Argillite

Argillite

Sedimentary rock
A piece of black argillite from Haida Gwaii
Composition
indurated clay particles

An argillite (pronunciation: /ˈɑːrdʒᵻlaɪt/) is a fine-grained sedimentary rock composed predominantly of indurated clay particles. Argillaceous rocks are basically lithified muds and oozes. They contain variable amounts of silt-sized particles. The argillites grade into shale when the fissile layering typical of shale is developed. Another name for poorly lithified argillites is mudstone. These rocks, although variable in composition, are typically high in aluminium and silica with variable alkali and alkaline earth cations. The term pelitic or pelite is often applied to these sediments and rocks. Metamorphism of argillites produces slate, phyllite, and pelitic schist.

The Belt Supergroup, an assemblage of rocks of late Precambrian (Mesoproterozoic) age, includes thick sequences of argillite, as well as other metamorphosed or semi-metamorphosed mudstones. It is exposed primarily in western Montana, including the Bitterroot Valley and Bitterroot Mountains, the Missoula area, Flathead Lake, and Glacier National Park, and in northern Idaho. There are also minor occurrences in northwestern Washington and western Wyoming. Excellent outcrops of deep purple, wine red, red, blue, turquoise, and green argillites of the Belt Supergroup can be seen in Glacier National Park in northwestern Montana and in Wolf Creek Canyon along US Interstate 15 in west-central Montana.