Burgess Shale Type Preservation - Post Burial

Post Burial

Organisms may have been shielded from oxygen in the ocean by a microbial mat, which could have formed an impermeable layer between the sediment and the oxic water column. There is no evidence for these mats in the higher stratigraphic units of the Burgess Shale Formation, so they can't be the whole story. However, cyanobacteria do appear to be associated with the preservation of the Emu Bay Shale, which was deposited beneath an oxygen-rich water column; by growing over carcasses, microbial mats held their soft tissue in place and allowed its preservation. It is possible that the sediments were not always anoxic, but that burrowing was prevented in oxic intervals by a high deposition rate, with new material provided faster than burrowers could keep up with. Indeed, a growing body of research indicates that sediment oxygenation is not related to preservation quality; the Burgess Shale itself appears to have been consistently oxic and trace fossils are sometimes found within body fossils.

Because of the great age of Cambrian sediments, most localities displaying Burgess Shale type preservation have been affected by some form of degradation in the following 500+ million years. For instance, the Burgess Shale itself endured cooking at greenschist-level temperatures and pressures (250-300°C, ~10 km depth), while the Chengjiang rocks have been deeply affected by weathering. The Burgess Shale has been vertically compressed by at least a factor of eight.