TIMING OF UPPER DEVONIAN MARINE FLOODING EVENTS AND EPISODES OF ORGANIC-RICH SWEETLAND CREEK, GRASSY CREEK AND SAVERTON SHALE DEPOSITION IN THE IOWA AND N.W. ILLINOIS BASIN

The Upper Devonian deeper-water organic-shale facies of the Sweetland Creek, Grassy Creek and lower Saverton Shales in southeastern Iowa and western Illinois comprise the shale-dominated portions of Late Frasnian and Famennian depositional sequences in deeper parts of the Iowa Basin where it opened into the northwestern margin of the Illinois basin. Organic-rich facies are associated with the first six of eight Upper Devonian sequences in the region.

The Lower Lime Creek-Sweetland Creek sequence onlaps a deeply eroded unconformity and consists of an expansive carbonate-evaporite platform (Lime Creek Fm.) in the main part of the Iowa Basin with condensed carbonates and organic-rich Sweetland Creek Shale restricted to southeastern Iowa during cycle IId-1 (Montagne Noire Zones 11-12). The very Late Frasnian Upper Lime Creek-Lower Grassy Creek-1 sequence is recorded by organic–rich Grassy Creek shales restricted to and persisting in southeastern Iowa and western the Illinois Basin during cycle IId-2 (M.N. Zones 13a-13c to Lower triangularis Zone). The Lower Grassy Creek-2 sequence is restricted to southeast Iowa where it includes organic-shale facies (lower IIe-1 cycle, Middle triangularis-Middle crepida zones). The Lower Sheffield-Lower Grassy Creek-3 sequence records a major onlap of older Frasnian strata and seaway expansion into central and northern Iowa within the Middle crepida Zone (middle IIe-1 cycle). The Upper Grassy Creek-Upper Sheffield sequence records the maximum expansion of organic-shale facies into southern Iowa (Lower marginifera Zone, lower IIe-2); it shallows upward and westward into fossiliferous green-gray shale facies. The Upper Grassy Creek-Lower Saverton-Aplington sequence includes the last organic-rich shales, oversteps the Sheffield edge, and onlaps older Frasnian strata across central and western Iowa (middle IIe-2 cycle, trachytera Zone). The English River sequence (cycle IIf-1, Lower-Middle expansa Zone) transgression is marked by widespread stratigraphic condensation (oolitic ironstone and phosphorite facies) and gray shale and siltstone deposition. Geographically restricted carbonate platform deposits of the Lousiana Limestone sequence (cycle IIf-2, Middle?-Upper praesulcata zones) abruptly overstep English River sequence deposits where preserved.