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Prairie du Chien Group

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Prairie du Chien Group
State Michigan
Name Prairie du Chien Group
Geologic age Early Ordovician
Lithologic constituents
Major
Sedimentary > Carbonate > Dolostone (Bed)
Minor
Sedimentary > Clastic > Sandstone (Bed)
Incidental
Sedimentary > Clastic > Mudstone > Shale (Bed)
Comments Secondary unit description from USGS Geologic Names lexicon (ref. MI016): Includes (descending) Shakopee or Willow River dolomite, approximately 40 feet thick; New Richmond sandstone, 15 to 130 feet thick; and Oneota dolomite, 200 to 225 feet thick. Rests on Cambrian and is unconformably overlain by St. Peter sandstone. Age is Early Ordovician. Nomenclature used in this report assigns Foster Formation to the Prairie du Chien Group. Foster includes the "Brazos shale" at the top and underlies the St. Peter Sandstone. Foster overlies Umlor Formation, also of Prairie du Chien. Foster is Early and Middle Ordovician and therefore its assignment modifies the age of the Prairie du Chien.
References

Reed, Robert C., and Daniels, Jennifer (compilers), 1987, Bedrock geology of northern Michigan: Geological Survey Division, Michigan Dept. of Natural Resources, scale 1:500,000.

USGS Geologic Names lexicon found at: http://ngmdb.usgs.gov/Geolex/

https://ngmdb.usgs.gov/Geolex/search
NGMDB product
Counties Alger - Chippewa - Luce - Marquette - Menominee - Schoolcraft