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Three Forks through Bighorn Formation: Three Forks, Jefferson, and Bighorn Formations (MTMDSOtb;0)

Geologic Formation in MT

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Lithology

Dolostone

Limestone

Shale

Other (199 mi²)

Geologic Age

Darriwillian – Tournaisian (468.4 Ma – 347.1 Ma ago)

Geologic Unit Label

MDSOtb

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). Bighorn Formation (Ob): Yellowish gray to very pale orange crystalline dolomite with local basal sandstone. Marine shelf. Thickness as much as 120 m (394 ft).

— USGS

Taxonomy

Classified As

Geologic Formation