Devonian (c. 388–375 MY) Horn River Group of Mackenzie Platform (Northwestern Canada) is an Open-Shelf Succession Recording Oceanic Anoxic Events

At least four horizons of enhanced anoxia (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 the 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 ICP elemental data from 1687 samples, siliciclastics-lean basinal mudrock units that host AHs are strongly enriched in Mo (median EFMo~97–172 EFMo/EFU≈3–3.5 X SW) compared to siliciclastic-rich units (median EFMo~17–37) and show strong EFU/EFMo covariation (r ≈ 0.8 in Canol Formation and Bluefish Member). (EFMo and EFU = Al-normalized Mo and U in enrichment factor notation; SW = average present-day seawater values.) 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 the model of Kidder & Worsley (2010), which involves a global shift to a warm greenhouse mode with slowed oceanic convection, expanded OMZs, 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.