Named for Fergus Co in central MT. Encompasses all formations between Late Mississippian (late Chesterian) Big Snowy Group (revised) and Middle Pennsylvanian (Atokan) Amsden Formation (revised). Type section of Fergus Group is composite of original types first assigned to its four subunits (ascending), lower Tyler Formation (revised), NE1/4 sec 25, T13N, R21E (Mundt, 1956); Bear Gulch Formation (revised), in Bear Gulch, SW1/4, sec 22, T14N R21E; upper Tyler Formation (revised), NE1/4 sec 30, T14N, R21E; Cameron Creek Formation (revised), in Cameron Creek, sec 31, T11N, R21E (Gardner, 1959); and Alaska Bench Formation (revised) near Beacon Hill, SW1/4 sec 36, T13N, R19E, in Fergus Co or adjacent Golden Valley Co), central MT. The Fergus consists of estuarine and restricted marine form sediments highlighted by red bed sequence (Gardner, 1959), a paleosol (unpublished work by author), and light colored marine to brackish water limestone. Maximum thickness of the Fergus 1000 ft (305 m), placed as it is, in context of Mississippian rift basin thought to contain about 3500 ft (1068 m) of Kinderhookian through Chesterian sediments. Unconformably overlies late Chesterian Heath Formation (revised) of Big Snowy Group and unconformably lies beneath Amsden Formation. Based on fauna, age is Late Mississippian (late Chesterian). Prevalent are thin shelled and dwarfed brachiopods, stromatolites, shrimp, pelecypods, ostracods, conodonts, gastropods, a few nautiloids, and fish. Early workers, and more recently, the U. S. Geological Survey, assigned all Fergus Group subunits to Pennsylvanian (Morrowan) age (Maughan, 1984). Within past fifteen years, however, extensive paleontological work with faunas of Bear Gulch Formation, as well as with limited faunas from Cameron Creek Formation and Alaska Bench Formation, have lead, instead, to the late Chesterian determination. Report includes historical nomenclature chart, depositional model data, columnar section, and fossil lists.

Source: GNU records (USGS DDS-6; Denver GNULEX).