Type Locality:

Ridge about 15 km (9 mi) west of Hayhook Lake in the Plateau thrust plate (63°33'30"N, 127°03'30"W).

Distribution:

Extends from central Mackenzie Mountains for 500 km (311 mi) northwestward, and ranges in thickness from 100-700 m (328-2,297 ft) in exposed sections and probably more in subsurface sections. It is 672 m (2,204 ft) thick at the type section.

Locality Data:

Thickness(m): Minimum 100, Maximum 700, Typical 672.

Lithology:

Near the base, dark pyritic shale or thinly laminated to bedded grey limestone. Shale, siltstone and internally parallel laminated sandstone. Numerous slump folds in the basal parts of the formation. Sandstones are quartzo-feldspathic arenites. Channels of quartz-chert pebble conglomerate. Towards the west (Snake River region) turbiditic sandstones and platform carbonate.

Relationship:

Rests in sharp but generally conformable contact on Shezal Formation with the base placed at the sharp contact between diamictite of the Shezal Fm and dark grey to black pyritic shale of the former. The angular unconformity postulated by Blusson (1971) to be between the Shezal and the Twitya formations was not substantiated by later work. The Twitya is gradation-ally overlain by the Keele Formation with the contact placed at the base of the first carbonate unit. The Twitya Formation corresponds to the upper unit of the Rapitan Formation of Upitis (1966) and Gabrielse et al. (1973), and the "shale" unit of the Rapitan Group of Eisbacher (1976). Yeo et al. (1978) and Yeo (1981) place the Twitya Formation in the Hay Creek Group. The assignment to the Rapitan Group is that of Eisbacher (1978b).

Other Citations:

Eisbacher, 1976, 1978a.

References:

Blusson, S.L., 1971. Sekwi Mountain map-area, Yukon Territory and District of Mackenzie (105 P); Geological Survey of Canada, Paper 71-22, 17 p. + "A" Series Map 1333A, Geology, Sekwi Mountain, Northwest Territories - Yukon Territories; Scale: 1:250 000.

Eisbacher, G.H., 1976. Sedimentology of the Dezadeash Flysch and its implications for strike-slip faulting along the Denali Fault, Yukon Territory and Alaska; Canadian Journal of Earth Sciences = Journal Canadien des Sciences de la Terre, vol. 13, no. 11 (November 1976), pp. 1495-1513.

Eisbacher, G.H., 1978a. Two major Proterozoic unconformities, northern Cordillera, in, Current research, Part A, Blackadar, R.G., Griffin, P.J., Dumych, H., and Irish, E.J.W. (Eds. and Compilers); Geological Survey of Canada, Paper 78-1A, pp. 53-58.

Eisbacher, G.H., 1978b. Re-definition and subdivision of the Rapita Group, Mackenzie Mountains; Geological Survey of Canada, Paper 77-35, 21 p.

Gabrielse, H., Blusson, S.L., and Roddick, J.A., 1973. Geology of the Flat River, Glacier Lake and Wrigley Lake map-areas, District of Mackenzie and Yukon Territory; Geological Survey of Canada, Memoir 366 (Parts I and II), 421 p.

Upitis, U., 1966. The Rapitan Group, southwestern Mackenzie Mountains, Northwest Territories: unpublished M.Sc. thesis, McGill University, Montreal, 70 p.

Yeo, G.M., 1981. The Late Proterozoic Rapitan glaciation in the northern Cordillera; in, Proterozoic basins in Canada, Proceedings of a Symposium held in Halifax, May 1980, sponsored by the Geological Association of Canada, Campbell, F.H.A. (Ed.); Geological Survey of Canada, Paper 81-10, pp. 25-46.

Yeo, G.M., Delaney, G.D., and Jefferson, C.W., 1978. Two major Proterozoic unconformities, Northern Cordillera: Discussion, in, Current Research, Part B, III: DIscussions and Communications, Blackadar, R.G., Griffin, P.J., Dumych, H. and Neale, E.R.W. (Eds. and Compilers); Geological Survey of Canada, Paper 78-1B, pp. 225-230.