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

The architecture of submarine monogenetic volcanoes – insights from 3D seismic data

Many prospective sedimentary basins contain a variety of extrusive volcanic products that are ultimately sourced from volcanoes. However, seismic reflection-based studies of magmatic rift basins have tended to focus on the underlying magma plumbing system, meaning that the seismic characteristics of volcanoes are not well understood. Additionally, volcanoes have similar morphologies to hydrothermal vents, which are also linked to underlying magmatic intrusions. In this study, we use high resolution 3D seismic and well data from the Bass Basin, offshore southern Australia, to document 34 cone- and crater-type vents of Miocene age. The vents overlie magmatic intrusions and have seismic properties indicative of a volcanic origin: their moderate–high amplitude upper reflections and zones of “wash-out” and velocity pull-up beneath. The internal reflections of the vents are similar to those found in lava deltas, suggesting they are composed of volcaniclastic material. This interpretation is corroborated by data from exploration wells which penetrated the flanks of several vents. We infer that the vents we describe are composed of hyaloclastite and pyroclasts produced during submarine volcanic eruptions. The morphology of the vents is typical of monogenetic volcanoes, consistent with the onshore record of volcanism on the southern Australian margin. Based on temporal, spatial and volumetric relationships, we propose that submarine volcanoes can evolve from maars to tuff cones as a result of varying magma-water interaction efficiency. The morphologies of the volcanoes and their links to the underlying feeder systems are superficially similar to hydrothermal vents. This highlights the need for careful seismic interpretation and characterization of vent structures linked to magmatic intrusions within sedimentary basins

How wide is a fault damage zone? Using network topology to examine how fault-damage zones overprint regional fracture networks

Topological analysis of networks of linear features has recently gained popularity in structural geology, as it provides a robust system of fracture network characterisation with consistent terminology. This approach has mainly been applied by using discontinuous sample areas to characterise topology of a region, or geological feature (e.g. Procter and Sanderson, 2018). We apply network topology to investigate spatial variation of a natural fracture network in the damage zone of the Castle Cove Fault, Otway Basin, Victoria. The Castle Cove Fault's associated fracture network occurs in the hanging-wall Eumeralla Formation, a fine-to medium-grained volcanogenic sandstone or Early Cretaceous age. Topological characterisation of the Castle Cove Fault damage-zone identified an increase in the two-dimensional intensity of fracturing in the hanging-wall at a distance 70 m from the fault, reaching a maximum ~40 m from the fault, indicating the extent of the fault-damage zone is between 40 and 70 m from the main fault. Maximum fracture intensity values decay with a power law relationship with respect to distance from the fault plane, averaging ~0.06 adjacent to the fault, and ~0.02 within what is interpreted as the regional fracture network, 70 m + outboard of the fault. Fracture orientations in the damage zone are synthetic and antithetic with respect to the ~60° dip to the NW of the of the Castle Cove Fault, with these fault-related fractures overprinting an existing regional fracture network, creating high fracture network connectivity within the fault-damage zone. Our results show that network topology is a powerful tool for quantifying and visualising the properties of fracture networks associated with crustal-scale faults.Rowan L. Hansberry, Rosalind C. King, Simon P. Holford, Martin Hand, Natalie Debenha

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

Determining paleo-structural environments through natural fracture and calcite twin analyses: a case study in the Otway Basin, Australia

The structural history of the Otway Basin has been widely studied; however, previous works have focussed on large kilometre scale, basin and seismic structures, or have over-simplified natural fracture analysis with an excessive focus on fracture strike direction and a disregard for 3D geometry, a crucial characteristic when considering states of stress responsible for natural fracture formation. In this paper, we combine techniques of natural fracture analysis and calcite twin stress inversion to investigate the meso (outcrop and borehole) and micro (crystal) scale evidence for structural environments that have contributed to basin evolution. Our results indicate that basin evolution during the post-Albian may be markedly more complex than the previously thought stages of late Cretaceous inversion, renewed rifting and long-lived mid-Eocene to recent compression, with evidence for up to six structural environments detected across the basin, including; NE–SW and NW–SE extension, NW–SE compression, a previously undetected regime of NE–SW compression, and two regimes of strike-slip activity. By constraining structural environments on the meso- and micro-scale we can deliver higher levels of detail into structural evolution, which in turn, provides better-quality insights into multiple petroleum system elements, including secondary migration pathways and trap formation. Our research also shows that the Otway Basin presents a suitable environment for additional micro-scale structural investigations through calcite twin analyses.Hugo B. Burgin, Khalid Amrouch, Mojtaba Rajabi, David Kulikowski and Simon P. Holfor

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

Cenozoic structural evolution of the Mount Lofty Ranges and Flinders Ranges, South Australia, constrained by analysis of deformation bands

Published online: 09 Feb 2020The presence of deformation bands in Upper Cretaceous–Pleistocene sediments of the St Vincent and Lake Eyre basins, South Australia supports the occurrence of intraplate normal and reverse faulting in this region throughout the Cenozoic. Current tectonic models for the development of this region lack structural evidence and underestimate the role and influence of extensional tectonics in the Cenozoic tectonic evolution of the Mount Lofty Ranges and Flinders Ranges. This lack of understanding has resulted in poor structural and chronological constraints of the regions paleostress models. For the first time, we use detailed mapping and structural analysis of deformation bands across different chronostratigraphic units to identify variations in paleostress. Deformation bands are discrete, measurable strain features that develop readily in clastic sedimentary rocks in response to regional stress. We present a comprehensive structural analysis of five study sites across six chronostratigraphic units adjacent to the Mount Lofty Ranges and Flinders Ranges. Here, we have recorded 737 cataclastic and dilation bands and 397 fractures adjacent to reactivated Cenozoic faults, which reveal step changes in structural sets, identifying temporal constraints for our paleostress model. Our analysis supports the notion of a regional late Miocene shift in the paleostress regime from N–S extension to N–S compression, followed by a rotation of the maximum horizontal stress (σH) from NW–SE to E–W. This coincides with a well-documented pulse of late Miocene–Pliocene compression and uplift observed throughout southeastern Australia. Our results indicate that deformation bands are a useful resource for constraining discrete changes in local stress regimes and enhance our understanding of dynamic intraplate strain accommodation within the Mount Lofty Ranges and Flinders Ranges. These have proven useful for hydrocarbon and mineral exploration in neighbouring Otway Basin and Gippsland Basin.D. C. Lubiniecki, R. C. King, S. P. Holford, M. A. Bunch, S. B. Hore and S. M. Hil

The effects of deformation bands on uranium-bearing fluid migration in sedimentary sequences, Flinders Ranges, South Australia

Luniniecki, D.C., King, R.C., Holford, S.P., Bunch, M.A., Hill, S.M. and Hore, S.B