23-4 Evaluating Diagenetic Alteration in Ordovician Brachiopods from the Nashville Basin Using SEM Microstructure

Session: Paleontology of North America (Posters)

Presenting Author:

Authors:

Abstract:

Brachiopod shells are widely used in paleoenvironmental and geochemical studies because their calcitic skeletons have the potential to preserve primary environmental signals. However, post-depositional diagenesis can alter shell microstructure, complicating interpretations of paleoenvironmental data. This project evaluates the degree of diagenetic alteration in Late

Ordovician brachiopods from the Nashville Basin using scanning electron microscopy (SEM). The Nashville Basin records a key interval that includes a transition from tropical to more temperate climatic conditions and a major biotic immigration event known as the Richmondian Invasion. Constraining associated environmental changes, including temperature, sediment input, and sea-level fluctuations, is critical for understanding links between environmental shifts and biotic turnover, making the identification and selection of minimally altered brachiopod material essential for robust geochemical reconstructions.

The primary objective of this study is to assess whether brachiopod shell microstructure reflects pristine preservation or diagenetic overprint, and to examine how preservation varies across stratigraphic members and lithologies. Brachiopods were collected at 30 cm intervals from the C3 to C5 stratigraphic sequences of the Katian Stage (Late Ordovician) in the Nashville Basin. Each specimen was sectioned, polished, and etched prior to SEM examination. SEM imaging is used to characterize shell microstructure, with particular attention to calcite fiber organization, crystal boundaries, and textural evidence of recrystallization.

The study indicates that most brachiopods preserve their primary shell microstructure, including calcite fibers, and that diagenetically altered zones, such as silicified portions of the shells, can be readily identified. Lithology appears to have a stronger influence on alteration than stratigraphic position, with broadly similar diagenetic patterns observed from C3 to C5. By integrating microstructural observations, this project develops criteria for identifying minimally altered brachiopod material, better constrains diagenetic processes affecting Ordovician carbonate successions, and targets the least altered shell regions for subsequent isotopic geochemical analyses.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

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