Sedimentation of the Phyllopod Bed within the Cambrian Burgess Shale Formation of British Columbia

Abstract

We provide the most detailed sedimentological log to date through the Phyllopod Bed of the mid-Cambrian Burgess Shale Formation of British Columbia, based on millimetre-scale logging of a suite of thin sections. The sedimentary facies is dominated by alternations of homogeneous mudstone and a coarser-grained, laminated, variably sandy and shelly mudstone that is locally micronodular. Most boundaries between these two lithologies are gradational, and discrete fining-upwards turbidite units were rarely recognized. Such a pattern is interpreted to indicate rapid sedimentation of up to decimetre-thick units at this location from pulsatory, quasi-continuous density currents consistent with earlier proposals of exceptional preservation through rapid burial; the density currents responsible were probably largely akin to mud-rich slurries, helping explain the transport and entombment of the fossils. The homogeneous mudstone units are characterized by numerous distinctive lenses of pyrite framboids or subeuhedral crystals, previously interpreted as small ripples. Their 3D shape, however, suggests an origin as subspherical early diagenetic aggregates; their present morphology is consistent with the high levels of compaction inferred from the preservation of fossils.