Three Forks and Jefferson Formations

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).
State Montana
Name Three Forks and Jefferson Formations
Geologic age Mississippian and Devonian
Lithologic constituents
Sedimentary > Carbonate > Dolostone (Bed)
Sedimentary > Carbonate > Limestone (Bed)
Sedimentary > Clastic > Mudstone > Shale (Bed, Calcareous)
Sedimentary > Clastic > Siltstone (Bed)
Sedimentary > Clastic > Sedimentary-breccia (Bed)argillaceous limestone breccia and shale breccia
Metamorphic > Metasedimentary > Metaclastic > Quartzite

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 - Broadwater - Gallatin - Jefferson - Madison - Meagher - Silver Bow