MASS EXTINCTION AND DIACHRONISM: RECONCILING THE END ORDOVICIAN ROCK RECORD

New analytical methodologies are rapidly advancing our understanding of Earth’s litho-, hydro-, and biosphere as well as the severe disruption to these systems that occur during mass extinctions. Yet, interpretations of fossil diversity trends and geochemical information used to reconstruct ancient oceans, atmosphere, and ecosystems are biased by the completeness of the stratigraphic record. Identifying temporal gaps in the stratigraphic record has been a vexing problem since the inception of modern geology, limiting robust interpretation of this new data. Here we integrate these same geochemical data within a sequence stratigraphic framework to determine the temporal completeness of the Upper Ordovician (Katian-Hirnantian) record.

Eastern North America is an ideal place to resolve questions regarding Upper Ordovician stratigraphy. This region features an unrivaled tropical marine record that was deposited over ~4.5 million km². Exposed by highways that stretch from the foreland basin deposits of the Appalachian Mountains to the quiescent carbonate platforms of the continental interior, this Ordovician succession is also penetrated by hundreds of publically available subsurface drill cores. Integrated biostratigraphy, high-resolution chemostratigraphy, and facies analysis from hundreds of these localities allow a high-resolution assessment of the Upper Ordovician stratigraphic record. Through highly detailed and integrative analysis of closely spaced sections, we can now detect the most cryptic of unconformities. Along multiple regional cross sections we also identify onlap-offlap patterns that reflect severe regional diachronism that juxtaposes the records of unrelated Late Ordovician events within individual sections.

The complexity of the Upper Ordovician record has stymied regional to global correlation for over a century and is now the major limiting factor in the interpretation of new geochemical proxies. Our holistic reconstruction of the Upper Ordovician of eastern North America demonstrates a methodology to identify temporal gaps and provide precise stratigraphic correlations critical in reconstructing the timing and interrelationships of earth-system processes that led to the end Ordovician mass extinction.