NEWLY DISCOVERED EVIDENCE FOR LATE ORDOVICIAN STRUCTURAL DEFORMATION IN OGLE COUNTY, ILLINOIS

During recent mining operations near Oregon, Ogle County, Illinois, a high wall was cut through the Late Ordovician St. Peter Sandstone and overlying Glenwood Formation, both assigned to the Mohawkian Series, Turinian Stage. This outcrop is remarkable because it reveals a previously unknown episode of Late Ordovician structural deformation in northern Illinois. The exposure is approximately 30 m high, 300 m long and extends in an east-west direction along a north facing wall. The St. Peter Sandstone is folded into a broad north-northeast trending syncline and anticline. Structurally tilted up to 20 degrees, the St. Peter strata were subsequently erosionally truncated and covered by flat-lying shale, dolomite and sandstone of the Glenwood Formation, forming a pronounced angular unconformity. Stratigraphic relations indicate that at least 30 m of the St. Peter Sandstone were eroded from the crest of the anticline prior to deposition of the Glenwood. This site is situated along the northwestern termination of the Sandwich Fault Zone, which begs the question of whether the folds are structurally associated with the fault zone, perhaps along a zone of crustal weakness that was episodically reactivated by tectonic forces during the Paleozoic.