|Résumé||(disponible en anglais seulement)|
The study area lies within Canada's Northern Interior Platform, around the Norman Wells area in the Northwest Territories (Fig. 1). Due to heavy hydrocarbon
exploration, several wells were drilled and cored, a small subset of which were chosen for study by the first author. We used geochemical analysis, sedimentology, and conodont biostratigraphy from wells and outcrop sections to infer the
paleo-environment of deposition of black shale called the Bluefish Member, and consider its possible correlation to global episodes of sea-level change.
The area preserves Paleozoic strata, from Cambrian siliciclastics to
Devonian carbonates overlain by Devonian siliciclastics (Fritz et al., 1991). Within the Devonian siliciclastics, the Bluefish Member of the Hare Indian Formation, a succession of black, bituminous, Middle Devonian-age shale, is the first to be
deposited (Pugh, 1983). This shale, part of the Horn River Group, sharply overlies the Hume Formation, an Eifelian limestone that was deposited in oxic waters. The Hume Formation contains a variety of well-preserved macrofossil assemblages including
corals, brachiopods, and bivalves. The sharp limestone-to-shale contact is visually obvious. It is also present in spectral gamma ray logs and geochemical character. Above the Bluefish Member lies grey shale of the Francis Creek Member, which grades
into the black shale of the Canol Formation overlain by Imperial Formation siltstone and shale.
Biostratigraphically, the Bluefish Member probably encompasses the base of the Givetian, where samples from the lower part of the Hare Indian
Formation contain conodonts Polygnathus linguiformis weddigei and Icriodus difficilis, indicating an Early Givetian age (Gouwy, 2016; Uyeno, 2008). Given this age constraint, the onset of the Bluefish Member potentially coincides with the global
Kacák Episode, a global phase of black shale deposition (Walliser & Bultynck, 2011).
MATERIAL AND METHODS
The ConocoPhillips Canada Resources Corporation Mirror Lake N-20 well, drilled approximately 30 km south of Norman Wells and completed in
2013, contains a cored interval preserving the contact between the Middle Devonian Hume Formation and the overlying Hare Indian Formation (including the basal Bluefish Member). Approximately 50 metres of this core was described, including the Hume
and Hare Indian formations, and the lower part of the overlying Canol Formation.
A geochemical database containing 28 well and outcrop sections was compiled from publications of the Geological Survey of Canada (GSC) and Northwest Territories
Geological Survey (NTGS). Data consists of ICP-MS inorganic elemental geochemistry, Rock-eval, and X-ray Diffraction. Geochemical logs were created from calculations of redox and basin restriction proxies of two available nearby wells. Statistical
Primary Components Analysis, a method of evaluating elemental covariance, was performed using this data compilation. Spectral gamma ray logs, collected using handheld scintillometer from GSC field programs in the summer of 2015 and 2016 were
correlated to well gamma-ray logs.
Core descriptions of Mirror Lake N-20 show the preservation of the sharp Hume - Bluefish contact; a slickenslided surface marks the end of the Hume carbonates with the Bluefish black shales above. The
Hume Formation preserved many rugose and other coral species, stromatoporoids, and shell fragments. The Bluefish Member preserves many calcareous, tentaculite-rich laminae in the lower third, pyritized bivalve shells and organic walled orthoconic
cephalopods in the middle, and an arcritarch rich upper third which grades into the Francis Creek Member grey shales.
Geochemical analysis normalizing Molybdenum, Copper, and Nickel to TOC was used as a proxy for basin restriction (Tribovillard
et al., 2006). The Hume contains relatively low ratios, while the Bluefish sharply contrasts with higher ratios (Fig. 2). The higher ratios indicate greater nutrient flux, where elements listed above are introduced. The boundary between the Hume and
Bluefish also marks a strong shift from highly oxic conditions to euxinic (conditions with enrichment of hydrogen sulphide) waters when looking at the excess Mo and V (Fig. 2). The geochemical logs on Figure 2 show three distinct packages within the
Bluefish Member: highly anoxic/euxinic conditions (high excess Mo and V), overlain by more oxygenated conditions (low excess Mo and V), in turn overlain by another anoxic event.
The boundary from Hume to Bluefish shows a phase of
deepening water depth. This is evident in the change from oxic-zone macro-fossils of the Hume, to the appearance of more pelagic species and pyritization of the Bluefish Member. This may also explain the geochemical signatures, where the redox
boundary moves up when entering the Bluefish Member, as evidenced by the redox proxies of excess Mo and V. This could be attributed to the transgressive event, where the change in local water depth results in a shift in the redox boundary upward.
These anoxic/euxinic conditions also coincide with basin nutrient flux, where Mo, Cu, and Ni are replenished more in the Bluefish Member than in the Hume Formation. Given the preliminary biostratigraphic constraint of Bluefish Member, the anoxic
event may also coincide with the global Kacák Episode. This widespread event is attributed to a rise in global sea-level, which is also characteristic of the deposition of the Bluefish Member.
The Hume-Bluefish contact represents a
sharp boundary from shallow water oxic marine conditions to a transgressive anoxic deposition. Lithology from core descriptions shows a change from carbonate-shelf macrofossils in the Hume to pelagic rich shale in the Bluefish. This transgressive
surface probably coincides with the onset of a global sea-level rise, called the Kacák Episode.