Jennifer Totterdell

Early Permian East Australian Rift System

The Early Permian to Middle Triassic Bowen and Gunnedah Basins in eastern Australia developed in response to a series of interplate and intraplate tectonic events that occurred to the east of the basin system. The initial event was extensional and stretched the continental crust to form part of the major Early Permian East Australian Rift System that occurred at least from far-north Queensland to southern New South Wales. The most important of the rift-related features, in a commercial sense, are a series of half-grabens that form the Denison Trough, now the site of several producing gasfields. The eastern part of the rift system commenced at about 305 Ma and was volcanic dominated. In contrast, the half-grabens in, and to the west of, the Bowen Basin were non-volcanic, and appear to have initiated significantly later, at about 285 Ma. These half-grabens are essentially north–south in length with an extension direction of approximately east-northeast. Mechanical extension appears to have ceased at about 280 Ma, when subsidence became driven by thermal relaxation of the lithosphere. The extension occurred in a backarc setting, in response to far-field stresses that propagated from the west-dipping subduction system at the convergent plate margin of East Gondwanaland that was located to the east of the East Australian Rift System.

Contractional structures and deformational events in the Bowen, Gunnedah and Surat Basins, eastern Australia

During the Permian and Triassic, eastern Australia was part of an active Gondwanaland convergent plate margin. The Bowen and Gunnedah Basins formed in a backarc setting, which was initially extensional, but switched to contractional in the mid-Permian, leading to the development of a major west-directed retroforeland thrust belt in the New England Orogen, and the formation of a major foreland basin phase to the west in the Bowen and Gunnedah Basins. The contractional deformational style is asymmetric, changing from the eastern side of the basins, adjacent to the thrust belt, to the western side of the basins which was not physically affected by the retrothrust belt. In the east, new thrusts are hard-linked to the growing thrust wedge further to the east, which propagated westwards and cannibalised the eastern part of the basin system. In the western part of the basin, however, the transmission of far-field compressional stresses led to the inversion of Early Permian extensional faults as thrusts, along with the development of new thrusts and backthrusts, which are not hard-linked to the retrothrust belt in the east. During the sustained period of rapid subsidence and sedimentation driven by thrust loading in the Bowen and Gunnedah Basins in the Late Permian to Late Triassic, there are several short periods of non-deposition and contraction. The contractional events were usually short-lived, less than a few million years each in duration, in an overall period of subsidence that lasted for ∼30–35 Ma. It is suggested that shallow to flat subduction over much of this period produced strong coupling across the plate boundary, which allowed the transmission of compressive far-field stresses well into the distal part of the foreland, possibly during times of global plate boundary reorganisation. A final contractional event in the early Late Cretaceous corresponds with the cessation of sedimentation in the Surat Basin, uplift and reactivation of earlier structures.

Subsidence history and basin phases of the Bowen, Gunnedah and Surat Basins, eastern Australia

The Early Permian to Middle Triassic Bowen and Gunnedah Basins and the Early Jurassic to Early Cretaceous Surat Basin exhibit a complex subsidence history over a period of about 200 Ma. Backstripped tectonic subsidence curves, constructed by removing the effects of processes such as sediment loading, loading due to the water column and sediment compaction, allow the subsidence histories of the basin to be examined in terms of the tectonic drivers that caused the subsidence of the basins. In the Early Permian, rapid subsidence was driven by mechanical extension, forming a series of half-grabens along the western margin of the Bowen and Gunnedah Basins. Mechanical extension ceased at about 280 Ma, being replaced by a phase of passive thermal subsidence, resulting in more widespread, uniform sedimentation, with reduced tectonic subsidence rates. At the start of the Late Permian, the passive thermal subsidence phase was interrupted by the onset of lithospheric flexure during a foreland basin phase, driven by convergence and thrust loading to the east in the New England Orogen. Initially, dynamic loading, caused by viscous corner flow in the asthenospheric wedge above the west-dipping subducting plate, led to limited tectonic subsidence. Later in the Late Permian, the dynamic loading was overwhelmed by static loading, caused by the developing retroforeland thrust belt in New England, leading to very high rates of tectonic subsidence, and the development of a major retroforeland basin. Peneplanation in the Late Triassic was followed by sedimentation at the start of the Jurassic, forming the Surat Basin, where the tectonic subsidence can again be interpreted in terms of dynamically induced platform tilting. Subduction ceased at about 95 Ma, resulting in rapid uplift, due to the rebound of the lithosphere following either cessation of subduction, or it stepping well to the outboard of Australia.

Seismic stratigraphy of a large, Cretaceous shelf-margin delta complex, offshore southern Australia

Seismic images along Australias southern continental margin reveal the internal geometry and depositional history of the Hammerhead Delta, a Late Cretaceous shelf-margin delta complex in the Ceduna Subbasin of the Bight Basin. The Hammerhead Delta comprises the late Santonian to Maastrichtian Hammerhead supersequence, which is divisible into three, third-order sequence sets and their component sequences. Sequence set 1 (late Santonian to early Campanian) comprises three progradational sequences deposited after a major fall in sea level under a high sediment supply regime. Sequence set 2 (early Campanian to early Maastrichtian) comprises two, sandy progradational sequences. Basinward shifts in facies caused by forced regression are apparent between each progradational sequence. Sequence set 3 (early to late Maastrichtian) is a thick, aggradational succession deposited when the rates of creation of accommodation space and sediment supply were balanced. A transgressive episode at the top of sequence set 3 marks a rapid decrease in sediment supply and the end of deltaic sedimentation.A long-lived sand-rich sediment supply, most likely derived from erosion of the eastern Australian highlands to the northeast, was the major influence on delta formation. Rapid progradation and the formation of thick shelf-margin clinoforms resulted in slope instability, growth-faulting, and load-induced collapse of the shelf margin during the Campanian. The Hammerhead Delta is characterized by sandy, progradational clinoforms and lacks the thick coeval prodeltaic shales and shale tectonism that are common to many other large deltas. The results of this study, which included seismic facies mapping, well correlations, and comparisons to other large shale-poor deltas, suggest that the Hammerhead Delta has excellent reservoir potential.

Sequence stratigraphy and fill history of the Bowen Basin, Queensland

A regional seismic synthesis of the 10 km-thick continental and shallow-marine succession of the Bowen Basin has revealed the basin-filling episodes and nine depositional supersequences (A–I). An Early Permian extensional phase, characterised by thick volcanics and half-graben development in a number of separated troughs, allowed fluviolacustrine sediments, including coal, to accumulate (Supersequence A). In the subsequent thermal subsidence phase, four marine supersequences (B–E) were generated. The onsets of supersequences B and C appear to be primarily tectonically controlled, but there is no evidence of tectonism initiating D, E and F. Supersequences B, D and E start with marine-flooding events, and in the Denison Trough end with either non-marine progradation (B), or high-energy nearshore sands (D and E). The first evidence of the compressional deformation associated with the development of a foreland basin occurs at the base of deltaic to marine Supersequence C. The thrust-induced loading of the foreland caused rapid subsidence in the adjacent Taroom Trough. It took about 10 Ma for the effects of thrust loading to spread across the basin from east to west. The foreland loading first affected the entire basin at the time of Late Permian Supersequence F, and lasted until the end of Triassic deposition. Four supersequences (F–I) have been identified in this phase of basin evolution. The later part of Supersequence F is almost wholly non-marine coal measures. The tectonism and base-level fall that generated the sequence boundary between Supersequences F and G, and ended coal deposition, represents a significant hiatus, and in places is characterised by low-angle unconformity and erosion. The pattern of the remaining units of the foreland basin phase indicates episodic basin filling as accommodation space became available, related to pulses of thrust loading. The regional geometry of the units, and the overstepping of Supersequence G by H, and then by I, indicates continuing westward progradation of foreland loading and corresponding changes in accommodation. It is striking that although the late foreland-loading phase displays the greatest rate of subsidence affecting the basin since the initial rift phase, there is little evidence of widespread flooding by the sea. The reasons were probably that the Late Permian–Early Triassic was a time of global first-order lowstand, and that foreland loading was able to provide simultaneously high rates of subsidence in the basin and greatly increased sediment supply from erosion in the mountains thrust up to the east.

Pre-existing basement structure and its influence on continental rifting and fracture zone development along Australia’s southern rifted margin

Palaeogeographical reconstructions of the Australian and Antarctic margins based on matching basement structures are commonly difficult to reconcile with those derived from ocean-floor magnetic anomalies and plate vectors. Following identification of a previously unmapped crustal-scale structure in the southern part of the early Palaeozoic Delamerian Orogen (Coorong Shear Zone), a more tightly constrained plate reconstruction for these margins is proposed. This reconstruction places the Coorong Shear Zone opposite the Mertz Shear Zone in Antarctica and lends itself to a revised interpretation of continental rifting along Australia’s southern margin in which rift basin architecture, margin segmentation and the formation of ocean-floor fracture zones are all linked to pre-existing basement structure and the reactivation of a few deep-rooted crustal structures inherited from the Delamerian Orogeny in particular. Reactivation of the Coorong Shear Zone and other basement structures (Avoca–Sorell Fault Zone) during the earlier stages of rifting was accompanied by the partitioning of extensional strain and formation of late Jurassic–Early Cretaceous normal faults and half-graben in the Bight and Otway basins with opposing NE–SW and NW–SE structural trends. Previously, the Mertz Shear Zone has been correlated with the Proterozoic Kalinjala Mylonite Zone in the Gawler Craton but this positions Australia 300–400 km too far east relative to Antarctica prior to breakup and fails to secure an equally satisfactory match in both basement geology and the superimposed extension-related structures.

Sequence stratigraphy of the Bowen–Gunnedah and Surat Basins in New South Wales

A new tectonostratigraphic framework for the southern Bowen, Gunnedah and Surat Basins in New South Wales is developed, based on the sequence stratigraphic and structural interpretation of regional reflection-seismic data, and petroleum and stratigraphic wells. This framework provides an improved correlation between the Bowen and Gunnedah Basins, and delineates the relationship between tectonic events, basin phases, and the development of depositional sequences. The Early Permian–Cretaceous depositional history of the basins in this region was controlled by successive phases of tectonic subsidence driven by extension, thermal relaxation and lithospheric flexure, interrupted by periods of contraction and uplift. Early Permian extension was characterised by the eruption of volcanic rocks and the accumulation of dominantly lacustrine sediments, followed by the deposition of thick coal–conglomerate successions, particularly along the eastern margin of the basin. The succeeding thermal subsidence phase was quickly overwhelmed by flexural subsidence driven by foreland loading during the Early–Late Permian, and the deposition of marine, deltaic and non-marine rocks. Late Permian contraction and uplift in the Gunnedah Basin was followed by the accumulation of non-marine Triassic rocks derived from the New England Orogen. A dominantly fluvial Middle Triassic succession is present across the entire region. Effects of the Middle–Late Triassic contractional event that is evident throughout the Bowen–Gunnedah–Sydney basin system were concentrated along the basin-bounding Goondiwindi, Kelvin and Mooki Faults and also adjacent to the Boggabri Ridge.

Giant middle Eocene bryozoan reef mounds in the Great Australian Bight

This paper reports the discovery of extensive middle Eocene bryozoan reef complexes along the paleoshelf edge of the Great Australian Bight (GAB). The complexes form the earliest carbonate deposit in the GAB, which is the largest Cenozoic cool-water carbonate province on Earth. The bryozoan reef mounds, previously misidentifi ed as volcanic bodies, were deposited parallel to the shelf margin for more than 500 km along strike. Individual reef mound complexes are 60?150 km long, as wide as 15 km, and as thick as 200 m, and dwarf all previously described examples. Superimposed on the distal margin of an underlying Paleocene to mid-Eocene siliciclastic delta complex, the reef mounds provide a critical insight into changing paleoenvironments of the Australo-Antarctic Gulf ca. 43 Ma, coinciding with global and continent-wide climatic and tectonic events. The rapid growth and demise of reef mound?building bryozoans raises new questions regarding the interplay of Southern Ocean opening, ocean currents, and biosphere interactions.

Evolution of the Bowen, Gunnedah and Surat Basins, eastern Australia

This thematic issue presents a summary of the key results generated during the National Geoscience Mapping Accord’s project Sedimentary Basins of Eastern Australia, which studied the evolution and petroleum potential of the Early Permian–Middle Triassic Bowen and Gunnedah Basins and the Early Jurassic–Early Cretaceous Surat Basin in Queensland and New South Wales. It was a cooperative project between Geoscience Australia, the Geological Survey of Queensland (part of Queensland Department of Mines and Energy) and the New South Wales Department of Primary Industries— Mineral Resources (including the Geological Survey of New South Wales), with input from petroleum exploration companies, several universities and CSIRO. In addition, the Australian Geodynamics Cooperative Research Centre contributed, through its project on Tectonic framework of eastern Australia, by undertaking quantitative geodynamic modelling and initial construction of a three dimensional geological map of the basins. Key objectives of the project were to: (i) enhance our knowledge of, and develop models for, the origin and evolution of the Gunnedah, Surat, southern Bowen and associated basins in eastern Australia; (ii) relate these models to potential hydrocarbon occurrences as a basis for future exploration and assessment of resources; (iii) update the understanding of the geology of the basins; and (iv) provide information to explain the distribution of known, potential and undiscovered occurrences of fossil fuels. The initial paper in the thematic issue, by Klootwijk on Sedimentary basins of eastern Australia: paleomagnetic constraints on geodynamic evolution in a global context, uses paleomagnetism to relate the evolution of the Bowen, Gunnedah and Surat Basins to major loops in the Australian Phanerozoic pole path. Intraplate, basin-forming events are related to more global interplate tectonic events. The second paper by Waschbusch, Korsch & Beaumont on Geodynamic modelling of aspects of the Bowen, Gunnedah, Surat and Eromanga Basins from the perspective of convergent margin processes applies quantitative geodynamic modelling to examine possible subsidence mechanisms that were operating during the formation of the basins. They focus, in particular, on the foreland basin phase of the Bowen and Gunnedah Basins, demonstrating that the subsidence was driven by both dynamic and static loading, whereas the subsidence of the Surat and Eromanga Basins can be explained by dynamic loading alone. Using a different but complementary approach, Korsch & Totterdell’s paper Subsidence history and basin phases of the Bowen, Gunnedah and Surat Basins, eastern Australia utilises backstripped tectonic subsidence curves, generated from data from petroleum-exploration wells, to examine the subsidence mechanisms operating in the basins, and also the timing of the changes from one mechanism to another. Two-dimensional modelling of gravity data by Krassay, Korsch & Drummond in their paper Meandarra Gravity Ridge: symmetry elements of the gravity anomaly and its relationship to the Bowen–Gunnedah–Sydney basin system demonstrates that the source of the Meandarra Gravity Ridge, one of the major gravity features in eastern Australia, is most likely a dense mafic body in the upper crust, probably mafic volcanics, strongly suggesting a rift origin for the initiation of the basins in the Early Permian. One of the major tasks of the project involved the structural and sequence stratigraphic mapping of a regional grid of seismic reflection data in the Bowen, Gunnedah and Surat Basins (usually 4 s two-way travel time data, with about 15 000 line km of data on about 1200 individual seismic lines). The seismic mapping was used to define the interplate and intraplate tectonic events that have helped to create the accommodation space and also to define the stratal geometry of the sedimentary units. Thus, the mapping provided the overall geometry of the basin system as well as the geometry of several of the sequence boundaries, resulting in the development of a new sequence stratigraphic framework for the basins. These results were also compiled into a series of structure contour and isopach maps, which have been used to build a 3D geological map of the Bowen, Gunnedah and Surat Basins. The next five papers utilise the seismic interpretations to develop an understanding of the stratigraphic sequences and their deformational history. In their paper Early Permian East Australian Rift System, Korsch, Totterdell, Cathro & Nicoll show that the Bowen and Gunnedah Basins initiated in the Early Permian as a series of half-grabens in an extensional backarc setting, in response to far-field stresses that propagated from a west-dipping subduction system at the eastern, convergent margin of Gondwanaland. The paper by Brakel, Totterdell, Wells & Nicoll on Sequence stratigraphy and fill history of the Bowen Australian Journal of Earth Sciences (2009) 56, (271–272)

The 2017 offshore acreage release areas: petroleum geological overview

In 2017, 21 new offshore petroleum exploration areas have been released. The majority of the areas are located along the North West Shelf spanning the Westralian Superbasin from the Bonaparte Basin in the north-east to the Northern Carnarvon Basin in the south-west. New areas have been released in offshore south-eastern Australia with new opportunities provided in the Otway, Bass and Gippsland basins. Two large areas in the northern Perth Basin, an offshore frontier, complete the 2017 Acreage Release. All Release Areas are supported by industry nominations and one new cash bid area has been offered in the Dampier Sub-basin. Geoscience Australia continues to support industry activities by acquiring, interpreting and integrating pre-competitive datasets that are made freely available as part of the agency’s regional petroleum geological studies. A new regional 2D seismic survey was acquired in the Houtman Sub-basin of the Perth Basin, forming the basis of the latest prospectivity study carried out by Geoscience Australia. The results of the study are presented in the technical program of the 2017 APPEA conference. A wealth of seismic and well data, submitted under the Offshore Petroleum and Greenhouse Gas Storage Act 2006 (OPGSSA) are made available through the National Offshore Petroleum Information Management System (NOPIMS). Additional datasets are accessible through Geoscience Australia’s data repository.