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Book Chapter

Sudbury impact layer in the western Lake Superior region

By
Mark A. Jirsa
Mark A. Jirsa
Minnesota Geological Survey, 2642 University Avenue, St. Paul, Minnesota, 55114-1032, USA,
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;
Philip W. Fralick
Philip W. Fralick
Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada,
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;
Paul W. Weiblen
Paul W. Weiblen
Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota, USA,
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;
Jennifer L.B. Anderson
Jennifer L.B. Anderson
Geoscience Department, Winona State University, Winona, Minnesota 55987, USA,
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Published:
January 01, 2011

ABSTRACT

This field trip examines a sequence of ejecta and deformed substrate resulting from the 1850 Ma meteorite impact. An impact origin for the Sudbury structure in Ontario has long been accepted, but knowledge of the corresponding ejecta was limited to fall-back breccia in the relict crater at Sudbury. The more distant ejecta blanket was discovered only recently near Thunder Bay, Ontario, and later in other parts of the Lake Superior region. Known informally as the Sudbury impact layer (SIL), it occurs at and near the stratigraphic top of Paleoproterozoic iron-formation. The impact-related deposits in the western Lake Superior region include (1) autochthonous material interpreted to be seismically folded and shattered iron-formation and carbonate rocks (breccia), overlain by (2) strata composed largely of allochtho-nous material (ejecta) derived in part from target rocks, and (3) irregular layers that appear to be mixtures of locally and distally derived material. Definitive microscopic evidence of an impact origin includes the occurrence of accretionary lapilli, ash pellets, spherules, devitrified glass, and quartz fragments marked by planar deformation features. The SIL exhibits extreme lithologic variability from place to place within each exposure area and between exposure areas. Nevertheless, the stratigraphic relationships that are presented by these exposures can be used to devise a sequence of deformation and depositional events that is largely consistent with experimental and empirical evidence of impact processes. This field trip will demonstrate that the stratigraphic arrangement of facies in the SIL has important temporal implications for understanding mechanisms of ejecta delivery and deposition.

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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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