OCCURRENCE OF NAMA AND AVALON ASSEMBLAGE FOSSILS IN THE EDIACARA MEMBER OF SOUTH AUSTRALIA: THE SIGNIFICANCE OF FACIES IN THE RECORD OF THE DISTRIBUTION AND EXTINCTION OF THE EDIACARA BIOTA

Localities containing fossils of the Ediacara Biota are loosely considered as either part of the Avalon, White Sea or Nama Assemblages. These assemblages have been interpreted to have biostratigraphic significance as the fossils at Mistaken Point, Newfoundland (Avalon Assemblage) are established as the oldest fossils and the Nama Group as the youngest fossils respectively of the Ediacara biota dated from associated ash beds.

Well-known fossils of the Ediacara Member of the Rawnsley Quartzite (South Australia) include Dickinsonia, Spriggina, Parvancorina, Tribrichidium and other taxa typical of the White Sea Association from Russia. These fossils occur abundantly on the base of thin-bedded rippled sandstones representing deposition in shallow marine settings within wave-reworking depths though not activity reworked by storm events. This is the most common type of preservation of fossils within the Ediacara Member. With the exception of the form genus Aspidella, and some frondose fossils like Charniodiscus and Charnia, none of the fossils characteristic of the Mistaken Point or Nama Assemblages occur in this facies.

However, recent examination of two channel-fill facies; mass flow channel sandstones with no obvious sedimentary structures and planar laminated immature sandstones reveal fossils of both the Nama and Avalon assemblages. Pteridinium, Archaeichniim, Nasepia and Rangea occur with Charnia and Rangeomorphs of diverse geometry in these two facies. Only Aspidella and Dickinsonia overlap from the typical Ediacara Member assemblages. The channel-fill facies and the classic Ediacara rippled facies are interpreted to be of largely similar age.

These discoveries demonstrate the strong facies associations of these fossils and point to the significance of facies in evaluating the extinction of the Ediacara biota. While the absence of facies-restricted taxa from successions in Newfoundland and Namibia cannot be used as evidence of their late origin or early extinction, the absence of the facies-crossing Dickinsonia in these other associations is likely to be evidence of a more restricted time range and true extinction before the end of the Ediacaran Period.