6-1 Branching Calcite Tubules from the Late Ordovician Juniata Formation, Tennessee, and their Environmental Implications

Session: Advances in ichnology: walking in Memphis and other records of behavior

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Abstract:

Continental ecosystems of the Late Ordovician are still poorly understood, leaving a gap in understanding during a critical time in the development of life on and near land. Tubular branching (primarily) calcite structures found in redbed-containing successions of the Late Ordovician (Katian-Hirnantian) Juniata Formation of Tennessee offer possible insights. The Juniata Formation comprises the 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 traces from outcrops of the Juniata Formation are present in multiple outcrops along Clinch Mountain in northeastern Tennessee. These traces provide an opportunity to further investigate the environments represented by calcite and dolomite-bearing red siltstone facies in the Juniata Formation, possible biologically induced sedimentary structures that remain undocumented, and the diagenetic processes that led to their preservation.

Samples were collected from outcrop as well as float from two roadcuts along Clinch Mountain. Samples were cut with a water-cooled rock saw to provide lateral and vertical views of unweathered structures; serial thick sections were also cut where possible to observe tubular structures in three dimensions. Thin sections were prepared for description of matrix micromorphology and indications of the environmental context of the calcite structures. Scanning electron microscopy with X-ray dispersive spectroscopy is also being used to further study morphology of the structures and assess their origin and preservation through elemental mapping.

At least two expressions of branching structures have been discovered at both localities, varying in length (~5cm-20cm), with some presenting clear downward tapering. Large (2-10 mm wide), distinct halos around the calcite structures are present with concentric banding, although further testing is needed to fully understand definite mineralogical composition and the more specific geochemical causal processes. 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 for understanding the evolution of continental and marginal marine ecosystems during a critical episode of biotic change in the Late Ordovician.

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