Paper No. 260-6
Presentation Time: 2:55 PM
NEW CONSTRAINTS ON THE AGE OF THE ELLIS BAY FORMATION, ANTICOSTI ISLAND, CANADA: A REVISED FRAMEWORK FOR UNDERSTANDING THE LATE ORDOVICIAN MASS EXTINCTION
ZIMMT, Joshua, Department of Integrative Biology and Museum of Paleontology, University of California Berkeley, 1101 Valley Life Sciences Bldg, Berkeley, CA 94720, CAPPELLO, Mariko, School of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8P5C2, Canada, JONES, David S., Geology Department, Amherst College, Amherst, MA 01002, JIN, Jisuo, Department of Earth Sciences, University of Western Ontario, London, ON N6A 5B7, Canada, HUSSON, Jon M., School of Earth and Ocean Sciences, University of Victoria, 3800 Finnerty Road Bob Wright Center A405, Victoria, BC V8P 5C2, Canada, SCHOENE, Blair, Department of Geosciences, Princeton University, Guyot Hall, Princeton, NJ 08544, MITCHELL, Charles, Department of Geology, University at Buffalo, The State University of New York, 126 Cooke Hall, University at Buffalo, Buffalo, NY 14260, MELCHIN, Michael, St. Francis Xavier UniversityEarth Sciences, Antigonish, NS B2G2V5, CANADA, FINNEGAN, Seth, Department of Integrative Biology, University of California Berkeley, 5151A Valley Life Sciences, Be, CA 94720-3140 and DESROCHERS, André, Department of Earth and Environmental Sciences, University of Ottawa, Ottawa, ON K1N 6N5, Canada
Anticosti Island, Canada, exposes a well-preserved and fossiliferous Ordovician-Silurian boundary succession that is one of the best-studied records of climatic, environmental, and biological events associated with the Late Ordovician mass extinction. However, the base of the terminal Ordovician stage, the Hirnantian, has been difficult to confidently identify on Anticosti due to the paucity of diagnostic graptolites, the apparent absence of a typical Hirnantia fauna within the Upper Ordovician Ellis Bay Formation, and lateral facies variability among outcrops. This has led to differing interpretations of the age of the Ellis Bay Formation, which in turn has hindered efforts to incorporate paleobiological and geochemical data from Anticosti into our global understanding of the Late Ordovician mass extinction.
Here we take an integrative approach to addressing the geologic age of the Ellis Bay Formation, combining new paleobiological, geochemical, radiometric, and sequence stratigraphic constraints from ongoing fieldwork with existing biostratigraphic, geochemical, and palynological studies in the context of newly measured stratigraphic sections. We conclude that multiple lines of evidence favor a Hirnantian age for the entire Ellis Bay Formation, including newly recognized occurrences of Hirnantia and Hindella in the lower Ellis Bay Formation, a two-phased positive carbon isotope excursion, with the second phase reaching ~6‰ in the Laframboise Member, and a U-Pb TIMS age of 443.61±0.52 Ma from zircons in a bentonite from the mid-Ellis Bay Formation. Graptolite and conodont biostratigraphy supports this age model, while determination based on chitinozoan biozonation is more equivocal. Conodont, brachiopod, and chemostratigraphic data additionally suggest that the Hirnantian Stage may extend slightly into the lower Becscie Formation on the western end of Anticosti and well into the lower Becscie Formation in the eastern part of Anticosti. Our reappraisal of a classic early Paleozoic section has important implications for understanding the sequence of climatic, environmental, and biological events across the Ordovician-Silurian boundary and provides a new basis for the integration of Anticosti Island into global assessments of the Late Ordovician mass extinction.
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