Relative sea-level cycles and organic matter accumulation in shales of the Middle and Upper Devonian Horn River Group, northeastern British Columbia, Canada; insights into sediment flux, redox conditions, and bioproductivity

The integration of geochemistry and sequence stratigraphic models in the study of shale formations is critical to the development of robust stratigraphic correlations and paleoenvironmental interpretations. The Middle and Upper Devonian Horn River Group is a prominent organic-rich shale sequence in northeastern British Columbia, Canada, and it also hosts major natural gas reserves. The availability of high-resolution geochemical data sets on five continuous cores and an independent core-based sequence stratigraphic framework provide new insights into the effects of relative sea-level changes on redox conditions, productivity, detrital flux, and organic matter enrichment patterns and their geographic variation. Sequence stratigraphic analysis documents three third-order transgressive-regressive cycles within the Horn River Group. Organic carbon content is typically enriched in transgressive systems tracts and depleted in regressive systems tracts. Correlations among total organic carbon (TOC) content and multiple geochemical proxies indicate that organic matter accumulation was controlled by redox conditions, bioproductivity, and detrital dilution, all of which were directly affected by relative sea-level fluctuations. Redox proxies are strongly correlated to TOC content, suggesting that redox conditions exerted a major control over organic carbon accumulation. Redox-sensitive trace-elemental ratios and Corg-Fe-S relationships suggest that bottom water was less oxygenated during transgressions than regressions and in distal areas than in proximal parts of the basin. Productivity may have been linked to anoxia via mineralization of organic matter during periods of high productivity, resulting in water-column oxygen depletion. However, biogenic silica concentrations demonstrate enhanced productivity during transgressions, perhaps related to enhanced recycling of nutrients under anoxic conditions. Relative detrital sediment flux to the basin, as measured by aluminum and titanium concentrations, varied systematically as a function of relative sea level and paleogeographic position. Specifically, elevated detrital flux accompanied regressions and was greatest in proximal areas of the basin, resulting in dilution of organic matter concentration at these times in these regions of the basin. Our study demonstrates that relative sea-level fluctuations exerted substantial control on patterns of organic carbon enrichment. However, geochemical signatures of sea-level cycles vary with respect to paleogeographic location within the basin and are most obvious at intermediate locations and water depths, where sea-level falls brought chemoclines to the seabed and forced clastic sediments to the basin floor.