Lombard Formation and Madison Group

Lombard Formation: Gray, silty limestone with minor black shale. Thickness 120 m (394 ft). Madison Group (Mm) - Mission Canyon Formation (Mmc): Gray, massive limestone with chert beds and nodules, and solution breccia zones. Shallow marine. Thickness as much as 520 m (1,706 ft). Lodgepole Formation (Ml): Woodhurst Member (upper part of formation): light gray, well-bedded limestone, typically with much dark chert, interbedded with thinner calcareous mudstone beds. Paine Member (middle part of formation):dark gray, thin-bedded, silty or fossiliferous limestone. Cottonwood Canyon Member (lower part of formation): black shale with basal conglomeratic lag deposit. Shallow marine. Thickness as much as 305 m (1000 ft). In Northwest Montana - Castle Reef Formation: Medium to light gray, thick-bedded limestone or dolomite. Sun River Member (upper part of formation): light gray dolomite with thick fossiliferous lenses. Shallow marine. Thickness as much as 300 m (984 ft). Allan Mountain Formation: Dark gray, thinly bedded limestone with thin mudstone and shale partings, and nodular chert. Shallow marine. Thickness as much as 200 m (656 ft).
State Montana
Name Lombard Formation and Madison Group
Geologic age Mississippian
Lithologic constituents
Major
Sedimentary > Carbonate > Dolostone (Bed)
Sedimentary > Carbonate > Limestone (Bed)
Sedimentary > Clastic > Mudstone > Shale > Black-shale (Bed)
Minor
Sedimentary > Clastic > Mudstone (Bed, Calcareous)
Incidental
Sedimentary > Clastic > Conglomerate (Bed)basal conglomeratic lag deposit
Sedimentary > Clastic > Mudstone > Shale (Bed)shale partings
Sedimentary > Chemical > Chert (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 Beaverhead