Type Locality:

Type section - section 2 - 62°7.9'N; 129°11.3'W. Reference section for top of formation - section 6 - 62°15.5'N; 129°40.2'W (Gordey and Anderson, 1993).

Distribution:

The Yusezyu Formation occurs in the Yukon-Northwest Territories and British Columbia (Gordey and Anderson, 1993; Cecile, 2000; Barnes et al., 2007; Gabrielse, 2003). It is a thick sequence (3000 m) which constitues the oldest strata known within the Selwyn Basin region. Unfortunately, there is no locality that displays both the upper contact of the formation and a significant thickness of Yusezyu strata. Therefore, a type section that portrays the thickest, best exposed section of representative lithologies and a separate reference section for the top contact of the formation are described separately (Gordey and Anderson, 1993).

Locality Data:

Thickness(m): Typical 3000.

Lithology:

In stratigraphic position the top of the type section is estimated to lie 1500 m below the top of the formation. The base of the type section is estimated to lie about 500 m above the oldest exposed Yusezyu strata in the map area. The type section (section 2 - 62°7.9'N; 129°11.3'W) is located 25 km east-southeast of Mount Pike, and about 13 km north of the west-flowing headwaters of the Yusezyu (pronouced yoo'says'yoo) River, after which the formation is named. The basal 273 m consist of white weathering, grey, locally gritty massive quartz sandstone. Sandstone beds are commonly graded, and rarely their upper parts are parallel laminated. Beds range from 0.5 to 1.5 m thick and are stacked without intervening slate to form "sandstone-only" members up to 30 km thick. Intervals of pale green slate up to 5 m thick separate adjacent coarse clastic units. Grey weathering, black, fine crystalline, thin(?)- bedded limestone occurs from 272 to 330 m. In its middle part are 14 m of grey, very coarse grained, locally gretty quartz sandstone. Orange weathering laminated blue-grey slate composes the overlying interval from 330 to 361 m. The coarsest clastics in the section occur at 361 to 600 m. In this interval three coarse members from 37 to 100 m thick are recognized. Two members of brown-green to rusty blue-grey weathering slate less than 30 m thick separate the three coarse members. The coarse clastics are composed of grey-white weathering, grey, coarse to very coarse grained quartz sandstone and minor quartz-pebble conlomerate; outsize shale clasts are common. The rocks are typically massive, but locally show a 2 to 3 m thick bedding-parallel parting. At least 7 m of erosional relief is demonstrated below the middle coarse member. The upper part of the section from 660 to 1140 m comprises the light green to rusty blue-grey weathering, pale green to blue-green slate. Scattered beds and thick-bedded members up to 30 m thick of fine to coarse grained, locally gritty quartz sandstone occur with the late. Orange weathering, blue-black argillaceous to silty limestone forms the interval from 1038 to 1089 m. The highest beds in the section from 1089 to 1140 m, are medium- to thick-bedded quartz sandstone with rare interbeds of pale green slate. The upper contact of the Yusezyu Formation is well exposed 6 km north-northwest of Mount Pike (section 6 - 62°15.5'N; 129°40.2'W), about 29 km northwest of the type section. Upward from the base of measurement the Yusezyu Formation consists of 8 m of grey-green laminated shale, 27 m of grey weathering, grey, medium grained sandstone with a 1 to 2 m thick parting, 60 m of pale to dark green weathering shale and siltstone, with minor sandstone, and 12 m of blue-grey to white weathering, dark grey to black, very fine crystalline limestone in beds 1.0 to 1.5 m thick. The limestone is designated the limestone member of the Yusezyu Formation. The basal contact of the limestone member against the siltstone is sharp. The upper contact with the overlying Narchilla Formation is defined at the top of the highest limestone bed. In a 2 m interval beneath the top of this bed, limestone and shale interfinger in beds about 0.3 m thick. In this zone the limestone is white weathering, and light to dark grey on fresh surface. The interbedded shale is pale green on weathered and fresh surfaces. The highest limestone bed is overlain by maroon weathering shale (Narchilla Formation). The maroon shale is punctuated by pale green shale interbeds 1 to 3 cm thick, spaced every 5 to 10 cm. Several other sections within the Yusezyu Formation were measured (sections 1 to 7) and all are essentially similar. The Yusezyu Formation is a thick succession of grey-brown weathering, gritty quartz sandstone and evenly interbedded shale. The proportion of coarse to fine clastics varies from place to place, but at least 50 percent of any given section may be fine to coarse grained sandstone. Quartz is the dominant constituent, and many of the coarse grains are opalescent blue. Limestone is a minor lithology, forming a thin discontinuous member at the top of the formation. Beds are even and continuous, and sandstone-shale bedding contacts are sharp and planar. Sole markings, including grooves and load casts, are rare. Many beds show normal size grading. Outsized shale clasts, some to a few tens of centimetres in diameter, are common and usually occur along discreet horizons near the bases of beds. Most coarse sandstone beds are massive or graded. Rarely the tops of beds are planar laminated, and locally ripple cross-laminated. Large scale cross-bedding is absent. Some thin fine grained sandstone beds show climbing ripple lamination, and locally exhumed bedding planes show well-developed ripple marks. Amalgamation of sandstone beds into shale-free units up to 100 m thick is common; quartz- pebble conglomerate is restricted to these intervals. In many places these units are massive, the only discernible structure being a 1 to 2 m thick parting. Scour of several metres into underlying shale is also locally seen at their base. Concentrated near the top of the formation are members to 50 m thick of medium to coarse grained sandstone, in massive, even, medium to thick beds. This sandstone contains little interbedded shale and virtually no tother associated sedimentary structures. The uppermost Yusezyu Formation is variably calcareous. In many places there are abrupt and irregular changes from carbonate to silica cement. The limestone member at the top of the Yusezyu Formation consists of fine crystalline light to dark grey limestone that varies from 0 m to as much as 15 m thick (section 7). Where this member is absent uppermost Yusezyu sandstone is highly calcareous , and is in sharp contact with overlying shale of the Narchilla Formation (Gordey and Anderson, 1993).

Relationship:

The Yusezyu Formation belongs to the Hyland Group. The base of the formation is not exposed. It is overlain sharply and conformably by maroon or dark grey shale of the Narchilla Formation (Gordey and Anderson, 1993). On the basis of stratigraphic position and composition, the Algae Formation is considered a direct correlative of the limestone member at the top of the Yusezyu Formation (Cecile, 2000). Part of the Windermere Supergroup in Goodfellow (2007).

References:

Barnes, E.M., Groat, L.A., and Falck, H., 2007. A review of the Late Cretaceous Little Nahanni Pegmatite Group and associated rare-element mineralization in the Selwyn Basin area, Northwest Territories; in, Mineral and Energy Resource Assessment of the Greater Nahanni Ecosystem Under Consideration for the Expansion of the Nahanni National Park Reserve, Northwest Territories; Geological Survey of Canada, Open File 5344, p. 191-202.

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.

Gabrielse, H., 2003. Geology, Kechika River, British Columbia; Geological Survey of Canada, Open File 1633, 2 sheets.

Goodfellow, W.D., 2007. Base metal metallogeny of the Selwyn Basin, Canada; in, Goodfellow, W.D., ed., Mineral Deposits of Canada: A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods: Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5, p. 553-579.

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.