GEOLOGY OF THE LEAPWOOD 7.5-MINUTE QUADRANGLE, MCNAIRY COUNTY, TENNESSEE

To determine the spatial relationships of Late Cretaceous lithofacies along the eastern Gulf Coastal Plain in western Tennessee, the Leapwood 7.5-minute quadrangle in McNairy County is mapped at the 1:24,000-scale. Formal stratigraphic units within the quadrangle include the Late Cretaceous Demopolis Formation, the Coon Creek Formation, and the McNairy Sand. The Demopolis Formation consists of light gray to greenish-gray, medium-grained, fossiliferous calcareous sandstone interbedded with thin light gray and tan siltstone-claystone- and mudstone layers that weather to a yellowish-brown-red color. Molluscan, bivalve, and gastropod fossils are common and typically preserved as external casts. The stratigraphic contact between the Demopolis and Coon Creek formations is gradational with similar lithofacies. We interpret the contact at the top of the stratigraphically highest marl, a 3-5 ft thick light gray silty marl with an abundant presence of Exogyra. The Coon Creek Formation can be separated into lower and upper parts with distinctive facies. The lower portion consist of thin-to-medium bedded, predominantly fine-grained, sporadically calcareous silty clay and clayey silt. The silty clay is abundant in bivalve (Exogyra) and gastropod (Belemnites and Turritella) fossils. There is an interbedding sequence of dark and white silty clay/clayey silt towards the midsection of the formation. The upper portion of the unit consists of laminated, thinly bedded, light gray to green to dark gray, fine to medium-grained, micaceous, moderately to highly weathered, fractured sandstone with occasional iron concretions. Sandstone is interbedded with thin strata of shale and has a ferruginous appearance. The overlying McNairy Sand consists of thin to thick bedded, fine-to-very coarse grained, mostly medium to coarse grained, light gray to brown, dark brown to red, laminated, and cross-bedded, ferruginous sand. Bedding measurements and lithostratigraphic relationships show that Cretaceous strata strike north-south and dip approximately 30 ft per mile to the west (0.3°). Detailed field relationships of lithofacies will advance renewed interests in critical mineral exploration of Cretaceous sand beds in the eastern Gulf Coastal Plain.