Geochemical analysis of the McArthur and Tawallah Groups, McArthur Basin: chemostratigraphy & palaeo-redox proxies using shales and carbonates

Abstract

The lower portion of the McArthur Basin includes the Palaeoproterozoic to Mesoproterozoic McArthur and Tawallah Groups. During this time, Earth had comparatively less oxygen following the beginning of ‘The Great Oxygenation Event’ (GOE, Palaeoproterozoic ca. 2.34 Ga) and organisms were simple. In conjunction, the ‘Boring Billion’ (1800 to 800 Ma) is an acknowledged period following the GOE due to reported flat carbon and oxygen isotope trends which is evidence for a relatively stable climate and delayed organism evolution. The aim of this thesis is to gather a coupled dataset using conventional geochemistry and organic geochemistry through the use of carbon/oxygen isotopes, redox sensitive trace elements (Mo, V and U), rare earth elements and organic carbon content in order to understand the oxygen conditions (through palaeo-redox techniques) within the lower portions of the Palaeoproterozoic McArthur Basin. Shale units of the McArthur and Tawallah groups (Mallapunyah, Wollogorang and Wuraliwuntya Formations) are shown to be deposited in a stratified oxygenation and sulfidic environment. These episodes of anoxia or euxinia are shown by elevated concentrations of rare earth elements (REE) and trace elements at specific drill core depth. Subtle and fluctuating cerium and europium anomalies along with trace and major elemental data were coupled by total organic carbon (TOC) to show a relationship with the organic matter within the Wollogorang Formation specifically. The trace elements seemingly concentrated within the regions of high organic content, which resulted in euxinia events causing elevated levels of trace metals. Potential trace metal elevations within the Wollogorang and Mallapunyah Formations specifically are compared against the reported evidence for sedimentary exhalative deposits (SedEX deposits). The Amelia Dolostone recorded carbon isotope values of 0 to -2‰ and oxygen isotope values of -5 to -9‰ - expected carbon and oxygen isotopic concentrations for the Palaeoproterozoic to Mesoproterozoic Era.