Type Locality:

The type section (section 45 - 63°8.0'N; 130°1.5'W) is located in southeastern Niddery Lake map area, about 22 km northwest of the lake after which the formation is named (Gordey and Anderson, 1993).

Distribution:

The best exposures occur along Margaret Syncline, north of O'Grady Batholith, in the general vicinity of Placer and Don creeks, and 18 km southwest of Pelly River pluton (Gordey and Anderson, 1993).

Lithology:

The Portrait Lake Formation comprises shale, chert, and minor sandstone and conglomerate. The type section, totalling 897 m, is divisible into three members; lower and upper fine clastic members are separated by a middle member of chert conglomerate. The lower member, 419 m thick, consists mostly of dark brown weathering, silty shale and shale in beds 0.1-0.3 m thick. The middle member, 196 m thick, consists of black weathering, massive pebble conglomerate. The upper member comprises 261 m of light gun-blue weathering, black platy siltstone (Gordey and Anderson, 1993).

Relationship:

The Prevost Formation is part of the Earn Group. Near Broken Skull River, the Portrait Lake Formation conformably(?) overlies the Nahanni Formation. Toward the southwest of the Nahanni map area it sharply and unconformably overlies carbonate of the Funeral, Grizzly Bear, Sapper, and locally Haywire formations. Farther to the southwest near Summit Lake, the base rests conformably above orange weathering mudstone of the Steel Formation. It is overlain sharply and unconformably by brown weathering, blue-grey shale scree of the Prevost Formation (Earn Group). The Lower and Middle Devonian parts of the Portrait Lake Formation correlate with all or part of shallow water Mackenzie Platform carbonates of the Delorme, Camsell, Sombre, Arnica, Natla, Landry, Headless, and Nahanni formations. 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). The Earn Group in the northeastern Niddery Lake map area consists of two formations, the Misfortune and the overlying Thor Hills. These two units are generally equivalent to the Portrait Lake Formation (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.

Chi, B.I. and Hills, L.V., 1974. Stratigraphic and paleoenvironmental significance of Upper Devonian megaspores, type section of the Imperial Formation, Northwest Territories Canada; pp. 241-257: in Aitken, J.D. and Glass, D.J. (eds.), Canadian Arctic Ge

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.