Greenschist-facies metamorphism of the Burgess Shale and its implications for models of fossil formation and preservation

Powell, W.G.

2003, Canadian Journal of Earth Sciences, v.40, p. 13-25

Metamorphosed silty mudstones of the Burgess Shale and Stephen Shale formations record a polymetamorphic history. An early greenschist-facies event associated with burial by Paleozoic strata produced a nearly ubiquitous bedding-parallel cleavage (S₁). Tectonic exhumation during the formation of the southern Canadian Rocky Mountains produced a domainal subgreenschist-facies retrograde overprint in which a high-angle crenulation cleavage (S₂) was developed. Whereas all rocks have experienced these two events, the degree of deformation and fossil preservation varies with position relative to the Cathedral Escarpment. This paleo-submarine cliff resulted in a zone of reduced deformation within adjacent strata by buttressing them during burial and deflecting deformation during orogenesis. Fossil-bearing strata are composed of a typical greenschist-assemblage of muscovite-chlorite-quartz-albite, are devoid of clays, and contain an average of 0.28% organic carbon. This typical meta-mudstone assemblage is consistent with the typical whole-rock composition of these rocks which tends to be richer in K and Al and poorer in Fe relative to the Post-Archean Average Shale. These mineralogical/compositional characteristics suggest that the pre-metamorphic clay assemblage was likely illite-smectite-kaolinite, with no evidence of highly reactive species such as nontronite or Na-montmorillonite. This is contrary to the required conditions for taphonomic models involving organic preservation due to clay-related suppression of decomposition-related reactions. Metamorphism of the Burgess Shale has also reduced the total organic carbon content to < 20% of initial values. This must be considered in any models that involve interpretation of organic carbon in diagenetic processes (e.g., fossil formation and determination of paleo-redox conditions).