Michigan Formation (MIMmi;0)

State	Michigan
Name	Michigan Formation
Geologic age	Mississippian
Original map label	Mmi
Comments	Secondary unit description from USGS Geologic Names lexicon (ref. MI016): Unit, which is mostly shale, is Osagean and Meramecian in age and conformably overlies the Marshall Sandstone. Underlies the Meramecian Bayport Limestone. Michigan Formation reaches its maximum thickness of 152 m in Missaukee Co. to the north of the central basin area. Thicknesses of 91 to 122 m are more typical. Although generally composed of greenish gray and dark gray shale, this formation is lithologically variable and includes discontinuous beds of sandstone, limestone, dolostone, gypsum, and anhydrite. One of these carbonate beds, called the "Brown Lime," is a 3- to 6-m thick dolostone that can be traced in the subsurface across much of the basin.
Primary rock type	shale
Secondary rock type	black shale
Other rock types	sandstone; dolostone (dolomite); limestone; evaporite
Lithologic constituents	Major Sedimentary > Clastic > Mudstone > Shale (Bed) Minor Sedimentary > Clastic > Mudstone > Shale > Black-shale (Bed) Incidental Sedimentary > Chemical > Evaporite > Gypsum (Bed) Sedimentary > Chemical > Evaporite > Anhydrite (Bed) Sedimentary > Carbonate > Limestone (Bed) Sedimentary > Carbonate > Dolostone (Bed) Sedimentary > Clastic > Sandstone (Bed)
Map references	Milstein, Randall L. (compiler), 1987, Bedrock geology of southern Michigan: Geological Survey Division, Michigan Dept. of Natural Resources, scale= 1:500,000
Unit references	Milstein, Randall L. (compiler), 1987, Bedrock geology of southern Michigan: Geological Survey Division, Michigan Dept. of Natural Resources, scale= 1:500,000 USGS Geologic Names Lexicon (GEOLEX) [http://ngmdb.usgs.gov/Geolex/]
Geographic coverage	Allegan - Arenac - Barry - Calhoun - Crawford - Eaton - Genesee - Gladwin - Grand Traverse - Huron - Ingham - Ionia - Iosco - Jackson - Kalkaska - Kent - Lake - Lapeer - Livingston - Manistee - Mason - Missaukee - Muskegon - Newaygo - Oakland - Oceana - Ogemaw - Oscoda - Ottawa - Roscommon - Sanilac - Shiawassee - Tuscola - Washtenaw - Wexford