CHRONOSTRATIGRAPHY OF LATE DEVONIAN SANDSTONES FROM SOUTHWEST ILLINOIS AND EAST-CENTRAL MISSOURI

While Devonian eustatic fluctuations described by Johnson and others (1985) have been instrumental in improving chronostratigraphy, the relative importance of eustasy vs far-field tectonics in forming the sedimentary record in the U.S. Midcontinent is poorly understood. Here we present preliminary results on our efforts to refine the chronostratigraphy and provenance of Devonian sandstone units from southwestern Illinois and east-central Missouri (southwest margin of the Illinois Basin) using conodonts, detrital zircon analysis, bulk mineralogy and geochemistry, and petrographic properties. In southwestern Illinois, the Devonian Cedar Valley Group is overlain by very thin to thin-bedded sandstones of the Sylamore Sandstone Member of the New Albany Shale that contain conodonts, fish material, and phosphate pebbles. Previous studies have determined the Sylamore Sandstone is diachronous across the basin, spanning various intervals within the Frasnian, but most likely being deposited as part of the IIc Devonian T-R Cycle. Recently collected samples of the Sylamore Sandstone have so far yielded an icriodid dominated fauna. In East-Central Missouri, the Ordovician Cape Limestone, Kimswick Formation or Macquoketa Shale is overlain by the Devonian Sulphur Springs Group; the latter consisting of the Glen Park Limestone and overlying Bushberg Sandstone. The Glen Park Limestone is a fossiliferous grainstone containing abundant conodonts, fish material, and phosphate pebbles. The Bushberg Sandstone is an iron-stained fine-grained quartz sandstone containing meter-scale cross-beds that transition upwards to planar bedded burrowed sandstone. Previous studies have concluded that the Sulphur Springs Group is late Famennian in age, probably deposited during the IIf T-R Cycle. Recently collected Glen Park Limestone samples have so far yielded conodont faunas of polygnathids, palmatolepids, and rare icriodids as well as scolecodonts. Bushberg Sandstone samples have yet to yield conodonts. Interestingly, preliminary detrital zircon results for the Sylamore and Bushberg sandstones show a provenance signature similar to the Ordovician St. Peter Sandstone. This study will ultimately better constrain the age of these Devonian sandstones and the role of eustasy vs tectonics in the study area.