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Piegan group

Piegan group: most of the Piegan group is subdivided into formations that are in part equivalent to each other. Where correlations are relatively doubtful the group designation is retained. Carbonate-bearing rocks predominate in the group, but the proportions and character of the impurities in these rocks and the relations to non-carbonate-bearing rocks vary from place to place.
State Montana
Name Piegan group
Geologic age Proterozoic | Mesoproterozoic
Comments This unit nomenclature is now obsolete, replaced on more modern geologic maps by individually mapped formations of the Belt Supergroup. As mapped, probably consists mostly of the Helena Formation.
Primary rock type carbonate
Secondary rock type meta-argillite
Other rock types quartzite
Lithologic constituents
Major
Sedimentary > Carbonate (Bed)Weakly metamorphosed limestones and dolomites.
Minor
Metamorphic > Metasedimentary > Metaclastic > Argillite (Greenschist)
Metamorphic > Metasedimentary > Metaclastic > Quartzite (Greenschist)
Map references
Ross, P.R., Andrews, D.A., and Witkind, I.J., 1955, Geologic map of Montana: U.S. Geological Survey, 2 sheets, scale 1:500,000.
Unit references
Lopez, D.A., 2002, Geologic map of the Lonesome Lake 30' x 60' quadrangle, north-central Montana: Montana Bureau of Mines and Geology Open File Report 446, 4 p., scale 1:100,000.
Mudge, M.R., Earhart, R.L., 1983, Geologic and structure maps of the Choteau 1° x 2° quadrangle, western Montana: United States Geological Survey Miscellaneous Geologic Investigations Series Map I-1300, scale 1:250,000.
Harrison, J.E., Whipple, J.W., and Lidke, D.J., 1998, Geologic map of the western part of the Cut Bank 1° x 2° quadrangle, northwestern Montana: U.S. Geological Survey Miscellaneous Geologic Investigations Series Map I-2593, scale 1:250,000.
Harrison, J.E., Griggs, A.B., and Wells, J.D., 1986, Geologic and structure maps of the Wallace 1° x 2° quadrangle, Montana and Idaho: Montana Bureau of Mines and Geology Montana Atlas Series 4-A, scale 1:250,000.
Harrison, J.E., Cressman, E.R., and Whipple, J.W., 1992, Geologic and structure maps of Kalispell 1° x 2° quadrangle, Montana and Alberta, British Columbia: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-2267, scale 1:250,000.
Whipple, J.W., 1992, Geologic map of Glacier Park, Montana: U.S. Geological Survey Miscellaneous Investigations Series Map I-1508-F, 1 sheet, scale 1:100,000.
Counties Fergus - Flathead - Golden Valley - Lake - Lincoln - Meagher - Missoula - Powell - Ravalli