Controls on organic-rich shale formation in the Mesoproterozoic Beetaloo Sub-basin, Northern Territory, Australia: insights from biogeochemistry and mineralogy

Abstract

The Mesoproterozoic Velkerri shales (ca. 1400 Ma, million years) deposited in the McArthur-Yanliao Gulf of Nuna (preserved in northern Australia and north China) preserve high levels of organic carbon that have led to them being explored for hydrocarbons. In the Proterozoic-aged sub-basin in north of Australia, organic matter preservation coincides with mineralogical and chemical evidence for anoxic to euxinic bottom-water and/or sediment–water interface conditions, which were likely driven by the increased organic C content and associated O₂ consumption in the local environment. Whether this increase in organic matter formation/preservation is due to increased surface-water primary productivity, or simply because the seabed was starved of siliciclastic input due to relative sea-level change, remains controversial. To address these questions, here we present total organic carbon (TOC) data, organic carbon isotopic compositions (δ¹³C_org), and bulk mineralogy of shales from the sections through the Amungee Member of the Velkerri Formation, recorded and sampled from two correlative cores located in northeast and north of the basin (ca. 170 km apart). Similar δ¹³C_org and TOC profiles, lightest in the A-organofacies, peaking in the B-organofacies, and declining toward the C-organofacies top, suggest peak photosynthetic productivity in B-organofacies and dominant chemosynthetic input in A-organofacies. The high covariance between δ¹³C_org and TOC in Principal Component Analysis (PCA), particularly in PC1 (37.52 % of variations) and PC2 (20.9 % of variations) of B-organofacies in Marmbulligan-1, suggests a strong coupling between δ¹³C_org and TOC, indicating a common control on their variations. Stable δ¹³C_org values suggest steady bioactivity during the lean interval (intra B–C), though increased clastic input likely diluted TOC abundance. This is marked by an increased volume of presumably detrital clays/phyllosilicates (or rather their alteration products) such as chlorite content, particularly in the B–C interval. The role of detrital input in this interval is also evidenced by high correlation between chlorite and kaolinite minerals in PCA analysis. This heterolithic siliciclastic influx represents the distal Wyworrie delta, prograding northward across the basin.