9-10 Recent 1:24,000-scale Geologic Mapping of Upper Cretaceous Strata in West Tennessee, Part II: Regional Correlations and Implications to Mapping Conventions

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

Presenting Author:

Authors:

Abstract:

Upper Cretaceous strata in West Tennessee trend roughly north-south and record deposition across a range of shallow- to marginal-marine environments. Recently, three 7.5-minute quadrangles were mapped at the 1:24,000-scale through the U.S. Geological Survey (USGS) Educational Mapping Program in West Tennessee with the objective of determining the spatial-temporal distribution of lithostratigraphic units, specifically the Demopolis-Coon Creek-McNairy Sand interval. The conformable, gradational contact between the Demopolis and Coon Creek formations is difficult to delineate in Tennessee because of similar interbedded lithofacies, including gray clay, marl, silt and glauconitic sand. To the southeast, the contact can be determined based on lithology (i.e., marl versus sand) and the top of the range for associated Exogyra sp. biozones; however, this boundary is time-transgressive and not applicable to strata in Tennessee. Previous studies distinguished the contact using percent-clay abundance, the lowest glauconitic sand, or the highest marl bed. From a mapping perspective, these practices are problematic because beds are commonly non-traceable, inconsistently tied to lithology, influenced by topography, and frequently obscured by weathering or vegetation. The stratigraphic contact between the upper Coon Creek Formation and the overlying McNairy Sand is similarly gradational and placement is complicated by the interbedding of similar lithofacies, including clay, silt, sand, and sandstone. 

The Coon Creek Formation can be subdivided into lower and upper intervals on the basis of lithology; the lower Coon Creek is dominated by fine-grained, thinly- to medium-bedded, locally calcareous silty clay and clayey silt with abundant mollusk fossils, whereas the upper Coon Creek consists of micaceous and ferruginous, argillaceous to silty sand and fine- to medium-grained sandstone with common ironstone concretions. We suggest, from a 1:24,000-scale mapping perspective, that the lower Coon Creek interval should instead be a member of the Demopolis Formation and the upper Coon Creek interval should be considered part of the lower McNairy Sand. Therefore, the stratigraphic contact between the Demopolis Formation and the McNairy Sand would be a regionally traceable, lithologic contact. This interpretation aligns with observed trends in lithofacies (at both the local and regional scale), incorporates recently reported micropaleontologic data from the Coon Creek type section indicating a Campanian age, and improves consistency in identifying Upper Cretaceous contacts.

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