Isotope constraints on paleo-depositional environments and intra-basin correlation in the Proterozoic McArthur Basin, Northern Territory, Australia

Abstract

The Proterozoic McArthur Basin, Northern Territory, Australia, hosts some of the oldest proven hydrocarbon sources in the world including the organic rich black shale unit in the McArthur Group, the Barney Creek Formation. Stratigraphically positioned between dolomitic carbonate units, the depositional setting for the Barney Creek Formation has been in debate for at least three decades. Published literature supports both an anoxic coastal lagoon/lacustrine environment and also a relatively deep-marine low-energy deposition. Here we employ high-resolution records of stable isotope ratios (δ13C, δ18O,87Sr/86Sr) and elemental concentrations (Ba/Ca) in the carbonates rich sedimentary record of the McArthur Group (cores LV09001, McA5, Bj2, BCF 4) as proxies to constrain paleo-depositional environments. Using samples from the LV09001 core, we were able to create a high-resolution, geochemical and isotope ratio profile of the Glyde Package, McArthur Group, ranging from the Emmerugga Dolomite to the Donnegan Member. Acquired δ13C, 87Sr/86Sr and Ba/Ca stratigraphic trends indicate a changing paleo-environment at the time of sediment deposition and carbonate precipitation. In particular a systematic negative δ13C anomaly, magnitude around 2.5‰, is recorded in the Barney Creek Formation where Ba/Ca ratios are also anomalously high and a coeval 87Sr/86Sr trend indicates a ‘non-marine’ restricted environment. Observed light carbon (12C), and elevated barium concentrations (Ba/Ca ratios) are likely controlled by local cycling of organic matter in a redox-stratified restricted basin analogous to modern Black Sea. A restricted setting also aids in chemical and redox stratification, creating deep water anoxic and low energy conditions for the deposition of the Barney Creek Formation. Finally, our constraints from coupled C and Sr isotope data, collected from the most ‘marine’ carbonate intervals, suggest that the inferred δ13C and 87Sr/86Sr signatures of Proterozoic paleo-seawater (at about 1640 Ma) were close to -0.5 to 0 per mil, and 0.705 to 0.706, respectively.