Niobrara Formation

Gray to dark gray shale with numerous thin bentonite beds. Chalky aggregates of coccoliths and rhabdoliths in upper part. MacGowan Concretionary Bed (middle part of formation): grayish brown concretionary dolostone and limestone with phosphatic pellets and gray to black chert pebbles. Marine. Lower part noncalcareous. Thickness as much as 9–23 m (30–75 ft).
State Montana
Name Niobrara Formation
Geologic age Cretaceous
Lithologic constituents
Major
Sedimentary > Carbonate > Dolostone (Bed)
Sedimentary > Carbonate > Limestone (Bed)
Sedimentary > Clastic > Mudstone > Shale (Bed)
Minor
Sedimentary > Clastic > Mudstone > Claystone > Bentonite (Bed)
Incidental
Sedimentary > Carbonate > Limestone > Chalk (Bed)Chalky aggregates of coccoliths and rhabdoliths
Sedimentary > Chemical > Chert (Bed)
Sedimentary > Chemical > Phosphorite (Bed)grayish brown concretionary dolostone and limestone with phosphatic pellets
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 Big Horn - Carbon - Carter - Fergus - Garfield - Musselshell - Petroleum - Rosebud - Stillwater - Yellowstone