Argillite () is a fine-grained sedimentary rock composed predominantly of Friability clay particles. Argillaceous rocks are basically lithified muds and Pelagic sediment. 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 . The term pelite is often applied to these and rocks. Metamorphism of argillites produces slate, phyllite, and pelitic schist.
Belt Supergroup
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.
[Schieber, J. 1990. Significance of styles of epicontinental shale sedimentation in the Belt basin, Mid-Proterozoic of Montana, U.S.A. Sedimentary Geology, v. 69, p. 297-312.] 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 northeastern 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 Interstate 15 in west-central Montana.
[Alt, D.D. and Hyndman, D.W. 1986. Roadside geology of Montana. Mountain Press Publishing Co., Missoula, Montana, 427 p. .]
"Black slate"
The Haida carvings of
Haida Gwaii along the coast of
British Columbia are notable aboriginal art treasures created from a type of a hard, fine black silt argillite, sometimes called "black
slate". The black slate occurs only at a quarry on a Slatechuck Mountain in the upper basin of
Slatechuck Creek, near the town of Skidegate on
Graham Island. At one time, around 1900, it was shipped to Victoria for manufacturing; today the
Haida people have a monopoly on use of the argillite. Argillite carvings are synonymous with Haida artwork and are one of the few art forms on the Northwest Coast that is the exclusive right of one cultural group. This artwork has been of high quality and prized around the world since the Haida first began carving it to trade and sell to sailors around 1800. Contemporary Haida carvers continue the tradition.
See also
