Geologic Unit: Amsden

Usage:

Amsden Formation (ID*,MT*,ND,WY*)
Amsden Group* (MT*,ND*)

Subunits:

GROUP STATUS (alphabetical): Alaska Bench Limestone (MT*,ND*), Devils Pocket Formation (MT*), Tyler Formation (MT*).
FORMATION STATUS: Alaska Bench Limestone Member (MT,ND), Darwin Sandstone Member (MT*,WY*), Horseshoe Shale Member (WY*), Moffat Trail Limestone Member (WY*), Ranchester Limestone Member (WY*) or Ranchester Dolomite Member (WY*).

Geologic age:

Early to Middle Pennsylvanian (Morrowan to Atokan)*
In northwestern Wyoming, age is Late Mississippian to Middle Pennsylvanian (Meramecian to Atokan)*

Type section, locality, area and/or origin of name:

Named from Amsden Branch of Tongue River, west of Dayton, in sec. 32, T. 57 N., R. 86 W., Sheridan Co., WY (Darton, 1904).
Type section: on Amsden Creek, in SE/4 sec. 33, T. 57 N., R. 87 W., 4 mi west of Dayton, [Dayton South 7.5-min quadrangle], Sheridan Co., WY (Mallory, 1967).
Reference section: west flank of Bighorn Mountains, near lower end of Tensleep Canyon, near Tensleep [Ten Sleep], about 0.5 mi upstream from Wigwam Trout Rearing Station on north side of U.S. Highway 16, [border of Ten Sleep and Old Maid Gulch 7.5-min quadrangles], Washakie Co., WY (Mallory, 1967).

AAPG geologic province:

Bighorn basin*
Central Montana uplift*
Green River basin*
Idaho Mountains province*
Montana folded belt*
Powder River basin*
Sweetgrass arch*
Williston basin*
Wind River basin*
Yellowstone province*