The nature of the Precambrian-Cambrian transition in the northern Flinders Ranges, South Australia

This item is only available electronically.Previous investigations into the Ediacara Member of the late Proterozoic Rawnsley Quartzite in the Flinders Ranges have produced differing interpretations of the depositional environment. Studies at Nilpena Hills indicate that deposition was influenced by back barrier lagoonal conditions with the intermittent influx of fluidised sands which mantled lagoonal muds. Re-interpretation of the Ediacara assemblage shows a hitherto unrecognised benthonic bias. This abundance of sessile, benthonic forms supports a sub-tidal depositional environment. However, the increase in the numbers of motile forms compared with sessile forms, preserved towards the top of the member, accords well with one of two inferred shallowing upward cycles within the sequence. A recent re-evaluation of the nature of the Precambrian-Cambrian boundary in the Flinders Ranges suggests a conformable relationship between the Pound Subgroup and the overlying Early Cambrian beds. This is at odds with previous interpretations, which proposed that a regional disconformity occurs at the boundary. Mapping at Mt. Scott Range, Puttapa Syncline and Red Range provided ample evidence that several periods of at least partial lithification occurred within the Pound Subgroup, before the onset of Cambrian deposition. Erosive downcutting marks the contact of the Pound Subgroup-Uratanna Formation at Mt. Scott Range, Red Range and Puttapa Syncline. Erosive downcutting of the Parachilna Formation into the Uratanna Formation was mapped at Mt. Scott Range. The Pound Subgroup-Parachilna Formation contact was mapped as a disconformity which becomes a high angle unconformity near the Beltana Diapir.Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 199

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 (Lagerstätten) 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 (δ13Corg). We also establish its degree of thermal alteration as a datum for use in taphonomic comparisons with other Cambrian Lagerstätten. 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.D. M. McKirdy, P. A. Hall, C. Nedin, G. P. Halverson, B. H. Michaelsen, J. B. Jago, J. G. Gehling and R. J. F. Jenkin