Greenhorn and Belle Fourche Formations

Greenhorn Formation (Kgr): Gray to light gray calcareous shale and shaly marl with thin beds of limestone. Shale contains white to pink calcareous specks. Marine. Thickness as much as 100 m (30–75 ft). Belle Fourche Formation (Kbf): Gray to black shale with ironstone concretions and numerous bentonite beds. Mosby Sandstone Member (upper part of formation in north- and east-central Montana): brown sandstone locally with chert pebbles, interbedded with gray shale. Big Elk Sandstone Member (lower part of formation in southwest-central Montana): light gray, chert-rich sandstone commonly stained dark red, interbedded with thin, dark gray to black clayey shale. Marine shelf. Thickness as much as 260 m (853 ft).
State Montana
Name Greenhorn and Belle Fourche Formations
Geologic age Cretaceous
Lithologic constituents
Sedimentary > Clastic > Sandstone (Bed)
Sedimentary > Clastic > Mudstone > Shale (Bed, Calcareous)
Sedimentary > Carbonate > Limestone (Bed)
Sedimentary > Chemical > Chert (Bed)
Sedimentary > Clastic > Mudstone > Claystone > Bentonite (Bed)

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 - Blaine - Chouteau - Hill - Petroleum - Phillips - Yellowstone