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TitleDevonian (c. 388-375 Ma) Horn River Group of Mackenzie Platform (NW Canada) is an open-shelf succession recording oceanic anoxic events
Associated Datahttps://doi.org/10.6084/m9.figshare.c.4212428
 
AuthorKabanov, PORCID logo
SourceJournal of the Geological Society vol. 176, 2018 p. 29-45, https://doi.org/10.1144/jgs2018-075
Image
Year2018
Alt SeriesNatural Resources Canada, Contribution Series 20180027
PublisherGeological Society of London
Documentserial
Lang.English
Mediapaper; on-line; digital
RelatedThis publication is accompanied by Devonian (c. 388-375 Ma) Horn River Group of Mackenzie Platform (NW Canada) is an open-shelf succession recording oceanic anoxic events
File formatpdf
ProvinceNorthwest Territories
NTS106F; 106G; 106I; 106J; 106K
AreaMackenzie Platform; Mackenzie River
Lat/Long WENS-136.0000 -130.0000 68.0000 66.0000
Subjectstectonics; geochronology; lithostratigraphy; tectonic setting; tectonic interpretations; Horn River Group; Devonian
Illustrationsplots; cross-plots; location maps; stratigraphic columns
ProgramGEM2: Geo-mapping for Energy and Minerals Mackenzie Corridor, Shield-to-Selwyn geo-transect, Mackenzie-Selwyn sub-activity
Released2018 10 12
AbstractAt least four horizons of enhanced anoxia (anoxic horizons; AHs) are recognized in the uppermost Eifelian-Middle Frasnian mudrock-dominated strata of the Mackenzie Valley and Peel area of NW Canada. Aluminium-normalized Mo and U logs in two cored sections reveal AH-I at the Eifelian-Givetian boundary, AH-II in basal Frasnian, and AH-III and AH-IV bundled in the Middle Frasnian interval. These four horizons are characterized by attenuated siliciclastic components. Spectral gamma-ray K+Th and U are the best tools to trace these horizons in wells and outcrops. AHs are biostratigraphically correlated with 'black-shale events' in several basins of the world. Depositional environment is depicted as a stratified basin where the water-column chemocline defined co-sedimentation of anoxic mudrocks in topographic lows and oxic grey shales and carbonate banks on seafloor elevations. Based on inductively coupled plasma elemental data from 1687 samples, siliciclastic-lean basinal mudrock units that host AHs are strongly enriched in Mo (median EFMo~97-172 EFMo/EFU~ -(3-3.5)×SW, where EFMo and EFU are respectively Al-normalized Mo and U in enrichment factor notation and SW is average present-day seawater value) compared with siliciclastic-rich units (median EFMo~17-37) and show strong EFU/EFMo covariation (r~ 0.8 in Canol Formation and Bluefish Member). Supported by a lack of geological evidence for an oceanographic barrier, this enrichment indicates unrestricted water exchange with Panthalassa. At the same time, development of oligotrophy is indicated by a lack of P enrichment and weak to non-existent enrichment in Zn and Cu. These features are reconciled through a model by earlier workers that involves a global shift to a warm greenhouse mode with slowed oceanic convection, expanded oxygen minimum zones and a failure of nutrient resupply from the upwelling. The onset of mass degassing in continental large igneous provinces represents a potential trigger for this mid-Devonian shift. Devonian black-shale events in this scenario represent genuine oceanic anoxic events marking hothouse episodes in their nascent form.
Summary(Plain Language Summary, not published)
Four horizons of enhanced anoxia (AHs) are recognized in the latest Eifelian ¿ Middle Frasnian strata. Alumina-normalized Mo, U, V, logs in two sections reveal the AH-I at the Eifelian/Givetian boundary, AH-II in basal Frasnian, and AH-III and AH-IV in the Middle Frasnian. AHs are traced in wells and outcrops and correlated with global black-shale events. Depositional environment is depicted as a stratified basin where water-column chemocline defined anoxic deposition in topographic lows and growth of carbonate banks on seafloor highs. Lack of evidence for oceanographic barrier, basin-centered Mo enrichment and strong U/Mo covariation indicate unrestricted water exchange with Panthalassa. Oligotrophy is indicated by depleted inventory of P and micronutrient trace metals. This is consistent with mid-Devonian global shift to the warm greenhouse triggered by the onset of degassing in LIPs. Devonian black-shale events in this model represent genuine OAEs marking nascent hothouse episodes.
GEOSCAN ID308173

 
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