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).