Muddy and Thermopolis Formations

Muddy Formation (Kmd): Gray to brown fine- to medium-grained sandstone with interbedded dark gray locally carbonaceous shale. Deltaic, shoreface, and offshore marine. Thickness as much as 50 m (164 ft). Thermopolis Formation (Kt): Dark gray to black shale with subordinate siltstone beds. Middle part of formation contains one or more thin sandstone beds with chert pebble horizons where Muddy Sandstone is not present. Offshore marine. Thickness as much as 305 m (1,000 ft).
State Montana
Name Muddy and Thermopolis Formations
Geologic age Cretaceous
Lithologic constituents
Sedimentary > Clastic > Sandstone (Bed)
Sedimentary > Clastic > Mudstone > Shale > Black-shale (Bed, Carbonaceous)
Sedimentary > Clastic > Siltstone (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 Gallatin - Madison