HIGH RESOLUTION GRAVITY STUDY OF THE GRAY FOSSIL SITE, WASHINGTON COUNTY, TENNESSEE

The Gray Fossil Site, Washington County, Tennessee is a remarkable sink hole fill deposit that has produced the only known Mio-Pliocene fauna and flora in the Appalachian region. It was discovered by accident during a Tennessee Department of Transportation (TDOT) road modification project in 2000. After discovery, TDOT drilled a series of auger holes to determine the nature of the fossil bearing strata and its aerial extent. The fossil bearing strata fill occurs as dark, organic rich, lacustrine sediments deposited in a paleo-karst basin developed on Cambro-Ordovician carbonates. This unit has not been recognized anywhere else in the region. Since the site has no known correlative in the region it is vital to understand its structure, how it formed and how it filled. To this end a high resolution gravity survey which included 1104 gravity measurement stations was conducted across the site. Analysis of the gravity data in the form of complete Bouguer and residual gravity anomaly maps, and three gravity models indicate the presence of a northwest-southeast trending basin with a center delineated by a string of gravity lows. The linear structure of the basin compares favorably with a basement contour map created using TDOT auger data but contains more detail. Other gravity anomalies define lineations perpendicular to the basin axis and parallel to structural strike. The presence of multiple gravity lows and highs within the basin clearly indicate the presence of a complex basement topography intimately tied to structure. The gravity lows were modeled using drill hole density and depth data as constraints to determine that the depth to carbonate basement varied between 25 and 50 m. It is possible that the basin formed as the result of coalescence of multiple sink hole structures.