AN ANOMALOUS GLAUCARENITE AT THE BASAL STEPTEOAN EXTINCTION HORIZON IN CENTRAL IOWA: THICKENING OF THE WIND-MIXED LAYER?

by
Matthew R. Saltzman and Robert M. McKay

The Geological Society of America
33rd Annual North-Central Section Meeting
Clarion Hotel and Convention Center, Champaign, IL, April 22-23, 1999
1999 Abstracts with Programs, v. 31, no. 5, p. A-69

ABSTRACT


Carbon isotope-based stratigraphy from an Upper Cambrian core (Rhinehart A-1) in central Iowa records a large positive excursion (SPICE event) that reaches peak ratios of +3.8‰. The predominantly carbonate section contains abundant glauconite and hardgrounds and is clearly condensed: 12 meters record the SPICE excursion in Iowa versus some 200 m further west at Shingle Pass in Nevada. The beginning of the excursion in Iowa coincides with mass extinction at the base of the Steptoean Stage, which has been narrowed to 30 cm in the core. Significantly, the crisis fauna (Coosella as identified by A.R. Palmer) occurs within an anomalous, oxidized glauconitic arenite bed that contains highly abraded gastropod, trilobite, echinoderm and brachiopod fragments. We interpret this to indicate a prolonged ‘agitation event,’ perhaps indicative of a thickening of the base of the surface wind-mixed layer during an increase in storm activity. Regression could also have caused the base of the mixed layer to impinge on the seafloor, although some coeval outcrops in the clastic facies of Wisconsin and Minnesota record a flooding surface at the extinction horizon. The formation of this glaucarenite may signal the disruption of a stratified water column that ultimately resulted in extinctions.

The peak of the SPICE excursion coincides with an interval of cross-stratified trilobite grainstones and minor coarse-grained, rounded quartz sand. This interval in the core is interpreted to reflect maximum regression associated with the formation of the Sauk II-Sauk III subsequence boundary. The peak of the excursion elsewhere in North America also correlates with evidence for sea-level fall during Dunderbergia Zone time (mid-Pterocephaliid biomere/Steptoean Stage). This establishes a firm link between sea level and carbon cycling that likely reflects drawdown of atmospheric carbon dioxide and global cooling during enhanced primary productivity/organic matter burial.