Mille Lacs and North Range Groups and equivalent sedimentary rocks

Includes the Mahnomen and Rabbit Lake Formations and Trout Lake marble. Unit descriptions from GEOLEX - Mahnomen Formation: Upper member described as gray to light-gray siltstone, argillite, and fine-grained quartzose sandstone containing irregularly distributed beds and lenses of iron-formation and allied anomalously iron-rich sedimentary rocks; base of unit defined arbitrarily as the top of the lowermost mappable iron-formation layer of the middle, iron-formation member. Middle, iron-formation member--interlayered cherty and slaty iron-formation that is similar lithologically to the smaller iron-formation lenses within the upper and lower members; contains rocks formerly assigned to the Trommald Formation and the Emily Member of the Rabbit Lake Formation (Morey, 1978). Lower member--predominantly siltstone, fine- to medium-grained quartzose graywacke, and argillite; also includes minor quartz arenite and irregularly distributed layers and lenses of anomalously iron-rich sedimentary rocks. Rabbit Lake Formaiton: gray to very dark-gray slate, siltstone, and very fine-grained graywacke; includes irregularly distributed layers and lenses of carbonaceous slate and also carbonate- and sulfide-facies iron-formation, chiefly in the lower part of the unit.
State Minnesota
Name Mille Lacs and North Range Groups and equivalent sedimentary rocks
Geologic age Paleoproterozoic
Lithologic constituents
Major
Sedimentary > Clastic > Sandstone > Graywacke (Bed)sandstone and graywacke
Sedimentary > Clastic > Siltstone (Bed)
Metamorphic > Metasedimentary > Metaclastic > Slateargillite to slate
Minor
Sedimentary > Chemical > Banded-iron-formation (Bed, Carbonaceous)carbonate- and sulfide-facies iron-formation and allied anomalously iron-rich sedimentary rocks
Incidental
Metamorphic > Metasedimentary > Metacarbonate > MarbleTrout Lake marble of Mille Lacs Group
Comments Part of the Mille Lacs and North Range Groups in the fold-and-thrust belt of the Penokean orogen (twice-deformed). Twice-deformed metasedimentary rocks at the apparent base of the Animikie basin. Part of the Geon 18 intrusions and supracrustal rocks (syn- to late Penokean orogenesis).
References

Jirsa, Mark A., Boerboom, Terrence J., Chandler, Val W., Mossler, John H., Runkel, Anthony C., and Setterholm, Dale R., 2011, S-21 Geologic Map of Minnesota-Bedrock Geology; Minnesota Geological Survey, State Map Series S-21. Scale 1:500,000.

USGS Geologic Names lexicon found at: http://ngmdb.usgs.gov/Geolex/

https://ngmdb.usgs.gov/Geolex/search

NGMDB product
Counties Aitkin - Benton - Carlton - Cass - Crow Wing - Mille Lacs - Morrison - Sherburne - Todd