Type Locality:

Midway between Cranbrook and Fort Steele, southeastern British Columbia (no section designated).

Distribution:

Exposed near 49 deg 30'N, between the Moyie and St. Mary faults in the Purcell Mountains and, directly across the Rocky Mountain Trench between the comparable Dibble Creek and Boulder Creek faults in the Hughes Range of the Rocky Mountains. This disrupted wedge of Cranbrook strata thickens westward from less than 150 m (492 ft) in the Hughes Range to about 300 m (984 ft) in the Purcell Mountains near 116 deg W, and about 500 m (1,640 ft) near 116 deg 20'W. North of a zone in which such rocks are missing identical strata in the western edge of the Rockies near and north of 50 deg N have been referred to the Cranbrook Formation (Leech, 1954). Their maximum thickness is about 300 m (984 ft). They pinch out northward against the Windermere High. Southward they disappear 6 km (3.75 mi) south of 50 deg N as a result of depositional thinning (?) and intra-Cambrian erosion.

Locality Data:

Thickness(m): Maximum 500.

Lithology:

Predominantly siliceous quartzite, medium- to coarse-grained, in part containing sporadic quartz pebbles with gritty quartzite and lenticular beds of quartz-pebble conglomerate. White, cream and grey colors prevail, but pale green, rose and pale tan are common. Weathered surfaces are typically smooth and white or yellowish. The formation is generally distinctly bedded in thicknesses of 0.3 to 1.25 m (1 to 4 ft), but instances of indefinite and/or much thicker beds occur. Beds are mostly massive or have repetitive slight color variations that in part reflect grain size variations. Cross-bedding is common. The basal beds vary with locality but are generally quartzite, pebbly quartzite or quartz-pebble conglomerate, with or without fragments of the underlying formation. These beds are commonly darker than higher ones and in places are red-brown. Locally the lowest beds are thin alternations of siltstone and quartzite, but there are instances of a thin basal conglomerate of angular and partly rounded Precambrian fragments in a sandy matrix. The coarsest and thickest conglomerates are in western exposures. [Coarse conglomerates north of Boulder Creek in the Rocky Mountains, originally assigned to the Cranbrook Formation (Rice, 1937) are probably younger.] In the Purcell Mountains, near 49 deg 35'N, 116 deg W the highest part of the formation consists of thin interbeds of quartzite and carbonate and locally contains magnesite.

Relationship:

Unconformably overlies Precambrian strata. Evidence of angular unconformably is difficult to discern locally but obvious regionally. The wedge exposed near 49 deg 30'N rests on progressively older strata as it thickens westward. The wedge rests on the Gateway or the Phillips Formation (upper Purcell) in the Rocky Mountains or the Kitchener Formation (lower Purcell) in the Purcell Mountains, where western exposures contain clasts of the still older Creston Formation. The Cranbrook Formation is overlain, apparently conformably by the pelitic Lower Cambrian Eager Formation. Cranbrook strata in the Rocky Mountains near and northward from 50 deg N rest on the Roosville Formation (upper Purcell), in the south and the Horsethief Creek Group (Windermere) in the north. They are overlain conformably by arenaceous and calcareous Lower Cambrian beds and, where these are lacking, unconformably by conglomerate, dolomitic sandstone and sandy dolomite beneath typical strata of the Upper (?) Cambrian Jubilee Formation.

References:

Leech, G.B., 1954. Canal Flat, British Columbia; Geological Survey of Canada, Paper 54-7.

Rice, H.M.A., 1937. Cranbrook map-area, British Columbia; Geological Survey of Canada, Memoir 207.

Schofield, S.J., 1922. Relationship of the Precambrian (Beltian) terrain to the Lower Cambrian strata of southeastern British Columbia; Geological Survey of Canada, Museum Bulletin 35.