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Mineral Resources > Online Spatial Data > Geology > by state > Iowa Geology

Geologic units in Clayton county, Iowa

Cambrian, undivided (Cambrian) at surface, covers < 0.1 % of this area

Cambrian, undivided - Sandstone with some dolomite and shale, undivided; includes Trempealeau, Tunnel City, and Elk Mound Formations

Lithology: sandstone; dolostone (dolomite); shale

Prairie du Chien Formation (Phanerozoic | Paleozoic | Ordovician-Early [Beekmantown]) at surface, covers 3 % of this area

Sandy dolomite in upper postion; sandstone in middle portion; dolomite with oolitic and tripolitic chert in lower portion.

Lithology: dolostone (dolomite); sandstone; chert

St. Croixan Series (Phanerozoic | Paleozoic | Cambrian-Furongian) at surface, covers 0.6 % of this area

Jordan Sandstone- fine- to medium-grained, well sorted, frosted sandstone (includes Madison Sandstone). Approx thickness 120 ft. St. Lawrence Formation- coarsely crystalline, gray, silty dolomite; glauconite common (includes Lodi Sandstone).Approx thickness 230+ ft Franconia Sandstone- glauconitic, dolomitic siltstone and shale and glauconitic sandstone. Approx thickness 280 ft Galesville Sandstone- medium- to coarse-grained, white to gray sandstone. Eau Claire Sandstone- fissile, gray, silty shale and dolomitic siltstone and fine-grained sandstone. Mt. Simon Sandstone- medium- to coarse-grained sandstone with minor shale stringers. Galesville+Eau Claire+Mt. Simon=Approx thickness 1000 ft

Lithology: sandstone; siltstone; shale; dolostone (dolomite)

St. Peter Sandstone (Phanerozoic | Paleozoic | Ordovician-Early [Chazyan]) at surface, covers 3 % of this area

Coarse to fine, rounded, frosted sand grains; minor green shale stringers.

Lithology: sandstone; shale

Niagaran Series (Phanerozoic | Paleozoic | Silurian) at surface, covers 8 % of this area

Gower Dolomite- LeClaire (reef phase) tough, greenish-blue dolomite; Anamosa (inner-reef) soft, yellowish-brown, thin-bedded dolomite. Hopkinton Dolomite- light colored dolomite with nodular chert.

Lithology: dolostone (dolomite); chert

Alexandrian Series (Phanerozoic | Paleozoic | Silurian) at surface, covers 10 % of this area

Kankakee Formation- light colored dolomite; contains much interbedded chert. Approx thickness 155 ft. Englewood Dolomite- gray, sandy, silty, argillaceous dolomite. Approx thickness 55 ft

Lithology: dolostone (dolomite); chert

Maquoketa Formation (Phanerozoic | Paleozoic | Ordovician-Late [Cincinnatian]) at surface, covers 34 % of this area

Predominantly grayish-green shale in east-central area; predominantly brown dolomite and chert in the subsurface of north-central and western areas; red shale with limonite or hematite pellets (Neda) occurs locally at top.

Lithology: shale; dolostone (dolomite); chert

Prairie du Chien Group (Ordovician) at surface, covers < 0.1 % of this area

Prairie du Chien Group - Dolomite with some sandstone and shale; includes Shakopee and Oneota Formations

Lithology: dolostone (dolomite); sandstone; shale

Ancell Group (Ordovician) at surface, covers < 0.1 % of this area

Ancell Group - Orthoquartzitic sandstone with minor limestone, shale and conglomerate; includes Glenwood and St. Peter Formations

Lithology: sandstone; limestone; shale; conglomerate

Sinnipee Group (Ordovician) at surface, covers < 0.1 % of this area

Sinnipee Group - Dolomite with some limestone and shale; includes Galena, Decorah, and Platteville Formations

Lithology: dolostone (dolomite); limestone; shale

Galena-Decorah-Platteville Formations (Phanerozoic | Paleozoic | Ordovician-Middle [Mohawkian]) at surface, covers 41 % of this area

Galena FM- dolomite, minor limestone; chert in lower half. Approx thickness 230 ft. Decorah FM- brown limestone and dolomite; gray-green and brown shales at top and base. Approx thickness 60 ft. Platteville FM- fossiliferous gray limestone and brown dolomite; grayish-green shale at base (Glenwood). Fine- to medium-grained sandstone occurs above the Glenwood in the subsurface of southeastern Iowa. Approx thickness 110 ft

Lithology: dolostone (dolomite); limestone; shale; sandstone; chert