Helena Formation

Northwestern Montana (formerly “lower Wallace”): cycles of basal white quartzite or intraclast beds overlain by couplets of green siltite and argillite, capped by dolomite beds. Calcite pods and ribbons (molar tooth structure) common. West-central Montana: gray to dark gray limestone and dolomitic limestone with siltite partings. Thickness as much as 2,000 m (6,562).
State Montana
Name Helena Formation
Geologic age Middle Proterozoic
Lithologic constituents
Major
Metamorphic > Metasedimentary > Metaclastic > Argillite
Metamorphic > Metasedimentary > Metaclastic > Metasiltstonesiltite
Metamorphic > Metasedimentary > Metaclastic > Quartzite
Minor
Sedimentary > Carbonate > Dolostone (Bed)
Sedimentary > Carbonate > Limestone (Bed)
References

Vuke, S.M., Porter, K.W., Lonn, J.D., and Lopez, D.A., 2007, Geologic Map of Montana - Compact Disc: Montana Bureau of Mines and Geology: Geologic Map 62-C, 73 p., 2 sheets, scale 1:500,000. This map was digitized in 2012 as a result of a contract between the U.S. Geological Survey and the Montana Bureau of Mines and Geology.

NGMDB product
Counties Flathead