JOINT CONTROLS ON CAVE FORMATION AND MORPHOLOGY: SNAIL SHELL CAVE, TENNESSEE

Fracture data collected from within Snail Shell cave, located in the Ridley Limestone of central Tennessee, are combined with surface data in order to better understand the role of jointing in this shallow cave system’s development. The cave is an extensive and not yet fully mapped network of passages that are partially flooded depending on local river stage. Snail Shell cave is one of the most biologically significant caves in the southeastern US. Its proximity to Murfreesboro, TN may allow pollutants from surface waters to enter the cave through fractures, which act as the main recharge structures for the cave. The purpose of this study is to identify areas characterized by large joints and relatively high fracture density, that could act as recharge pathways.

Data collected from the interior walls and roof of the cave include joint orientation, spacing, local scanline counts, and observations of fracture aperture and vein fill. There is sample bias related to passage orientation with respect to fracture strike and fracture size associated with data collected within the cave. In order to address the sampling bias and add to the dataset, we collected data from outcrops and mapped joints using LIDAR data that was provided to us by Rutherford County. The combined dataset was then analyzed using the MATLAB toolbox FracPaQ. Fracture density, intensity, orientation, and size-frequency distributions were generated and compared with the plan-view pattern of the cave network using ArcGIS.

Maps of fracture intensity and orientation from the surface data reflect both the known subsurface extent and passage orientations within the cave system. This method has the potential to help correct inaccurate cave maps and identify undiscovered passages of Snail Shell cave. In addition, the approach of combining LIDAR data with structural data obtained from outcrops and the subsurface may help delineate other shallow cave systems in this region and elsewhere.