TAPHONOMY OF A NEW MIDDLE CAMBRIAN (SERIES 3) FOSSIL LAGERSTÄTTE FROM THE MACKENZIE MOUNTAINS, NORTHWESTERN CANADA

A new Lagerstätte in the Rockslide Formation, Mackenzie Mountains of northwestern Canada, is a Burgess Shale-type (BST) deposit of Drumian age (Bolaspidella Biozone). A 1 m thick mudstone (composed of illite, quartz, clinochlore and dolomite) contains a low-diversity biota similar to that of the Wheeler and Marjum formations of Utah, and to some extent the Burgess Shale itself. The lithologically heterogeneous composition of the formation (shale, dolomite, lime mudstone, sandstone, mudstone) and absence of metamorphism offer an opportunity to analyze the depositional environment and taphonomy of this deeper water unit via organic/inorganic geochemistry (trace element, ∂¹³C_org, biomarker, XRD and synchrotron XRF). Similar to other BST Lagerstätten, the environment could not have been deposited under stable anoxic conditions, as the amount of organic matter is too low (TOC= 0.10–0.32%). Trace element ratios, especially V/Sc, indicate low oxygenation of the bottom waters during the deposition of the fossil-bearing interval but a higher oxygen content thereafter. The results differ from most other BST deposits, which suggest oxygenated bottom waters were maintained throughout sedimentation. In addition, organic matter in the sediments and ∂¹³C_org values (-22.3–26.9) suggest the presence of benthic microbial mats. Heavy elements are concentrated mostly in nm-µm-sized particles, but some can reach up to 50 µm in size. These are most likely due to bioaccumulation by microbes as they only occur in circular organic residues several centimeters in diameter and there is no indication of hydrothermal activity. These organic residues and the evidence for hyoliths feeding on them suggest an in situ preservation of the biota, perhaps from sudden and short anoxic events and quick burial under either microbial mats and the muddy sediment. This is also supported by the presence of only very rare trace fossils.