Origin of Green Shales in the Devonian Swan Hills Formation, Alberta, Canada
Date
2022-09-19
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Abstract
Throughout its growth history, the Devonian Swan Hills Reef Complex at Swan Hills has been subjected to variably long episodes of subaerial exposure, storm events, strong wind, oceanic currents, base level fluctuations and/or a long post-depositional burial history. For nearly sixty years, several generations of carbonate specialists have invoked one or more of these mechanisms to explain the presence of fine-grained green shale sediment observed sporadically throughout the carbonate-dominated reef complex. Through core examination, petrography and geochemical analysis, this study proposes a new model which reinterprets green shale from the above processes to a coproduct of dolomitizing and potentially non-dolomitizing fluids. The terrigenous clays are suggested to be sourced from underlying clastic formations, in particular, the Watt Mountain Formation, and advected upwards alongside a mixture of carbonate mud, organic matter, pyrite and carbonate debris which accumulated onto the reef environment through hydrothermal mud volcanism. Advection and extrusion of such green shale slurries were associated with NW-SE trending basement faults that initiated buildup nucleation on the widespread platform and acted as an important segment of a large hydrothermal convection cell driven by regional tectonism. In addition to being reducing and toxic, these gaseous slurries were likely episodic in nature, acting in a fault-valve type manner, and resulted in the diverse depositional spectrum of green shale across various facies, diagenetic environments and stratigraphic intervals. The hydrothermal mud volcanism model thus provides a new and testable explanation for the origin, transportation, sedimentation and diagenetic transformations of green shales in the Devonian succession of the Western Canada Sedimentary Basin.
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Keywords
Devonian, Swan Hills, green shale, carbonate, reef
Citation
Howrish, M. R. (2022). Origin of green shales in the Devonian Swan Hills Formation, Alberta, Canada (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.