Terminal Proterozoic mid-shelf benthic microbial mats in the Centralian Superbasin and their environmental significance

A combined sedimentological and biogeochemical study has been conducted on several Terminal Proterozoic mid-shelf microbial mat facies from the Centra lian Super-basin. Isotopic and organic geochemical analysis of the bitumen and kerogen indicated that two sources of organic matter from 'planktonic' and 'benthic microbial-mat' populations contributed to the sediment. The 'p lanktonic' source provided a suite of n-alkanes with <C-20 predominance and the odd n-alkanes >C-20, whereas, the 'benthic' source contributed an over lay of n-alkanes >C-20, with a strong even preference, together with mid-ch ain methyl alkanes. Kerogen and biomarkers derived from the microbial mat w ere found to be depleted in C-13 relative to planktonic material. Pyrite in the micorbial mats was also found to be depleted in S-34 compared to surro unding facies. The combination of these observations suggested that the mat s may have been at least partly composed of sulfide oxidising bacteria. The se organisms have specific environmental tolerances that set limits on pala eoenvironment. Their requirement for oxygen indicates that the water column above the mid-shelf could not have been anoxic. Accordingly, from the resu lts and age determinations reported here, it would appear that mid-shelf en vironments of the Centralian Superbasin of Australia were seeing significan t levels of oxygen through the Ediacarian. Copyright (C) 1999 Elsevier Scie nce Ltd.