Abstract The Sauk megasequence in the far inboard region of the cratonic interior of North America (Minnesota, Wisconsin, and Iowa) is divisible into two packages that fundamentally differ from one another in facies and stratigraphic attributes. A lower Sauk succession package, Marjuman–early Skullrockian in age, is characterized by deposits of the traditional inner detrital belt (IDB) that interfinger hundreds of kilometers seaward with the middle carbonate belt or cratonward margin of the central mid-continent great American carbonate bank (GACB). The IDB contains a typical suite of nearshore siliciclastic facies containing features that document the importance of both wave- and tide-dominated currents in the depositional system. The transitional area between the IDB and the GACB in the Cambrian and earliest Ordovician was a moat, characterized by relatively deep-water deposition, which served as a catchment for mud that was winnowed from landward parts of the shelf and then deposited near the stormwave base. Mixed carbonate and siliciclastic facies in the moat are characterized by condensation features and other attributes indicative of suppressed carbonate productivity and starvation of siliciclastic sand. These facies contrast with shallower water facies that commonly filled available accommodation space in both seaward (central part of the GACB) and landward (cratonic shoreline) directions, the former dominated by typical stacks of oolitic, ribbon-rock, and microbialite lithofacies, and the latter by stacks of nearshore siliciclastic sand-dominated parasequences. Our chronostratigraphic framework provides temporal constraints that support the long-postulated hypothesis that these two depositional systems expanded and contracted in reciprocating fashion: substantial landward migration and expansion of the GACB occurred when siliciclastic input was diminished during the most rapid rates of transgression (marked by maximum flooding intervals in the IDB). Retreat and diminishment in the extent of the GACB corresponded to falls in sea level that led to major progradations of nearshore siliciclastics of the IDB and terrigenous poisoning of the carbonate factory. An overlying upper Sauk succession package records the establishment of a fundamentally different depositional system in the far inboard regions of the cratonic interior beginning in the later Skullrockian. The Prairie du Chien Group and its equivalents represent a major landward migration and perhaps cratonwide distribution of the oolitic, ribbon-rock, and micro-bialite lithofacies that were previously restricted mostly to the GACB of Missouri and adjacent areas. This change was triggered by a pronounced continental-scale flooding event that led to onlap across much, or all, of the cratonic interior. The resultant burial of terrigenous source regions by carbonate strata is in part responsible for this fundamental change in de-positional conditions.

Abstract The oldest Paleozoic rocks exposed in Iowa are found in the northeastern corner of the state in Allamakee County. Cambrian exposures in the county form part of the classic reference area in the Upper Mississippi Valley for the Upper Cambrian Croixan Series. Overlying dolomites of the Prairie du Chien Group are exposed as bold cliffs in the bluffs along the Mississippi River and its tributaries. Cambrian and Lower Ordovician strata collectively encompass the Sauk Sequence, the first large-scale transgressive-regressive cratonic marine cycle of the Phanerozoic (Sloss, 1963). A major episode of erosion followed deposition of the Sauk Sequence. A widespread unconformity separates Lower and Middle Ordovician strata in the region, marking the boundary between the Sauk Sequence and the succeeding Tippecanoe Sequence. The most complete sequence of Cambrian and Ordovician strata in northeast Iowa is exposed between Lansing and Church as one ascends from the Mississippi River up the valley of Clear Creek to the crest of Lansing Ridge along Iowa 9 (Fig. 1) 1). Vertical relief along this profile is 630 ft (192 m), and the composite stratigraphic section measured totals 619 ft (189 m). The base of the section begins along Iowa 26 north of the Mississippi River bridge (loc. 1, Fig. 1), and continues through a series of roadcuts and natural exposures in Mount Hosmer Park (loc. 4, Fig. 1). Strata equivalent to the lower half of the section at locality 1 can be seen behind Knopf’s Standard Station