9-11 Recent 1:24,000-scale Geologic Mapping of Upper Cretaceous Strata in West Tennessee, Part I: Field Observations and Lithostratigraphy

Session: Advances in Geologic Mapping, Databases, and Dissemination: Student Posters

Presenting Author:

Authors:

Abstract:

The Leapwood, Enville, and Life 7.5-minute quadrangles were mapped at the 1:24,000-scale through the U.S. Geological Survey (USGS) Educational Mapping Program. These quadrangles represent 3 of the remaining 6 unmapped quadrangles (58 total) in the state containing Cretaceous bedrock units. Upper Cretaceous units mapped include the Demopolis Formation, the Coon Creek Formation, and the McNairy Sand. A total of 348 field observations were identified across the study area, in which sedimentological and stratigraphic relations were recorded.

The Demopolis Formation is between ~ 80 – 100 ft thick and consists of gray to greenish-gray, fine- to medium-grained, fossiliferous, calcareous clayey sands interbedded with silts. Bioturbation is present towards the top. The contact between the Demopolis and overlying Coon Creek formations is conformable, gradational, and typically concealed at the surface. The Coon Creek Formation is 140 ft thick and informally divided into upper and lower parts based on lithology. The lower part consists of gray to brown, very fine- to medium-grained, thin- to thick-bedded, micaceous, fossiliferous marls, clayey silts, and sands. The upper part consists of fine- to medium-grained, frequently ferruginous, clayey-silty sands with frequent ironstone concretions. Fossils are typically sparse in the lower Coon Creek, but the Leapwood quadrangle contains the Coon Creek Lagerstätte. Common invertebrate fossils include bivalves, gastropods, ammonites, and crustaceans. Vertebrate fossils are uncommon but include fish, marine reptiles, pterosaurs, and dinosaurs. The contact between the Coon Creek and the overlying McNairy Sand is gradational. The lower McNairy Sand consists of variable colored, fine- to medium-grained, laminated to thin-bedded with some cross bedding, micaceous, argillaceous sand, and silty sand, and is commonly interbedded with clay and sparse bioturbation. A regional unconformity is present between the lower and middle parts of the McNairy Sand. The middle part of the McNairy Sand consists of red to tan to black, medium- to coarse-grained, sparsely pebbly, thin- to thick-bedded, frequently ferruginous sands and sandstones. Overall, the McNairy Sand is ~ 200 ft thick. Together, these formations represent a regressive package of strata that shift from a marine environment (Demopolis formation) to a nearshore transitional environment (Coon Creek Formation) to a shoreline environment (McNairy Sand). Strata dip less than 1˚ to the west and locally contain systematic joints that mostly strike ~ N-S.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.