Munising Group

Potsdam Supergroup,

Cambrian System

Type locality and use of name: Sandstones, 200 to 250 feet (61 to 76 m) thick and exposed in bluffs near Munising, Alger County, Mich., were designated the Munising Sandstone by Lane and Seaman (1907, p. 680, 692). This unit was proposed to help clarify the stratigraphic relationships in rocks that were included earlier in the Lake Superior Sandstone. East of Copper Range, Upper Peninsula of Michigan, the light-colored rocks of the Munising lie above the brightly colored red and brown sandstone of the Jacobsville Sandstone. Use of the term Munising was extended into the Lower Peninsula of Michigan by Catacosinos (1973) to include there the Eau Claire Formation, the Galesville Sandstone, and the Franconia Formation. The name was further extended to Indiana use, with group status, by Droste and Patton (1985), where it includes the Eau Claire, Davis, and Franconia Formations and the Galesville and Ironton Sandstones.

Description: The Munising Group of the Potsdam Supergroup in Indiana (Droste and Patton, 1985) is conformable with the Mount Simon Sandstone below. It ranges from less than 600 feet (183 m) to more than 1,500 feet (457 m) in thickness. In southwestern Indiana the only formation of the Munising Group present, the Eau Claire Formation, lies with gradational contact below the Potosi Dolomite. Throughout much of Indiana the Munising consists of the Eau Claire Formation below and the Davis Formation above, the Davis also lying conformably below the Potosi. In northwestern Indiana the Munising consists in ascending order of the Eau Claire Formation, the Galesville and Ironton Sandstones, and the Franconia Formation. Here, too, the Potosi Dolomite conformably over-lies the Munising rocks. Southward and eastward from northwestern Indiana, therefore, the top of the Munising descends stratigraphically through a lateral transition zone with the Potosi Dolomite.

Correlation: Given the relations noted above, the upper part of the Munising in northwestern Indiana is presumably late Franconian in age, whereas its upper part in southwestern Indiana is presumably late Dresbachian in age

The overall Munising Group of Indiana correlates with a sequence of siliciclastic and carbonate rocks in Illinois that has a southward, stratigraphically descending transition zone between the two basic lithologies similar to that noted above for Indiana. In part of northern Illinois, therefore, the Munising-correlative rocks consist dominantly of siliciclastic rocks of the Eau Claire Formation and overlying rocks through the Franconia Formation, there assigned to the Potsdam Sandstone Megagroup by Buschbach (1975). In much of the rest of Illinois the Munising correlatives, although known by the same formation names, have an increased carbonate content and were assigned to the Knox Dolomite Megagroup by Buschbach. The southern Indiana Munising rocks, however, consist only of the Eau Claire and therefore correlate only with the Eau Claire of Illinois.

The Munising of Indiana also correlates with rocks in Wisconsin ranging upward from the bottom of the Eau Claire through the Tunnel City Group, and it coextends with the Munising Formation in southern Michigan. Further, the Munising of Indiana correlates with rocks in Ohio called the Eau Claire, Rome, Conasauga, and Kerbel Formations by Janssens (1973). The Indiana correlative in Kentucky includes the same siliciclastic-carbonate transitional zone as noted above for Indiana and Illinois. The Eau Claire representative of the Munising of southern Indiana therefore correlates directly with the Eau Claire Formation of western Kentucky, but the higher and northward part of the Munising of Indiana correlates with carbonate rocks in Kentucky called the Elvins Formation (Droste and Shaver, 1983; Shaver and others, 1985).

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