Evolution of the Southwest Australian Rifted Continental Margin During Breakup of East Gondwana: Results from IODP Expedition 369

International Ocean Discovery Program Expedition 369 drilled four sites on the southwestern Australian continental margin, in the deep water Mentelle Basin (MB) and on the neighboring Naturaliste Plateau (NP). The drillsites are located on continental crust that continued rifting after seafloor spreading began further north on the Perth Abyssal Plain (PAP) between magnetochrons M11r and M11n (133‐132 Ma), ending when spreading began west of the NP between chrons M5n and M3n (126‐124 Ma). Drilling recovered the first in‐situ samples of basalt flows overlying the breakup unconformity on the NP, establishing a magnetostratigraphically constrained eruption age of >131‐133 Ma and confirming a minimal late Valanginian age for the breakup unconformity (coeval with the onset of PAP seafloor spreading). Petrogenetic modeling indicates the basalts were generated by 25% melting at 1.5 GPa and a potential temperature of 1380‐1410 °C, consistent with proximity of the Kerguelen plume during breakup. Benthic foraminiferal fossils indicate that the NP remained at upper bathyal or shallower depths during the last 6 Myr of rifting and for 3‐5 Myr after breakup between India and Australia. The limited subsidence is attributed to heat from the nearby Kerguelen plume and PAP spreading ridge. The margin subsided to middle bathyal depths by Albian time and to lower bathyal (NP) or greater (MB) depths by late Paleogene time. Periods of rapid sedimentation accompanied a westward jump of the PAP spreading ridge (108 Ma), rifting on the southern margin (100‐84 Ma), and opening of the southern seaway between Australia and Antarctica (60‐47 Ma)

Expedition 369 methods

This chapter documents the procedures and methods used in the shipboard laboratories during International Ocean Discovery Program (IODP) Expedition 369. This introductory section in particular provides a rationale for the site locations and an overview of IODP depth conventions, curatorial procedures, and general core handling/analyses during Expedition 369. Subsequent sections describe specific laboratory procedures and instruments in more detail. This information only applies to shipboard work described in the Proceedings volume; methods used in shore-based analyses of Expedition 369 samples and/or data will be described in various scientific contributions in the open peer-reviewed literature and the Expedition Research Results chapters of this Proceedingsvolume

Early Jurassic palaeoenvironments in the Surat Basin, Australia - marine incursion into eastern Gondwana

Interpretations of palaeodepositional environments are important for reconstructing Earth history. Only a few maps showing the Jurassic depositional environments in eastern Australia currently exist. Consequently, a detailed understanding of the setting of Australia in Gondwana is lacking. Core, wireline logs, two‐dimensional and three‐dimensional seismic from the Precipice Sandstone and Evergreen Formation in the Surat Basin have been used to construct maps showing the evolution of depositional environments through the Early Jurassic. The results indicate the succession consists of three third‐order sequences (Sequence 1 to Sequence 3) that were controlled by eustatic sea level. The lowstand systems tract in Sequence 1 comprises braidplain deposits, confined to a fairway that parallels the basin centre. The strata were initially deposited in two sub‐basins, with rivers flowing in different orientations in each sub‐basin. The transgressive systems tract of Sequence 1 to lowstand systems tract of Sequence 3 is dominated by fluvio–deltaic systems infilling a single merged basin centre. Finally, the transgressive and highstand systems tracts of Sequence 3 show nearshore environments depositing sediment into a shallow marine basin. In the youngest part of this interval, ironstone shoals are the most conspicuous facies, the thickness and number of which increase towards the north and east. This study interprets a corridor to the open ocean through the Clarence–Moreton Basin, or the Carpentaria and Papuan basins, evidence of which has been eroded. These results challenge a commonly held view that eastern Australia was not influenced by eustasy, and propose a more dynamic palaeogeographic setting comprising a mixture of fluvial, deltaic and shallow marine sedimentary environments. This work can be used to unravel the stratigraphic relationships between Mesozoic eastern Australian basins, or in other basins globally as an analogue for understanding the complex interplay of paralic depositional systems in data poor areas

Mires in the dark: high latitude coals in the Jurassic Walloon Subgroup of the Surat Basin, Australia

Carmine Wainman and Peter McCab

Inherited morphobathymetric controls over contourite drift deposition: a case study from the late Cenozoic Mentelle Basin, Australia

Deep-sea sedimentary deposits are important archives of the geologic past that preserve the records of past environmental changes in earth’s ocean. The detailed analysis of deep-sea sedimentary archives, in particular of contourite drifts, can help elucidate past changes in ocean circulation and the stratigraphic evolution of continental margins. However, the bathymetric profile of an oceanic basin can shape and modify the architecture of contourite drifts via the interaction between down-slope and along-slope processes. The identification of local bathymetric influence on depositional architectures is therefore important to help decipher local versus regional influences on deep-sea sedimentary signatures. Seismic data from Mentelle Basin in the southeast Indian Ocean integrated with deep-sea core data reveal a calcareous-siliciclastic mixed contourite-turbidite system developed during the late Cenozoic, starting in the middle Miocene. Current winnowing led to the formation of regional hiatuses, ferromanganese crusts, and siliciclastic lag deposits. The main locus of sediment deposition occurred on the shallower parts of the basin, whereas sediment preservation remained low in the deeper areas. Seismic analysis shows that inherited topography influenced the architecture of contourite deposits within the basin, with elongate-mounded and sheeted drifts forming preferentially at shallower depths on the continental slope and the Naturaliste Plateau, while channel incision occurred in the deepest parts of the basin. These results suggest that the intensification of current transport occurred preferentially within the deeper and spatially constrained parts of the basin, whereas current deflection around the slope and plateau enhanced drift deposition and preservation at shallower depths. Therefore, the basin topography at the time of deposition controlled the distribution of deep-sea deposits and drift morphologies within the mixed contourite-turbidite system in the Mentelle Basin.G. Tagliaro, C.C. Wainman, and C.S. Fulthorp

Nature and origin of tuff beds in Jurassic strata of the Surat Basin, Australia: implications on the evolution of the eastern margin of Gondwana during the Mesozoic

Volcanogenic rocks in the Great Australian Superbasin provide one of the principal records of contemporaneous volcanism in eastern Australia during the Mesozoic. However, the paucity of primary Jurassic to Early Cretaceous-age extrusive or intrusive igneous bodies on the Australian continent makes it particularly challenging to deduce their source and character. This, in turn, makes it difficult to ascertain how the eastern margin of Gondwana evolved during this timeframe. Despite some studies of this enigmatic volcanism, there have been little or no detailed analyses of these Jurassic to Early Cretaceous-aged sediments. Based on the multidisciplinary analyses of age-constrained air-fall tuffs (168 to 148 Ma) from the Jurassic Walloon Coal Measures of the Surat Basin, we suggest from zircon grains of a similar age that the tuffs were erupted from volcanoes fed by intermediate to felsic magmas supported by their quartz and feldspars-rich composition, and from zircon grains with low to moderate Nb values (0.5 to 100 ppm) and high U and Th values (30 to 1000 ppm). The mapping of tuff isopachs and a mean zircon crystal size of 170 μm supports the source being from volcanoes approximately 280 to 1000 km from the palaeoeast-southeast with a volcanic explosivity index (VEI) of 8. Our results indicate the tuffs originated from a continental arc setting associated with the Whitsunday Igneous Association, and the long-lived (late Palaeozoic to Cretaceous) westward subduction of the palaeo-Pacific oceanic crust beneath eastern Australia. Such a tectono-magmatic environment would help constrain the timing of the transition of eastern Gondwana from a convergent to a divergent margin.Carmine C. Wainman, Peter Reynolds, Tony Hall, Peter J. McCabe, Simon P. Holfor