23-8 Paleocommunity and Paleoenvironmental Change in the Little Cove Point Member of Maryland’s Calvert Cliffs

Session: Paleontology of North America (Posters)

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Abstract:

Fossil populations preserved in coastal marine deposits provide insight into paleoenvironmental change through Earth history. Molluscan communities serve as particularly useful paleoenvironmental proxies due to their lifestyle habits and feeding strategies, which reflect characteristics such as water depth, energy conditions, and salinity levels. This study examines genus-level taxonomic distributions within the Little Cove Point (LCP) member of the St. Marys Formation at Calvert Cliffs, Maryland, a Late Miocene (~8.7 Ma) deposit characterized by abundant fossil material and complex lateral facies variations.

Seven 1-L bulk stratigraphic samples were collected in place from distinct stratigraphic beds. In the lab, samples were disaggregated and mollusks were identified.

We found 24 gastropod and 15 bivalve genera from 3,979 individual specimens (1,568 gastropods and 2,410 bivalves). Lower stratigraphic intervals (samples 1-3) exhibit gastropod-dominated assemblages with Turritella as the most abundant genus (63%, 43%, and 49% in samples 1-3, respectively), with Ilyanassa, Euspirella, Mangelia, and Laevihastula as the remaining majority. Gastropods comprise 88-93% of individuals in these samples, suggesting deeper, more open marine conditions. Upper intervals (samples 4-7) demonstrate a dramatic reversal, with bivalves making up 62-85% of assemblages. Most significantly, the bivalve genus Spisula exhibits exponential increase from 8% of the total individuals in sample 3, to 75% of total individuals in sample 5, establishing overwhelming single-species dominance while Turritella drops from 49% to less than 1% of the total population across the same interval. Other notable bivalves present were Mercenaria, Mesodesma, Chesacardium, and Nuculana.

These patterns suggest environmental change toward shallower, higher-energy conditions with increased environmental stress. The transition from diverse gastropod-dominated to Spisula-dominated communities reflects adaptation to shallow, moving substrate environments with increased water movement. Reduced diversity and single-species dominance is consistent with prior paleoenvironmental research: the presence of iron monosulfides in upper sequences suggest increasingly restricted circulation and potential brackish influence, and the coarsening-upward lithology and cross-bedding support the interpretation of shoaling, or at the very least shallowing, conditions.

This evidence may demonstrate an ecosystem-scale response to chronic environmental change in Miocene coastal settings, illustrating how this molluscan community structure records paleoenvironment-driven habitat reorganization in ancient marginal marine environments.

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

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