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

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.