143-17 Chemostratigraphy of the Zilpha clay as a confining layer in the Memphis Aquifer, Northern Mississippi embayment.

Abstract:

Confining units regulate groundwater flow and aquifer vulnerability, yet their effectiveness is often difficult to quantify due to their heterogeneity. This study uses chemostratigraphy to evaluate the confining nature of the Zilpha Clay, a regionally extensive fine-grained interval, within the Middle Claiborne Group of the Eocene Memphis Aquifer system in the Northern Mississippi Embayment. Major, trace, and rare-earth element (REE) geochemistry from 185 samples collected from three continuously cored wells (AEM, MSA, EPA) across Shelby County, Tennessee, was integrated with sedimentologic descriptions of cores to construct a chemostratigraphic framework. Six lithofacies grouped into channelized, hemipelagic, and pelagic facies associations were identified, from river mouth to brackish water environment. Immobile element proxies and REE patterns indicated a broadly uniform felsic to intermediate upper continental crustal source across all wells, with minimal provenance variability. Sand- and silt-dominated channel facies showed elevated SiO₂ contents and reduced trace-element retention, whereas clay-rich facies show enrichment in Al₂O₃, TiO₂, Fe₂O₃, large-ion lithophile elements, and total REEs. Chemostratigraphic correlations identify laterally persistent marker horizons that distinguish the Zilpha clay beyond areas where the continuity was previously uncertain. Paleodepositional conditions indicate predominantly oxic to suboxic environments, with short-lived anoxic episodes. The regional extent and distinct geochemical signature of the Zilpha Clay suggest it may serve as a secondary protective layer for production wells screened below it, with implications for groundwater management.

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