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Terminal Ordovician carbon isotope stratigraphy and glacioeustatic sea-level change across Anticosti Island (Québec, Canada)

Jones, David S. and Fike, David A. and Finnegan, Seth and Fischer, Woodward W. and Schrag, Daniel P. and McCay, Dwight (2011) Terminal Ordovician carbon isotope stratigraphy and glacioeustatic sea-level change across Anticosti Island (Québec, Canada). Geological Society of America Bulletin, 123 (7-8). pp. 1645-1664. ISSN 0016-7606.

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Globally documented carbon isotope excursions provide time-varying signals that can be used for high-resolution stratigraphic correlation. We report detailed inorganic and organic carbon isotope curves from carbonate rocks of the Ellis Bay and Becscie Formations spanning the Ordovician-Silurian boundary on Anticosti Island, Québec, Canada. Strata of the Anticosti Basin record the development of a storm-dominated tropical carbonate ramp. These strata host the well-known Hirnantian positive carbon isotope excursion, which attains maximum values of ~4.5‰ in carbonate carbon of the Laframboise Member or the Fox Point Member of the Becscie Formation. The excursion also occurs in organic carbon, and δ^(13)C_carb and δ^(13)C_org values covary such that no reproducible Δ^(13)C (= δ^(13)C_carb – δ^(13)C_org) excursion is observed. The most complete stratigraphic section, at Laframboise Point in the west, shows the characteristic shape of the Hirnantian Stage excursion at the global stratotype section and point (GSSP) for the Hirnantian Stage in China and the Silurian System in Scotland. We therefore suggest that the entire Hirnantian Stage on Anticosti Island is confined to the Laframboise and lower Fox Point Members. By documenting discontinuities in the architecture of the carbon isotope curve at multiple stratigraphic sections spanning a proximal to distal transect across the sedimentary basin, we are able to reconstruct glacioeustatic sea-level fluctuations corresponding to maximum glacial conditions associated with the end-Ordovician ice age. The combined litho- and chemostratigraphic approach provides evidence for the diachroneity of the oncolite bed and Becscie limestones; the former transgresses from west to east, and the latter progrades from east to west. The sea-level curve consistent with our sequence-stratigraphic model indicates that glacioeustatic sea-level changes and the positive carbon isotope excursion were not perfectly coupled. Although the start of the isotope excursion and the initial sea-level drawdown were coincident, the peak of the isotope excursion did not occur until after sea level had begun to rise. Carbon isotope values did not return to baseline until well after the Anticosti ramp was reflooded. The sea-level–δ13Ccarb relationship proposed here is consistent with the “weathering” hypothesis for the origin of the Hirnantian δ^(13)C_carb excursion.

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URLURL TypeDescription DOIArticle
Fike, David A.0000-0003-2848-0328
Fischer, Woodward W.0000-0002-8836-3054
Additional Information:© 2011 Geological Society of America. Received 26 April 2010; Revision received 1 November 2010; Accepted 9 November 2010. First published online April 1, 2011. We are grateful to Bob Criss and Liz Hasenmueller for help with the δ^(13)C)carb and δ^(18)O_carb measurements at Washington University. Greg Eischeid supervised the mass spectrometry laboratory at Harvard University. We benefited from Jon Husson’s assistance in the field and thank Megan Rohrsson, Tim Raub, and Priya Nayak for valuable discussions at the outcrops. We thank the owners of the Salmon River Lodge for access to outcrops on the Salmon River. Dave Boulet and SEPAQ Anticosti granted permission to work in Anticosti National Park. We thank Paul Copper for suggesting outcrops to visit and for generously sharing his extensive knowledge of Anticosti geology. We thank Associate Editor Maya Elrick, Michael Joachimski, André Desrochers, and an anonymous referee for their helpful and thorough reviews of the manuscript. Paul Hoffman provided inspiration and guidance in the early phases of the project. The project was supported by a grant from the Agouron Institute to Fike and Fischer and a grant from the Henry Breck Fund through the Harvard University Center for the Environment to Jones.
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Agouron InstituteUNSPECIFIED
Harvard University Center for the Environment Henry Breck FundUNSPECIFIED
Issue or Number:7-8
Record Number:CaltechAUTHORS:20110621-120025387
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:24157
Deposited By: Jason Perez
Deposited On:21 Jun 2011 21:32
Last Modified:13 Dec 2019 00:48

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