Research articlePhotic-zone euxinia and anoxic events in a Middle-Late Devonian shelfal sea of Panthalassan continental margin, NW Canada: Changing paradigm of Devonian ocean and sea level fluctuations
Introduction
The second half of the Devonian (~388–359 My) is marked by spread of anoxic/dysoxic fine-grained sediments (a.k.a. black shales) in many shelfal basins of the World (Fig. 1; Falkowski et al., 2011; Kabanov, 2019). This period of anoxic shelfal sedimentation protracted into the Lower Mississippian to comprise one of six Phanerozoic time bins of major source rock deposition (Klemme and Ulmishek, 1991). In Devonian-Mississippian sedimentary systems, black shales commonly associate with isolated carbonate banks (reefs, pinnacles, mud mounds) outposting carbonate shelves far into the basin (Playford, 1980; Stoakes, 1980; Meijer Drees, 1993; Stanton et al., 2000; Gatovskii et al., 2016; Knapp et al., 2017; Kabanov and Gouwy, 2017). Major sea level fluctuations have been proposed to temporally separate depositional phases of carbonate buildups and basinal sediments (Jewell, 1994; House et al., 2000) and accommodate the evidence of hydrodynamic disturbance in black shales and their stratigraphic juxtaposition with oxic, apparently shallow-water fossiliferous facies (Brett et al., 2011; Knapp et al., 2017). However, debate seems to be everlasting in the shortage of rigorous evidence of eustatic fluctuations (Ettensohn, 1994; Hallam and Wignall, 1999; Bond and Wignall, 2008; Smith et al., 2019, Smith et al., 2020; Ver Straeten et al., 2020).
Global and local processes beyond this clearly non-actualistic condition of shelfal seas are poorly understood except for their strong causal relation with the greenhouse state of our planet (Algeo and Scheckler, 1998; Racki, 2005; Bond and Grasby, 2017; Carmichael et al., 2019; Percival et al., 2020). Most substantiated models involve atmosphere/ocean systems maintained in warm-greenhouse mode by successive and/or combined degassing in Vilyui, Dnieper-Donets, and Kola continental large igneous provinces (LIPs), as well as in a number of smaller continental magmatic centers and volcanic arcs (Fig. 1: Kidder and Worsley, 2010; Kravchinsky, 2012; Bond and Wignall, 2014; Ernst et al., 2020; Racki, 2020), which gains support in the evidence that Kellwasser anoxic events and the collapse of marine ecosystems at the Frasnian-Famennian boundary associate with bursts of effusive activity (Racki et al., 2018). An alternative explanation links the spreads of shelfal anoxia to the “top-down” eutrophication of shelfal seas caused by the erosion of soils which during the second half of the Devonian experienced dramatic expansion due to land afforestation (Algeo et al., 1995; Algeo and Scheckler, 1998; Carmichael et al., 2016, Carmichael et al., 2019; Percival et al., 2019), which, however, does not find support in paleosol and paleobotanical research (Retallack and Huang, 2011). Noteworthy, the known absolute ages of effusives in the Devonian continental LIPs are younger than the mid-Devonian (latest Eifelian) onset of widespread black shale sedimentation (Fig. 1), which is probably accounted for the incompleteness of absolute age records (Bond and Wignall, 2014; Ernst et al., 2020). This mid-Devonian shift in global condition is referred to as the Kačák or Kačák-otomari Event (House, 1996; Van Hengstum and Gröcke, 2008).
Extreme spreads of anoxic facies have been correlated globally as the Devonian black shale or anoxic events (House, 1983, House, 2002; Sandberg et al., 2002; Bond et al., 2004; Racki, 2005; Becker et al., 2016). Some, but not all, of these black shale events are coupled with biotic perturbations (Fig. 1) ranked as major in ecological severity (McGhee Jr. et al., 2013) and minor or moderate except for the Frasnian/Famennian in net taxonomic loss (Melott and Bambach, 2014). Basins where Devonian anoxic events are well-documented include those with signatures of impeded watermass exchange and those with evidence of oceanographic openness (Table 1; Carmichael et al., 2019). In the Devonian eustatic sea-level curve of Johnson et al. (1985) and its later modifications, anoxic horizons are seen as a result of genuine deepening or global transgressions (Sandberg et al., 2002; Arthur and Sageman, 2005; Ver Straeten et al., 2011; Brett et al., 2011), and this sea-level curve is the most common reference in case studies today (e.g., Pyle and Gal, 2016; Fraser and Hutchison, 2017; Dong et al., 2018; Philp and DeGarmo, 2020).
On the other hand, insights into the nature of the Middle Paleozoic anoxic events indicate possible links with profound change of circulation pattern and expansion of oxygen minimum zones (OMZs) of the ocean into shallower depth (Meyer and Kump, 2008), which is consistent with extensive biomarker evidence of photic-zone euxinia (PZE) at and between the stratigraphic levels of Middle Paleozoic biotic perturbations (Table 1 and references therein). In should be noted that the idea of oceanic circulation reversal as a critical component of the greenhouse planetary condition had century-long incubation period, but only recently growing knowledge pushed it into mainstream science (Hay and Floegel, 2012). Here we report more evidence from a Panthalassa-facing shelfal sedimentary system and contend that factors other than sea level rise controlled the spreads of black shales in Middle Paleozoic shelfal seas.
Section snippets
Geologic and palaeogeographic context
The study area is situated in the northwestern part of the Ancestral North America (ANA; Fig. 2A). The ANA refers to the main part of Laurentia as opposed to cordilleran terranes accreted during Meso-Cenozoic (Gabrielse and Yorath, 1991; Nelson et al., 2013). The Peel Platform (a.k.a. Peel Shelf, Mackenzie or Mackenzie-Peel Platform) refers to the Lower Paleozoic – Eifelian shallow-water carbonates of the northwestern ANA (Fallas et al., in press). The Peel Platform is bordered from the west by
Samples and methods
This paper reports on new organic geochemistry and stable organic-carbon isotope results from diamond-drill cores and sums up observations on the stratigraphic distribution of sponge spicules. Supplementary materials for this paper include stable carbon isotope data of organic matter (δ13Corg, ‰ VPDB notation) and their acquisition protocols, as well as SEM and light-microscope identification of sponge spicules and character of their pyritization.
Elemental geochemistry and horizons of enhanced anoxia
At least four horizons of enhanced anoxia (AHs) were revealed in the HRG in Little Bear N-09 and Loon Creek O-06 sections based on Al-normalized Mo and U logs from ICP elemental data (Kabanov, 2019), as these trace metals are widely used in elemental proxies interpreting anoxic sediments (Tribovillard et al., 2006). The revealed AHs, except for AH-I in the HRG base, are characterized by attenuated siliciclastic components as attested by major oxides Al2O3, K2O, TiO2, and Fe2O3 that are known to
Sponge spicules in the Horn River Group
Hand-lens observations of bedding/fissility planes reveal presence of hyalosponge spicules in cores (Kabanov et al., 2016b; Kabanov and Borrero Gomez, 2019) and outcrops (Kabanov et al., 2019). In dark laminated mudrocks and cherts of the HRG and in similar facies of the basal Imperial Formation, spicules are notably small in size and completely replaced by pyrite (Fig. 5 and Supplementary Appendix 2). No other benthic shelly fossils or metazoan-produced ichnofossils were encountered in anoxic
Organic geochemistry results
Table 2 presents key biomarker parameters indicative of organic inputs, depositional environments and thermal maturity for shale samples. With their C32 hopane 22S/(22S + 22R) ratio being around 0.58–0.62, the Devonian shale intervals from both wells have entered oil generation window, with East Mackay I-78 well being at slightly higher maturity than the Mackenzie River E-27 based on their C29 ααα steranes 20S/(20S + 20R) ratios (Table 2). All these black shale samples have a pristane over
Devonian anoxic events in the Horn River Group
Conodont age brackets available to date (data of S.A. Gouwy reviewed by Kabanov, 2019; Gouwy, in press) place the AH-I close to the Eifelian/Givetian boundary and the global Kačák Event (Fig. 1). The AH-II finds correspondence in the Frasnes event of the basal Frasnian, and AH-II and AH-IV are bundled within the Middle Frasnian, the time of remarkable spread of anoxic sedimentation over continental shelves resulted in the deposition of major hydrocarbon source rocks (Fig. 1; Kabanov, 2019). The
Conclusions
The Horn River Group (HRG) of the latest Eifelian – Frasnian Panthalassa-facing continental shelf of Laurentia is an excellent archive of paleoceanographic signals imprinted in oxic and anoxic facies. The HRG basin featured pronounced facies zonation with grey-shale clinoforms and carbonate banks outgrowing these clinoforms. These carbonate banks maintained their growth close to sea level, while laminated black shales and pelagic cherts were deposited in adjacent off-bank (fondoformic)
Acknowledgements
This work is a contribution to the Geoscience for New Energy Supplies (GNES) Programme, Geological Survey of Canada (Lands and Minerals Sector, Natural Resources Canada) publication No. 20190543. Edward Little and Margaret Ferguson (both GSC Calgary) provided administrative support, and M. Ferguson also performed style editing in the early version. Thorough improvement of the final version was made possible thanks to peer-reviews by three anonymous reviewers and the extensive editorial advice
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