Burgess Shale Type Preservation - Before Burial

Before Burial

The majority of the decay process occurred before the organisms were buried.

While the Chengjiang fauna underwent a similar preservational pathway to the Burgess Shale, the majority of organisms there are fossilised on their flattest side, suggesting that they were swept to their final resting place by turbidity currents. The location at which an organism ultimately comes to rest may depend on how readily it floats, a function of its size and density. Organisms are much more randomly arranged in the Burgess Shale itself.

Turbidity currents have also been posited as the depositional system for the Burgess Shale, but mud-silt flows seem more consistent with the available evidence. Such "slurry flows" were somewhere between a turbidity current and a debris flow. Any such flows must have enveloped free-swimming as well as bottom-dwelling organisms. In either case, additional processes must have been responsible for the exceptional preservation. One possibility is that the absence of bioturbation permitted the fossilisation, but some Burgess Shale fossils contain internal burrows, so that can't be the whole story. It is possible that certain clay minerals played a role in this process by inhibiting bacterial decay. Alternatively, reduced sediment permeability (a result of lower bioturbation rates and abundant clays) may have played a role by limiting the diffusion of oxygen.