Depositional environment of the middle proterozoic velkerri formation in Northern Australia: Geochemical evidence

The organic-matter-rich shales of the Middle Proterozoic Velkerri Formation in the McArthur Basin of northern Australia are part of the essentially unmetamorphosed Roper Group, which has many units showing remarkable lithological consistency over vast areas. Jackson et al. reported the discovery of indigenous ‘live’ oil in the Velkerri Formation, thereby raising considerable interest in its depositional environment. Based on sedimentology alone this unit was suggested to have formed in a deep-water low-energy open marine environment. A geochemical study of the silty mudstones of the Velkerri Formation from Urapunga 3 corehole, which intersects the upper half of this unit, was carried out to examine some aspects of this depositional model. Organic C contents in the core are up to 7.1% and maturity determinations indicate that the sediments have experienced a relatively low organic metamorphic grade. Pyrite S/organic C ratios vary from very low (< 0.01) in the bottom section of the core to high (up to 1.52) in the top part of the core. Disseminated pyrite δ34S values range from + 3.6 to + 34.4‰. The geochemical results from the upper half of the Velkerri Formation indicate an initial lowsulfate oxidizing environment. This environment may have been a large lake or a bay which became barred early in its depositional history and later became sulfate-poor as a result of the activities of sulfate-reducing bacteria. Some influx(es) of seawater resulted in increased organic C (and pyrite S) contents in the anoxic sediments, while the lake or barred bay eventually became euxinic, with the preservation of significant amounts of organic matter. These data indicate the need for an integrated approach to the interpretation of Proterozoic sedimentary environments.