Silurian System undivided

Silurian System undivided, includes Sexton Creek Limestone, St. Clair Limestone, and Moccasin Springs Formation in southern Illinois; includes Wilhelmi Formation, Elwood Dolomite, Kankakee Dolomite, Joliet Dolomite, Sugar Run Dolomite, and Racine Dolomite in northeastern Illinois; includes Mosalem, Tete des Morts, Blanding, Sweeney, Marcus, and Racine Dolomites in northwestern Illinois.
State Illinois
Name Silurian System undivided
Geologic age Silurian (Llandovery to Pridoli)
Lithologic constituents
Sedimentary > Carbonate > Dolostone (Bed, Glauconitic)Dolomite can be cherty, silty, shaly or argillaceous, or glauconitic.
Sedimentary > Carbonate > Limestone (Bed, Reef)Moccasin Springs Formation and Racine Dolomite contain limestone reefs. Limestone can be cherty, silty, or shaly or argillaceous.
Sedimentary > Clastic > Mudstone > Shale (Bed)Shale can be dolomitic.
Comments Thickness from stratigraphic columns (listed from oldest to youngest): Bowling Green Dol (0-50'), Sexton Creek Ls (0-125’), St. Clair Ls (0-150’) (of which the seventy-Six Shale Mbr occurs at the base), Moccasin Springs Fm (0-1,000’) in southern Illinois (south of 40 degrees north latitude); Wilhelmi Fm (0-100’), Elwood Dol (0-30’), Kankakee Dol (0-50’), Joliet Dol (0-80’), Sugar Run Dol (0-30’), and Racine Dol (0-300') in northern Illinois (north of 40 degrees north latitude); Mosalem Fm (0-100’), Tete des Morts Dol (0-24’), Blanding Dol (0-50’), Sweeney Dol (0-55’), Marcus Dol (0-45’), and Racine Dol (0-300’) in northwestern Illinois. The Schweizer Mbr occurs at the base of the Wilhelmi Fm and the Birds Mbr at the top; the Kankakee Dolomite is comprised, from oldest to youngest, of the Drummond, Offerman, Troutman, and Plaines Members; and the Joliet Dolomite is comprised, from oldest to youngest, of the Brandon Bridge, Markgraf, and Romeo Members. Unit name in original GIS (Esri shapefile IL_Geologic_Units_500K_2005_Py) is 'Silurian System'; name on Legend of geologic map is 'Silurian System undivided'.
Stratigraphic units Bowling Green Dolomite, Seventy-Six Shale Member (of the St. Clair Ls), Schweizer Member and Birds Member (of the Wilhelmi Fm), Drummond Member, Offerman Member, Troutman Member, and Plaines Member of the Kankakee Dolomite), Brandon Bridge Member, Markgraf Member, and Romeo Member (of the Joliet Dolomite)

Kolata, D.R., comp., 2005, Bedrock Geology of Illinois: Champaign, Ill., Illinois State Geological Survey, Illinois Map Series 14, scale 1:500,000.

NGMDB product
Counties Alexander - Boone - Bureau - Calhoun - Carroll - Champaign - Cook - DeKalb - Douglas - DuPage - Ford - Greene - Henry - Iroquois - Jersey - Jo Daviess - Kane - Kankakee - Kendall - Lake - Lee - McHenry - Ogle - Pike - Rock Island - Scott - Stephenson - Union - Whiteside - Will