INVESTIGATION OF PLIOCENE AND PLEISTOCENE FLUVIAL-TERRACE AND ALLUVIAL DEPOSITS IN SHELBY COUNTY, TENNESSEE, AND THEIR RELATIONSHIP TO THE SHALLOW AQUIFER SYSTEM

Pleistocene and Pliocene fluvial-terrace and alluvial deposits have been widely studied concerning the climatic, tectonic, and geomorphic processes that lead to their formation. To better understand the relationships of these deposits to subsurface water flow, we evaluated stratigraphic and sedimentological characteristics and lateral distribution of fluvial-terrace deposits in Shelby County, Tennessee. Lithological interpretation of geophysical and geologic log records and surface site analyses are used to construct cross-section profiles across the study area within a geomorphic context. The Pliocene Upland Complex, a remnant of the ancestral Mississippi River, caps the interfluves areas of the western Tennessee tributaries (WTT) and appears as the high-level terrace. The Finley, Hatchie, and Humboldt terrace deposits lie at successively higher elevations above the modern floodplains of WTT to the Mississippi River. In eastern Shelby County, these terrace deposits are laterally disconnected from each other and the associated alluvium compared to western Shelby County. Moreover, the thickness of the individual terrace deposits is greater in the eastern part of the county than in the western. These differences may be due to Quaternary up-to-east offset along the White Station fault, which displaces the Pliocene Upland Complex. Sedimentological analysis was conducted by field observations and grain-size analysis of samples from exposed terrace deposits and borehole cores. The results of this work suggest a heterogeneous and locally disconnected shallow aquifer in contrast to previous studies that indicate a single homogeneous and continuous shallow aquifer in the study area.