USING HORIZON ANNEALING, AN INTEGRATIVE QUANTITATIVE SEQUENCING APPROACH, TO PRODUCE A MORE PRECISE TIME SCALE FOR THE MOHAWKIAN STAGE (UPPER ORDOVICIAN)

For over a hundred years, biostratigraphy has been used for refining time scales. However, traditional zone-based methods provide only a limited means to refine the geologic timescale. Biozones typically are millions of years in duration, are limited to particular facies in which the target organisms are common, and are qualitative, relatively subjective interpretations of often conflicting species occurrence patterns. As we seek more detailed questions about precise causes for precise events those questions require much more precise timescales in which to locate geological events in space and time.

Sheets and Mitchell have developed an approach to automated sequencing of events, Horizon Annealing (HA), which shares much with CONOP (both seek to minimize species ranges) but complements that approach by ordering samples based on their fossil content rather than by ordering the first and last appearances of species. This approach yields a high resolution event sequence that integrates multiple data types and facies sets and also provides a means to probabilistically assess the confidence levels associated with event correlations. Here we used HA to sequence events from a set of more > 30-Mohawkian (specifically the C. bicornis– G. pygmaeuszones interval, 460-447-MA, within the Late Ordovician) stratigraphic sections in eastern Laurentia (New York State, Pennsylvania, Ohio, Ontario, and Quebec) as a framework to test these techniques. The data set comprises 1) biostratigraphic data from graptolites, conodonts, and chitinozoans; 2) accepted dates, and 3) occurrences of more than ten fingerprinted K-bentonite beds.

The Mohawkian interval is relatively well studied in terms of biostratigraphy and tectonic history, yet many questions remain related to the detailed history of events and time-transgressive facies changes (Sell et al. 2015). This dataset will contribute to the construction of a more accurate global timescale, which in turn will be used to better constrain timing rates of events associated with the evolution of this foreland basin and its interaction with eustacy and global climate change during the Late Ordovician shift from greenhouse to icehouse conditions.