THE SAUK SEQUENCE OF THE CRATONIC INTERIOR: IS IT REALLY SO "SPECIAL"?

It is ironic that the Paleozoic strata of the North American cratonic interior that gave birth to the concept of stratigraphic sequences by L.L. Sloss 40 years ago are now commonly regarded as so anomalous that they are inscrutable to modern sequence stratigraphic practices. As recently as 1996 Sloss urged great caution in the application of sequence stratigraphic methodology to strata deposited in the "special conditions" of hypsometry, bathymetry, and tectonism of the cratonic interior.

Our work on what is commonly regarded as the most enigmatic part of the cratonic interior succession, the Upper Cambrian siliciclastics of the Sauk Sequence (particularly its sheet sandstones), has revealed remarkable similarity to much younger stratal packages that are regarded as textbook examples amenable to modern sequence stratigraphic analysis. The key to deciphering stratal architecture is regional investigation, use of high-resolution biostratigraphy, core and petrophysical logs, and deployment of multidisciplinary techniques to identify subtle sequence-bounding unconformities. For example, high-resolution timelines and subsurface geometry demonstrated that sheet sandstones internally consist largely of a relatively "ordinary" arrangement of shingled, progradational parasequences that show typical and predictable stacking patterns. The only difference between these parasequences and those of most younger nearshore successions is that the systems tracts are stretched laterally great distances in proportion to their thickness.

Our results refute a number of widely held misconceptions of the cratonic interior Sauk Sequence that are in large part responsible for its enigmatic reputation. These include: it is substantially incomplete temporally (most and potentially all common Upper Cambrian biozones are represented, but shingled laterally rather than stacked vertically); the sheet sandstones lie atop regional unconformities (they downlap onto maximum flooding surfaces); it contains remarkably little shale (there is abundant shale in subsurface sections); and operative marine processes differed fundamentally from those that influenced deposition outside of the cratonic interior (current, wave, and tide activity can all be documented).