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.
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).
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).
Prairie du Chien Group includes Oneota Fm (Coon Valley, Hager City members) and Shakopee Fm (New Richmond Sandstone, Willow River members); St. Peter Sandstone includes Readstown and Tonti members. Sub-St. Peter erosional surface bevels and locally truncates Prairie du Chien strata. Primary lithologies: dolomite, part cherty, part sandy, mostly unfossiliferous, common voids and vugs (Prairie du Chien); sandstone, quartzose, very fine to medium grained (St. Peter, Shakopee). Secondary lithologies: dolomite, laminated to stromatolitic, part intraclastic; dolomite, fossil molds (primarily mollusks); interbedded very fine to fine sandstone and dolomite (Coon Valley, Shakopee); conglomerate, dolomite and chert clasts in sandy matrix (Readstown). Minor: oolitic dolomite and oolitic chert (Shakopee); shale, green, part sandy (Shakopee); shale, green-brown to gray, part laminated, silty to sandy, part fossiliferous (locally in St. Peter of Winneshiek Co.); coarse sand grains (St. Peter, Shakopee); breccia, dolomite-chert clasts; chalcedony/quartz nodules. Maximum thicknesses total interval: generally 300-400 ft (90-120 m), may reach thicknesses to 700 ft (210 m) (locally thick St. Peter in Winneshiek Co.). Unit thicknesses: Prairie du Chien Group, 225-335 ft (69-102 m), locally thinner where overlain by thick St. Peter; Oneota Fm, 130-225 ft (40-69 m), locally thinned beneath thick St. Peter; Shakopee Fm, 55-100 ft (17-30 m), locally thinned to absent beneath thick St. Peter; St. Peter Sandstone, generally 40-75 ft (12-23 m), varies 25-225 ft (8-69 m), may locally reach thicknesses to 700 ft (210 m) in Winneshiek Co.
Surface exposures limited to Allamakee and northeast Clayton counties; subcrops beneath Mississippi River alluvium in southern Allamakee and Clayton counties. Primary lithologies: sandstone, very fine to coarse grained, quartzose to feldspathic (Jordan); silty dolomite to dolomitic siltstone (St. Lawrence); sandstone, very fine to fine grained, glauconitic to very glauconitic, common greensands (Lone Rock). Secondary lithologies: siltstone, part argillaceous (St. Lawrence, Lone Rock). Minor: silty shale partings (St. Lawrence, Lone Rock); dolomite, part sandy; intraclastic dolomite, sandstone, and siltstone. Maximum thicknesses of total interval: 260-310 ft (79-95 m). Formation thicknesses: Lone Rock Fm, 110-140 ft (34-43 m); St. Lawrence Fm, 60-80 ft (18-24 m); Jordan Sandstone, 80-110 ft (244-34 m).
Orthoquartzitic sandstone with minor limestone, shale and conglomerate; includes Glenwood and St. Peter Formations
Dolomite with some limestone and shale; includes Galena, Decorah, and Platteville Formations
Interval includes Pinicon Ridge, Otis, Spillville, and Bertram formations. Otis and Bertram formations restricted to east-central Iowa only; Spillville Formation in northeast Iowa only. Pinicon Ridge Formation oversteps Spillville and Otis edges in Fayette, Bremer, Black Hawk, Buchanan, southern Chickasaw counties to directly overlie Sh, Sw, or Om. Primary Lithologies: dolomite, part laminated, variably argillaceous, part fetid; limestone, dense, “sublithographic”, part laminated to intraclastic; dolomite, fossil-moldic to vuggy (Otis-Spillville fms). Secondary Lithologies: limestone/dolomite breccia (evaporite collapse); dolomitic shale and shaly dolomite, gray to green-gray, part silty-sandy; limestone, part peloidal to fossiliferous (Otis Fm). Minor: chert and chalcedony nodules (Pinicon Ridge Fm); sandstone; oolitic limestone (Otis Fm). Extensive anhydrite/gypsum in subsurface outside of outcrop belt. Maximum thicknesses in outcrop belt: east-central Iowa, 60-160 ft (18-49 m); northern Iowa, 10-130 ft (3-40 m), locally absent in Bremer County beneath Dc.
Sandstone with some dolomite and shale, undivided; includes Trempealeau, Tunnel City, and Elk Mound Formations
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.
Dolomite with some sandstone and shale; includes Shakopee and Oneota Formations