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Thermopolis Formation

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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 Thermopolis Formation
Geologic age Cretaceous
Lithologic constituents
Major
Sedimentary > Clastic > Mudstone > Shale (Bed)
Minor
Sedimentary > Clastic > Siltstone (Bed)
Incidental
Sedimentary > Chemical > Chert (Bed)
Sedimentary > Clastic > Sandstone (Bed)
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 Beaverhead - Big Horn - Fergus - Jefferson - Judith Basin - Madison - Musselshell - Petroleum - Wheatland