Phosphoria through Amsden Formation: Phosphoria, Quadrant, and Amsden Formations

Phosphoria Formation (Pp): Gray and tan dolomite, cherty limestone, bedded chert, siltstone, sandstone, and gray to black phosphatic and dolomitic shale. Marine shelf and offshore marine. Thickness as much as 255 m (837 ft). Quadrant Formation (PAq): Very light gray, yellowish or pinkish, well-sorted sandstone or quartzite, locally interbedded with subordinate limestone beds. Marine. Generally, thickness as much as 140 m (460 ft), but as much as 800 m (2,625 ft) in southwestern-most Montana. 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).
State Montana
Name Phosphoria through Amsden Formation: Phosphoria, Quadrant, and Amsden Formations
Geologic age Permian through Mississippian
Lithologic constituents
Major
Sedimentary > Carbonate > Limestone (Bed)
Sedimentary > Clastic > Sandstone (Bed)
Sedimentary > Carbonate > Dolostone (Bed)
Minor
Sedimentary > Clastic > Siltstone (Bed)
Sedimentary > Clastic > Mudstone > Shale (Bed)
Sedimentary > Clastic > Mudstone > Shale > Phosphatic-shale (Bed)
Sedimentary > Chemical > Chert (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 Broadwater - Carbon - Deer Lodge - Gallatin - Granite - Jefferson - Lewis and Clark - Madison - Park - Powell - Stillwater - Sweet Grass