The Gray Fossil Site: A Spectacular Example in Tennessee of Ancient Regolith Occurrences in Carbonate Terranes, Valley and Ridge Subprovince, Southern Appalachians U.S.A.

In May, 2000, the Tennessee Department of Transportation (TDOT) initiated a road-widening construction project on State Route 75 in Washington County in a region underlain by folded and faulted carbonate bedrock. The project involved deepening an existing shallow cut through a small hill (~20-30 m high) to remediate a blind intersection. During the course of excavation, the contractor removed several meters of yellowish-brown, chert-rich regolith across the crest of the hill and encountered an underlying soft darkbrown to black, fine-grained, graded to laminated, organic- and clay-rich deposit that presented a stability problem for road construction. Later drilling delimited thicknesses of at least 30 meters of these sediments surrounded and overlain by the yellowish-brown cherty regolith. Geological results suggest a steep-walled structure of karstic origin at one time existed within the original bedrock. Subgrade stability failures were anticipated and a remedial plan was recommended. In June, TDOT and Tennessee Division of Geology personnel made a major paleontological discovery on the road project. Unusually well preserved vertebrate fossils including tapir, elephant, rhinoceros, alligator, and a number of smaller vertebrates including fish, frogs, and turtles were identified by scientists from The University of Tennessee and East Tennessee State University. These are the first fossils of probable Late Miocene age discovered in Tennessee and constitute the largest and best-preserved terrestrial Late Miocene to Early Pliocene (ca. 7-3 Ma) localities known in the United States east of the Mississippi River. The site is a highly significant scientific discovery, containing new species, including an animal related to the red panda, and the largest assemblage of tapir ever discovered. Abundant plant remains include leaves and leaf impressions, tree trunks, limbs, and twigs and seeds. In September, 2000, the Governor of Tennessee announced that TDOT would relocate a portion of the SR 75 road project to bypass the fossil-bearing sediments and preserve the site, which had been designated by the State Archeologist as the Gray Fossil Site. This action saves the site for scientific research and educational purposes, under the auspices of East Tennessee State University, but has mandated relocation efforts that are both expensive and time-consuming. Mounting evidence—especially in finds of fossil plants, but also lignite and other regolith—suggests that similar regolith deposits may exist in karst belts elsewhere within the Appalachians, where they may be encountered by future construction activities. Increased knowledge of localization factors for such features will aid in prospecting for infilled deposits so that they can be avoided during planning for future projects.