LITHOSTRATIGRAPHY OF THE MEMPHIS (SPARTA) SAND IN THE NORTHERN MISSISSIPPI EMBAYMENT

The Memphis (Aquifer) Sand is the principal water source for millions of people in the Northern Mississippi Embayment. This study evaluates aquifer compartmentalization and connectivity in a 29 county area in west Tennessee, northwest Mississippi and east Arkansas. Approximately 250 geophysical logs were collected from well libraries at the USGS, Arkansas Soil and Water Division, private contractors, and the University of Memphis Ground Water Institute. The logs were used to interpret formation and intraformation units in preparing seven cross sections, one parallel to the Mississippi River and six perpendiculars to the first. Structure contour, isopachous, and lithofacies maps indicate a gradual thickening and fining of the Memphis Sand toward the axis of the embayment and down structural dip to the south. Clay units become thicker and more common south of the 35th parallel, into north Mississippi and southeast Arkansas. Structural offsets of strata are minimal and mainly associated with the SW-NE structural grain of the basin. Although regional continuity dominates, internal partitioning increases toward the center and to the south of the Mississippi Embayment. Furthermore, sandy incised valley-fill deposits with regional confining units are observed toward the eastern margin of the embayment and provide potential avenues for water transfer between the successive aquifers. Our findings are consistent with a source area to the northeast of the embayment, deposition in a fluvial environment north of the 35th parallel, transitioning to mixed marine-nonmarine enrolments in north Mississippi and southeast Arkansas.