Maquoketa formation

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.
State Iowa
Name Maquoketa formation
Geologic age Upper Ordovician; Richmondian (upper Katian)
Lithologic constituents
Major
Sedimentary > Carbonate > Dolostone (Bed)argillaceous, part shaly, variably fossiliferous (part with common trilobites); part argillaceous, part cherty to very cherty (northern Iowa); incidental phosphatic to very phosphatic, argillaceous (Elgin Mbr.)
Sedimentary > Clastic > Mudstone > Shale (Bed)green-gray, variably dolomitic, mostly unfossiliferous; minor brown to brown-gray, organic, part graptolitic, part finely laminated (lower strata); minor interbedded dolomite and shale, part nodular, part fossiliferous; incidental red shale (Neda Mbr.)
Minor
Sedimentary > Carbonate > Limestone (Bed)dolomitic limestone to limestone, argillaceous, part cherty to very cherty, variably fossiliferous, part crinoidal (northern Iowa)
Incidental
Sedimentary > Chemical > Phosphorite (Bed)granular to massive (basal unit)
Sedimentary > Chemical > Banded-iron-formation (Bed)ooidal, (Neda Mbr.)
References

Witzke, Brian J., Anderson, Raymond R., and Pope, John P., 2010, Iowa Geological and Water Survey, DNR, Bedrock Geologic Map of Iowa, OFM-2010-1. Scale 1:500,000.

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Counties Allamakee - Black Hawk - Bremer - Buchanan - Chickasaw - Clay - Clayton - Clinton - Delaware - Dubuque - Emmet - Fayette - Howard - Jackson - Jones - Kossuth - Palo Alto - Scott - Winnebago - Winneshiek