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TitleNew fossil constraints on the Ediacaran-Cambrian boundary in the northwestern Mackenzie Mountains, NWT and Yukon
AuthorMacNaughton, R BORCID logo; Fallas, K MORCID logo
SourceC.P.C. 2018: Canadian Paleontology Conference, proceedings volume; Canadian Paleontology Conference Proceedings vol. 15, 2018 p. 19-20
Alt SeriesNatural Resources Canada, Contribution Series 20180425
PublisherPaleontology Division, Geological Association of Canada
MeetingCPC 2018 - Canadian Paleontology Conference; Saskatoon, SK; CA; September 21-24, 2018
Mediapaper; digital
File formatpdf
ProvinceNorthwest Territories; Yukon
AreaMackenzie Mountains; Bonnet Plume Lake
Lat/Long WENS-132.0000 -130.0000 65.0000 64.2500
Subjectspaleontology; stratigraphy; fossils; ichnology; trace fossils; stromatolites; biostratigraphy; bedrock geology; lithology; sedimentary rocks; sandstones; siltstones; limestones; carbonates; quartzites; Lower Cambrian; Canadian Cordillera; Neoproterozoic; Ediacaran; Ediacaran-Cambrian boundary; Terreneuvian; Backbone Ranges Formation; Beltanelliformis; Nimbia; Cyclomedusa; Ediacaria; Spriggia; Aspidella; Treptichnus pedum; Psammichnites gigas; Skolithos; Phanerozoic; Paleozoic; Cambrian; Precambrian; Proterozoic
ProgramGEM2: Geo-mapping for Energy and Minerals Mackenzie Corridor, Shield-to-Selwyn geo-transect, Mackenzie-Selwyn sub-activity
Released2018 09 01
AbstractThe Mackenzie Mountains of northwestern Canada preserve a thick, well-exposed Ediacaran to Cambrian succession. However, current knowledge of the position of the Ediacaran-Cambrian boundary in the region is uneven. To the southwest, relatively outboard sections at Sekwi Brook and June Lake (NTS 105P: Sekwi Mountain map area) preserve a combination of Ediacaran fossils and ichnofossils that have permitted the base of the Cambrian to be delineated with ever-increasing precision. By contrast, the Ediacaran to basal Cambrian fossil record is extremely sparse in the proximally deposited Backbone Ranges Formation, which is widely distributed in the northern and eastern Mackenzie Mountains. In its type area (NTS 95L: Glacier Lake map area), this unit is more than 1 km thick and consists of a basal siliciclastic member, a middle carbonate member, and an upper member dominated by cliff-forming quartz arenite. Various workers have considered it to be entirely early Cambrian (Terreneuvian) or to contain the Ediacaran-Cambrian boundary at a karst surface between the middle and upper members. New fossil collections from western Bonnet Plume Lake map area (NTS 106B) constrain the age of the Backbone Ranges Formation. In this area, the lower and middle members resemble their typical manifestations in other regions but lack fossils. However, the upper member is heterolithic and divisible into several mappable, formation-scale units, some of which are fossiliferous. The basal unit consists mainly of maroon- and grey-weathering sandstone and siltstone. The only biological remains are possible low-relief stromatolites in a thin package of grey, platy limestone. Overlying the basal unit is a recessive succession dominated by brown siltstone. These strata contain a low-diversity assemblage of Ediacaran fossils, including numerous Beltanelliformis with rare examples of Nimbia. Single specimens of Cyclomedusa, Ediacaria, and Spriggia also have been recovered, all of which may be preservational variants of the form-genus Aspidella. This unit is capped by a grey- to orange-weathering, resistant carbonate that may be only a few metres thick in the project area but thickens markedly to the south and west. Above the carbonate is a second recessive succession consisting of brown siltstone and sandstone, locally with abundant trace fossils. The diagnostic Cambrian ichnotaxon Treptichnus pedum is common in the upper part of this unit. The second recessive package grades upward into a succession of cliff-forming quartzites that comprise the uppermost of the newly recognized units. These sandy strata contain Treptichnus pedum, as well as the early Cambrian ichnofossil Psammichnites gigas and at least two morphotypes of Skolithos. Contrary to existing models, these fossil data place the Ediacaran-Cambrian boundary within the upper member of the Backbone Ranges Formation. The authors intend to formalize the newly documented subdivisions of the upper member as units for use on updated GSC maps of NTS 106B, and the new age constraints will help to correlate the Backbone Ranges Formation more accurately with Ediacaran-Cambrian successions elsewhere in northwestern Canada.
Summary(Plain Language Summary, not published)
New fossil occurrences radically improve the age constraints on a widely distributed geological unit in the Mackenzie Mountains, NW Canada. The fossils show that the boundary between the Ediacaran and Cambrian aged rocks (corresponding to roughly 541 million years ago) is not where previous models placed it. This improves our ability to understand and correlate these rocks, and will lead to more accurate basin models.

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