Type Locality:

The type section (section 15 - 62°26.8'N; 129°18.1'W) is located about 5 km west-southwest of the divide between Placer and Don creeks, about 50 km southeast of the headwaters of the Prevost River after which the formation is named. Unfortunately it includes neither upper nor lower formational contacts (Gordey and Anderson, 1993).

Distribution:

Exposures in the northwestern part of the Nahanni map area (105I) are extensive; the best outcrops occur southwest of the headwaters of Placer and Don creeks and south of Peller River pluton (Gordey and Anderson, 1993).

Lithology:

The Prevost Formation comprises brown weathering shale, resistant grey to grey-brown weathering chert pebble conglomerate, and dark grey to black chert-quartz sandstone. For descriptive purposes the type section, totalling 555 m thick, is divided into three members. The lower member, 160 m thick consists of grey weathering, dark grey, medium to coarse grained chert-quartz sandstone and rare chert granule to pebble conglomerate. The middle member, 90 m thick, comprises brown weathering, dark grey, thin-bedded shale and siltstone. The upper member comprises massive, coarse grained, chert-quartz sandstone and chert-pebble conglomerate totalling 305 m (Gordey and Anderson, 1993).

Relationship:

The Prevost Formation is part of the Earn Group. It rests sharply and unconformably above gun-blue weathering siliceous shale and chert of the Portrait Lake Formation (Earn Group). It is overlain unconformably(?) by resistant, clean quartz sandstone and black shale of the mid-Mississippian Tsichu Group. Clastic formations correlative with Devono-Mississippian parts of the Earn Group blanket the northern Cordillera. These include the Giventian Hare Indian, Frasnian Canol (Aitken et al., 1982), and Frasnian to early mid-Famennian Imperial formations (Chi and Hills, 1974) of northern Mackenzie Mountains and northern Yukon. The Besa River Formation (Pelzer, 1966) and the Fort Simpson Formation (Belyea and McLaren, 1962) are correlatives in northern British Columbia and western Northwest Territories respectively. MacIntyre (1983) and Jefferson et al. (1983) describe correlative chert-pebble conglomerate, shale and sandstone, and stratiform barite-sulphide deposits of the Gataga District of northeastern British Columbia. In Pelly Mountains, clatic strata and felsic volcanic rocks of Earn Group age have been described by Gordey (1981) and Mortensen (1982) (Gordey and Anderson, 1993). Gordey and Anderson (1993) recognized Prevost Formation in southeastern Niddery Lake map area. However the unit they designated as Prevost? is identical to the Hawthorne Creek Formation, which is defined by Cecile (2000) and included with the Tsichu Group because it contains quartzite and limestone (Cecile, 2000).

History:

The Earn Group was first proposed by Campbell (1967) for chert, conglomerate, and limestone in Glenlyon map area (105L), about 250 km northwest of the Nahanni area. In Nahanni map area two regionally mappable formations compose the Earn Group (Gordey et al., 1982). The lower is called the Portrait Lake Formation and the upper Earn Group is the Prevost Formation (Gordey and Anderson, 1993).

References:

Aitken, J.D., Cook, D.G., and Yorath, C.J., 1982. Upper Ramparts River (106 G) and Sans Sault Rapids (106 H) map areas, District of Mackenzie, Northwest Territories; Geological Survey of Canada, Memoir 388, 48 p.

Belyea, H.R. and McLaren, D.J., 1962. Upper Devonian formations, southern part of Northwest Territories, northeastern British Columbia and northwestern Alberta; Geological Survey of Canada, Paper 61-29, 74 p.

Campbell, R.B., 1967. Geology of Glenlyon map-area, Yukon Territory (105L); Geological Survey of Canada, Memoir 352, 92 p.

Cecile, M.P., 2000. Geology of the northeastern Niddery Lake map area, east-central Yukon and adjacent Northwest Territories; Geological Survey of Canada, Bulletin 553, 120 p.

Gordey, S.P. and Anderson, R.G., 1993. Evolution of the northern Cordilleran miogeocline, Nahanni map area (105I), Yukon Territory and District of Mackenzie; Geological Survey of Canada, Memoir 428, 214 p.

Gordey, S.P., 1981. Stratigraphy, structure and tectonic evolution of southern Pelly Mountains in the Indigo Lake area, Yukon Territory; Geological Survey of Canada, Bulletin 318, 44 p.

Gordey, S.P., Abbott, J.G., and Orchard, M.J., 1982. Devono-Mississippian (Earn Group) and younger strata in east-central Yukon; in, Current Research, Part B; Geological Survey of Canada, Paper 82-1B, pp. 93-100.

Jefferson, C.W., Kilby, D.B., Pigage, L.C., and Roberts, W.J., 1983. The Cirque barite-zinc-lead deposits, northeastern British Columbia; in, Sediment-Hosted Stratiform Lead-Zinc Deposits, edited by D.F. Sangster, Mineralogical Association of Canada, Short Course Handbook, v. 9, p. 121-140.

MacIntyre, D.G., 1983. Geology and stratiform barite-sulphide deposits of the Gataga District, northeast British Columbia; in, Sediment-Hosted Stratiformed Lead-Zinc Deposits, edited by D.F. Sangster; Mineralogical Association of Canada, Short Course Handbook, vol. 9, pp. 85-120.

Mortensen, J.K., 1982. Geological setting and tectonic significance of Mississippian felsic metavolcanic rocks in the Pelly Mountains, southeastern Yukon Territory; Canadian Journal of Earth Sciences, v. 19, no. 1, p. 8-22.

Pelzer, E.E., 1966. Mineralogy, geochemistry, and stratigraphy of the Besa River Shale, British Columbia; Canadian Society of Petroleum Geologists (CSPG), Bulletin of Canadian Petroleum Geology, vol. 14, no. 2 (June), pp. 273-321.