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Tracing the Manson impact event across the Western Interior Cretaceous Seaway

By
David J. Varricchio
David J. Varricchio
Department of Earth Sciences, Montana State University, Bozeman, Montana 59717, USA
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Christian Koeberl
Christian Koeberl
Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
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Russell F. Raven
Russell F. Raven
6324 Wisteria Way, San Jose, California 95129, USA
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Wendy S. Wolbach
Wendy S. Wolbach
Department of Chemistry, DePaul University, Chicago, Illinois 60014-3214, USA
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William C. Elsik
William C. Elsik
The MycoStrat Connection, P.O. Box 549, Snook, Texas 77878, USA
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Daniel P. Miggins
Daniel P. Miggins
U.S. Geological Survey, Denver Federal Center, Box 25046, MS 963, Denver, Colorado 80225, USA
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Published:
September 01, 2010

The Campanian Manson impact structure of Iowa represents the best-preserved, large-diameter complex crater within the continental United States. The related bolide struck from the southeast at a low angle, potentially distributing ejecta downrange to the northwest across the Western Interior Cretaceous Seaway. Here, we (1) examine possible correlation of Manson impact horizons across the Cretaceous seaway to terrestrial formations of Montana, and (2) test a large hadrosaur bone bed from the Two Medicine Formation for evidence indicative of the Manson impact. The study includes geochronology; palynomorph, soot, and geochemical analyses; and physical searches for impact ejecta.

The impact ejecta–bearing Crow Creek Member of the marine Pierre Shale can be correlated to the SB2 discontinuity in the Judith River and Two Medicine Formations of Montana based on radiometric dates, ammonite zonation, and an association with the onset of the Bearpaw transgression. A 40Ar/39Ar analysis of an associated bentonite bed dates the hadrosaur bone bed (TM-003) to 75.92 ± 0.32 Ma referenced to MMhb-1 at 523.1 Ma. This bentonite and associated lacustrine units suggest a potential correlation with the SB2 and the Crow Creek Member. However, our examination of the bone bed produced no definitive impact evidence. The combined analyses did reveal three unusual aspects: (1) an abundance of Ulmoideipites sp., (2) a high soot content, and (3) elemental and mineralogical changes suggestive of distinct geochemical units. A major wildfire followed by a postcatastrophe bloom dominated by Ulmoideipites sp. likely preceded the eventual debris flow that generated the bone bed. The SB2 discontinuity and the 33n.3r magnetic subzone represent traceable stratigraphic markers that could serve as guides in future exploration for Manson impact evidence in terrestrial formations west of the seaway.

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Contents

GSA Special Papers

Large Meteorite Impacts and Planetary Evolution IV

Roger L. Gibson
Roger L. Gibson
Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa
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Wolf Uwe Reimold
Wolf Uwe Reimold
Museum für Naturkunde-Leibniz Institute at Humboldt University, Berlin, Germany
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Geological Society of America
Volume
465
ISBN print:
9780813724652
Publication date:
September 01, 2010

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