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出典:『Wikipedia』 (2011/07/13 23:23 UTC 版)

英語による解説

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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. [1] 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.