Reconnaissance stratigraphy of the Prichard Formation (Middle Proterozoic) and the early development of the Belt Basin, Washington, Idaho, and Montana
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 in 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.
- OSTI ID:
- 6034271
- Journal Information:
- United States Geological Survey, Professional Paper; (USA), Vol. 1490
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
GEOLOGIC FORMATIONS
STRATIGRAPHY
IDAHO
MONTANA
WASHINGTON
AGE ESTIMATION
ARGILLITE
BIOTITE
CARBON
CHLORITE MINERALS
COMPRESSION
DEPOSITION
DRAINAGE
GEOLOGIC HISTORY
IRON SULFIDES
ISOTOPE DATING
LEAD ISOTOPES
MAPPING
METAMORPHIC ROCKS
METAMORPHISM
MOUNTAINS
QUARTZITES
RIVERS
SCHISTS
SEDIMENTARY BASINS
SEDIMENTS
SILTSTONES
THICKNESS
URANIUM ISOTOPES
ACTINIDE ISOTOPES
CHALCOGENIDES
DIMENSIONS
ELEMENTS
FEDERAL REGION VIII
FEDERAL REGION X
GEOLOGIC STRUCTURES
GEOLOGY
IRON COMPOUNDS
ISOTOPES
MICA
MINERALS
NONMETALS
NORTH AMERICA
ROCKS
SEDIMENTARY ROCKS
SHALES
SILICATE MINERALS
STREAMS
SULFIDES
SULFUR COMPOUNDS
SURFACE WATERS
TRANSITION ELEMENT COMPOUNDS
USA
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