Isotopic and trace element constraints on the paleodepositional environment and redox history of the Greater McArthur Basin, Northern Territory

Abstract

The Limbunya Package of the Greater McArthur Basin, Northern Territory, hosts the oldest known hydrocarbon reservoirs. Despite the prospective nature of the Fraynes Formation (ca. 1638 Ma), the depositional environment and redox conditions for these organic-rich sediments and the associated carbonates is not well understood. Utilising high resolution stable and radiogenic isotope proxy records (δ13C, Sr87/Sr86 and Nd143/Nd144 ), we show that basin restriction was prevalent during deposition of the Fraynes Formation. As basin restriction has been suggested for the correlative Barney Creek Formation, we argue that this event affected the entire Greater McArthur Basin. The δ13C and Sr87/Sr86 isotopic signatures measured in this study (Manbulloo S1 core) are compared to corresponding stratigraphic units from the central McArthur Basin (LV09001 core). The results show evidence for coherent basin-wide C and Sr isotopic patterns that in turn could be used for the chemostratigraphy purposes during the deposition of the organic rich shales (Fraynes and Barney Creek Formations) and the overlying Reward dolomite sequences. Differences in δ13C isotope signal in the underlying Campbell Springs Dolomite (Teena Dolomite equivalent) are proposed to be related either to differences in the C isotope composition of local DIC pools at these two paleo-locations (i.e., LV09001 and Manbulloo sites), or the isotopically lighter δ13C signal in the latter core could reflect deeper depth of the deposition for dolomite at this site. Linking redox-sensitive trace element enrichments with total organic carbon data indicates that the water column was anoxic to euxinic during the deposition of the Fraynes Formation, with episodes of euxinia driven by the relatively high input of organic matter. Overall, knowledge gained here, particularly in regard to the links between paleo-depositional environments, redox conditions, and the occurrences of organic-rich shales, can be applied to other Precambrian frontier basins.