL SITE, EAST TENNESSEE

The Gray Fossil Site(GFS) is a sinkhole lake deposit of Mio-Pliocene age within a Cambro-Ordovician carbonate terrain. Over 200 plant and animal taxa have been found in this lagerstaette. Microscopic examination of water-screened residue from 20 kg matrix samples have revealed rare, polished, silica-rich stones averaging 4 mm in size. Approximately 600 kg of field material from one area (TP 1-2012) produced ~35 gm of polished stones (~1 gm/18 kg or 0.0065% of the total matrix volume). Over 400 stones (N=437) from thirty matrix samples were studied for maximum size, median size, sphericity, roundness, and number per sample. They averaged 14 stones per sample with no “stoneless” samples. These samples are less than five meters from a cluster of semi-articulated alligator elements. To the north of these elements 296 stones occur along a north-dipping, fifty centimeter thick slope. Three alligator teeth and three oolitic stones were collected within 2.5 m of the semi-articulated alligator suggesting a possible association with this carcass. The polished oolitic stones are lithologically and stratigraphically restricted to the uppermost Knox Group sinkhole bedrock; therefore it is likely they were polished in the GFS lake. By contrast, a separate area (TP2-2004) which is 30m from the semi-articulated alligator revealed no significant alligator remains. 108 samples from 46 meter squares revealed only 18 squares with polished stones and therefore 60% of the squares were “stoneless”.