COMPOSITE UPPER ORDOVICIAN CHRONOSTRATIGRAPHY IN EASTERN NORTH AMERICA—DEVELOPING A LATE ORDOVICIAN GEOCHRONOLOGIC REFERENCE STANDARD OF ENVIRONMENTAL RECORDS

Geochronologic standards provide references for physical aspects of deep-time (e.g., sea level history, global temperature change, etc.)—powerful tools for understanding the processes that drive evolution of the surface Earth. However, the initial development and refinement of those standards requires compilation of extensive chronostratigraphic records. Our ongoing studies have generated geochemical and biostratigraphic data sets for the Upper Ordovician across major portions of eastern North America. While many authors have argued that Late Ordovician geologic history is well known, our work suggest otherwise.

Our chronostratigraphic approach integrates a variety of data within a sequence stratigraphic framework. Carbon isotopes, in particular, are heavily utilized—the total unpublished Upper Ordovician results exceed 7000 with an additional 3000+ awaiting analysis. These sample sets are primarily from continuous drill core and stratigraphically contiguous outcrops. Closely spaced sections and iterative sampling capture the full expression of the carbon isotope record and provides confidence in interpretation and the stratigraphic correlation of excursions. These sections are arranged as transects across basins and are integrated with biostratigraphy and facies analysis.

Previously unrecognized or disputed unconformity surfaces are clearly resolved using the integrated sequence stratigraphic method. Unconformities plotted at the 100 kyr-scale are abundant and time-transgressive and their correspondence with carbon isotope excursions suggests a genetic link. Longer duration unconformities form major onlap-offlap surfaces that are resolvable at the 1 Myr-scale and correspond to the development of large paleotopography and paleokarst features. In total, our comparison of Upper Ordovician chronostratigraphic records across major portions of eastern North America demonstrates the highly discontinuous nature of temporal preservation locally, but compilation of these records regionally provides a never-before seen continuum of events that shaped the chemistry of the oceans and atmosphere in Late Ordovician world and provides new perspective on the nature of the second largest extinction in the fossil record.