Unit Name: Risky Formation
Unit Type: Lithostratigraphic
Usage: Currently in use
Age Interval: Late Proterozoic (900 - 542 ma)
Age Justification: From its position relative to underlying and overlying fossiliferous formations, the Risky Formation is either latest Proterozoic (Ediacaran) or, less probably, earliest Cambrian. The only fossils yet recovered from it are the trace fossisl Planolites sp., and cf. Torrowangea sp. or cf. Gordia sp. The conformably underlying Blueflower Formation contains elements of an Ediacaran fauna (Hofmann, 1981). Trace fossils and shelly fossils in the overlying Ingta Formation may be either latest Proterozoic or earliest Cambrian, possibly "pre-trilobite" (Aitken, 1989b).
Province/Territory: Arctic Offshore; Northwest Territories; Nunavut; Yukon Territory
Originator: Aitken, 1989b.
The type section of the Risky Formation (Section 81AC-9) is continuous with the type section of Blueflower Formation, in the Sekwi Brook area of northeatern Sekwi Mountain map area. The mid-point of the section is at latitude 63°21'10''N, longitude 128°38'00''W. The section was chosen because of the superior preservation of protolith fabric in the dolomites. A thicker section (167 m versus 114 m) is present 3 km south of Risky Peak, but preservation of protolith fabrics there is poor (Aitken, 1989b).
Central Mackenzie Mountains, Northwest Territories. Because of apparent southwestward depositional pinchout and northeastward erosional bevelling at one or more subsequent unconformities, the Risky Formation can be studied at few localities. In the Sekwi Brook area of Sekwi Mountain map area, its preserved thickness varies from less than 114 m (type section) to at least 167 m. In the June Lake anticline, its thickness in the east limb is 84 to 97 m, but it is missing in the west limb, apparently by downslope, depositional pinchout. In central Nadaleen River map area, the most recently assigned boundaries of Map unit 11, that is, the Risky Formation, enclose 342 m of strata (Narbonne et al., 1985) (Aitken, 1989b).
The Risky Formation is a mappable, resistant, feature-forming unit of dolomite, sandy dolomite, dolomitic sandstone, sandstone and minor shale. In the Sekwi Brook area, the dominant weathered colour is pale pinkish orange ("peach-coloured"), but, in the June Lake panel farther west, where sandstone and quartzite are dominant and the carbonate beds are limestone, the formation weathers pale grey. Protoliths of the characteristic, fine and very fine crystalline, thick bedded, massive dolomites, which mostly have a significant content of quartz sand (locally granules), include oncoid wackestone/floatstone, algal stromatolites, and minor fenestral lime mudstone with teepee structures. A common, kind of dolomite bears a multitude of millimetre-scale, "sorted", perfectly smooth-walled, spherical and ovoid vugs; these appear to be leached ooids, and indeed, oolitic dolomite has been seen locally. The sandstones are mainly fine grained; uncommon, coarser beds range up to very coarse grained sandstoen and grit. They are mainly medium and thick bedded, with both plane-parallel lamination and crosslamination. A peculiarity of the formation is the presence at several levels of algal stromatolites composed of dolomitic sandstone. The dolomite-cemented beds weather pale orange; those cemented with a ferrous carbonate weather brown. A distinctive unit of breccia is common to the Sekwi Brook and southern June Lake panel areas. It consists of angular, decimetre-scale and larger blocks of dolomite and sandstone in a matrix of sandstone that is partly crosslaminated and partly contorted and slump-folded. This appears to be a slide-mass of extraordinary extent, that has not completely lost coherence. It is excellently exposed in the topographically lowest outcrops immediately north of Risky Peak where it is 8 m thick (Aitken, 1989b).
The Risky Formation of the Windermere Supergroup conformably overlies the Upper Proterozoic Blueflower Formation, and is unconformably overlain, wherever studied to date, by strata correlative with some part of the (?)uppermost Proterozoic and Lower Cambrian Backbone Ranges Formation (Aitken, 1989b, 1991).
The formation is named after Risky Peak, an eye-catching peak, elevation 2100 m-plus, that overlooks a pass between the drainage of Sekwi Brook and Gametrail Creek. The peak is composed of Risky Formation (Aitken, 1991).
Aitken, J.D., 1989b. Uppermost Proterozoic Formations in Central Mackenzie Mountains, Northwest Territories; Geological Survey of Canada, Bulletin 368, 26 pp.
Aitken, J.D., 1991. The Ice Brook Formation and Post-Rapitan, Late Proterozoic Glaciation, Mackenzie Mountains, Northwest Territories; Geological Survey of Canada, Bulletin 404, 43 pp.
Narbonne, G.M., Hofmann, H.J., and Aitken, J.D., 1985. Precambrian-Cambrian boundary sequence, Wernecke Mountains, Yukon Territory; in Current Research, Part A, Geological Survey of Canada, Paper 85-1A, p. 603-608.
Source: GSC file of geological names; T.E. Bolton and J. Dougherty (compiler)
Contributor: Michael Pashulka
Entry Reviewed: No
Name Set: Lithostratigraphic Lexicon
LastChange: 31 Mar 2011