H. B. Willman and T. C. Buschbach
The Cincinnatian Series (Meek and Worthen, 1865, p. 155), named for Cincinnati, Ohio, is the uppermost series of the Ordovician System (fig. O-4).
The Cincinnatian strata in Illinois were long believed to be equivalent to only the uppermost (Richmond) strata in the type region at Cincinnati (fig. O-27) but now are correlated with the entire series and are subdivided into the Edenian (earliest), Maysvillian, and Richmondian Stages (Templeton and Willman, 1963). The fauna of the Cincinnatian strata in Illinois has not been studied in sufficient detail to determine the precise position of the stage boundaries.
Extent and Thickness
Where not affected by sub-Silurian erosion (fig. O-26), the series is 180-350 feet thick.
The Cincinnatian Series is separated by unconformities from the Champlainian Series below and the Silurian Alexandrian Series above. In Illinois the Cincinnatian Series includes the Cape Limestone, the Maquoketa Group, and the Girardeau Limestone (fig. O-25).
Despite unconformities at the top and bottom, it appears that almost the entire Cincinnatian Series is represented in Illinois. The Cape Limestone at the base of the Cincinnatian in southern Illinois is as old or older than the oldest Cincinnatian strata at Cincinnati. The Fort Atkinson Limestone, in the middle of the Maquoketa Shale Group, has a fauna that closely matches that of the Richmondian Waynesville Limestone of the type region (Savage, 1925a). The Cornulites Zone, which occurs near the top of the Richmondian, is present in the upper part of the Brainard Shale in Illinois. The Neda Formation and the Girardeau Limestone at the top may be younger than the youngest strata in the type Cincinnatian. The unconformity at the base of Cincinnatian strata is marked by the sharp truncation of the upper half of the Trentonian Galena Group limestone and dolomite by the Cincinnatian strata in an east-west belt across central Illinois. The Dubuque and Wise Lake Formations and the upper part of the Dunleith Formation were eroded in this belt and are not known south of there, although they may be present in subsurface in the southeastern part of the state where the Galena Group thickens. The Lower Depauperate Zone at the base of the Maquoketa Group is continuous across the truncated formations. Impact of a large meteorite near Glasford in western Illinois probably accounts for the intensive deformation of the Champlainian and older strata before deposition of the Maquoketa Group in that area (Buschbach and Ryan, 1963).
BUSCHBACH, T. C., and ROBERT RYAN, 1963, Ordovician explosion structure at Glasford, Illinois: American Association of Petroleum Geologists, v. 47, p. 2015-2022; Illinois State Geological Survey Reprint 1964-B.
MEEK, F. B., and A. H. WORTHEN, 1865, Descriptions of new species of Crinoidea, etc. from the Paleozoic rocks of Illinois and some of the adjoining states: Philadelphia Academy of Natural Sciences Proceedings, v. 17, p. 143-155.
SAVAGE, T. E., 1925a, Correlation of the Maquoketa and Richmond rocks of Iowa and Illinois: Illinois Academy of Science Transactions, v. 17, p. 233-247.
TEMPLETON, J. S., and H. B. WILLMAN, 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p.
|Stratigraphic Code||Geo Unit Designation|