FACIES AND SEQUENCE STRATIGRAPHIC FRAMEWORK OF THE SAUK II SUPERSEQUENCE, THE CAMBRIAN MIAOLINGIAN-EARLY FURONGIAN POTSDAM GROUP, ILLINOIS BASIN

The Cambrian-Lower Ordovician Sauk megasequence in Illinois comprises a succession of mixed carbonates and siliciclastics, but dominantly carbonate deposits. It was mainly deposited in carbonate platform and the associated northern inner detrital facies belt that periodically shifted on the Laurentian continent because of global sea level fluctuations. Here, the Sauk succession is subdivided into Potsdam and Knox groups comprising the Sauk II and Sauk III supersequences, respectively. This study focuses on facies and sequence stratigraphic analyses of the Potsdam Group in the Illinois Basin.

The unconformity bounded Potsdam Group in Illinois comprises Sauk II supersequence, which began to form on the Precambrian basement and ended with the late Dresbachian regression and correlates with the middle part of the Potsdam Group in northeast New York. It includes Mt. Simon Sandstone, Eau Claire Formation, and the overlying Galesville-Ironton sandstone interval in northern Illinois and their coeval carbonate-siliciclastic strata in southern Illinois. Following the establishment of the intracratonic Illinois basin in the Middle Cambrian, gradual sea level rise toward the end of Mt. Simon deposition resulted in the development of an extensive mixed siliciclastic-carbonate platform, part of the “great American carbonate bank”, in southern Illinois that transitioned northward to a siliciclastic belt. The platform was a distally steepened ramp in which carbonate sediments accumulated in the inner ramp, ramp margin ooid-bioclastic shoals, or exported and accumulated in the basinal setting of the Reelfoot-Rough Creek Graben. The Potsdam succession in Illinois is characterized by a long-term second-order transgressive-regressive cycle comprised of continental deposits (fluvial, playa lake/lacustrine, and eolian facies) in the lower part of Mt. Simon that transition to shallow marine facies upward through the overlying Eau Claire/Bonneterre Formation. It is superimposed by third-order depositional sequences with the uppermost sequence consisting of the upper Eau Claire/Bonneterre transgressive tract and the highstand package comprised of coarsening upward Galesville-Ironton interval in northern Illinois and its basinward carbonate equivalent in the deeper part of the Illinois Basin.