Morrison through Chugwater Formation: Morrison, Swift, Rierdon, Piper, and Chugwater Formations

Morrison Formation (Jm): Green, gray, or red mudstone and marlstone with subordinate limestone and sandstone beds. Upper part Neocomian age in central Montana and contains carbonaceous black shale and coal. Fluvial, paludal, and lacustrine. Thickness as much as 180 m (590 ft). Swift Formation (Jsw): Orangish brown, glauconitic, flaggy-bedded, commonly fossiliferous, fine-grained sandstone or sandy coquina with subordinate dark gray shale interbeds; chert pebbles common. In west-central and northwestern Montana, a dark gray, noncalcareous, micaceous shale forms the lower part of the formation, commonly with a basal chert-pebble conglomerate or conglomeratic sandstone as much as 3 m (10 ft) thick. Shallow marine. Thickness as much as 70 m (230 ft). Rierdon Formation (Jr): Gray, locally fossiliferous limestone that may contain floating grains of quartz sand, interbedded with greenish gray limy shale. Lagoonal and marine shelf. Thickness as much as 105 m (344 ft). Piper Formation (Jp): Upper part: red mudstone and gypsum; middle part: gray shale, limestone, and dolomite; lower part: red mudstone and gypsum. Marine and restricted coastal evaporite basins. Thickness as much as 75 m (246 ft). Chugwater Formation (TRc): Red siltstone, sandstone, and shale with subordinate gypsum and local limestone bed at top. Shallow restricted marine and nonmarine. Thickness as much as 180 m (590 ft).
State Montana
Name Morrison through Chugwater Formation: Morrison, Swift, Rierdon, Piper, and Chugwater Formations
Geologic age Jurassic and Triassic
Lithologic constituents
Major
Sedimentary > Clastic > Sandstone (Bed, Glauconitic)
Sedimentary > Carbonate > Limestone (Bed)
Sedimentary > Clastic > Mudstone (Bed)
Minor
Sedimentary > Carbonate > Dolostone (Bed)
Sedimentary > Clastic > Mudstone > Shale (Bed)
Sedimentary > Clastic > Siltstone (Bed)
Sedimentary > Carbonate > Marlstone (Bed)
Sedimentary > Clastic > Conglomerate (Bed)
Sedimentary > Chemical > Evaporite > Gypsum (Bed)
Incidental
Sedimentary > Clastic > Mixed-clastic > Conglomerate-sandstone (Bed)
Sedimentary > Carbonate > Limestone > Coquina (Bed)sandy coquina
Sedimentary > Coal (Bed)
Sedimentary > Chemical > Chert (Bed)
Sedimentary > Clastic > Mudstone > Shale > Black-shale (Bed, Carbonaceous)
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 Carbon - Park - Stillwater - Sweet Grass