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 amount of time missing due to a lack of index fossils. Therefore, this study aims to address the issue by conducting a detailed sedimentologic (polished hand sample) and petrographic analysis on samples from the 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 Beekmantown - New Market contact.

Sedimentologic analysis found that samples from the Beekmantown are dominated by dolomitic mudstones/wackestone with little to no fossiliferous clasts, whereas samples from the New Market contain fenestral lime mudstones interspersed with isolated packstones/grainstones. Both Beekmantown and New Market samples bordering the hypothesized Knox unconformity contain lithoclasts indicative of an erosional event. A detailed thin section analysis is required to further characterize these features. Furthermore, εNd(t) data collected in this study show 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 a missing time in the rock succession. This study also provides evidence that the shift in provenance which caused the Nd values to increase was related to the uplift that caused the Knox unconformity.