Microflora of the H.Y.C. Pyritic Shale Member of the Barney Creek Formation (McArthur Group), middle Proterozoic of northern Australia

The H.Y.C. Pyritic Shale Member of the Barney Creek Formation (ca 1 500 my old; northern Australia) contains several stratiform base metal sulfide deposits of economic significance. Black cherts within these mineral deposits preserve a diverse assemblage of bacterial and algal microfossils. The assemblage differs from most other Precambrian biotas so far described in that it was deposited in deep water, it is not associated with stromatolites or algal mats, and it is dominated by filamentous bacteria, most of which are pyritized. Analysis of the assemblage suggests that the depth of the depositional basin exceeded that of the photic zone, that the bacteria inhabited the basin floor where they maintained anoxic conditions through heterotrophic degradation of detrital organic matter, and that the algae inhabited overlying near surface waters. Most of the algal fossils have been assigned to the Cyanophyta, although two of the described species are potentially referable to the eukaryotic green or red algae. Differences between this assemblage and other biotas described from the McArthur Group suggest that a workable system of biostratigraphic zonation for the Group is feasible.

Fossils in the H.Y.C. assemblage are here referred to 21 species and 16 genera, of which 14 species and 6 genera are new. The new taxa are: Bacteria, Biocatenoides incrustata sp. nov., B. pertenuis sp. nov., Ramacia carpentariana gen. et sp. nov., Coleobacter primus gen. et sp. nov., Ferrimonilis variabile gen. et sp. nov.; Chroococcales (Cyanophyta), Nanococcus vulgaris gen. et sp. nov., Bisacculoides tabeoviscus gen. et sp. nov., B. vacua gen. et sp. nov., B. grandis gen. et sp. nov.; Nostocales (Cyanophyta), Oscillatoriopsis schopfii sp. nov., Cyanonema inflatum sp. nov., C. minor sp. nov.; Incertae sedis, Clonophycus elegans gen. et sp. nov., Globophycus minor sp. nov. In addition, the new combination Gunflintia septata (Schopf) is proposed.