176-2 Ambiguous Calcite Structures from the Late Ordovician Juniata Formation, Tennessee: Environmental and Ecological Implications

Session: Laws of the Grave: Advances in Taphonomy Across the Paleontologic Record

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Abstract:

Continental ecosystems from the Late Ordovician are still poorly understood, leaving a gap in understanding during a critical time in the development of life on land. Narrow and tubular branching calcite structures found in the Late Ordovician (Katian-Hirnantian) Juniata Formation of Tennessee offer a narrow window into the ancient critical zone. The Juniata Formation comprises foreland basin deposits of the Taconic Orogeny interpreted as a mix of shallow marine, tidal, and subaerial environments. Sets of unusual calcite structures and other differentially preserved branching structures from outcrops of the Juniata Formation along Clinch Mountain in northeastern Tennessee provide an opportunity to further investigate the environments represented by calcite-cemented red mudstone and sandstone facies in the Juniata Formation, possible biotic elements that remain undocumented, and the diagenetic processes that led to their preservation.

This study utilized several basic methods of petrology to assess the calcite structures and associated matrix. Samples were collected from outcrop as well as float from two roadcuts along Clinch Mountain in northeastern Tennessee. Samples were cut with an oil-cooled rock saw to provide lateral and vertical views of unweathered structures, also creating serial thick sections to observe three-dimensional structures, when optimal. Additional planned work includes making thin sections for description of substrate micromorphology and indications of the origin of the calcite structures. Scanning electron microscopy with X-ray dispersive spectroscopy will be used to further study morphology of the structures and assess their origin and preservation through elemental mapping.

Multiple expressions of branching structures have been discovered at both localities, varying in size, but commonly presenting downward tapering and branching pattern. Large and distinct halos around the calcite structures are present at the hand-specimen scale, although further testing is needed to fully understand their elemental composition and likely causes. The morphology of these structures is suggestive of either complex animal activity or rooting structures, although the latter have not previously been observed in deposits of this age. Regardless, the implications of these structures are broad and important to understanding the evolution of terrestrial ecosystems from their rise in the Ordovician.

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

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