AN INVESTIGATION OF THE LITHOLOGIC AND HYDROSTRATIGRAPHIC CHARACTERISTICS IN THE UPPER CLAIBORNE CONFINING UNIT, SW TENNESSEE: PRELIMINARY RESULTS

The extent and stratigraphic characteristics of the upper Claiborne confining unit, which overlies the Memphis aquifer and protects its water resources, have not been well-defined, especially outside of Shelby County. In this paper, we evaluate the extent and stratigraphy of the upper Claiborne confining unit within Shelby County, Tennessee, and adjacent counties in Tennessee and Mississippi. Three hundred twenty-two geophysical logs were examined, mainly from collections at the Ground Water Institute (GWI) at the University of Memphis. Each log was first evaluated to determine the quality of the log signals, which typically include natural gamma, spontaneous potential, and one or more resistivity curves. Information from the logs was entered into an Excel spreadsheet and subsequently used to create shapefiles in ARC GIS. Logs were considered good quality if they had all of the following characteristics: geographic location, ground elevation, a minimum of 90 m of log signal (dependant on location), more than one log signal, and a good signal-to-noise response. Forty-four of the 322 logs were found to be of less than desirable quality for one or more reasons. However, the geographical distribution of the low-quality logs shows no clustering or pattern. The log signals were interpreted in regard to lithology and hydrostratigraphic unit, especially noting stratigraphic relationships within the upper Claiborne confining unit and enclosing hydrostratigraphic units. The hydrostratigraphic units identified include: loess and other surface materials, sandy alluvial or fluvial terrace aquifer (includes sand, silt, and clay), clay-dominated upper Claiborne confining unit, and Memphis aquifer. Preliminary results indicate that the lithology of the upper Claiborne confining unit includes clay, silt, and sand over much of the western part of the study area. Thinning of the confining unit is not uniform across the study area, but rather shows outliers that extend much further east than expected. The results of this study have important implications for the extent and competence of the protective character of the upper Claiborne confining unit for the underlying water resources of the Memphis aquifer.