SEDIMENTOLOGY AND STRATIGRAPHY OF THE EOCENE COCKFIELD FORMATION AT MEEMAN-SHELBY FOREST STATE PARK, WESTERN TENNESSEE

A field mapping and stratigraphic study of the Eocene Cockfield Fm. of the Claiborne Group was conducted at Meeman-Shelby Forest State Park, western Tennessee. The Cockfield Fm. is one of two confining units overlying the Memphis aquifer, a major regional aquifer in the northern Mississippi embayment. Continuous exposures of the Cockfield Fm. are rare in western Tennessee and the exposures at Meeman-Shelby Forest provide some of the best exposures with which to describe lateral continuity of facies and assess depositional environments. Valley-bottom exposures as thick as 7 m were mapped and described for their sedimentological characteristics, post-depositional weathering, and stratigraphic relationships. Thirty-two samples were collected from twenty-four sites for detailed descriptions and grain-size analysis.

The sedimentary facies of the Cockfield Fm. at Meeman-Shelby Forest include organic-rich interlaminated silty mud and ripple-crosslaminated very fine-grained sand, thickly bedded massive to burrowed organic-rich sandy mud, massive to cross-bedded silty very fine- to fine-grained sand, and lignitic mud. The interlaminated mud and sand show soft-sedimentary deformation and synsedimentary faulting characteristic of liquefaction processes. The lignitic mud facies overlies weak paleosol development in underlying massive muds. The mud-dominated facies show the greatest lateral continuity throughout the study area, although sand facies are locally continuous. Post-Eocene weathering as much as 6 m deep obscures depositonal textures at several locations. The depositional environments for the Cockfield Fm. encompass an array of delta plain settings ranging from shallow lakes, wetlands, and floodplains to distributary channels. Given that the Cockfield Fm. forms a significant confining unit for the Memphis aquifer, the sandy channel deposits may be an important avenue for groundwater leakage through the confining unit.