GEOLOGICAL AND GEOPHYSICAL INVESTIGATIONS OF MIDDLE AND UPPER CLAIBORNE STRATA IN WESTERN TENNESSEE AND HYDROGEOLOGIC IMPLICATIONS.

Sonic borehole and geophysical studies along with geologic mapping in western Tennessee have clarified the stratigraphy of middle and upper Claiborne strata, especially that of the Eocene Memphis Sand (MS) and, Cook Mountain (CM) and Cockfield (CF) formations. The latter formations form the upper confining unit for the Memphis aquifer, comprised mainly of the MS, a regionally important source of high-quality groundwater. Borehole cores from sonic drilling demonstrate that the contact of the MS with the overlying CM is conformable, with fine-grained distributary-channel sand and lower-delta-plain interbedded silt and clay abruptly grading upward to delta-front and pro-delta laminated mud and graded, rippled silty very fine-grained sand. Delta-front and pro-delta facies define the CM throughout western Tennessee and grade eastward to proximal deltaic silty mud and sand truncated by overlying Pliocene and Pleistocene deposits. Where present, the contact of the CM with the overlying CF is disconformable, commonly above a pronounced paleosol beneath a coal or at the base of incised sandy to conglomeratic fluvial-channel deposits. The CF is heterolithic, including fine-grained to conglomeratic fluvial- and distributary-channel sand, delta-front and pro-delta mud and very fine-grained sand, and bioturbated bay mud and nearshore sand. The upper MS records primarily fluvial to lower delta plain deposition prior to broad inundation by relative sea-level rise and fine-grained pro-delta deposition of the CM. Evidence of post-CM paleosol development across the region suggests widespread exposure and ample opportunity for incision into and through the CM by streams that would ultimately be filled with CF channel sand. The CF forms a network of discontinuous aquifers and confining layers. Incision through the CM following regional exposure during the late Eocene led to development of hydrogeologic breaches, some of which follow their formative channel networks, to hydrologically connect the CF aquifers with the underlying Memphis aquifer. These conditions create semi-confined conditions in the underlying Memphis aquifer, which, along with localized downward hydraulic gradients, increase vulnerability to recharge of poor-quality water to the Memphis aquifer.