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Decorah Shale
The Decorah Shale is a fossiliferous shale that makes up the lower most formation in the Galena Group. The Decorah lies above the Platteville Limestone and below the Galena Limestone in the sedimentary sequence that formed from the shallow sea that covered central North America during Ordovician Time. The Decorah consists of three members (from bottom to top): Spects Ferry, Guttenberg, and Ion.〔(Thompson, Thomas L., 2001, ''Lexicon of Stratigraphic Nomenclature in Missouri,'' Missouri Department of Natural Resources, Division of Geology and Land Survey, Report of Investigation Number 73, p. 80 )〕 The Spects Ferry member is organic-rich and suggests a large influx of terrigenous sediment during deposition. The Guttenberg is characterized by nodular calcareous beds and contains several K-bentonite deposits. The Ion Member, present in the southern Decorah in Iowa, is characterized by alternating beds of shale and limestone.
Where it crops out in the Upper Midwest, especially in the Twin Cities, the Decorah is a popular place for amateur fossil collecting. It contains trilobites, brachiopods, horn corals, gastropods, crinoids, and large numbers of bryozoans.〔Mossler, J. and Benson, S., 1995, 1999, 2006, (Fossil Collecting in the Twin Cities Area ). Minnesota at a Glance: Minnesota Geological Survey: University of Minnesota.〕 Cephalopods may also be found in the lower layers of the Decorah Shale.
==References==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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