Gunnedah Basin Research Articles

A revised Permian–Triassic stratigraphic framework for the northeastern Galilee Basin, Queensland, Australia, and definition of a new Middle–Upper Triassic sedimentary unit

The upper Paleozoic–lower Mesozoic strata in the northeastern Galilee Basin are important for their hydrocarbon and groundwater potential; however, despite numerous previous investigations, the nature of this stratigraphic interval remains enigmatic. This study integrates lithostratigraphy, palynology and U–Pb zircon geochronology of well-exposed Permian and Triassic sedimentary rocks and nearby stratigraphic cores from the Hughenden district to determine their maximum depositional ages and then revises the stratigraphy where necessary. The newly defined Galah Tuff Bed was discovered at the top of the Betts Creek beds in Porcupine Gorge and was dated at 251.9 ± 3 Ma. This provides an important age constraint for the Betts Creek beds and is a tie-point for correlations with coeval units in the adjacent Bowen Basin and the Sydney and Gunnedah basins to the southeast. The Galah Tuff Bed is interpreted to correlate with the Yarrabee Tuff in the Bowen Basin. Unconformably overlying the Betts Creek beds is a newly recognised stratigraphic unit, defined herein as the Porcupine Gorge Formation. Detrital zircon maximum depositional ages and palynology indicate a significant depositional hiatus of up to 20 million years between the Betts Creek beds and Porcupine Gorge Formation. The hiatus indicates that the lower part of the Porcupine Gorge Formation is at least upper Middle Triassic, whereas the upper part of the unit extends into the Late Triassic. This age also constrains the overlying Warang Sandstone, which is considerably younger locally than previously reported and is likely diachronous across the northeastern Galilee Basin. KEY POINTS An integrated lithostratigraphy, palynology and U–Pb detrital zircon geochronology methodology is applied to the Permian–Triassic succession the northeastern Galilee Basin. Two new stratigraphic units were recognised: the upper Permian Galah Tuff Bed (251.9 ± 3 Ma) and the late Middle–Late Triassic Porcupine Gorge Formation (238.7 ± 3 Ma). The Galah Tuff Bed, correlative of the Yarrabee Tuff in the Bowen Basin, is evidence for widespread volcanism in the late Permian. The age of the Porcupine Gorge Formation indicates diachroneity of the Triassic succession in the Galilee Basin, warranting further examination.

Subsidence prediction versus observation in Australia: A short comment

Over-prediction of subsidence levels of mined land may be an issue for getting mining approval and tighter conditions in some places can be imposed to better inform mining operations and planning. Under-prediction of subsidence, present safety, social and environmental consequences during or after longwall extractions have occurred. In this study we collected subsidence data from reports related to underground coal mining environmental impact statements (EIS) and end of panel reports, and compared predicted subsidence data with the observed data to compare the accuracy of prediction. From the limited data, while it was difficult to substantiate a strong relationship between the predicted and observed subsidence data and mining characteristics, we found that where mining is less than 220 m below the surface subsidence prediction generally matches well with the observed subsidence and the ratio is closer to 1 than those mined at greater depths. Subsidence prediction analysed for mines in the Gunnedah Basin are found to be generally better than those in the Sydney Basin.The results presented here are based on limited data available in the public domain and while more data is needed to prove or disprove the discussed hypotheses, we consider there to be value in extending EIS requirements. Importantly, publication of both prediction and observed subsidence measurements would provide suitable transparency for improved, future predictions, and to provide social and stakeholder assurances that would meet growing legislative requirements.

Controls on hydrocarbon accumulation by facies and fluid potential in large-scale lacustrine petroliferous basins in compressional settings: A case study of the Mesozoic Ordos Basin, China

Large-scale lacustrine basins in compressional settings are an important type of petroliferous basin, and the study of the characteristics of hydrocarbon accumulation in such basins is of great significance. This synthesis paper takes the Mesozoic Ordos Basin (central China) as a case study to establish lateral and vertical hydrocarbon accumulation patterns in this basin type, and highlights how facies and fluid potential control hydrocarbon accumulation in such basins. Based on systematic analyses of the Mesozoic source rocks, sedimentary facies, migration and accumulation patterns, and characteristics of the oil reservoirs, the hydrocarbon accumulation characteristics in the Mesozoic Ordos Basin are discussed. The analysis indicates that sedimentary facies controlled the characteristics and distribution of hydrocarbon source rocks and reservoirs, while diagenetic facies controlled the quality and distribution of the reservoirs. Excess pressure and fluid potential controlled hydrocarbon migration directions and paths, whereas facies and potential controlled the locations of oil accumulations and traps. It is concluded that source rocks controlled oil-bearing regions, sedimentary facies controlled oil-bearing plays, and fluid potential controlled the locations of accumulations. For shallow intervals, because fluid migration is mainly controlled by buoyancy, paleotopographic highs played an important role in controlling hydrocarbon accumulations. For deep layers (Chang 7 to Chang 10 members), since buoyancy is less than the capillary pressure, excess pressure caused by the undercompaction of mudstone and hydrocarbon generation at depth provides the driving force for lateral and vertical migration of oil, and becomes the main migration force. The reservoir accumulation characteristics differ laterally between regions of the basin; at the periphery of the basin, reservoir units are stacked in a step-wise manner, whereas at the center of the basin, multiple reservoir intervals are vertically stacked in the form of multistorey accumulations. Vertically, layers with different burial depths have different accumulation characteristics; the paleotopography controlled hydrocarbon accumulation in shallow intervals; instead excess pressure, fluid potential, and paleotopography controlled accumulation in deep intervals. This understanding could be used to inform prospecting in other large-scale lacustrine petroliferous basins in compressional settings such as the Eocene Green River Basin in the U.S.A. and the Middle Triassic Gunnedah Basin in Australia.

Warrumbungle Volcano: facies architecture and evolution of a complex shield volcano

We present a new geological map of Warrumbungle Volcano created from volcanic facies field mapping, new geophysical, geochemical and geochronological data as well as data from previous studies. Field mapping and petrography defined 19 volcanic and two mixed volcanic--sedimentary facies. Facies identification and distribution in conjunction with geochemical analyses indicate an early shield-forming phase of predominantly mafic and intermediate lavas and pyroclastic deposits, and minor felsic lavas deposited on an irregularly eroded basement of Surat and Gunnedah basin rocks. The shield was subsequently intruded by felsic intermediate to felsic magmas that formed dykes and other intrusions, including possible cryptodomes, and also erupted as lava domes and block-and-ash-flow deposits. A radial dyke swarm cross-cuts most units, although it is concentrated within basement sandstone surrounding the central area of the volcano, suggesting late inflation accompanied by dyke emplacement. Geochemistry indicates differentiation of a single although repeatedly recharged alkaline magmatic suite. Fractionation of olivine, Ti-magnetite and clinopyroxene occurred in mafic magmas, and after reaching 62 wt% SiO2, crystallisation of apatite and alkali feldspar took place. A new U–Pb zircon SHRIMP magmatic crystallisation age of 16.25 ± 0.12 Ma on a felsic block-and-ash flow deposit is in agreement with the recalculated 40Ar/39Ar isochron dates of previous workers. Based on our mapping and the use of volcanic facies to define mappable units, we recommend that the previous Warrumbungle Volcanics be elevated from formation to group status and renamed the Warrumbungle Volcanic Complex. KEY POINTS New geological map of Warrumbungle Volcano based on volcanic facies mapping. The volcanic facies and their distribution suggest a complex alkaline shield volcano erupted along regional-scale basement lineaments over an intracontinental ‘hot-spot’ or heat plume, and intruded by lava domes, cryptodomes, dykes and a radial dyke swarm. New and previous geochemical data suggest differentiation of a single, although repeatedly recharged, alkaline magmatic suite. A new U–Pb zircon SHRIMP date of 16.25 ± 0.12 Ma is in agreement with the recalculated 40Ar/39Ar isochron dates of previous workers.

Integrated Earth Data Interpretation Workflow - A Recipe for Success in Onshore Frontier Hydrocarbon Exploration

The interior of Australia plays host to a series of vast sedimentary basins spanning c. 2.5 billion years of the island continents geological history. Many of these basins contain significant reserves of both conventional and unconventional hydrocarbons. In addition to being active offshore, Santos Ltd has a long history of hydrocarbon exploration (generally as Operator) in several of the onshore basins, which notably include the Bowen, Gunnedah, Cooper, Amadeus and McArthur Basins. Frontier exploration involves various regional geological studies, these being geared towards deriving an early assessment of hydrocarbon potential and directing the geographical focus of future exploration work. Due to the general lack of data in the early exploration phase, much effort is expended in maximising the interrogation and understanding of all available open source and proprietary datasets. These typically include surface geology, surface elevation, surface vegetation, Landsat, gravity, magnetics, radiometry, existing seismic and existing boreholes (including water bores). The key to extracting every ounce of useful geological information from these data is through data integration and co-visualisation. To this end, Santos has developed a tried-and-tested regional exploration workflow, which often involves some novel visualisation techniques (e.g. use of 3D anaglyphs). This presentation will include discussion of basic workflows, the various data types and principles of data integration and interpretation, illustrated with numerous real-world examples from Santos’ extensive exploration experience in several of the aforementioned basins.

Quartz-Helium Method to Estimate Fluid Flow in Thick Aquitards, Gunnedah Basin, Australia.

The hydraulic integrity of aquitards is generally assumed and relies on a few core‐scale permeability measurements, drill‐stem tests, or textbook values. This approach is because hydraulic data across the full aquitard thickness is generally lacking. Proper assessment of aquitard integrity should be studied at the formation (spanning its entire thickness at a single point) or regional (formation properties at multiple locations throughout the basin) scale. One formation‐scale approach uses environmental tracers and advection‐dispersion modeling to constrain fluid flow rates. This study demonstrates the use of helium concentrations in quartz as a method of constraining the rate of fluid flow in a 520‐m thick aquitard in the Gunnedah Basin, NSW, Australia. Quartz was separated from existing core samples in the Watermark and Porcupine Formations at depths from 750 to 1200 m. The helium was released from these samples by heating and select samples were impregnated with helium to determine the rate of helium diffusion through the quartz. One‐dimensional advection‐dispersion modeling of the helium profile accounting for diffusive helium exchange between quartz and pore water revealed, that (1) vertical fluid velocity has been on the order of 0.02 mm/year or less for tens to thousands of years, (2) helium is in equilibrium between quartz and pore water, and (3) the helium profile is transient indicating that helium concentrations in the underlying Maules Creek Formation has varied over geological time. Further modeling identified aquitard conditions (thickness and temperature) for which equilibrium exists, a precondition for deriving formation‐scale permeability.

The effect of magmatic intrusions on coalbed methane reservoir characteristics: A case study from the Hoskissons coalbed, Gunnedah Basin, Australia

Magmatic intrusions can deteriorate coalbed methane reservoir quality by precipitating minerals in natural fracture and cleat system. To date, the effect of magmatic intrusions on coal rank and maturity has been studied extensively. However, their impact on fluid flow capacity and gas content is poorly investigated. This study evaluates the impact of magmatic intrusions on reservoir characteristics of the Hoskissons coal interval in the Gunnedah Basin (eastern Australia) where numerous coal-intrusion associations exist. Drill stem test (DST), borehole image logs and core data are used to determine fluid flow characteristics, gas content and quality in 14 wells across the Gunnedah Basin. The integration of borehole image logs and DST data analysis enables us to determine the existence, openness, and hydraulic conductivity of natural fracture and cleat systems in the Hoskissons coal interval. In addition, available desorption canister data, gas composition data, and conventional well logs are interpreted to investigate probable thermal effect on gas content.Our analyses of different datasets reveal that the thickness of intrusions and their positions with respect to the Hoskissons coal interval are variable in the studied wells. Permeability varies from 1091mD down to zero owing to heterogeneous fracture and cleat systems. Interpreted natural fracture/cleat systems are well correlated with measured permeability from DST data analysis. This highlights the role of open natural fractures/cleats in fluid flow characteristics of the Hoskissons coal interval. Results indicate that the mineralizing effect of hydrothermal fluids derived either from magmatic intrusions or coal itself is not a controlling factor in fluid flow capacity of the Hoskissons coal interval in the studied wells. This is described by either the distance between coal section and major intrusions in some wells or perhaps emplacement of intrusive bodies prior to development of cleat and natural fracture networks. The destructive impact of intrusions on permeability is observed in one of the studied wells in which in-situ stress perturbation is large (due to presence of magmatic intrusions in sedimentary rocks). Variable in-situ stress orientation can decrease fracture connectivity and consequently fluid flow properties are affected. Gas content largely varies in heat affected coal intervals. This signature is the result of thermal effect fading with distance and is more pronounced when intrusions are in close proximity to coal intervals. Gas composition is variable in the studied wells. Gas composition data indicate that high quality desorbed gas with methane concentration higher than 90% could be found even in coal intervals which are heavily intruded.

Cisuralian coals of the Mullaley Sub-basin, northern New South Wales

ABSTRACTThe Mullaley Sub-basin of the Gunnedah Basin extends from Quirindi in the southeast, to north of Narrabri, to west of Dunedoo in northern New South Wales. There have been more than 100 boreholes sunk to basement investigating the (lower Permian) Cisuralian coal and coal seam gas resources of the Mullaley Sub-basin since the early 1990s. A desktop review of this open file information has allowed the formal correlation and naming of six Cisuralian coal members attaining a maximum 35 m of cumulative thickness within an upward coarsening sedimentary package totalling no more than 150 m. In ascending order, the coal members are: Bibblewindi (0–10 m), Bohena (3–18 m), Collygra (0.5–3 m), Coxs (1.5–4 m), Tullamullen (0.5–4 m) and Mooki (0.5–3 m).Cisuralian coal seams in the Maules Creek Formation of the southern Mullaley Sub-basin are here correlated with those of the Greta Coal Measures at Werris Creek and Muswellbrook. It is apparent that basement paleotopography played a significant role in the Cisuralian coal development as coals are best developed where the sedimentary sequence is greater than 60 m thick, as there the thick seams (Bohena and Bibblewindi coal members) occur towards the base of the sequence. The maximum western limit of the Cisuralian coals (Rocky Glen Ridge) is further east than previously inferred with new drilling information showing the Porcupine Formation directly overlying the barren pelletoidal claystones of the Leard Formation or the underlying volcanics (Boggabri Volcanics/Werrie Basalt). Early marine transgressions at the top of the Maules Creek Formation have stopped development of the Mooki, Tullamullen and Coxs coal members in the northern and eastern Mullaley Sub-basin and allowed the development of localised paraconglomerate (diamictite) intervals up to 10 m thick. Thick (>20 m cumulative) coal occurrences are localised to the Jacks Creek and Pilliga East State Forest areas southwest of Narrabri. The coal resource potential of the Mullaley Sub-basin is estimated at 13–28 billion tonnes.

The present-day stress field of New South Wales, Australia

ABSTRACTThe Australian continent displays the most complex pattern of present-day tectonic stress observed in any major continental area. Although plate boundary forces provide a well-established control on the large-scale (>500 km) orientation of maximum horizontal stress (SHmax), smaller-scale variations, caused by local forces, are poorly understood in Australia. Prior to this study, the World Stress Map database contained 101 SHmax orientation measurements for New South Wales (NSW), Australia, with the bulk of the data coming from shallow engineering tests in the Sydney Basin. In this study we interpret present-day stress indicators analysed from 58.6 km of borehole image logs in 135 coal-seam gas and petroleum wells in different sedimentary basins of NSW, including the Gunnedah, Clarence-Moreton, Sydney, Gloucester, Darling and Bowen–Surat basins. This study provides a refined stress map of NSW, with a total of 340 (A–E quality) SHmax orientations consisting of 186 stress indicators from borehole breakouts, 69 stress measurements from shallow engineering methods, 48 stress indicators from drilling-induced fractures, and 37 stress indicators from earthquake focal mechanism solutions. We define seven stress provinces throughout NSW and determine the mean orientation of the SHmax for each stress province. The results show that the SHmax is variable across the state, but broadly ranges from NE–SW to ESE–WNW. The SHmax is approximately E–W to ESE–WNW in the Darling Basin and Southeastern Seismogenic Zone that covers the west and south of NSW, respectively. However, the present-day SHmax rotates across the northeastern part of NSW, from approximately NE–SW in the South Sydney and Gloucester basins to ENE–WSW in the North Sydney, Clarence-Moreton and Gunnedah basins. Comparisons between the observed SHmax orientations and Australian stress models in the available literature reveal that previous numerical models were unable to satisfactorily predict the state of stress in NSW. Although clear regional present-day stress trends exist in NSW, there are also large perturbations observed locally within most stress provinces that demonstrate the significant control on local intraplate sources of stress. Local SHmax perturbations are interpreted to be due to basement topography, basin geometry, lithological contrasts, igneous intrusions, faults and fractures. Understanding and predicting local stress perturbations has major implications for determining the most productive fractures in petroleum systems, and for modelling the propagation direction and vertical height growth of induced hydraulic fractures in simulation of unconventional reservoirs.

Palynofacies changes and their reflection on preservation of peat accumulation stages in the Late Permian coal measures of the Bowen Basin, Australia: A new system for coal palynofacies characterisation

A high-resolution study (n=66) was conducted on Late Permian coal measures of the Bowen Basin, Australia, to observe palynofacies changes during peat accumulation over time. Palynofacies were determined using polytopic vector analysis (PVA) to derive end members with variable compositions that reflected fine scale environmental changes, within and between seams, of the Kaloola Member. Palynofacies reflect a three-stage succession within the palaeo- peat-forming environments from stable, anoxic conditions at the base, progressing to variable, oxic conditions toward the top of each seam. In cases where a seam was capped by tuff, rather than siliciclastic sediment, anoxic conditions similar to the bottom of each seam occurred. The results corroborate previous interpretations of palaeofloral communities and provide a new palynofacies model that integrates environmental progression within the coal seams in the Late Permian coal measures of the Bowen Basin. A complementary study of palynological assemblages recognised a shift from Dulhuntyspora sp., to Protohaploxypinus sp. in the lower Kaloola Member, indicating a change in pollen/spore source. Associations between dispersed palynomorphs and coal phyterals of parent flora remain tenuous beyond class rank, due to poor preservation and taxonomic bias inherent within the sample space. This is the first such palynofacies study to be conducted in the Bowen–Sydney–Gunnedah basin complex, thus opening the way for future work examining regional changes in palynofacies, expressed in both coal and siliciclastic sediments.

CO 2 degassing and trapping during hydrothermal cycles related to Gondwana rifting in eastern Australia

Intensive carbonate and clay mineral authigenesis took place throughout the Late Permian Bowen–Gunnedah–Sydney basin system in eastern Australia. We conducted isotopic and trace element analyses of carbonate and clay minerals from clastic sedimentary rocks of the Gunnedah Basin and the Denison Trough in the Bowen Basin. Rb–Sr isochron age data of the illitic clays are consistent with episodic hydrothermal fluid flow events that occurred in association with Gondwana rifting accompanied by alkaline magmatism at ∼85 Ma and ∼95 Ma. Stable isotope data of carbonate and clay minerals from the Gunnedah Basin are indicative of meteoric waters from a high-latitude environment as the main fluid source, whereas trace element, Sr and Nd isotope data highlight mixing of meteoric fluids with magmatic and/or crustal components, with a possible input from marine carbonates for some samples. Trace metals, oxygen and strontium isotopes of dawsonites from the Denison Trough are interpreted to have been mobilised by fluids that interacted with evolved clastic sedimentary and marine carbonate end members. According to the carbon isotope data, CO 2 for calcite and ankerite precipitation was sourced mainly from thermal degradation of organic matter and magmatism, whereas the CO 2 used for dawsonite formation is inferred to have been derived from magmatic and marine sources. In the low permeability environments (particularly in coal seams), the increasing accumulation and oversaturation of CO 2 particularly promote the precipitation of dawsonite.

Orogenesis without collision: Stabilizing the Terra Australis accretionary orogen, eastern Australia

The Neoproterozoic to end-Paleozoic Terra Australis orogen extended along the Gondwana margin of the paleo–Pacific Ocean, and it now provides a detailed record of orogenic activity and continental stabilization within an ongoing convergent, accretionary plate margin. New geochronological data from end-Paleozoic plutonic and volcanic rocks associated with the Gondwanide orogeny in the New England region of eastern Australia, integrated with information on the nature and timing of associated sedimentation, deformation, and metamorphism, allow resolution of a high-fidelity record of orogenesis. At the end of the Carboniferous, around 305 Ma, convergent margin magmatism, which had been active along the western margin of the New England region, terminated and was followed by a short pulse of regional compressional deformation and metamorphism, marking the commencement of the Tablelands phase of Gondwanide orogenesis. Deformation was almost immediately followed by the onset of clastic sedimentation and local calc-alkaline volcanism, dated at 293 Ma, in the extensional Barnard Basin. Emplacement of the two New England S-type granitic suites, the Bundarra and the Hillgrove suites, along with localized high-temperature, low-pressure metamorphism, was essentially contemporaneous, ranging in age from 296 to 288 Ma, and overlapped in time with I-type magmatism and the switch from regional compression to extension and Barnard Basin rifting. The Hunter-Bowen phase of the Gondwanide orogeny commenced with contractional deformation, resulting in termination of sedimentation in the Barnard Basin and regional deformation and metamorphism across New England and into the Sydney and Gunnedah basins to the west at around 265–260 Ma. Contractional loading of the Sydney and Gunnedah basins resulted in their conversion from extensional to foreland basins, which received ongoing pulses of sediment from the New England orogenic welt until 230 Ma. The Hunter-Bowen phase was associated with widespread I-type plutonism and volcanic activity in New England that ceased around 230 Ma, marking the termination of Gondwanide orogenesis. Orogenesis occurred in an evolving convergent plate-margin setting. S- and I-type magmatic activity ranging in age from ca. 300 to 230 Ma represents a stepping out of arc magmatism from the western margin of New England (prior to 305 Ma) into the preexisting arc-trench gap. There is no evidence that deformation was related to the collision of the convergent margin with a major lithospheric mass, and the widespread development of extensional basins in the eastern third of Australia in the Early Permian indicates control by phenomena acting on a continental scale, probably changing plate kinematics associated with the amalgamation of Pangea.

Adsorption and mineral trapping dominate CO2 storage in coal systems

High-CO2 coal seams occur in many sedimentary basins world-wide and provide natural analogues of the processes likely to occur as a result of CO2 injection and storage in coal systems. CO2 is stored in coal predominantly as adsorbed molecules on micropore surfaces (adsorption trapping) that allow higher densities and greater volumes of CO2 at shallower depths than in sandstone and carbonate reservoirs. In the longer term CO2 will dissolve in formation water and react with minerals in the host formation (solubility/ionic trapping) and may be precipitated as carbonate minerals (mineral trapping). A recent study suggests that solubility trapping is the predominant CO2 sink in natural gas fields (Gilfillan et al., 2009), whereas our natural analogue studies in the Bowen and Gunnedah Basins, eastern Australia, indicate that CO2 has been stored in coal and sandstone formations since the Mesozoic through a combination of adsorption and mineral carbonation reactions. These differences reflect the different mineralogies and hydrogeology of the CO2-rich natural gas fields studied by Gilfillan et al. (2009) and coal systems. Our work shows that natural analogue studies of sites that have stored CO2 over geological time scales are essential to determine the mechanisms of CO2 storage and the level of characterisation required for storage site equivalents. (C) 2011 Published by Elsevier Ltd.

Managing the cumulative impacts of coal mining on regional communities and environments in Australia

The expansion and contraction of the coal mining industry in Australia has placed pressure on regional communities and environments and multiplied the extent, magnitude and profile of cumulative impacts. While some mining communities have benefited from the expansion of the coal industry through the creation of jobs and the investment in economies, the compounding impacts of multiple mining operations have stretched environmental, social, human and economic systems and rendered conventional mine-by-mine governance approaches ineffective. In this paper we draw from examples in the Bowen Basin, Hunter Valley and Gunnedah Basin to traverse the range of cumulative impacts resulting from mining activities, and detail working examples of management and assessment practices that aim to enhance positive, and avoid and mitigate negative, cumulative impacts.

Gunnedah Basin 3D architecture and upper crustal temperatures

The Gunnedah Basin in New South Wales has long been an important coal and gas resource, but limited information exists on the temperature structure or crustal architecture at depth to enable development of its geothermal potential. Here we combine gravity modelling, seismic-reflection surveys and borehole drilling results to develop a 3D depth to basement structural map and geological model of the basin. The 3D structure of the Gunnedah Basin is characteristic of a typical intracontinental rift basin. Gravity modelling of the Lachlan Fold Belt basement, using borehole and seismic-reflection controls, shows a 2–3.5 km-deep approximately north–south-oriented channel between the basement highs of the Rocky Glen Ridge in the west and Boggabri Ridge in the east. Extensional basal volcanics during the Late Carboniferous–Early Permian fill this channel. Borehole data and gravity modelling show up to 1 km of Permian to Jurassic sedimentary rocks overlying the rift volcanics. Preliminary thermal modelling, incorporating the geological model and limited deep borehole temperatures, indicates temperatures at the top of basement are in the range 105–165°C.

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.

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.

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.

Meandarra Gravity Ridge: symmetry elements of the gravity anomaly and its relationship to the Bowen–Gunnedah–Sydney basin system

The Meandarra Gravity Ridge is one of the major gravity features in eastern Australia. The source(s) for this gravity anomaly is problematic because the ridge, a gravity high, coincides for much of its length with the thickest parts of the Bowen and Gunnedah Basins, areas that would normally be expected to be regional gravity lows. Furthermore, the Meandarra Gravity Ridge is continuous across numerous structural elements and regional terranes, and there is no obvious expression of its source in the surface geology. Construction of a series of gravity profiles across the Meandarra Gravity Ridge in this study has provided detailed analyses of changes in the size and shape of the Bouguer anomaly along the ridge axis. These data suggest that the ridge is composed of a series of offset colinear anomalies, and demonstrate that the magnitude and symmetry of the Bouguer anomaly are different for each segment of the ridge. 3D-mesh diagrams of the total-field Bouguer gravity illustrate that the ridge may be subdivided into at least five segments, each of which is characterised by particular gravity values and shapes for the anomaly profile. 2D gravity modelling of selected Bouguer anomaly profiles suggests that the source for the Meandarra Gravity Ridge anomaly is a dense mafic body in the upper crust. This source has a preferred density contrast of +0.25 tm−3, and probably consists of a pile of mafic volcanics up to 9 km thick. The size and geometry of the modelled anomalous mass supports a rift-type origin for the source of the Meandarra Gravity Ridge. Our modelling predicts that the magnitude and symmetry of the Meandarra Gravity Ridge gravity anomaly are controlled by the geometry of an underlying mafic source and not by interference effects due to density contrasts between sedimentary and metamorphic rocks of adjacent orogens.

Geodynamic modelling of aspects of the Bowen, Gunnedah, Surat and Eromanga Basins from the perspective of convergent margin processes

Geodynamic modelling of selected aspects of the Bowen, Gunnedah, Surat and Eromanga Basins provides possible explanations of the mechanisms that were operating during their formation. For the Bowen and Gunnedah Basins, a quantitative analysis of the early Late Permian to Middle Triassic foreland loading phase examined the relative roles of static loading vs dynamic loading associated with the convergent plate margin. Subsidence in the initial foreland phase in the early Late Permian is consistent with platform tilting due to corner flow in the mantle associated with west-directed subduction. Later in the Late Permian, platform tilting probably continued to be the dominant cause of subsidence, but increasing amounts of subsidence due to foreland loading occurred as the thrust front in the New England Orogen migrated westward. In the latest Permian and Early Triassic, static flexural loading due to foreland loads is dominant and may be the sole cause for basin subsidence. For the Surat and Eromanga Basins, the tectonic subsidence across an east–west transect is modelled to assess the contribution of dynamically induced platform tilting, due to viscous mantle corner flow, in basin subsidence. The modelling suggests that subsidence was again controlled by dynamic platform tilting, which provides a mechanism for both the near-field and far-field effects. Uplift of the Eastern Highlands in the mid-Cretaceous may also be related to viscous corner flow driven by west-directed subduction beneath eastern Australia, with the uplift being due to rebound of the lithosphere after the cessation of subduction.