Three Forks through Maywood Formation: Three Forks, Jefferson, and Maywood Formations or Palliser and Alexo Formations, and Fairholme Group in northwestern Montana near Canadian border

Three Forks Formation (MDt): Sappington Member (upper part of formation):yellowish orange and yellowish gray siltstone. Trident Member (middle part of formation): greenish gray and light olive gray, fossiliferous, calcareous shale that contains interbeds and nodules of fossiliferous, argillaceous limestone. Logan Gulch Member (lower part of formation): yellowish gray and grayish red, argillaceous limestone breccia and shale breccia that may include dolomitic siltstone. Marine to restricted marine with evaporite basins. Thickness as much as 185 m (607 ft). Jefferson Formation (Dj): Birdbear Member (upper part of formation): light to medium gray, sucrosic dolomite. Lower part of formation: grayish black, commonly petroliferous dolomite or limestone that may be interbedded with light gray quartzite. Marine. Thickness as much as 520 m (1,706 ft). Maywood Formation: Thin-bedded dolomitic limestone and dolomite interbedded with calcareous shale. Shallow marine. Thickness as much as 120 m (394 ft). In northwestern Montana near Canadian border - Palliser Formation: Gray and greenish gray limestone and dolomitic limestone with locally abundant black chert. Pinkish gray, very thinly laminated, calcareous mudstone at base. Marine. Thickness as much as 300 m (984 ft). Alexo Formation: Pale yellowish orange, medium grained, calcareous siltstone with calcareous mudstone at the base. Shallow marine. Thickness as much as 595 m (1,952 ft). Fairholme Group: Brownish black, bioturbated, calcareous mudstone, gray algal limestone and dolomite, and pale yellowish orange, medium-grained, calcareous sandstone at the base. Thickness as much as 550 m(1,805 ft).
State Montana
Name Three Forks through Maywood Formation: Three Forks, Jefferson, and Maywood Formations or Palliser and Alexo Formations, and Fairholme Group in northwestern Montana near Canadian border
Geologic age Mississippian and Devonian
Lithologic constituents
Major
Sedimentary > Carbonate > Limestone (Bed)
Sedimentary > Clastic > Mudstone > Shale (Bed, Calcareous)
Sedimentary > Carbonate > Dolostone (Bed)
Minor
Sedimentary > Clastic > Mudstone (Bed, Calcareous)
Sedimentary > Clastic > Siltstone (Bed)
Incidental
Metamorphic > Metasedimentary > Metaclastic > Quartzite
Sedimentary > Clastic > Sandstone (Bed, Calcareous)
Sedimentary > Clastic > Sedimentary-breccia (Bed)argillaceous limestone breccia and shale breccia
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 - Blaine - Broadwater - Cascade - Deer Lodge - Flathead - Gallatin - Granite - Jefferson - Judith Basin - Lewis and Clark - Lincoln - Madison - Meagher - Missoula - Phillips - Pondera - Powell - Silver Bow - Teton - Wheatland