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.
Maquoketa Formation or Group, includes Cape Limestone, Cape La Croix Shale, Thebes Sandstone, Orchard Creek Shale, Girardeau Limestone, and Leemon Formation in southern Illinois; includes Scales Shale, Fort Atkinson Limestone, Brainard Shale, and Neda Formation in northern Illinois; includes Noix Oolite in western Illinois.
Galena Group (Trenton Limestone), includes Decorah, Dunleith, Wise Lake and Dubuque Formations in northern Illinois.
Includes Elgin, Clermont, Fort Atkinson, Brainard, and Neda members. Overlain by Silurian strata in east-central Iowa, incised paleovalleys beneath Mosalem Fm.; truncated beneath Devonian strata in northern-most counties. Primary lithologies: shale, green-gray, variably dolomitic, mostly unfossiliferous; dolomite, argillaceous, part shaly, variably fossiliferous (part with common trilobites); dolomite, part argillaceous, part cherty to very cherty (northern Iowa). Secondary lithologies: shale, brown to brown-gray, organic, part graptolitic, part finely laminated (lower strata); dolomitic limestone to limestone, argillaceous, part cherty to very cherty, variably fossiliferous, part crinoidal (northern Iowa); interbedded dolomite and shale, part nodular, part fossiliferous. Minor: phosphorite, granular to massive (basal unit); dolomite, phosphatic to very phosphatic, argillaceous (Elgin Mbr.); ooidal ironstone and red shale (Neda Mbr.); pyrite, finely disseminated to nodular, pyrite cements. Maximum thicknesses: generally 200-275 ft (60-85 m) beneath Silurian, locally 100-200 ft (30-60 m) beneath Mosalem Fm.; 75-180 ft (23-55 m) where capped by Devonian strata in northern Iowa.
Mosalem Formation infills erosional paleovalleys incised into Maquoketa Shale; absent outside of paleovalleys. Lower Hopkinton Formation locally contains carbonate mound facies in Bremer County. Blanding Formation oversteps Tete des Morts-Mosalem edge in southern outcrop area. Primary Lithologies: dolomite, fossil-moldic to vuggy, fine to coarse crystalline; dolomite, dense to porous, cherty to very cherty, nodular to bedded chert (especially Blanding Fm). Secondary Lithologies: dolomite, dense, sparsely fossiliferous; dolomite, argillaceous, part laminated (Mosalem Fm). Minor: chalcedony, silicified fossils. Maximum thicknesses total interval: 160-330 ft (49-100 m), thickest in Dubuque, Jackson, Jones counties where Mosalem present; southern outcrop total thickness 65-130 ft (20-40 m). Formation thicknesses: Hopkinton, 65-160 ft (20-49 m); Blanding, 25-65 ft (8-20 m); Tete des Morts, 0-35 ft (11 m); Mosalem, 0-100 ft (30 m).
Galena Group includes Decorah, Dunleith, Wise Lake, Dubuque formations; Platteville Formation includes Pecatonica, McGregor members; Glenwood Shale forms thin basal shale unit. Primary lithologies: dolomite, part porous to vuggy, fossiliferous, part cherty; limestone and dolomitic limestone, fossiliferous, part cherty (note: Galena Group entirely limestone to north, entirely dolomite to south, interstratified limestone-dolomite in intervening areas). Secondary lithologies: shale, green-gray, calcareous, with thin to nodular limestone, part fossiliferous (Decorah Sh); limestone, wavy-bedded to nodular, part fossiliferous (Decorah-Platteville); dolomite and limestone, slightly argillaceous. Minor: shale partings and thin beds, green-gray to brown-gray, part fossiliferous (Dubuque Fm); shale partings and thin beds, light to dark brown, part organic-rich (Decorah-Platteville); dolomite, sandy to very sandy (Pecatonica Mbr); shale, green-gray, noncalcareous, unfossiliferous, part sandy (Glenwood Sh); thin sandstone to siltstone (Glenwood Sh); dark phosphatic grains (primarily Glenwood, Pecatonica, Decorah); phosphate-pyrite-iron oxide crusts (on some hardground surfaces); pyrite nodules; thin K-bentonite beds (Galena Gp); ironstone ooids (Decorah Sh). Maximum thicknesses total interval: 330 ft (100 m); Galena Group, 250-280 ft (76-85 m); Platteville Formation, 25-55 ft (8-17 m); Glenwood Shale, 3-10 ft (1-3 m).
Formation includes Welton, Buck Creek Quarry, Waubeek, Palisades-Kepler, Johns Creek Quarry members; Palisades-Kepler and Johns Creek Quarry members contain carbonate mound facies. Erosionally beveled and truncated beneath Dw. Primarily Lithologies: dolomite, porous, fossil-moldic to vuggy, part very crinoidal (includes Welton Mbr); dolomite, cherty to very cherty, dense (Buck Creek Quarry Mbr). Secondary Lithologies: dolomite, sparsely fossil-moldic, dense, part vuggy (includes Waubeek Mbr); dolomite, coarsely crystalline, part very crinoidal (within Johns Creek Quarry, Palisades-Kepler mbrs); dolomite mudstone, dense. Minor: dolomite, slightly argillaceous; quartz druse, chalcedony, silicified fossils. Maximum thicknesses: 94-240 ft (29-73 m); may reach thicknesses to 300 ft (90 m); beveled and truncated beneath Dw.