NAVIGATING MUDDY WATERS: EXPERIMENTAL MODELLING OF BURGESS SHALE FLOW TYPE AND ITS IMPLICATIONS FOR THE PRESERVATION OF SOFT-BODIED ORGANISMS

The middle-Cambrian Burgess Shale Lagerstätte holds key insights into the Cambrian explosion. Its exquisite fossil preservation has enabled insights into the anatomy of early soft-bodied organisms and their evolution. Experimental taphonomy has focussed primarily on the importance of post-depositional decay and preservation of these soft-bodied animals. Often overlooked is the sedimentology of the fossil-enclosing deposits of this assemblage. Recent advances in our understanding of sediment-density flow processes, and how they are related to deposit type, has given us new information to investigate just how crucial transport can be for the overall completeness and preservation potential of soft-bodied organisms in different kinds of sediment-density flows. In the case of the Burgess Shale, there is still a fundamental debate with regards to the transport these organisms have undergone. Namely, whether they were living within or close to the environment of deposition, or could they have been transported from one environment to another? Understanding the sedimentology of the Burgess Shale Formation, and combining this with analogue experiments allows us to constrain the problem.

Samples were collected from three Burgess Shale localities to enable re-evaluation of the flow type, which could then be directly fed into the experimental design to replicate Burgess Shale flow conditions. Sedimentary structures included laminations, microscours, large floating quartz grains and shelly fragments; while grain size analysis identified clay to fine-grained sand grade material. This suggests that these deposits were most likely the product of a quasi-laminar to upper transitional plug flow.

In a two-factorial design using an annular flume tank, experiments were conducted to explore the effect of both pre-transport decay (up to 48 hours) and transport duration (equating to distances ranging from less than 1 km to 21.6 km) on the overall completeness and preservation potential of the polychaete, Alitta virens. The flow consisted of 11 % volume of kaolinite at 0.4 ms^-1 to create quasi-laminar to upper transitional plug flow conditions. The results of these experiments are used make direct comparisons to the preservation state of Burgessochaeta and Canadia from the Burgess Shale.