57-3 Trace fossils of the Liberty and Saluda intervals of the Cincinnatian Series (Late Ordovician) at Madison, IN: Assessing bioturbation levels and environmental controls

Abstract:

The Ordovician Cincinnatian Series (OH, IN, KY) is one of the most documented and analyzed geological sequences in the world, renowned for its pristinely preserved fossils from the Katian (Late Ordovician). This project aims to analyze the trace fossils and their depositional context in the Liberty and Saluda intervals of the Cincinnatian Series at Madison, IN. Our goals are to assess the amount of sediment mixing by animals during this time period and evaluate possible environmental stress factors affecting bottom waters and the seafloor. A 15 m-thick section was logged, and 20 samples were collected. Samples were embedded in epoxy, cut, and polished to improve trace fossil characterization. An ichnological analysis consisting of ichnotaxa and ethological characterization and quantification of the bioturbation index provided the basis for an ichnofabric analysis. The studied succession displays calcareous and mixed mudstone, bioclastic floatstone and rudstone, coral framestone, and dolostone, representing a prograding succession that ranges from shoreface mixed (carbonate-siliciclastic), shell beds and mudstones, through coral buildups, lagoonal organic-rich silty carbonates and mudstones, and culminating in intertidal laminated dolomitic siltstones. Six ichnofabrics have been identified within the logged sections: highly bioturbated with Chondrites, Nereites, Palaeophycus, Planolites, and Teichichnus; moderately bioturbated with Nereites, Palaeophycus, and Planolites; moderately bioturbated with Chondrites; sparsely to moderately bioturbated with Skolithos; sparsely bioturbated with Teichichnus; and sparsely bioturbated with Zoophycos. Preliminary findings show lower ichnofossil diversity within the intertidal zone and higher in the subtidal facies, with the highest in the shoreface mixed. Furthermore, the description of highly bioturbated ichnofabrics supports the idea that mixed carbonate and siliciclastic seafloors were fully mixed by the Late Ordovician.

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