The Middle and Upper Devonian Horn River Group in northeastern British Columbia, Canada, consists predominantly of organic-rich mudstones that are typically described as black shales. This stratigraphic unit has seen substantial exploration and development for natural gas during the last decade. Although black shales such as the Horn River Group have historically been interpreted as deposited in anoxic deep-water basins, detailed sedimentological and ichnological analyses of eleven Horn River Group cores indicate that the depositional conditions varied significantly with respect to paleoenvironments, redox conditions, and physical processes, resulting in distinctive sedimentary facies. This study integrates data on lithology and physical and biogenic sedimentary structures from detailed core descriptions combined with total-organic-carbon content to better understand depositional processes and conditions.
Ten mudstone lithofacies and three lithofacies associations are identified. Massive mudstones and pyrite-rich mudstones display very rare planar lamination and graded beds and scarce bioturbation (BI 0–1), and are interpreted to represent anoxic waters, below storm wave base (SWB), with conditions of relatively low to moderate energy. Heterolithic, laminated, and more bioturbated lithofacies are interpreted to represent oxygenated waters below SWB with relatively high-energy conditions. Overall bioturbation intensity varies between moderate to intense (BI 3–6). Where these lithofacies are not intensely bioturbated, oxygenated lithofacies show combinations of well-preserved physical sedimentary structures, including horizontal parallel lamination, soft-sediment deformation, double mud drapes, graded beds, and current ripples. A third lithofacies association represents conditions intermediate between anoxic and oxygenated lithofacies associations, mainly based on local changes in the ichnological characteristics. These lithofacies show either intense or sparse bioturbation (BI 4–6 or BI 0–1, respectively) depending on the core location, possibly indicating local changes in the depositional conditions and processes (e.g., variations in the oxygen content and/or sedimentation rate). They can also be found in thick anoxic and oxygenated deposits. These lithofacies display intervals of faint lamination, amalgamated current ripples, and normally graded bedding.
The cores collectively display significant spatial and stratigraphic heterogeneity in lithology, sedimentary structures, and bioturbation that are related to relative water depth, oxygen content, sediment input, and deep-water currents. We interpreted bioturbation intensity to vary mainly as a function of oxygen content related to proximity to the basin margin and sea–level fluctuations, although secondary stress conditions (e.g., nutrient content and sedimentation rate) may also be important. Unbioturbated and massive fabrics represent anoxic to dysoxic conditions. Current-generated structures are present, mainly in the central and distal parts of the basin, and are interpreted to represent the presence of contour currents. In the proximal parts of the basin, laminated and heterolithic units are more common and more bioturbated than in the central and distal basin. This study thus can provide significant insights into other shale basins that show similar lateral and vertical variations. Integration of multidisciplinary studies, particularly sedimentology, ichnology, and total organic carbon aids in understanding and modeling the complexity of these systems.