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Distal signatures of Late Ordovician oceanic anoxia—New data from a classic epeiric ramp transect

By
Patrick I. McLaughlin
Patrick I. McLaughlin
Wisconsin Geological and Natural History Survey, Madison, Wisconsin 53705, USA
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Norlene Emerson
Norlene Emerson
Department of Geography and Geology, University of Wisconsin–Richland, 1200 Highway 14 West, Richland Center, Wisconsin 53581, USA
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Brian Witzke
Brian Witzke
Department of Geoscience, University of Iowa, Iowa City, Iowa 52242, USA
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Bryan Sell
Bryan Sell
University of Geneva, Rue de Maraîchers 13, 1205 Geneva, Switzerland
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Poul Emsbo
Poul Emsbo
U.S. Geological Survey, Federal Center, Denver, Colorado 80225, USA
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Published:
January 01, 2011

ABSTRACT

New data and review of classic sections from the Middle and Upper Ordovician North American Midcontinent in the Upper Mississippi Valley provide a refined picture of the age, stable isotope geochemistry, faunal composition, and—ultimately—origin of this epeiric ramp succession. Sequence stratigraphic analysis reveals a series of unconformity-bounded, genetically related facies packages. Shallowing and deepening trends are sometimes difficult to resolve due to a paucity of hydrodynamic indicators, yet unconformity surfaces are well marked by hardgrounds and confirmed by negative C-isotope spikes. Recent conodont biostratigraphy, new U-Pb radioisotopic ages for K-bentonites, and correlation of C-isotope profiles to global trends suggest that the succession spans the Darriwilian to Hirnantian epochs. Focus on Platteville to lower Galena Group strata (Sandbian to early Katian) provides a temporally high-resolution look at the onset and evolution of a long-term (>2 m.y.) positive carbon-isotope excursion, short-term perturbations in that record, and relationship to the preservation and diversity of the enclosed fauna and strata. Major changes in authigenic mineral suites and organic carbon content throughout the Upper Ordovician Upper Mississippi Valley suggest at least three major redox cycles. The combined evidence for globally recognized, positive carbon-isotope excursions coincident with these redox cycles, as well as high-frequency, sea-level fluctuations and successive faunal turnover events, suggests far-field responses to multiple global oceanic anoxic events.

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Contents

GSA Field Guide

Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America

James D. Miller
James D. Miller
Department of Geological Sciences and Precambrian Research Center University of Minnesota-Duluth Duluth, Minnesota 55812 USA
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George J. Hudak
George J. Hudak
Precambrian Research Center Natural Resources Research Institute 5013 Miller Trunk Highway Duluth, Minnesota 55811 USA
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Chad Wittkop
Chad Wittkop
Department of Chemistry and Geology Minnesota State University Mankato, Minnesota 56001 USA
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Patrick I. McLaughlin
Patrick I. McLaughlin
Wisconsin Geological and Natural History Survey Madison, Wisconsin 53705 USA
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Geological Society of America
Volume
24
ISBN electronic:
9780813756240
Publication date:
January 01, 2011

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