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High Resolution Stratigraphic Variability in Black Shale Geochemistry: Horn River Group, British Columbia

Abstract

The geochemical composition of black shales provides critical information about paleoceanographic conditions at the time of deposition and mechanisms for accumulation of organic matter. Specifically, the concentration and ratios of various major, minor and trace elements can be used to assess redox conditions, the relative inputs of carbonates and siliciclastics into the basin and bioproductivity. However, typical sampling strategies either yield sparse results or individual samples composite such thick intervals – representing large periods of time – that mask important relationships and feedback loops between the processes controlling organic carbon deposition. We report the results of high resolution geochemical profiles measured on shale cores from the Upper Devonian Group, British Columbia. This shale section is a major shale gas resource and is time-equivalent to several other black shale sequences in North America. It is comprised of three sections, the lower carbonate-rich Evie Member, the middle clay-rich Otter Park Member and the upper quartz-rich Muskwa Formation. Organic carbon contents are highest in the Muskwa, intermediate in the Evie and lowest in the Otter Park. Geochemical data were obtained at 2 mm spacing with a portable XRD analyzer that achieves a spatial resolution of approximately 1 mm on core slabs that averaged 10 cm in length. A whole chemical analysis was obtained by ICP and ICP-MS for the entire slab that provided calibration for the XRD data. The high resolution geochemical provide insights into both the duration of geochemical excursions and triggering mechanisms that are not apparent when stratigraphically longer core slabs are analyzed. Major redox excursions represented by Mo/Al or Th/U ratios are manifested by intervals of core 1 cm or less in thickness. While our chronostratigraphy is not very well constrained, intervals this short must represent durations on the order of 500 to a few thousand years. We have also examined the geochemical record for synchronicity and diachronocity between different indicators. Commonly decreases in bioproductivity (low biogenic silica, and phosphate) are associated with elevated concentrations of redox condition, indicating that anoxia does not result from feedback loops associated with bioproductivity. Instead, slightly decreased iron flux into the basin reduces the magnitude of the sink for hydrogen sulfide, enhancing reducing conditions.