SAUK SEQUENCE DIVISIONS: INTERNATIONAL IMPLICATIONS

Sequences were originally defined by Sloss in 1963 as assemblages of strata bounded by major interregional unconformities. If stratigraphic history had first been worked out in Laurentia, these would have most likely been our Systems. Sequences now come in a myriad of sizes accompanied by the elaborate reductionist vocabulary of sequence stratigraphy. There are perhaps important differences between the boundaries of these relatively minor features and the sequence and subsequence boundaries of the “classical” Sloss type.

When Sloss defined the “classical” Sauk Sequence, there was not sufficient synoptic data on Cambrian deposits to recognize that there were two intervals within Cambrian time that deserved recognition as major sequence boundaries. By the early 1980’s the outlines of these subsequences had come into focus and Sauk I, II and III were born. Further examination of Cambrian successions on other Cambrian continents has demonstrated that the events represented by these subsequence boundaries are widespread. However, the subsequence boundary intervals are far from straightforward. Rather than being simple unconformities, some boundary intervals represent periods of minimal sedimentation resulting in condensed stratigraphic sections; others may contain multiple disconformities.

In the past decade, the Sauk II/III boundary has been shown to coincide with the peak of a dramatic global positive shift in the d¹³C isotope curve now known as the SPICE event. Although a similar spike might be expected at the Sauk I/II boundary, one has not yet materialized. Perhaps the right stratigraphic intervals in the places where the stratigraphic record seems to be most complete have not yet been sampled. Nevertheless, intercontinental recognition of unusual stratigraphic perturbations represented by the Sauk subsequence boundaries, and the accompanying SPICE event for Sauk II/III, strongly suggest these reflect important events in early Phanerozoic history on a supra-continental scale.