LITHOFACIES AND ND ISOTOPE STRATIGRAPHY OF THE KNOX UNCONFORMITY IN THE CENTRAL APPALACHIAN BASIN

The Middle to Late Ordovician Period (470 Ma - 450 Ma) was characterized by major changes in global climate as well as 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. Previous studies have focused extensively on the stratigraphic and sedimentological descriptions of formations above and below the unconformity, but have been unable to precisely constrain the position of the unconformity and amount of time missing, due to a lack of index fossils. This study aims to address the issue by conducting a detailed sedimentologic (polished hand sample) and petrographic analysis on samples from the Knox-Beekmantown and New Market formations from Collierstown (VA), in order to identify the stratigraphic position of the unconformity in this section, as well as using neodymium isotope chemostratigraphy (εNd(t)) to constrain the absence of deposition across the Knox-Beekmantown/New Market contact. The εNd(t) data collected in this study shows a “jump” of ~+7 epsilon units (εNd(t) = ~ –17 to –10) across the hypothesized Knox unconformity horizon, which is in contrast with existing records of shale εNd(t) in the Appalachian basin that show gradually increasing values, suggesting missing time. However, sedimentologic observations of the bulk rock polished hand samples found little evidence of an erosional contact at this interval. Therefore, within this study we attempt to correlate the geochemical evidence of the unconformity with physical evidence gathered through analysis of thin sections such as intraclasts/extraclasts which are indicative of an erosional event.