Upper Cretaceous stratigraphic correlations in West Tennessee based on recent 1:24,000-scale mapping

Document Type

Article

Publication Date

1-1-2026

School

Biological, Environmental, and Earth Sciences

Abstract

Upper Cretaceous strata in West Tennessee (USA) have been investigated for approximately two centuries, and conventions for subdividing units have evolved during this period based on lithologic and biostratigraphic observations. The current convention is a byproduct of the 1966 Geologic Map of Tennessee, where the Upper Cretaceous units Coon Creek and McNairy Sand were raised to formation status. To understand stratigraphic contacts and regional correlations, we present sedimentological and stratigraphic field observations from the Leapwood, Enville, and Life quadrangles associated with a multi-year mapping campaign through the U.S. Geological Survey (USGS) Educational Mapping Program and in collaboration with the Tennessee Geological Survey. The Demopolis Formation consists of light-gray to greenish-gray, very fine- to fine-grained, glauconitic siltstone interbedded with light- to dark-gray and yellowish-brown silty clay and clayey silt that weather to a yellowish brown-red color. The contact between overlying Coon Creek Formation is gradational and is characterized by similar, interbedded lithofacies consisting of silt, glauconitic sand, marl, and clay beds. The lower Coon Creek Formation consists of thin- to medium-bedded, predominantly fine-grained, sporadically calcareous silty clay and clayey silt. The upper part of the Coon Creek Formation consists of light-gray to dark bluish-gray, orange- to reddish-brown to red, very fine- to coarse-grained, thinly bedded to massive, micaceous, silty to sandy clay. These clays interbed with white, red to purple, and yellow, fine- to medium-grained97 laminated to thickly bedded, highly micaceous, clayey to silty sand with regular ironstone concretions. The gradational contact between the upper Coon Creek Formation and McNairy Sand records a coarsening upward sequence of argillaceous sand and silt interbedded with localized clay horizons typical of a progradation of shoreline lithofacies. The McNairy Sand consists of pink, purple, white, tan, reddish-brown to rust-red, and black, fine- to coarse-grained, laminated to massively bedded, cross-bedded, frequently ferruginous and micaceous, argillaceous-sand, silty-sand, and sandstone with many cut-and-fill structures toward the upper part of the lower McNairy Sand.Previous studies used Exogyra fossils to distinguish between the Demopolis and Coon Creek formations, commonly delineating the contact at the highest glauconitic sand or the highest marl bed. However, the lack of exposure, interbedded nature of lithofacies, and environment-dependent reinterpretations of Exogyra presence show that these approaches are impractical. The use of percent clay abundance and presence of biotite grains have also been employed to delineate the contact, both of which present difficulties in reproducibility. The contact between the upper Coon Creek and McNairy Sand is likewise difficult to determine because of gradational interbedding of similar lithofacies of silt, sand, sandstone, and clay. This contact has commonly been denoted based on topographic benches and then traced horizontally along contour lines. Our field observations show this approach is problematic and does not integrate well with modern perspectives regarding the development of stratigraphic architecture. Therefore, we propose a new mapping convention, based on 1:24,000-scale mapping, in which the lower Coon Creek Formation is reassigned as a member of the Demopolis Formation and the upper part of the Coon Creek Formation is included as part of the lower McNairy Sand. This approach leverages local to regional trends in lithofacies, addresses new micro-paleontology data reported for the type-section of the Coon Creek Formation, and allows for more straightforward assignment of Upper Cretaceous stratigraphic contacts.

Publication Title

GSA Field Guides

Volume

75

First Page

97

Last Page

139

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