2011 GSA Annual Meeting in Minneapolis (912 October 2011)
Paper No. 225-12
Presentation Time: 11:00 AM-11:15 AM

THE GREAT UNCONFORMITY: A SMOKING GUn FOR THE CAMBRIAN EXPLOSION?

PETERS, Shanan E., Department of Geoscience, University of Wisconsin-Madison, 1215 W Dayton St, Madison, WI 53706, peters@geology.wisc.edu and GAINES, Robert R., Geology, Pomona College, 185 East 6th Street Claremont, Claremont, CA 91711

The Great Unconformity (GUn) is a globally-occurring stratigraphic surface that typically separates Cambrian marine sedimentary rocks deposited on Earth’s surface from much older continental crystalline basement rocks that were formed and/or metamorphosed at depth. Here we use macrostratigraphic, sedimentological and geochemical data to characterize the GUn and patterns of physical and chemical sedimentation during the Cambrian-Early Ordovician Sauk Transgression. We find that early Paleozoic sediments record both a dramatic expansion in the area of shallow epicontinental seas and anomalous patterns of chemical sedimentation that are indicative of elevated oceanic alkalinity and enhanced chemical weathering of continental crust, including a Phanerozoic peak in glauconite abundance in siliciclastic sediments, Phanerozoic maxima in shelf carbonate sedimentation rates, and the occurrence of abundant authigenic carbonate cements in offshore mudstones that have ∂13C values indicative of direct precipitation from seawater. Such geochemical conditions were caused by a protracted period of widespread continental denudation during the Neoproterozoic followed by sustained shoreface erosion during the first continent-scale marine transgression of the Phanerozoic. Although Darwin and others have interpreted the resultant widespread hiatus in sedimentation on the continents as a failure of the stratigraphic record, the GUn in fact represents a fundamentally unique physical environmental boundary condition that strongly affected seawater chemistry during a time of expanding shallow epicontinental seas. Thus, the formation of the GUn may have served as an important environmental trigger for a very real Cambrian “Explosion” of biomineralized marine animals.

2011 GSA Annual Meeting in Minneapolis (912 October 2011)
General Information for this Meeting
Session No. 225
Paleontology III: Diversity, Extinction, and Origination
Minneapolis Convention Center: Room 205AB
8:00 AM-12:00 PM, Wednesday, 12 October 2011

Geological Society of America Abstracts with Programs, Vol. 43, No. 5, p. 543

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