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July-August 2013

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Some Representative Fossils and Minerals of the Prairie du Chien Group in the Upper Mississippi Valley

In particular, the rugged terrain in counties bordering the upper Mississippi River Valley from St. Paul, Minnesota, south to Prairie du Chien, Wisconsin, provides many productive collecting opportunities. Bluffs, quarries, and stream valleys expose abundant natural and artificial sections of Paleozoic formations ranging in age from Late Cambrian through Late Ordovician. Although the Upper Ordovician strata are the source of most of the region's well-known fossils and minerals, the underlying Lower Ordovician rocks contain a frequently overlooked, unique fossil fauna as well as collectible specimens of some common minerals. The Lower Ordovician of this area is composed of the Prairie du Chien Group, roughly 488–478 million years old, named after the type locality at Prairie du Chien in Crawford County, Wisconsin. This rock sequence was deposited in a shallow, warm sea that covered portions of the North American continent when it was located near the equator during Ordovician time. The environment was locally stressed and hypersaline, tolerated only by some mollusks and microorganisms. The Prairie du Chien Group is divided into two formations (Sloan 2005). In ascending order, they are the Oneota Formation and the Shakopee Formation. The Oneota Formation is a sequence of dolostone with chert and minor sandstone and shale. It varies in thickness from about 21 meters in the north to 52 meters in the southern part of the area. The Oneota is resistant to erosion and forms bold cliffs fronting the major stream valleys. The overlying Shakopee Formation consists of dolostone, sandstone, and shale and varies from zero to about 46 meters in thickness. It is usually covered; the only good exposures occur in road cuts and quarries, where the massive microbialites (stromatolites) it contains stand out in sharp relief.

Phil Burgess has been a lifelong collector of mostly Paleozoic fossils and Mississippi Valley–type (MVT) sulfide minerals. He is especially interested in the Lower Ordovician strata in the region where Wisconsin, Iowa, and Minnesota meet; he is currently collecting and studying a number of sites in the Oneota Formation there that contain a unique suite of Early Ordovician mollusk fossils.


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