Morrison through Dinwoody Formation: Morrison, Swift, Rierdon, Sawtooth, Thaynes, Woodside, and Dinwoody 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). Sawtooth Formation: Western Montana: dark gray, platy to shaly, dense limestone with local basal conglomerate. Central Montana: upper calcareous siltstone, middle dark gray shale with thin limestone interbeds, and lower fine-grained sandstone. Three local members Bowes, Firemoon, and Tampico, in descending order. Bowes Member: dark gray to medium gray, calcareous mudstone, limestone, and quartzose sandstone. Firemoon Member: dark to medium gray, limestone and calcareous mudstone. Tampico Member: very light gray, well-sorted quartz sandstone and siltstone, and chert-pebble conglomerate. Shallow marine. Thickness as much as 205 m (673 ft). Thaynes Formation: Gray and tan limestone with interbedded siltstone, shale, and tan sandstone. Locally with red shale tongues. Marine. Thickness as much as 290 m (950 ft). Woodside Formation: Maroon and red siltstone, dolomite, and shale. Tidal flat. Thickness as much as 250 m (820 ft). Dinwoody Formation (TRd): Interbedded green siltstone, shale, sandstone, and carbonate that grades eastward into red shale, siltstone, and anhydrite. Nearshore and restricted marine. Thickness as much as 330 m (1,083 ft).
State Montana
Name Morrison through Dinwoody Formation: Morrison, Swift, Rierdon, Sawtooth, Thaynes, Woodside, and Dinwoody Formations
Geologic age Jurassic and Triassic
Lithologic constituents
Major
Sedimentary > Carbonate > Limestone (Bed)
Sedimentary > Clastic > Sandstone (Bed, Glauconitic)
Sedimentary > Clastic > Siltstone (Bed, Calcareous)
Minor
Sedimentary > Clastic > Mudstone (Bed, Calcareous)
Sedimentary > Clastic > Conglomerate (Bed)
Sedimentary > Carbonate > Dolostone (Bed)
Sedimentary > Carbonate > Marlstone (Bed)
Sedimentary > Clastic > Mudstone > Shale (Bed)
Incidental
Sedimentary > Chemical > Evaporite > Anhydrite (Bed)
Sedimentary > Carbonate > Limestone > Coquina (Bed)sandy coquina
Sedimentary > Chemical > Chert (Bed)
Sedimentary > Coal (Bed)
Sedimentary > Clastic > Mudstone > Shale > Black-shale (Bed, Carbonaceous)
Sedimentary > Clastic > Mixed-clastic > Conglomerate-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.

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Counties Gallatin - Madison