Paleoredox status and thermal alteration of the lower Cambrian (Series 2) Emu Bay Shale Lagerstatte, South Australia

While exceptionally diverse fossil assemblages of non-biomineralised organisms (Lagerstatten) are rare, they are unusually common in marine sedimentary sequences of early and mid-Cambrian age. Their mode of preservation has been the subject of much debate. The lower Cambrian (Series 2) Emu Bay Shale biota, found at Big Gully on the north coast of Kangaroo Island, is by far the richest Burgess Shale-type (BST) fauna known in the southern hemisphere. Such fauna are preserved characteristically as two-dimensional compression fossils, comprising both carbonaceous and mineralised films on bedding surfaces of the host marine mudstones. The biotic diversity of the Big Gully assemblage suggests a habitat very favourable for life. Its preservation is exceptional, with gut remains and other soft parts quite common. Evidence of predation and scavenging is rare, and the finely laminated texture of the host mudstone attests to a lack of burrowing and bioturbation. Recent studies indicate that conservation of organic tissues, rather than authigenic mineralisation of their more labile components, is the principal taphonomic pathway responsible for BST deposits. In so far as such preservation requires suppression of the early diagenetic processes that normally result in the rapid decay of organic matter at or near the sea floor, the oxicity of the bottom waters, below which the Emu Bay Shale accumulated, becomes critically important. Here we determine the paleoredox status of the fossiliferous basal portion of the formation using selected trace element proxies, in combination with total organic carbon (TOC) concentrations and isotopic signatures (13Corg). We also establish its degree of thermal alteration as a datum for use in taphonomic comparisons with other Cambrian Lagerstatten. The Emu Bay Shale contains insufficient organic matter (TOC=0.25-0.55%) to have accumulated under stable anoxic conditions. Even allowing for the inevitable loss of organic carbon during the oil- and gas-generation phases of thermal maturation, to a present rank equivalent to 1.5% vitrinite reflectance, its original TOC content was 1%. Measurement of a series of redox-sensitive elemental ratios (viz. U/Th, V/Cr, Ni/Co and V/Sc) across the lower 8 m-thick fossiliferous section of the Emu Bay Shale confirms that it was deposited beneath an oxic water column. In this respect it is similar to the archetypical Burgess Shale. In the absence of an exaerobic zone, benthic cyanobacterial mats are likely to have mantled recently dead fauna and helped maintain the integrity of a sharp redox boundary at the sediment-water interface.