Isotope Constraints on Intra-Basin Correlation and Depositional Settings of the Mid-Proterozoic Carbonates and Organic-Rich Shales in the Greater McArthur Basin, Northern Territory, Australia

Abstract

The Greater McArthur Basin in northern Australia forms the world’s oldest potential unconventional gas play. It comprises Paleo- to Mid-Proterozoic sedimentary successions (i.e., the McArthur and Limbunya Groups) dominated by carbonate rocks (i.e., dolostones) deposited in various shallow marine to more restricted lagoonal and sabkha/playa evaporitic environments, while the associated organic-rich shales (i.e., the Barney Creek and Fraynes Formations) likely formed in relatively deeper and/or redox stratified depositional settings. Here we use a multi-proxy approach based on the isotope tracers of carbon (13C) and strontium (87Sr/86Sr), and selected paleo-redox proxies (cerium anomalies: Ce/Ce*) to further constrain (i) temporal and spatial changes in the paleo-depositional environments and redox-structure of the basin, and also (ii) to test the applicability of the above isotope proxies for intra-basin correlations in the Greater McArthur Basin. This study presents the first continuous high-resolution 13C and 87Sr/86Sr isotope records acquired from two drill cores: LV09001 and Manbulloo-S1 (located more than 400 km from each other), which intersected the above Proterozoic carbonate and organic-rich sedimentary sequences (i.e., the Barney Creek and Fraynes Formations, dated at ~1640 ± 5 million years) deposited in the central and western parts of the basin, respectively. Importantly, our composite isotope trends from the Greater McArthur Basin (based on data from LV09001 and Manbulloo-S1) show consistent and systematic variations in the carbonate-based 13C, 87Sr/86Sr and Ce/Ce* proxy records that are tightly coupled to changes in the local depositional environments, the latter interpreted as oscillations between relatively open marine (suboxic to anoxic) and more restricted (anoxic to euxinic) conditions. Overall, our results indicate coherent basin-wide isotope patterns with characteristic isotope anomalies during the purported basin restriction (i.e., the deposition of organic-rich shales) the measured 13C and 87Sr/86Sr trends shift to isotopically lighter and more radiogenic values, respectively. These coherent isotope trends acquired from LV09001 and Manbulloo-S1 cores thus supports the proposed connectivity of the central and western parts of the basin, and the suitability of our multi-proxy isotope approach for future intra-basin correlation studies in the Greater McArthur Basin.