Expression of Major and Trace Element Signatures of the Frasnian Duvernay Formation within a Sea Level Context: insights into the processes that control mudstone composition, paleoredox conditions and organic matter enrichment

• Author / Creator

  McMillan, Julia M

• This study evaluates the effects of sea level on redox conditions, shale composition and sediment provenance in the Late Devonian (Frasnian) Duvernay Formation of the Western Canada Sedimentary Basin (WCSB) of Alberta, Canada, an organic-rich ‘black’ shale localized between the reefs and platforms of the coeval Leduc Formation. The Duvernay was deposited in two separate large sub-basins, the East Shale Basin (ESB) and West Shale Basin (WSB), separated by a nearly continuous chain of reefs. Redox conditions, shale composition and sediment provenance can control the accumulation of total organic carbon (TOC). These controls on TOC accumulation, along with the effect sea level has on them, are significant because they dictate the geographic and stratigraphic distribution of organic rich targets for unconventional oil and gas exploration. Organic geochemistry and major, minor and trace element inorganic geochemistry were utilized in order to document stratigraphic and geographic variation in shale composition. Five long cores were sampled at high resolution in order to investigate the processes controlling geochemical composition, in particular the accumulation of organic carbon. Geochemical results were placed in the context of third order transgressive systems tract-regressive systems tract (TST-RST) sea level cycles. The Duvernay expresses three third order transgressive-regressive cycles: TST1-RST1 corresponds to the Lower and Middle Duvernay; TST2-RST2 corresponds to the lower half of the Upper Duvernay; and TST3-RST3 corresponds to the upper half of the Upper Duvernay. Major element oxides were used as proxies for bulk mineralogy. SiO2-Zr cross plots were used to determine the source of silica and contribution of biogenic productivity. The role of detrital input and influence of source terranes was investigated using Zr-TiO2 cross plots; C-Fe-S systematics and trace element ratios (Mo/ Al, U/Al and V/Al) were used in order to investigate redox conditions during sediment accumulation; Hydrogen Index (HI)-Tmax cross plots were used in order to determine organic matter (OM) type and maturity. Stratigraphic and geographic trends in proxies for silica source, source terranes and redox conditions were then tied to an independently formulated sedimentological and stratigraphic interpretation. The controls of organic matter accumulations were explore using from plots of TOC and key geochemical indicators in order to determine the relative influence of productivity, preservation and dilution. Both sub-basins are influenced by biogenic and detrital silica. Bulk mineralogy proxies indicate that the WSB is silica-dominated and the ESB is carbonate-dominated although reef-proximal locations show carbonate influence regardless of sub-basin. Multiple source terranes were identified. Possible detrital sources include the Peace River Arch (PRA), the craton beyond the Grosmont Platform and landmasses north of the entrance to the basin. Locations proximal to detrital sources exhibited the influence of more than one source and distal or geographically isolated locations showed single source characteristics. Carbon-iron-sulfur (C-Fe-S) relationships and trace element ratios indicate that conditions were dominantly marginally oxic to dysoxic with frequent anoxia, reaching oxic or euxinic ranges only occasionally. Restricted locations were characterized by less oxic conditions than more open locations which were characterized by more oxic conditions. Average TOC contents were higher in TSTs than in RSTs. The effect of redox conditions on TOC preservation were determined using cross plots of TOC with two redox proxies: the Fe/S and Mo/Al (ppm/%) ratios. The effect of dilution on TOC accumulation was determined using cross plots of TOC and Zr/TiO2 ratio. The potential effect of clay enhanced preservation on TOC accumulation was determined using cross plots of TOC with Al2O3. TOC accumulations were observed to be the result of a balance of redox enhanced preservation, possible clay enhanced preservation and possible nutrient delivery or productivity. The dominance of each of these controls varied between sub-basins and stratigraphic intervals.

• Subjects / Keywords

  • Trace elements
  • Duvernay Formation
  • Frasnian
  • Geochemistry
  • Devonian
  • Black Shale
  • Redox conditions
  • Organic matter
  • TOC

• Graduation date

  Spring 2016

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3HH6CH26

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.