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

Geologic units in Winneshiek county, Iowa

St. Peter Sandstone (Phanerozoic | Paleozoic | Ordovician-Early [Chazyan]) at surface, covers 6 % 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 0.6 % 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

Decorah Shale (Middle Ordovician) (Middle Ordovician) at surface, covers < 0.1 % of this area

Decorah Shale (Middle Ordovician) - Light-greenish-gray shale and lesser amounts of coquinoid limestone, especially in the upper half of the formation. Mapped as a separate unit where possible.

Lithology: shale; limestone

Prairie du Chien Formation (Phanerozoic | Paleozoic | Ordovician-Early [Beekmantown]) at surface, covers 7 % 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 0.1 % 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)

Cedar Valley Limestone (Phanerozoic | Paleozoic | Devonian-Middle) at surface, covers 14 % of this area

Finely crystalline and clastic limestone in upper part with coral and stromatoporoid-rich zones; brown, dolomitic limestone with biostromes of corals and stromatoporoids in middle part; gray argillaceous, fossiliferous limestone in lower part; dominantly dolomite with intervals of gypsum in the subsurface of central and southwestern Iowa.

Lithology: limestone; dolostone (dolomite)

Upper and Middle Ordovician rocks, undivided (Late and Middle Ordovician) at surface, covers 0.2 % of this area

Upper and Middle Ordovician rocks, undivided - Limestone and shaly limestone of the Maquoketa and Dubuque Formations, and limestone and dolostone of the Galena Group in the Hollandale embayment of southeastern Minnesota

Lithology: limestone; dolostone (dolomite)

Galena-Decorah-Platteville Formations (Phanerozoic | Paleozoic | Ordovician-Middle [Mohawkian]) at surface, covers 30 % 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

Lower Ordovician rocks, undivided (Lower Ordovician) at surface, covers < 0.1 % of this area

Lower Ordovician rocks, undivided - Shakopee and Oneota Formations of the Prairie du Chien Group in the Hollandale embayment of southeastern Minnesota. Unit consists dominantly of dolostone and dolomitic limestone. The Shakopee also contains intervals of quartz arenite, including a pronounced basal unit named the New Richmond Member

Lithology: dolostone (dolomite); limestone; sandstone

Middle Ordovician rocks, undivided (Middle Ordovician) at surface, covers < 0.1 % of this area

Middle Ordovician rocks, undivided - Decorah Shale; limestone of the Platteville Formation; shaly rocks of the Glenwood Formation; and St. Peter Sandstone in the Hollandale embayment of southeastern Minnesota

Lithology: sandstone; limestone; shale

Maquoketa Formation (Phanerozoic | Paleozoic | Ordovician-Late [Cincinnatian]) at surface, covers 41 % 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