Abstract

The Prichard Formation crops out in the area between Glacier National Park on the east and Spokane, Washington, on the west and from the United States-Canada border on the north nearly to the Idaho batholith on the south. The formation, commonly more than 6 kilometers thick, comprises the basal one-fourth to one-third of the Belt Supergroup, and the base is everywhere concealed. The Prichard differs from the rest of the Belt Supergroup in containing a great thickness of turbidite, abundant and widespread diorite sills, laminated carbon-rich silts, and abundant iron sulfide. The rocks, which have been regionally metamorphosed to the greenschist facies, are within the biotite to garnet zone everywhere except in Glacier National Park. Original metamorphism was static and resulted from load, but Mesozoic compression and intrusion superposed dynamic metamorphism on the rocks in the western and southwestern parts of the outcrop area. Rocks of the Prichard Formation, which are classified as quartzite, siltite, and argillite, consist mostly of quartz, plagioclase (mostly albite), sericite (2M illite), biotite, and chlorite. The quartzites are graywackes. The sills are mostly gabbro to quartz diorite, and their composition is characteristic of continental gholeiite. The Prichard Formation consists of an argillaceous facies, present inmore » the St. Joe, Coeur d'Alene, and Salish Mountains in the southern and eastern parts of the area, and a quartzite facies present in the Purcell, northern Cabinet, and Selkirk Mountains. Each facies is divided into informal members for purposes of mapping, and although the nomenclature is clumsy and not entirely in accord with the North American Stratigraphic Code, it is used in this report.« less

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