High-Resolution Geochemical Analysis of Controls on Organic Matter Accumulation in the Middle-Upper Devonian Horn River Group

• Author / Creator

  Zhou, Haolin

• Organic carbon-rich mudstones, or ‘black shales’, record highly anomalous perturbations of local or earth systems at critical times in earth history. Paleoenvironmental parameters, including primary productivity, redox conditions, and sedimentation rate, have been widely recognized as the controls for organic matter (OM) accumulation in shales. However, immediate triggers for organic carbon deposition in these formations are commonly difficult to identify because of feedback loops that cause geochemical proxies for these controls to vary apparently synchronously when sampled at decimeter to meter spacing. This thesis examines a set of 133 samples from a long, continuous core in the Middle and Upper Devonian Horn River Group in the northwestern Western Canadian Sedimentary Basin, a black shale and important shale gas reservoir. We compare a geochemical data set comprised of bulk geochemical analyses of 133 7-12 cm-long slabs to a novel set of millimeter-resolution geochemical profiles of the same slabs, based on energy dispersive X-ray fluorescence (EDXRF) and hyperspectral imagery (infrared scan) analyses.
  Proxies applied in this high-resolution analysis identify multiple patterns of controls (bioproductivity, redox conditions, and dilution by terrestrial or carbonate input) on organic richness and feedback loops between bioproductivity and redox conditions at different intervals. In these profiles, proxies for bioproduction and redox conditions, biogenic Si and S/Fe, show separate trends when compared to apparently synchronous variation between them observed in more coarsely sampled profiles. Bioproductivity and dilution are identified as the primary controls on OM accumulation in the Horn River Group based on the high-resolution profiles, while redox conditions are the less common trigger for organic matter accumulation in this shale. Comparison of geochemical profiles reveals bioproductivity-redox feedback loops developed on time scales of decades to centuries.
  Geochemical datasets do not always provide unambiguous constraints on interpretations of paleoenvironments, because different sources and processes can contribute minerals of similar geochemical composition. Through petrographic analysis, different biogenic inputs can be distinguished from detrital inputs and diagenetic mineralization, for example, carbonate bioclasts from carbonate detritus and authigenic carbonate minerals, and syndeposition from authigenic pyrite.
  Because sea-level change can influence bioproductivity, redox states, and dilution and therefore organic matter accumulation, this study compares both millimeter-scale and meter-scale geochemical analysis with second- and third-order sequence stratigraphy to develop the connection between relative sea level and OM accumulation. Results of meter-scale analysis show that sea-level changes can influence bioproductivity, redox conditions, terrestrial input, and carbonate input, but we cannot identify the immediate controls on organic richness because of the synchronous variations in proxies. Regression analysis based on the meter-scale dataset suggests that redox conditions are the primary control on organic richness. Results of millimeter-scale analysis show varying immediate triggers for organic matter accumulation during third-order sea-level changes, but may not effectively identify long-term, for example, timescale of hundreds of thousands of years, paleoredox variations that are effectively indicated by meter-scale analysis.

• Subjects / Keywords

  • Organic Matter Accumulation
  • Horn River Shale
  • Millimeter-Resolution Geochemistry

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-3qzk-k354

• License

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