Provenance and thermal history of the Beetaloo Basin using illite crystallinity and zircon geochronology and trace element data

Abstract

The Beetaloo Basin of the ‘greater McArthur Basin’, is a 15,000km2 Palaeoproterozoic depocenter which hosts shallow water, dominantly marine, clastic sedimentary rocks and is a large hydrocarbon reserve. Here I present LA-ICP-MS detrital zircon U-Pb age data, Rare Earth Elemental zircon and illite crystallinity XRD results and compare with existing studies to explore the variation in provenance throughout the basin and to better understand its temperature history as much of the basins’ history is still unknown. Nine sandstone and seventeen shale core samples were analysed. New constraints were placed on the depositional age for the Corcoran Formation to between 1390 ± 27 Ma and 1324 ± 4 Ma. The Velkerri Formation, Moroak Sandstone and Kyalla Formation of the Maiwok Sub-group all largely supported the results of previous studies yielding comparable maximum depositional ages. Zircon phosphorous concentrations revealed a largely I-type granitic source rock indicating the granites were formed in arc related settings.

Detrital zircon age data revealed possible origins of sediments showing that the Corcoran Formation has a major source of ca. 1600 Ma zircons which are not unlike rocks from Eastern Queensland orogens. The Velkerri Formations’ main age peak falls at ca. 1765 Ma which shows a change to older detrital source rocks with more similarities to the Arunta and Kathleen and Western Orogenies. Moving up-section to the Moroak Sandstone and Kyalla Formations, samples shift to younger ca. 1560 Ma peak ages at the base of the Moroak followed by a gradual increase in age with younger sequences where a maximum peak age of ca. 1795 Ma is found in the mid Kyalla Formation. This gradual increase shows a gradual shift in sediment source from E/SE sources to southern source regions. Illite crystallinity data show that the shales within the Beetaloo Basin have experienced much greater temperatures than at present. Altree 2 has an XRD calculated bottom-hole temperature of 155°C at1647m depth, the Jamison records 156°C at 1695m with the Elliot being the hottest at 194°C at 1697 deep. These values were then used to calculate the amount of cover removed from present day. Altree 2 returned an estimate of 2050m of cover removed, Jamison 1769m and Elliot with the most cover removed at 2680m showing that the southern region of the Beetaloo Basin has experienced the greatest uplift since maximum subsidence followed by the northern Altree 2.