188-5 Skeletal animal abundance and depositional environments of the Carboniferous (Visean) Castletown and Poyllvaaish formations, southern coast of the Isle of Man

Session: Linking Biodiversity Loss to Environmental Stressors Through Integrated Approaches (Posters)

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Abstract:

Following the later Devonian’s widespread marine extinctions, the early Carboniferous Period (Mississippian Subperiod) saw the development of widespread carbonate platforms in western Europe and North America. The Isle of Man (UK) hosts diverse Carboniferous carbonates ranging from subtidal mud mounds and to deeper, oxygen-poor basinal facies. To examine the skeletal abundance of marine animals in distinct depositional settings, 7 thin sections from the Visean Castletown Formation and 36 from the Visean Poyllvaaish Formation were examined. The Castletown samples came from ~3 m of section containing small mud mounds exposed at Port Saint Mary. Thirty-six thin sections were sampled through 94 m of Poyllvaaish stratigraphy including a Waulsortian mudmound at the base. Point count analysis of the sections provided a quantitative assessment of fossil and non-fossil carbonate components, and the presence/absence of skeletal grains was also noted for each thin section. The Castletown mud mound environment hosts lower diversity and higher fossil abundance than the Poyllvaaish Formation, with benthic forams contributing roughly 25% to all fossil points. The Poyllvaaish samples have higher diversity with abundant bivalves, brachiopods, and calcispheres throughout the section. Bryozoans compose nearly 10% of all Poyllvaaish fossil points but only significantly contribute to two thin sections. The Poyllvaaish samples also preserve a range of cement types including peloidal micrite, isopachous radial fibrous cements, and radiaxial fibrous cement fans. Echinoderms contribute nearly 50% of all fossil points in both formations, and neither environment preserves abundant skeletal reef frame builders like corals or sponges. The abundance and diversity of skeletal animals from the Castletown Formation thin sections suggest that these small-scale mud mounds were inhabited by benthic forams and echinoderms (mostly crinoids), but hosted a less diverse community than the larger Poyllvaaish mudmound and overlying facies. The Poyllvaaish Formation lacked significant skeletal contribution from corals and showed only two peaks of bryozoan abundance, suggesting this environment did not host animal reefs. The precipitation of radiaxial fibrous cements may suggest high Mg concentrations and elevated local alkalinity in Poyllvaaish porewater. These data point to a diversity of mudmound facies being present on the Isle of Man, unusual for the early Carboniferous, that require further study.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8951

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