Belt Supergroup

[2] It extends into Canada where the equivalent rocks, which are called the Purcell Supergroup, are exposed in southeastern British Columbia and southwestern Alberta.

Most have undergone weak metamorphism to greenschist facies, and as a result the mudrocks are commonly classified as argillites and the sandstones as quartzites.

[5] Paleogeographic reconstructions indicate that the Belt Supergroup accumulated in a fault-bounded rift basin that existed where the North American craton and another landmass were joined in a supercontinent called Columbia/Nuna.

[8] Evidence of the basin-bounding faults exists on all sides of the Belt basin except the west, which rifted away during subsequent continental breakup.

The Belt Supergroup rocks host a variety of economically significant deposits of lead, zinc, silver, copper, gold, and other metals.

The equivalent rocks of the Purcell Supergroup in British Columbia include the Sullivan ore body, which has also been a major producer of lead, zinc, and silver.

Geologic map of the Belt Supergroup in Idaho
Snowslip Formation, numerous beds of dolomitic siltite, dolomite, and stromatolites and gray to greenish-gray, very fine grained quartzite beds
Helena Formation, thin to thick beds of gray to dark gray argillaceous dolomite, gray limestone
Grinnell Formation, quartz-rich of interlayered red argillite and siltite characterized by mudcracks and interstratified with abundant white, crossbedded quartzite and sandstone.
Appekunny Formation , green and red argillite and siltite and moderate amounts of light-gray quartzite. Divided into five informal members.
Grinnell Formation, mudcracks in red siltite
Appekunny Formation with white bands of quartzite overlying the Altyn Formation of slope forming buff, light-gray, dark-gray, and orange dolomite, argillaceous dolomite, and stomatolitic limestone