Unit Name: Ingta Formation
Unit Type: Lithostratigraphic
Usage: Currently in use
Age Interval: Late Proterozoic - Early Cambrian (900 - 513 ma)
Age Justification: The Ingta Formation is earliest Cambrian at its top, but may include beds of latest Proterozoic age. The limestone member at its top has yielded a single specimen of the protoconodont Prothoherzine cf. P. anabarica (Conway Morris and Fritz, 1980; the source was reported as the Risky Formation). Conway Morris and Fritz (op. cit.) suggested that this occurrence indicates a position near the Precambrian-Cambrian boundary (Aitken, 1989b). The Ingta is currently regarded as Precambrian, but post-Ediacaran (G.M. Narbonne, pers. comm., 1989) (Aitken, 1991).
Province/Territory: Arctic Offshore; Northwest Territories; Nunavut; Yukon Territory
Originator: Aitken, 1989b.
The type section of the Ingta Formation (Section 81AC-15) is in the overturned, east limb of the major anticline passing through June Lake ("June Lake anticline"), 9 km south of Keele River, at latitude 63°21'10''N, longitude 128°38'00''W (Aitken, 1989b).
Central Mackenzie Mountains, Northwest Territories. The Ingta Formation is 256 m thick at the type section, in the east limb of June Lake anticline, and 220 m in the west limb, directly opposite; the limestone member thins from 39 to 5 m between the same points. The extent of the formaiton, though obviously limited, is unknown, because it has been included previously with other units in the course of mapping (Aitken, 1989b).
The Ingta Formation is a mappable unit of distinctively coloured shale, with subordinate beds of sandstone, and, at the top, a limestone member (informal). It is neither recessive weathering nor a feature-former, except for the limestone member. The shales of the Ingta Formation are mainly a distinctive green colour ("apple green"), with intervals of purple-red, and have a strongly developed platy fissility. Sandstones and minor quartzites are thin, very thin, and medium bedded, with rare thick beds; they are pale brown, green, and pale grey, and almost entirely very fine and fine grained, weathering mainly brown. The dominant primary structures are plane-parallel lamination and low-relief linguoid ripple marks, with a few groove casts and rare flute casts. A few beds display coarse-mode supply grading. Slump-folds are rare. The limestone (locally, dolomite) member at the top of the formation consists of a lower unit mainly pale-pellet grainstone, a middle unit sandstone with minor limestone, and an upper unit almost entirely of stromatolitic limestone and dolomite. In the June Lake panel, trace fossils are abundant and well preserved on bedding planes of both the green and the purple-red platy shales (Aitken, 1989b).
The Ingta Formation of the Windermere Supergroup overlies the uppermost Proterozoic Risky Formation at an erosional surface, and underlies another erosional surface overlain by a quartzite-dominated unit that corresponds to the Backbone Ranges Formation or some part (upper member?) thereof (Aitken, 1989b, 1991).
The formation is named after the Ingta River, a tributary that joins Keele River 25 km west of the type section (Aitken, 1989b).
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.
Conway Morris, S. and Fritz, W.H., 1980. Shelly microfossils near the Precambrian-Cambrian boundary, Mackenzie Mountains, northwestern Canada; Nature, v. 286, no. 5771, p. 381-384.
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