UPPER CAMBRIAN EXTINCTION, CARBON ISOTOPE SHIFTS AND THE SAUK II - SAUK III SEA LEVEL EVENT: NEW EVIDENCE FROM NORTHERN UTAH, NEWFOUNDLAND AND THE MIDCONTINENT

Three significant paleoceanographic events are juxtaposed in Upper Cambrian (Steptoean Stage) sequences of Laurentia: 1) a mass extinction of trilobites that marks the base of the Steptoean (Marjumiid-Pteroceaphliid biomere); 2) a large positive excursion in carbon isotope values that spans much of the Steptoean (SPICE event); and 3) an imprecisely dated craton-wide drop in sea level (Sauk II – Sauk III event) that was terminated by widespread flooding in the Late Steptoean (mid-late Elvinia Zone time). The first two events are clearly global in scope, but the scale and timing of the sea level drop is not known in detail outside Laurentia. We have recently undertaken a project aimed at evaluating whether there are demonstrable cause and effect relationships among these events in widely separated Laurentian sections.

New carbon isotope data from sequences in western Newfoundland and northern Utah indicate that the acme of the Sauk II – Sauk III regression and the peak of the SPICE excursion are essentially synchronous events. This pattern is identical to that seen in the Upper Mississippi Valley, as determined previously by two of us working independently on a core in central Iowa. The simplest explanation is glaciation, though we continue to explore alternatives. The precise relationship between the basal Steptoean extinction event and the beginning of the SPICE are more problematic, although they also appear to correlate closely in time. The rapid mixing of sub-thermoclinal waters onto the shelf remains a viable hypothesis. Perhaps the most challenging question relates to the nature of sea level changes during the period of time spanning the base of the Steptoean Stage and prior to the mid-Steptoean acme of regression. Our preliminary analysis suggests that a brief sea level rise marks the basal Steptoean interval, and that the more protracted Sauk II – Sauk III fall in sea level begins relatively early in the Steptoean.