TAXONOMY, TAPHONOMY AND PALEOECOLOGY OF A NEW BURGESS SHALE-TYPE LAGERSTÄTTE FROM THE MACKENZIE MOUNTAINS, NORTHWEST TERRITORIES, CANADA

Abstract

The middle Cambrian (Drumian) Rockslide Formation is a deeper-water succession of mixed carbonates and siliciclastics. At the Ravens Throat River location it hosts a Burgess Shale-type (BST) deposit. The BST units are found in two separate 1m thick horizons of green-coloured calcareous mudstone and contain a biota similar to that of the Wheeler and Marjum formations of Utah, and to some extent the Burgess Shale itself. The biota is low in diversity and preserves mainly robust soft-tissue parts. 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. Geochemical analysis including, trace elements, organic carbon, biomarker, and synchrotron were attempted and yielded varying results. Trace element ratios, particularly V/Sc, indicate low oxygenation of the bottom waters during the deposition of the fossil-bearing interval. These results differ from most other BST deposits, which suggest that oxygenated bottom waters were maintained throughout sedimentation. In addition, organic matter in the sediments and δ13Corg values suggest the presence of benthic microbial mats. Silver is concentrated mostly in nm to μm-sized particles in the fossilifereous calcareous mudstone, suggesting elevated levels in the depositional environment. Organic films 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 or the muddy sediment. This is also supported by the presence of only very rare trace fossils.