Lithofacies and Neodymium Isotope Stratigraphy of the Knox Unconformity in the Central Appalachian Basin
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Date
2022-12
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The Ohio State University
Abstract
The Middle to Late Ordovician Period (470 Ma-450 Ma) was characterized by major changes in global climate and tectonics. One such event, the Taconic Orogeny, occurred when a series of island arcs collided with the southern margin of Laurentia (present day east coast of North America) converting the formerly passive margin into a convergent margin. Evidence of this orogeny is expressed in the Knox Unconformity, an erosional surface apparent in carbonate successions throughout southwest and northern Virginia. This study aims to conduct a detailed sedimentologic and petrographic analysis of carbonate samples from the Beekmantown Group and New Market Formation at Collierstown (VA) in order to constrain the age and the stratigraphic position of the Knox Unconformity. This study also attempts to use neodymium isotope chemostratigraphy (εNd(t)) to improve age constraints on the unconformity and address the implications for changes in silicate weathering and climate during this large mountain building event.
Samples from the Beekmantown Group are dominated by dolomitic mudstones/wackestones with little to no fossiliferous clasts, whereas samples from the New Market Formation contain calcareous mudstones, packstones, and grainstones and fenestral facies. Both Beekmantown and New Market polished slab samples and petrographic thin sections bordering the hypothesized Knox Unconformity contain angular lithoclasts indicative of an erosional event. This correlates to the way the Knox Unconformity was originally defined as a layer of conglomerates and breccia separating the Beekmantown Group and New Market Formation.
Furthermore, εNd(t) data collected in this study shows an abrupt, discontinuous shift of ~ +7 epsilon units (εNd(t)= ~ –17 to –10) across the hypothesized Knox Unconformity horizon, suggesting missing time in the rock succession. The shift in provenance which caused the Nd values to increase was likely related to the uplift that caused the Knox Unconformity. Previous studies have also linked the shift in εNd(t) towards more radiogenic values to a shift to a more mafic weathering source, and thus increased silicate weathering and regional cooling.
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Keywords
Geochemistry, Stratigraphy, Sedimentology, Ordovician Period, Carbonate Rocks, Taconic Orogeny