Constructing a fine scale physiochemical depositional model for Devonian black shales: a petrographic study of the Hare Indian and Canol formations, Central Mackenzie Valley, Northwest TerritoriesTuesday, November 19, 2019 - 9:00am to 9:20am Theatre Three
The Hare Indian and Canol formations, making up part of the Horn River Group (HRG) in the Northwest Territories, primarily consists of organic-rich shales deposited during the Middle to Late Devonian. The formations are considered to represent marine basin fill accumulated in an oxygen starved distal shelf setting, evidenced by the shale’s organic-rich character, pyrite content, and lack of macro-scale bioturbation. This study aims to identify the small-scale fluctuations in both the physical and chemical conditions at and just below the sediment-water interface during deposition.
Detailed petrographic sedimentological and ichnological analyses were carried out on thin sections taken from several cored HRG intervals (MGM Shell East Mackay I-78, Husky Little Bear N-09 and H-64, and ConocoPhillips Mirror Lake N-20 and Loon Creek O-06). These organic-rich shales contain eight distinct microfacies representing three main sedimentation processes: (1) pelagic and hemipelagic suspension settling, (2) combined traction transport and suspension settling, and (3) bedload traction transport. Bedload traction transport dominates in what is interpreted to be the proximal microfacies and is the result of down-slope sediment gravity flows and bottom currents, likely initiated during intermittent or seasonal storms and NW to SE flowing winds. Suspension settling dominates in what is interpreted to be distal quiet waters below storm wave base. Several morphologically distinct microscopic biogenic-sedimentary structures (i.e. ichnofossils) have been identified throughout the HRG mudstones, indicative of sediment pore waters that were at least periodically partially oxygenated. Evaluation of total organic carbon (TOC) content against bioturbation and microfacies interpretation suggest that persistent anoxia was not the dominant factor in organic carbon preservation, but is rather a result of a combination of heightened sedimentation and burial rates and possible amplified rates of primary production.
The depositional model, paleo-oxygenation interpretations, and methods of organic carbon preservation presented in this study are in contrast to previous assumptions of the Horn River Group mudstones; where such previous interpretations involve a stagnant stratified water column dominated by hemipelagic suspension settling, with organic carbon preservation attributed to persistently anoxic bottom waters. The interpretations presented in this study are likely applicable in other fine-grained organic-rich deposits, where previously anoxia was attributed to high TOC content.