Amsden Formation and Big Snowy Group

Amsden Formation (PAMa): Red shale, light gray limestone, and cherty and sandy limestone. Coastal plain or marine. Thickness as much as 180 m (590 ft). Big Snowy Group (Mbs) - Heath Formation (Mh): Black, fissile, commonly petroliferous shale with subordinate sandstone, gypsum, and coal. Restricted marine to coastal plain. Thickness as much as 150 m (492 ft). Otter Formation (Mo): Green, grayish green, gray, and subordinate reddish brown shale interbedded with light brown or gray limestone. Open and semirestricted marine. Thickness as much as 150 m (492 ft). Kibbey Formation (Mk): Red, quartzose sandstone, siltstone, and shale, locally with subordinate thin gypsum beds and limestone bed in middle part. Intertidal and subtidal. Thickness as much as 105 m (345 ft).
State Montana
Name Amsden Formation and Big Snowy Group
Geologic age Pennsylvanian and Mississippian
Lithologic constituents
Sedimentary > Clastic > Sandstone (Bed)
Sedimentary > Carbonate > Limestone (Bed)
Sedimentary > Clastic > Mudstone > Shale (Bed)
Sedimentary > Clastic > Mudstone > Shale > Black-shale (Bed)
Sedimentary > Clastic > Siltstone (Bed)
Sedimentary > Coal (Bed)
Sedimentary > Chemical > Evaporite > Gypsum (Bed)
Sedimentary > Chemical > Chert (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 Broadwater - Judith Basin - Lewis and Clark - Meagher - Wheatland