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TitleHighly variable Precambrian fluvial style recorded in the Nelson Head Formation of Brock Inlier (Northwest Territories, Canada)
AuthorIelpi, A; Rainbird, R HORCID logo
SourceJournal of Sedimentary Research vol. 86, no. 3, 2016 p. 199-216,
Alt SeriesEarth Sciences Sector, Contribution Series 20150282
PublisherSociety for Sedimentary Geology
Mediapaper; on-line; digital
File formatpdf
ProvinceNorthwest Territories; Nunavut
NTS96; 97; 98; 86; 87; 88
AreaNelson Head Formation
Lat/Long WENS-128.0000 -112.0000 76.0000 64.0000
Subjectsgeneral geology; sedimentology; regional geology; Nature and Environment; deltaic sediments; fluvial deposits; fluvial systems; fluvial processes; fluvial transport; sedimentary environment; sedimentary rocks; sedimentary petrology; sedimentary structures; braided channels; mudstones; sandstones; carbonates; Plants; Precambrian
Illustrationsgeological maps; sketch maps; location maps; photographs; cross-sections; cross-sections, stratigraphic; photomicrographs; tables
ProgramGEM2: Geo-mapping for Energy and Minerals Mackenzie Corridor, Shield to Selwyn
Released2016 03 22
AbstractSpectacular canyon exposures of the ~ 1 Ga Nelson Head Formation along the modern Brock River, Northwest Territories, provide a rare opportunity to assess the deposits of pre-vegetation, braided to sinuous-channelized fluvial systems. We analyze the sedimentology, architecture, and depositional evolution of 16 stacked fluvial-channel belts at this site, demonstrating greater variability in sedimentary style and morphodynamics than is typically interpreted for Precambrian rivers. Inferences on fluvial planview based on integrated analysis of depositional architecture and paleoflow reveal complex patterns of channel-planform evolution, and four depositional stages are recognized: (1) floodbasin-splay progradation with bounding episodes of eolian reworking; (2) deposition in wandering-channel belts characterized by deep anabranches and prominent lateral accretion; (3) progressive shift towards shallow, braided-channel belts confined within alluvial valleys and dominated by downstream accretion; and (4) deposition by marine-influenced braided-channel belts with mixed downstream, lateral, and upstream accretion.
Overall, the studied fluvial-channel belts point to significant morphodynamic complexity and geomorphic variability, challenging the assumption that all pre-Silurian rivers shared poor channelization and low sinuosity. Observed channel bodies also have width:thickness ratios strongly overlapping with those of rivers postdating the rise of vegetation. In both wandering- and braided-channel belts, complex patterns of channel migration, bar accretion, and dissection were the result of co-acting processes such as development of transverse-velocity zones within the channels, local paleoflow disturbance induced by depositional topography, and avulsion. In concert with the regional extent and paleoflow, and increasingly distal character of correlative stratigraphic units towards the northwest, the magnitude of the observed trunk-fluvial channels is consistent with a mature drainage capable of transecting the Laurentian craton over thousands of kilometers.
Summary(Plain Language Summary, not published)
The Nelson Head Formation is a ~1 billion year-old fluvial to shallow-marine sandstone deposit exposed at several sites in the western Canadian Arctic. Spectacular canyon exposures along the modern Brock River, Northwest Territories, offer a rare opportunity to study the deposits of rivers, which existed before the appearance of land plants on Earth. We analyze the sedimentology, 3D architecture and depositional evolution of 16 stacked river-channel deposits at this site, demonstrating a variability in fluvial depositional style much greater than that previously portrayed for Precambrian rivers.

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