FAMENNIAN (UPPER DEVONIAN) STRATIGRAPHIC SEQUENCES, TRANSGRESSIVE EVENTS, AND DEPOSITIONAL PATTERNS IN THE IOWA AREA

The succession of onlapping Famennian sequences across Iowa records the transition from deeper-water organic-shale facies along the northwestern margin of the Illinois Basin to shallower-water mixed carbonate-siliciclastic facies of the central Iowa platform. In general, the initiation of marine transgression in each sequence correlates with several “transgressive starts” proposed by Johnson et al. (1989). 1) The Lower Grassy Creek sequence is restricted to southeast Iowa where it includes organic-shale facies (lower cycle IIe-1). 2) The Sheffield Shale unconformably onlaps Frasnian strata into central and northern Iowa (middle IIe-1). 3) The Upper Grassy Creek/upper Sheffield sequence records the maximum expansion of organic-shale facies in Iowa (Lower marginifera Zone, lower IIe-2); it shallows upward and westward into fossiliferous green-gray shale facies. 4) The Aplington carbonate-shale sequence oversteps the Sheffield edge to unconformably overlie Frasnian strata across all of central and western Iowa, where it includes fossiliferous carbonate and shale facies (middle IIe-2). 5) Regional depositional deepening is recorded by transgression of the English River sequence (cycle IIf); widespread stratigraphic condensation is marked by deposition of oolitic ironstone and phosphatic facies, capped by fossiliferous siltstone and shale facies. 6) The Lousiana Limestone forms the youngest Famennian sequence (Upper praesulcata Zone, upper IIf), but it is restricted to southeastern-most Iowa and adjoining areas of Missouri and Illinois. The stratigraphic succession in southeast Iowa forms one of the most complete Famennian sections known in North America, but certain palmatolepid conodont ranges appear to be discordant with those of the “Standard Zonation.”