WHY THE SAUK TRANSGRESSION IS NOT JUST ANOTHER TYPE OF SHORELINE ADVANCE

Cambrian epicratonic deposits of the Sauk Sequence are commonly expressed as a sandstone-shale-limestone motif nonconformably overlying older Precambrian basement. The Sauk transgression apparently did not encounter structural basins along its historically high east-west advance toward the continent center. This implies that facies stacking patterns of the epicratonic Sauk transgression arise from mechanisms that are fundamentally different from those that typify passive continental margin transgressive-regressive cycles. Sediment transport is not easily cast as moving basinward from a landward source (from onshore to offshore) on such low slopes. Mechanisms of offshore sediment advection and ravinement development, namely wave and storm action, longshore drift, and geostrophic currents are less significant, if not absent, in such shallow dissipative settings.

We present case studies typifying each phase of Sauk I-II-III transgression, in a transect from the southern Great Basin to the Upper Mississippi Valley to highlight ways in which passive margin facies stacking patterns successively depart from the classic Waltherian mode of shoreline shift to that of the less familiar epicratonic sequences. Whereas accommodation space was governed in the long term by subsidence from loading by water column weight, accommodation space was governed in the short term by migration of channelized advective foci rather than regional slope-actuated/wave-induced bottom shear. Nonmarine regolith remained resident or was transported locally through tides and winds, ultimately equilibrating to the local transient hydraulic regime.

A 1 m diurnal tide on a cratonic slope of 1:50,000 over a 1000 km^2 area, comparable to the tidally-influenced, sheet-like strata of the Bighorn Basin and Grand Canyon region, results in a minimum peak flow rate >23,000 m^3/s. This is ~136% the average constant flow rate of the Mississippi River. Exchange of this volume of water is untenable, and demonstrates that communication of epicratonic waters with well-mixed open ocean marine water cannot be assumed under conditions of such extreme tidal reach. Thus the notion of marine vs. nonmarine influences in Cambrian epicratonic deposits should be recast in terms of stability vs. transitiveness of various admixtures of marine and fresh waters.