The sedimentary geochemistry and geochronology of the Proterozoic greater McArthur Basin, northern Australia

Abstract

The greater McArthur Basin is a Proterozoic sedimentary system deposited in northern Australia and has been widely used to reconstruct ancient global biogeochemical systems. Sections within the basin are economically prospective, hosting petroleum supersystems and world-class metal deposits. Consequently, the geochemistry of the basin also plays a crucial role in the formation of energy and mineral resources. Identifying units with similar age and geochemistry is pivotal for finding analogous targets elsewhere in the region. However, dating Proterozoic sedimentary rocks can be difficult, as they are devoid of a diverse fossil record for biostratigraphy to be applicable. To better define these parameters, a new laser-based technique to date shales using in situ Rb–Sr geochronology was developed. Here, geochronological and geochemical information are collected concurrently. Triaging these datasets help differentiate between samples that record primary signatures versus those that were altered. Thermal constraints of the same samples can also be obtained using indicators used in petroleum research. The development of this method is the subject of the second and third chapters in this thesis. In situ Rb–Sr dating of mature, oil-prone shales from the Velkerri Formation gave ages ca. 1448–1421 Ma, agreeing with previous Re–Os geochronology. This is interpreted as a minimum age, recording the deposition of the formation. Oppositely, thermally overmature shales sourced from the unit were dated at ca. 1336–1322 Ma. This is younger than its expected age and instead overlaps with the Derim Derim-Galiwinku/Yanliao magmatic event. Geothermal modelling found that a Derim Dolerite sill would have enough influence on the surrounding shales to reset their chronometer and elevate their organic matter into overmaturity. This method was also applied to the lower formations of the greater McArthur Basin; establishing correlations between the McArthur Basin and the Birrindudu Basin in Chapters 4 and 5. We build on this framework by constraining their palaeo-water conditions using multiple proxies. Bioproductivity indices such as Ba, P, and total organic carbon content display a relationship with several redox-sensitive tracers in the Redbank and Glyde Packages. Further evidence from Ce, δ13Ccarb, and 87Sr/86Sr values suggest that more oxic and productive basin waters occur in open, marine settings as opposed to restricted palaeoenvironments. In addition, δ88/86Sr analysis show that these marine settings are more likely to source mantle-like provenance, elevating bioproductivity and producing a redox gradient in the water column. Comparatively, restricted basins experienced stunted bioproductivity and were largely oxygen-poor. These isotopic excursions were also used to chemostratigraphically correlate coeval units elsewhere in the basin and globally. Altogether, the geochronology of the greater McArthur Basin and similarly aged systems can benefit from the innovative dating method developed here. The thermochronological history of a sedimentary unit can be defined by timing its deposition or subsequent alteration events. The geochemical signatures of the basin also show that surface environments in the Proterozoic were deeply complex. Palaeo-water heterogeneity is controlled by basin-scale tectonic processes and would influence where and how economic resources form within the sediment.