PROVENANCE OF EARLY-MIDDLE PENNSYLVANIAN SANDSTONES OF THE MIDCONTINENT: DETRITAL ZIRCON EVIDENCE FOR THE UNROOFING OF APPALACHIAN OROGENIES IN IOWA

Detrital zircons have been used to identify detritus derived from the Appalachian margin in places as far away as the Grand Canyon in Arizona. Based on this distal location, a westward-flowing, transcontinental Late Paleozoic sediment transport system has been proposed. A data gap exists, however, within the basins of midcontinent where the provenance of Lower-Middle Pennsylvanian sandstones that fill large incised valleys is still poorly understood. To determine the relative contributions of various sources of detritus to the intracratonic basins of the Laurentian midcontinent, we analyzed detrital zircons from five sandstones in the Forest City Basin and three sandstones from the Illinois Basin. In these basins, Lower-Middle Pennsylvanian strata unconformably overlie Mississippian carbonate strata and reflect a renewed influx of detritus after a significant depositional hiatus. In the total combined dataset (n=2,278), U-Pb ages of approximately 66% of zircons matched ages interpreted to be derived from the Appalachian region, including Grenville (1.3-1.0 Ga), Iapetus synrift (530-620 Ma and 750 Ma), Taconic (440-490 Ma), Acadian (350-420 Ma), and Alleghenian (330-270 Ma) ages. Subordinate populations of grains consisted of Granite-Rhyolite (1300 -1500 Ma), Midcontinent Rift (1600- 1900 Ma), and Superior Province (2600-2800 Ma) ages. Proportions of grains matching Appalachian sources increased in the Illinois Basin from ~46% to ~79% from Early to Middle Pennsylvanian time. In the Forest City Basin, the proportions of Appalachian grains increased later, during Middle Pennsylvanian time, from ~52% to ~70%. These shifts are interpreted as an increased flux of erosional detritus from the Appalachian margin during each repsective interval. In general, our data preclude the sources to the north as major contributors of detritus to the Forest City and Illinois Basins, as had been previously suggested for Iowa’s portion of the Forest City Basin. These results supplement our understanding of large-scale sediment distribution during Early-Middle Pennsylvanian time within the context of tectonic events along the Appalachian margin.