GEOLOGICAL CONTROL OF RECHARGE PROCESSES IN THE MEMPHIS AQUIFER IN WESTERN TENNESSEE: FIELD MAPPING AND SEDIMENTOLOGICAL DATA

Detailed geologic mapping of two 7.5-minute quadrangles in Fayette County, Tennessee, and sedimentological analysis of obtained samples reveals that direct recharge to the Memphis aquifer is limited spatially by overlying Quaternary loess and likely impeded by paleosols capping thin Pliocene-Pleistocene fluvial-terrace deposits and the Eocene Memphis Sand. The Memphis Sand comprises the majority of the Memphis aquifer, the major water-supply aquifer in western Tennessee. Exposure of Memphis Sand in the outcrop belt is limited to upland valleys and steep topography, and increases from northwest to southeast (stratigraphically higher to lower). Upland surface are blanketed with as much as 6 m of loess. The alluvial valleys are filled with Quaternary-age pebbly sand grading upward to sandy silt. The Memphis Sand is as much as 141 m thick in the map areas and composed of unconsolidated to semi-consolidated very fine to very coarse-grained fluvial sand with minor interbedded white silty clay floodplain deposits. The Memphis Sand is overlain by 1 to 2 m of Pliocene-Pleistocene fluvial-terrace deposits, which are not mappable at the 1:24,000 scale. The upper 1 to 2 m of both the Memphis Sand and fluvial terrace deposits shows paleosol development as evident by extensive oxidation, clay accumulation, and blocky soil structure development. The underlying Memphis Sand is highly porous and largely unconsolidated. Recharge to the Memphis aquifer in the exposure belt is hypothesized to occur most extensively in upland stream channels and steep hillsides where the Memphis Sand is exposed. Substantial recharge may also occur in the alluvial valleys where Memphis Sand directly underlies the sandy alluvial deposits. Results from a field experiments in one of the Quadrangles (Moscow SE) indicate limited recharge through the loess-covered upland surfaces, which cover as much as 70 % of the land surface. The results of the study suggest that recharge in the outcrop belt of the Memphis Sand is more restricted than previously thought, requiring re-thinking of the overall water-budget for the highly utilized water resources of the Memphis aquifer. Student support for mapping was provided by a grant from the USGS EdMap Program and logistical support from the University of Memphis Ground Water Institute.