Unit Name: Duo Lake Formation
Unit Type: Lithostratigraphic
Rank: Formation
Status: Formal
Usage: Currently in use
Age Interval: Tremadocian - late Early Silurian (488.3 - 422.9 ma)
Age Justification: Biostratigraphy. Graptolites collected from the Duo Lake Formation in the northeastern Niddery Lake map area range in age from middle Early Ordovician (Late Tremadoc) to late Early Silurian in age. At least eight graptolite zones are represented in these collections (Cecile, 2000).
Province/Territory: Northwest Territories; Yukon Territory

Originator: Cecile, 1982.

Type Locality:
The type section (Section 6) is in the Misty Creek Embayment at 64°42'N and 130°47'W, 2 km southwest of Goober Lake (64°44'N, 130°45'W), in the Bonnet Plume Lake map area. There the formation, including upper and lower contacts, is well exposed along the sides and crest of a major south trending ridge (Cecile, 1982).

Distribution:
The Duo Lake Formation is up to 415 m thick and is thickest along the embayment axis. The formation is found over most of the embayment area but is generally missing in Franklin Mountain transition zone and platform facies (Cecile, 1982). The Duo Lake Formation outcrops over the eastern two thirds and the southern part of the northeastern Niddery Lake map area (Cecile, 1997a-d). It is also mapped throughout the Nahanni map area (Gordey and Anderson, 1993) where it is very similar in composition and thickness to the Duo Lake Formation in the type area but has virtually no interbedded limestone (Cecile, 2000).

Locality Data:
Thickness(m): Maximum 415.

Lithology:
The Duo Lake Formation comprises recessive weathering, black siliceous graptolitic shale and chert and minor limestone (Gordey and Anderson, 1993). All strata are thin bedded or laminated and graptolitic. In the type area the Duo Lake Formation is divided into two units, a basal, 240 m thick succession of thin bedded limestone and graptolitic shale, and an upper, 76 m thick unit of graptolitic siliceous shale and minor chert. Elsewhere in the Misty Creek Embayment the Duo Lake Formation is up to 415 m thick (Cecile, 1982). In the northeastern Niddery Lake map area the Duo Lake Formation consists of between 125 and 225 m of graptolitic, calcareous shale, shale, siliceous shale, minor dolomitic shale, and minor chert. It has a distinct, conformable contact with the Rabbitkettle Formation that can be placed to within a metre, where a distinct change from limestone-dominated to shale-dominated strata occurs (Cecile, 2000).

Relationship:
Gordey and Anderson (1993) included the Duo Lake Formation within the Road River Group. The Duo Lake Formation overlies the Rabbitkettle Formation conformably. In most places the contact, although defined in scree, is abrupt. The contact with the overlying Steel Formation is sharp and apparently conformable. The Duo Lake Formation is continuous north, northeast and east with Duo Lake Formation strata found in the Misty Creek Embayment. The upper Duo Lake Formation in the study area is also correlative with the Cloudy Formation in the Misty Creek Embayment. The Cloudy Formation is a distinctive succession of basin limestone and shale, that 'shales-out' to the southwest and merges into the northeastern Niddery Lake map area with time-equivalent shaly facies of the Duo Lake Formation. The Duo Lake Formation is laterally correlative and intertongues with the Marmot Formation. The Marmot Formation is a mappable succession of volcanic strata that occurs as a lens within the Duo Lake Formation of the northeastern Niddery Lake map area. To the southwest, the Duo Lake Formation is correlative with, and transitional into chert of the Elmer Formation, and argillite in the lower part of the Steel Formation (Cecile, 2000). In the southwestern embayment the upper Duo Lake Formation is diachronous, almost entirely shale and limestone in the northern embayment. Locally, however, a few metres of sooty grey weathering platy limestone, homotaxial with and lithologically similar to the Cloudy Formation are preserved as a unit at the top of the Duo Lake Formation. Because these rocks cannot be mapped but represent a thin, restricted continuation of Cloudy lithologies they are identified by Cecile (1982) as the Cloudy member of the Duo Lake. The Duo Lake including the Cloudy member in the southwestern embayment is unconformably overlain by the late Early Devonian Natla? Formation. Duo Lake strata are partly correlative with upper Franklin Mountain Formation and transitional Franklin Mountain dolostone (Cecile, 1982).

History:
Duo Lake is a geographic locality on the southwest side of the Misty Creek Embayment (Cecile, 1982).

Remark:
Thin bedding, dominance of shale and chert, and presence of gratolites indicate that the Duo Lake Formation is a deep-water facies unit. The presence of calcareous shale and limestone indicates it is, however, above the level of the carbonate compensation depth (Cecile, 2000).

References:
Cecile, M.P., 1982. The lower Paleozoic Misty Creek Embayment, Selwyn Basin, Yukon and Northwest Territories; Geological Survey of Canada, Bulletin 335, 78 p.
Cecile, M.P., 1997a. Geology of the Thor Hills map area (NTS 105-O/15); Geological Survey of Canada, Map 1899A, Scale: 1:50 000.
Cecile, M.P., 1997b. Geology of NTS 105-O/09 and northern part of Keele Peak map area (NTS 105-O/08); Geological Survey of Canada, Map 1902A. Scale: 1:50 000.
Cecile, M.P., 1997c. Geology of the Elmer Creek map area (NTS 105-O/10) and northern part of NTS 105-O/07; Geological Survey of Canada, Map 1901A. Scale: 1:50 000.
Cecile, M.P., 1997d. Geology of the Hailstone Creek map area (NTS 105-O/16); Geological Survey of Canada, MAp 1900A, Scale: 1:50 000.
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.

Source: GSC file of geological names; T.E. Bolton and J. Dougherty (compiler)
Contributor: A.A. Coyne; Michael Pashulka
Entry Reviewed: No
Name Set: Lithostratigraphic Lexicon
LastChange: 06 Dec 2010