A PROMINENT UNCONFORMABLE BOUNDARY WITHIN THE UPPER NIAGARAN RACINE FORMATION: RECORD OF A MAJOR MIDDLE SILURIAN TECTONO-EUSTATIC EVENT IN THE SANGAMON ARCH, WEST-CENTRAL ILLINOIS

The Racine Formation is the uppermost unit of the unconformity bounded Niagaran succession in western and northern Illinois. In the Sangamon Arch of west-central Illinois, it unconformably underlies the New Albany Shale Group (Upper Devonian-lowermost Mississippian) and sharply overlies the thick bedded limestone of the Joliet Formation. Examination of the geophysical log signatures and stratigraphic correlation in the study area has revealed a fairly uniform thickness and lithology (dominantly limestone) for the lower part of the Niagaran deposits, which suggests no differential erosion prior to the deposition of the upper Niagaran deposits. The upper portion of the Niagaran (the Racine Formation), on the other hand, is characterized by variable thickness and lithology (interbedded limestone, dolomite, argillaceous limestone or dolomite and shale) making well to well correlation difficult.

The difference in thickness and lithology is interpreted to be mainly the consequence of differential post depositional erosion, thus suggesting the presence of a major unconformity within the Racine Formation. Throughout the study area, the middle part of the Racine Formation has been truncated as a result of this erosional event and subsequently overlapped by rocks of younger Niagaran deposits. The differential erosion along this unconformity is as much as 25 meters in parts of the study area. This erosional unconformity records a major tectono-eustatic sea level fall during Middle Silurian time and divides the Racine Formation into two depositional sequences. It corresponds with the global sea level fall at the Wenlockian-Ludlovian boundary and could be, at least partly, the result of up warping in the Sangamon Arch area.