Decorah Shale

Paleozoic Stratigraphy of the Upper Midwest, USA Dates are approximate, and deposition occurred at slightly different times in different areas This box: view · talk · edit

Ordovician Maquoketa Group (446–440 Ma) ::Neda Formation Brainard Formation Fort Atkinson Formation Scales Formation Galena Group (454–446 Ma) ::Dubuque Formation (447–446 Ma) Wise Lake Formation (449.4–447 Ma) Dunleth Formation (452–449.4 Ma) Decorah Shale (454–452 Ma) Platteville Limestone / Group (455–454 Ma) Ancell Group (454–455 Ma) Glenwood Shale (~455 Ma) St. Peter Sandstone (~459–~455 Ma) --- Major Erosional Unconformity --- Knox Supergroup ::--- Major Erosional Unconformity --- Everton Dolomite (only in S. Indiana) --- Major Erosional Unconformity --- Dunleth Formation Prarie du Chien Group * Shakopee Dolomite * New Richmond Sandstone (only in S. Indiana) * Oneota Dolomite * Dunleth Formation * Gunter Sandstone Cambrian Knox Supergroup (ct'd) ::Jordan Sandstone Potosi Dolomite Potsdam Supergroup/Potsdam Sandstone Megagroup Munising Group Davis Formation Franconia Formation Ironton Sandstone Galesville Sandstone Eau Claire Formation Mount Simon Sandstone

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.

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.^[1] Cephalopods may also be found in the lower layers of the Decorah Shale.

References

1. ^ 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.