Depositional Setting of the Burgess Shale’s Greater Phyllopod Bed

Kevin Gostlin
University of Toronto, Toronto, ON

High resolution sedimentologic analysis of the Greater Phyllopod bed (GPB) of the Burgess Shale formation was utilized to critically contrast the various depositional scenarios postulated for the Burgess Shale. The sediments of the GPB were deposited at the foot of the Cathedral escarpment as a series of mudstone and calcisiltite interbeds. Sharp contacts divide these two lithologies indicating they are not genetically related. The famous soft-bodied fossils are entombed within the mudstone beds, although calcisiltites also contain fossils with soft-bodied preservation. Fossils preserved in the calcisiltites are typically found on bedding plains where the diversity and quality of preservation is significantly lower.

The distribution of pyrite indicates that the sediments were variably oxygenated, but were only very rarely colonized. The vast majority of organisms, including cyanobacteria, are allochthonous. The brittle nature of several of the fossils, such as the long-spiculed sponge, Halichondrites, suggests that transport within high density mud flows was unlikely. Further, the chaotic orientation of the majority of the fossils indicates they were not transported to the GPB by bottom currents, or low density turbidity currents traveling along the escarpment front. The most likely scenario is that the bulk of Burgess Shale organisms that are now fossilized in high detail lived on the platform above the escarpment, were churned up in storm events and carried out over the platform rim where they subsequently settled into the deeper water of the basin.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005