Impacts of Neogene-recent compressional deformation and uplift on hydrocarbon prospectivity of the passive southern Australian margin

The passive southern margin of the Australian continent, which formed following Cretaceous-Palaeogene separation from Antarctica, contains a rich record of Neogene-Recent compressional deformation and uplift. This deformation and uplift is manifested by reversal of displacement along syn-rift extensional faults, folding of mid-late Cenozoic post-rift sediments, and regional unconformities that can be traced for distances of up to 1500 km along the margin. Palaeothermal data from onshore and offshore exploration wells indicate that erosion associated with deformation and uplift locally exceeds 1 km in the eastern Otway Basin. Both neotectonic palaeostress trends inferred from these structures and present-day stress orientations are consistent with NW-SE directed compression controlled to first-order by plate boundary forces. The critical role of the relative timing of trap formation and source rock maturation in controlling hydrocarbon prospectivity in the southern Australian margin is investigated by comparing two structures that formed during Neogene-Recent deformation in the Otway Basin, the Minerva and Nerita anticlines. Whilst the Minerva Anticline hosts a major gas field (558 BCF GIP), the Nerita Anticline was found to be dry. A combination of apatite fission track analysis (AFTA), vitrinite reflectance (VR) and present-day temperature data show that all units intersected in the Minerva-1 well are currently at their maximum post-depositional temperatures, and are currently mature for hydrocarbon generation. In contrast, similar data collected from the preserved section at Nerita-1 indicate cooling from maximum post-depositional temperatures prior to formation of the Nerita Anticline in the late Miocene. Based on regional AFTA data the underlying early Cretaceous source rocks probably reached maximum palaeotemperatures and ceased hydrocarbon generation during mid-Cretaceous uplift. These results indicate that areas of the southern margin that were deformed during the Neogene-Recent have the greatest potential to trap hydrocarbons where potential source rocks are presently at their maximum post-depositional temperatures

Cenozoic post-breakup compressional deformation and exhumation of the southern Australian margin

We present results from a margin-wide analysis of the history of post-breakup Cenozoic compressional deformation and related exhumation along the passive southern margin of Australia based on a regional synthesis of seismic, stratigraphic and thermochronological data. The Cenozoic sedimentary record of the southern margin contains regional unconformities of intra- Lutetian and late Miocene-Pliocene age, which coincide with reconfigurations of the boundaries of the Indo-Australian Plate. Seismic data show that post-breakup compressional deformation and sedimentary basin inversion, characterised by reactivation of syn-rift faults and folding of post-rift sediments, is pervasive from the Gulf St Vincent to Gippsland basins, and occurred almost continually since the early-to-mid Eocene. Inversion structures are absent from the Bight Basin which we interpret to be the result of both the unsuitable orientation of faults for reactivation with respect to post-breakup stress fields, and the colder, stronger lithosphere that underlies that part of the margin. Compressional deformation along the southeastern margin has mainly been accommodated by reactivation of syn-rift faults resulting in folds with varying ages and amplitudes within the post-rift Cenozoic succession. Many hydrocarbon fields in the Otway and Gippsland basins are located within these folds, the largest of which are often associated with substantial localised exhumation. Our results emphasise the importance of constraining the timing of Cenozoic compression and exhumation in defining hydrocarbon prospectivity of the southern margin

Preservation of late Paleozoic glacial rock surfaces by burial prior to Cenozoic exhumation, Fleurieu Peninsula, Southeastern Australia

The antiquity of the Australian landscape has long been the subject of debate, with some studies inferring extraordinary longevity (>108 myr) for some subaerial landforms dating back to the early Paleozoic. A number of early Permian glacial erosion surfaces in the Fleurieu Peninsula, southeastern Australia, provide an opportunity to test the notion of long-term subaerial emergence, and thus tectonic and geomorphic stability, of parts of the Australian continent. Here we present results of apatite fission track analysis (AFTA) applied to a suite of samples collected from localities where glacial erosion features of early Permian age are developed. Our synthesis of AFTA results with geological data reveals four cooling episodes (C1-4), which are interpreted to represent distinct stages of exhumation. These episodes occurred during the Ediacaran to Ordovician (C1), mid-Carboniferous (C2), Permian to mid-Triassic (C3) and Eocene to Oligocene (C4). The interpretation of AFTA results indicates that the Neoproterozoic−Lower Paleozoic metasedimentary rocks and granitic intrusions upon which the glacial rock surfaces generally occur were exhumed to the surface by the latest Carboniferous−earliest Permian during episodes C2 and/or C3, possibly as a far-field response to the intraplate Alice Springs Orogeny. The resulting landscapes were sculpted by glacial erosive processes. Our interpretation of AFTA results suggests that the erosion surfaces and overlying Permian sedimentary rocks were subsequently heated to between c. 60 and 80°C, which we interpret as recording burial by a sedimentary cover comprising Permian and younger strata, roughly 1 km in thickness. This interpretation is consistent with existing thermochronological datasets from this region, and also with palynological and geochronological datasets from sediments in offshore Mesozoic−Cenozoic-age basins along the southern Australian margin that indicate substantial recycling of Permian−Cretaceous sediments. We propose that the exhumation which led to the contemporary exposure of the glacial erosion features began during the Eocene to Oligocene (episode C4), during the initial stages of intraplate deformation that has shaped the Mt Lofty and Flinders Ranges in South Australia. Our findings are consistent with several recent studies, which suggest that burial and exhumation have played a key role in the preservation and contemporary re-exposure of Gondwanan geomorphic features in the Australian landscape.Simon P. Holford, Paul F. Green, Ian R. Duddy, Richard R. Hillis, Steven M. Hill and Martyn S. Stoke