GEOLOGIC MAPPING OF THE EOCENE MEMPHIS SAND, WESTERN TENNESSEE, AND IMPLICATIONS FOR RECHARGE PROCESSES FOR THE MEMPHIS AQUIFER

A detailed geologic map of the 7.5-minute Macon Quadrangle is Fayette County, Tennessee, was completed to investigate the distribution of sedimentary facies in the Eocene Memphis Sand and evaluate the potential for recharge. The Memphis Sand comprises the majority of the Memphis aquifer, the major water-supply aquifer in western Tennessee. Field mapping was completed during April through November 2010 using topographic maps and investigating available field exposures. The map units identified in the map area include the Eocene Memphis Sand and Cockfield Formation, Quaternary loess, and Quaternary to modern Alluvium. Quaternary loess covers almost all upland surfaces, except where deeply incised. In those areas the Eocene units are exposed. The Memphis Sand is primarily fine- to coarse-grained quartz sand with minor thin, discontinuous white silty clay beds or clay intraclast breccias beds in sandy matrix. In subcrop beneath the loess, Memphis Sand is overlain by a thin (generally< 1 m thick) gravelly medium- to very coarse-grained sand that is cross-bedded and contains chert and limonite gravel. Geomorphic relationships as well as crossbed orientations that follow the modern drainage network suggest that the gravelly sand is a terrace deposit, perhaps correlative to the Pliocene-Pleistocene Upland Complex. The Cockfield Formation comprises interbedded white clay, massive or cross laminated white to light gray silty mudstone, and fine to coarse-grained quartz sand and gravel. It crops out only in the northwestern part of the Macon Quadrangle. The Quaternary to modern Alluvium comprises gravelly sand to clayey silt and fills modern creek valleys. Upland stream channels commonly have sand bottoms and are dry, whereas the silty bottom portions of upland as well as lowland streams often have standing water. Given that the loess presents an impediment to recharge, we suggest that the sandy bottom upland stream valleys, which commonly are incised through the loess, act as a primary recharge pathway for 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.