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Identification of Triggers for Organic Matter Burial of the Middle and Upper Devonian Horn River Shale, Northeastern British Columbia, Canada


Organic richness in black shales is directly related to the oil and gas generation. Organic matter (OM) enrichment in turn is mainly controlled by a combination of productivity, redox conditions and dilution. These factors have multiple influences on organic matter accumulation. However, interplays or feedback loops between productivity and redox controls result in proxies for these parameters varying synchronously in many formations, for example such that shifts to both higher bioproductivity and more anoxic conditions may be indicated at points in a shale section where TOC increases. At the scale we commonly sample drilled cores, it is typically impossible to resolve what factor triggers organic carbon enrichment. In this study, we are building a high-resolution geochemical dataset of the Horn River Shale, Western Canada Sedimentary Basin. The geochemical dataset includes high-resolution inorganic geochemical analyses by benchtop EDXRF and high-resolution TOC analyses by hyperspectral imagery, supplemented and calibrated by whole rock geochemical analyses by ICP-MS and Leco TOC analyses. To analyze the interplay between productivity and redox conditions, sampling resolution should reflect the time lag corresponding to the reactions that describe feedbacks, for example when enhanced bioproductivity induces bottom water anoxia. The lag duration may be tens to hundreds of years, which can correspond to millimeters of vertical section due to the low sedimentation rate in shales. The benchtop EDXRF collected inorganic geochemical data with 1 mm ~ 2 mm resolution and total organic carbon (TOC) was measured by hyperspectral imagery with 0.5 mm resolution, corresponding to approximately 31 yr ~ 85 yr and 16 yr ~ 20 yr respectively in the Horn River Shale. Geochemical analysis indicates the three stratigraphic units, the Evie Member, Otter Park Member and Muskwa Formation, were characterized by diverse dilution, redox conditions and bioproductivity. Carbonate input is common in the Evie Member and terrigenous input (aluminum concentration) is richer in the Otter Park Member, while the Muskwa Formation has the highest bioproductivity due to the high biogenic silica content. We apply biogenic silica and S/Fe ratio as proxies for productivity and redox conditions respectively. Examples include increases in S/Fe followed upward with a several-millimeter lag by increased biogenic silica suggesting that anoxia may have enhanced bioproductivity probably. In other examples, increased biogenic Si followed upward by increased S/Fe indicates organic content decay causes oxygen depletion in the bottom water. Our results provide the evidence for the interplay between productivity and redox conditions and identify triggers among productivity, redox conditions and dilution for OM accumulation in different intervals of the Horn River Shale.