Gunnedah Basin Research Articles

PROSPECTIVITY OF THE OFFSHORE SYDNEY BASIN: A NEW PERSPECTIVE

Approximately 40 per cent of the 52,000 km2 Sydney Basin lies in shallow waters (less than 200 m) off the central New South Wales coast. Containing more than 5,000 m of Permo-Triassic marine and non-marine sediments, and having been the subject of several previous exploration campaigns, no wells have been drilled in the offshore despite widespread numerous occurrences of oil and gas onshore.The Sydney Basin, together with the Bowen and Gunnedah basins, form a major longitudinal Permo-Triassic basinal complex stretching 2,500 km down the eastern margin of Australia. Whereas the onset of this basinal development may have been extensional, reinterpretation of seismic and other geophysical data highlight the potential role played in the early development of the Sydney Basin by easterly directed compression. A compressional style is to be contrasted with the dominantly extensional style interpreted by others for the adjacent onshore areas. The most conspicuous structural element in the offshore, the Offshore Uplift, is interpreted to represent the western overthrust edge of the Currarong Orogen. Accepting the Panthalassan margin geometry of Veevers and Powell (1994) it follows that the Offshore Uplift and restored Dampier Ridge would have constituted a 'greater Currarong Orogen'. A series of progressive westerly directed thrust fronts may have been established across the Panthalassan margin, including the uplifted western margin of the Currarong Orogen, which over-rode and created a thrust load onto the eastern margin of the Lachlan Fold Belt. Much of the Early Permian development of the Sydney Basin therefore could have resulted as a consequence of foreland loading. This is consistent with depositional trends including the overall westerly directed marine transgression which dominated the sedimentary record of the Early Permian. Alternatively, this marine transgression may represent the sag phase induced along a segment of the Bowen-Sydney rift system that had been offset by the Hunter River Transverse Zone from the Gunnedah Basin to a site coincident with the Offshore Syncline.Previous interpretations identified structural development of the Currarong Orogen as either a Cretaceous (Tasman Sea rift related) or Middle to Late Permian phenomena. Early Permian structural growth of the offshore Uplift has important implications for petroleum exploration. The major impediment to exploration appears to be the perception that the Sydney Basin lacks suitable reservoir targets and is gas-prone. Potential source and seal sequences occur extensively within both Early Permian marine shales and siltstones and Early and Late Permian coal measure sequences. The emerging uplift provided a major sediment provenance area and represented a barrier behind which restricted anoxic conditions flourished, conditions favouring the preservation of organic matter. Late Permian and Triassic sequences are absent across the crestal portions of the uplift. However, the emerging, sea-ward facing flank of the uplift would have been subject to marginal and shallow marine, wave-base, barrier and strand bar deposition during the Lower Permian, conditions known in the onshore to favour better reservoir development.Gas demand to the greater Sydney region is anticipated to exceed supply by the year 2000, and new gas markets are being eagerly sought in time for the expiration, in 2006, of the current contract under which gas is supplied to Sydney via the Moomba pipeline.Cretaceous, Tasman Sea rift related, structuring is subordinate to that of the earlier compressional and wrench related structuring. Several new structural targets have been added to the existing inventory of prospects and leads, including some now considered optiminally located with respect to source rock and reservoir development.

Authigenic fluid inclusions in lithic sandstone: A case study from the Permo‐Triassic Gunnedah Basin, New South Wales

The Gunnedah Basin is a structural subdivision of the Permo‐Triassic Sydney‐Bowen Basin, a major Permian coal province in eastern Australia. Coarse‐grained sediments of the Permian Maules Creek, Porcupine and Watermark Formations within the basin are predominantly volcanolithic to quartz‐lithic conglomerate and sandstone that were deposited in alluvial, marine fan delta and marine shelf environments. Triassic strata of the Digby and Napperby Formations were deposited in alluvial and lacustrine environments respectively. Fluid inclusions in authigenic quartz overgrowths and healed grain fractures identified in sandstone from these rocks comprise four distinct types (Types 1–4) generated during three discrete diagenetic episodes (GI [oldest]‐GIII [youngest]). Two of the inclusion types (Types 3 and 4) are hydrocarbon‐bearing and their distribution indicates that migration of hydrocarbons occurred late in the diagenetic history of the rocks, although homogenisation temperatures imply that emplacement of these inclusions pre‐dated maximum burial temperature. Microthermometric analysis of 57 aqueous inclusions gave homogenisation temperatures ranging from 62°C to 140°C. The majority of samples are now located at depths shallower than those at which the fluids were entrapped and data are used to constrain the palaeothermal and tectonic history of the basin. Freezing runs yielded first melts at between −65°C and −31°C suggesting the presence of a Ca‐Na‐Cl system. Ice melts ranged from −8.5°C to −0.2°C and are interpreted to represent much lower salinities than would have been present in the original marine depositional environment of the Porcupine and Watermark Formations.

Utility of Coal Seams as Genetic Stratigraphic Sequence Boundaries in Nonmarine Basins: An Example from the Gunnedah Basin, Australia: Reply

Utility of Coal Seams as Genetic Stratigraphic Sequence Boundaries in Nonmarine Basins: An Example from the Gunnedah Basin, Australia: Reply

Integrated Fluid History Investigations in the Sydney and Gunnedah Basins, Australia: ABSTRACT

Integrated Fluid History Investigations in the Sydney and Gunnedah Basins, Australia: ABSTRACT

Utility of Coal Seams as Genetic Stratigraphic Sequence Boundaries in Nonmarine Basins: An Example from the Gunnedah Basin, Australia: Discussion

Utility of Coal Seams as Genetic Stratigraphic Sequence Boundaries in Nonmarine Basins: An Example from the Gunnedah Basin, Australia: Discussion

Utility of Coal Seams as Genetic Stratigraphic Sequence Boundaries in Nonmarine Basins: An Example from the Gunnedah Basin, Australia

Concepts of genetic sequence stratigraphy evolved in marginal marine basin settings where widespread episodic marine flooding events provide readily recognizable correlative surfaces that bound successive progradational clastic wedges. However, application of these concepts to nonmarine aggradational basins has been limited. In cases where marine flooding surfaces are absent, different criteria for sequence recognition are necessary. This paper addresses the utility of regionally extensive coal seams as one type of sequence boundary. Peat accumulation and preservation as coal can only occur in the absence of significant clastic deposition. Formation of coal seams of regional extent requires processes capable of interruption in sediment supply at a basin-wide scale and can define times of major reorganization in basin tectonics or climate. Regionally extensive coals can exhibit the essential attributes of sequence boundaries. In many cases, they are readily recognized and correlated by their unique seam signature, or profile, which is a function of the original peat-forming plants and the physical and chemical conditions imposed during peat swamp evolution. The unique seam profile also records numerous isochronous events, further emphasizing their time equivalence and potential for accurate dating. The concept that regionally extensive coals can serve as genetic stratigraphic sequence boundaries is well illustrated by coal seams from the Gunnedah basin, Australia. The Hoskissons Seam marks the initial change to nonmarine conditions in the basin from underlying deltaic and shallow marine depositional systems. The stratigraphically higher Breeza Seam divides the nonmarine sequence into an underlying quartzose, lacustrine, bed load fluvial complex and an overlying volcanic-lithic, mixed load fluvial, alluvial fan complex.

POTENTIAL PETROLEUM SOURCE ROCKS IN TRIASSIC LACUSTRINE‐DELTA SEDIMENTS OF THE GUNNEDAH BASIN, EASTERN AUSTRALIA

Integration of lithofacies, inferred depositional environments and organic petrology of a lacustrine delta sequence in the Permo‐Triassic Gunnedah Basin of eastern Australia has provided a logical explanation for the patterns of distribution of the dispersed organic matter. The data show that: (i) sedimentary processes, such as size sorting and transportation, control the association of maceral assemblages in the dispersed organic matter with depositional environments; (ii) the extent of oxidation of organic matter increases downstream, from flood‐plain lake to interdistributary bay, delta front, prodelta and off‐shore lake deposits; (iii) coarse, woody material, mostly oxidised to semifusinite, is concentrated in the delta‐front environment; (iv) leaf material is transported preferentially to the prodelta, via the delta front. On the basis of organic matter type, the prodelta was probably the most favourable environment in this depositional system for the accumulation of oil‐prone source beds.

POTENTIAL PETROLEUM SOURCE ROCKS IN TRIASSIC LACUSTRINE-DELTA SEDIMENTS OF THE GUNNEDAH BASIN, EASTERN AUSTRALIA

Integration of lithofacies, inferred depositional environments and organic petrology of a lacustrine delta sequence in the Permo-Triassic Gunnedah Basin of eastern Australia has provided a logical explanation for the patterns of distribution of the dispersed organic matter. The data show that: (i) sedimentary processes, such as size sorting and transportation, control the association of maceral assemblages in the dispersed organic matter with depositional environments; (ii) the extent of oxidation of organic matter increases downstream, from floodplain lake to interdistributary bay, delta front, prodelta and off-shore lake deposits; (iii) coarse, woody material, mostly oxidised to semifusinite, is concentrated in the deta-front environment; (iv) leaf material is transported preferentially to the prodelta, via the delta front. On the basis of organic matter type, the prodelta was probably the most favourable environment in this depositional system for the accumulation of oil-prone source beds.

Sedimentological and Geostatistical Characterization of Reservoir Heterogeneities of the Triassic Fluvial and Lacustrine-Deltaic Sequences, Gunnedah Basin, NSW.

Sedimentological and Geostatistical Characterization of Reservoir Heterogeneities of the Triassic Fluvial and Lacustrine-Deltaic Sequences, Gunnedah Basin, NSW.

Sediment Input and Evolution of Deltaic Systems in the Triassic Napperby Formation, Gunnedah Basin, N.S.W.: Implications for Reservoir Description and Characterization of Deltaic Systems

Sediment Input and Evolution of Deltaic Systems in the Triassic Napperby Formation, Gunnedah Basin, N.S.W.: Implications for Reservoir Description and Characterization of Deltaic Systems

Organic petrological composition of Triassic source rocks and their clastic depositional environments in some Australian sedimentary basins

Kerogen from terrestrial plant debris (type III) has commonly been considered to be a good source for hydrocarbon gas, but not for oil, compared with types I and II kerogen from marine and lacustrine sediments. The Gippsland Basin, Australia, contains giant oil fields producing from organic matter of land plant origin. Clearly some terrestrial paleodepositional environments have produced organic matter of land plant origin with the potential to generate large volumes of oil. An attempt has been made here to identify some environments that contain organic matter of terrestrial origin with the potential to generate oil. The dispersed organic matter (DOM) in sediments from various paleodepositional environments in the Northern Carnarvon, Clarence-Moreton, Simpson Desert, Bowen and Gunnedah Basins of Australia has been analysed petrographically. To reduce variations in organic matter type due to differences in geological age, examples of Triassic age only have been compared. DOM with relatively high contents of liptinite, which is widely accepted as having a better potential to generate oil than vitrinite and inertinite, was found in the following environments: fluvio-deltaic (Bowen Basin), proximal lacustrine (Gunnedah Basin) and fluvio-deltaic (Northern Carnarvon). Relationships between Triassic DOM types and paleodepositional environments found in one basin did not necessarily hold true for other basins. It is not valid to infer a unique paleodepositional environment from DOM type, but within a given basin, DOM type may be predicted from environment.

Genetic stratigraphy of the Gunnedah Basin, NSW

A genetic approach to stratigraphic analysis which recognizes clastic sediments as facies components of sedimentary basins was applied to the Permo‐Triassic Gunnedah Basin sequence. The genetic units of widespread correlatability are unified by a common origin and deposited during discrete episodes of genera! tectonic, climatic and/or base level stability. Such units are the fundamental time stratigraphic increments of the basin‐fill and form the most logical basis for lithostratigraphic subdivision. Six depositional episodes and their stratigraphic products, the depositional systems, are recognized from detailed analysis of drill core and geophysical well logs within the Gunnedah Basin. Contrasts of regional extent in the gamma and neutron logs proved essential to the delineation of the depositional systems. The onset of sedimentation in the basin is recorded by alluvial and lacustrine deposition of the Leard‐Maules Creek alluvial/lacustrine system. The basin was then inundated by a regional transgressio...

Stable-isotope evidence for low-temperature kaolinitic weathering and post-formational hydrogen-isotope exchange in permian kaolinites

Depleted δ 18O- and δD-values of +6.1 to +9.4‰ and −107 to −94‰, respectively, have been obtained from kaolinite samples of early Permian (Sakmarian-Artinskian) age from the Gunnedah Basin, eastern Australia. The samples come from kaolinite clayrocks representing both in situ kaolinitic weathering profiles and the eroded and re-deposited products of this weathering developed in the Quirindi-Wingen and Boggabri-Gunnedah areas of New South Wales, Australia. The clayrocks were produced by the intense surficial weathering of an underlying volcanic sequence, with subsequent erosion and re-deposition in basins flanking the volcanic pile. The calculated isotopic composition of meteoric water in equilibrium with kaolinite from the clayrocks had δ 18O of ⩽ −17‰ and δD of ⩽ −125‰, indicating polar or sub-polar temperatures of condensation. As kaolinite formation cannot occur below the freezing point of water, the highly depleted isotopic results obtained from the samples suggest equilibrium with waters partly derived from deglaciation of the Gondwanan landmass, an event which consequently must have been well underway in eastern Australia by the Sakmarian-Artinskian. The climate prevailing at the time of kaolinite formation is unlikely to have been warmer than cool temperate, a conclusion which is difficult to reconcile with the classical interpretation of kaolinitic weathering as a tropical or subtropical phenomenon. This suggests that high rates of infiltration, efficient leaching of soluble cations, and good drainage are of more importance to kaolinite formation than the temperature at which the process operates. Oxygen-isotope exchange in the kaolinite samples since formation is thought to have been negligible, however, marked post-formational hydrogen-isotope exchange is demonstrable at temperatures not exceeding ∼ 80°C and probably considerably lower in some cases.

Field Trials of Non-Seismic Geophysical Techniques for Petroleum Exploration in New South Wales

The Wilga Park 1 gas discovery well, drilled in 1985, was the first valid structural test for hydrocarbons in the Gunnedah Basin in northern NSW. A further untested structure (Nyora) is located some 30 km to the west. Both structures were located and delineated solely by the use of seismic reflection techniques. Both structures are relatively small (7?10 sq km) and to assist in evaluation of optimal survey procedures the Department of Mineral Resources conducted trial surveys with alternative geophysical methods. The shallow Wilga Park structure (600 m to gas trap) was delineated by resistivity profiling due to the resistive Pilliga Sandstone (70 to 100 ohmm) contrasting with the deeper Purlawaugh Formation (7 to 10 ohmm). The resistivity data were duplicated using the dipole?dipole array (galvanic) and coincident loop SIROTEM (inductive) method. An induced polarization anomaly was observed at depth over strong faulting, which may have been a source of hydrocarbon gas seepage. The gravity method at Wilga Park was only partially successful as a regional gradient masked the anomaly. The untested Nyora structure was well delineated by gravity and magnetic methods. The magnetic anomaly was interpreted to be due to a fault bounded teschenite sill at 400 m depth, which was also defined by strong seismic reflections. The gravity anomaly, however, was modelled to be due to a basement high. Induced polarization data indicated anomalous responses over the fault margins of the structure and a depleted zone over the sill, which conforms with the model for an alteration/sulphide plume over gas and oil pools. It is concluded that potential field techniques could be cost effectively applied to exploration for these small structures, to supplement seismic data. Electric methods provide a useful adjunct data set that has positive exploration incentives if drilling proves the 'plume' to be a hydrocarbon indicator.

THE PETROLEUM POTENTIAL OF THE GUNNED AH BASIN AND OVERLYING SURAT BASIN SEQUENCE, NEW SOUTH WALES

The Permo-Triassic Gunnedah Basin has good potential for the discovery of commercial petroleum. Gas shows have been reported from the Porcupine-Watermark, Black Jack and Digby Formations, and from the basal sandstone of the Purlawaugh Formation in the overlying Surat Basin sequence. Gas flowed on drill stem test from the Porcupine-Watermark Formation in the Wilga Park No. 1 discovery well although the find was sub-commercial. An oil show was observed in Lower Permian volcanics, and oil staining has been observed in the Pilliga Sandstone in several wells. The origin of oil staining in the Pilliga Sandstone is unknown, however, and may have been the result of diesel contamination during drilling operations.Structural style within the basin sequence is characterised by north-south and north-north-west/south- south-east trending anticlines which formed in response to periodic compressive and left lateral strike-slip movements along the main Hunter Mooki Thrust Fault. These anticlines are attractive exploration targets.Westerly-derived quartz-rich sandstones occur at several stratigraphic levels within the Black Jack Formation and within the upper Digby Formation. Sandstones of the western bed-load fluvial system (lower Black Jack Formation) are most prospective with thick sections (up to 8 m) giving permeabilities from several hundred to several thousand millidarcies. Marine reworked easterly-derived sandstones up to 12 m thick in the Black Jack and Watermark Formations have minor reservoir potential with permeabilities in the order of tens of millidarcies. All potential reservoirs within the sequence are considered to be adequately sealed. Regionally extensive shaly units deposited either by marine incursion or lacustrine inundation overlie most reservoir horizons; remaining reservoirs are capped by intraformational shales.Organic petrology and geochemistry indicate the best potential source rocks within the Gunnedah Basin are floodplain, lacustrine and shallow marine facies of the Purlawaugh, Napperby, Watermark, Maules Creek and Goonbri Formations. The shallow marine Arkarula Sandstone Member within the Black Jack Formation also has significant potential for oil generation. Vitrinite reflectance, liptinite auto-fluorescence and TAI values indicate Lower Permian sediments are marginally mature to mature for oil generation. Combining the data on source quality and quantity with thermal maturity, the Permian sediments - in particular the Watermark Formation - have the best potential for generating oil.

Oil and Gas Developments in Australia in 1985

Emphasis in exploration drilling in Australia in 1985 continued to move onshore and concentrate in the Eromanga/Cooper and Surat basins. So although the 267 exploration wells drilled established a new record, the total number of wells of all classes drilled (359 wells) fell below the 1984 tally, with fewer development and offshore wells (both exploration and development) drilled. Offshore appraisal drilling in the Gippsland and Carnarvon basins proved discouraging. Most new-field discoveries were made in the onshore Eromanga/Cooper basin and, once again, an encouraging spread of shallow Eromanga accumulations provided incentive to move away from established areas of production. Muteroo 1 flowed 1,720 BOPD; Cook 1 flowed 880 BOPD; Watson South 1 flowed 3,112 BOPD; and Coro a 1 recovered 12 m of oil on a drill-stem test. Poolowanna 2 proved a disappointment in the western Eromanga basin. A new-field gas discovery was made in the New South Wales Gunnedah basin. An oil discovery, West Terrace 1, which flowed 760 BOPD, was made near existing production in the Canning basin. Offshore, Yolla 1 was an encouraging oil, gas, and condensate discovery in the little-explored Bass basin. Further successes were recorded in the Barrow subbasin by Saladin 1 and by the Harriet C1 appraisal well, whereas the Dampier subbasin just to the north yielded discouraging results in appraisal drilling near the Talisman and Wilcox discoveries, made during 1984 and 1983, respectively. An appraisal well, Goodwyn 7, with a condensate flow of 123 bbl/mmcf of gas on a test rate of 38 MMCF D, gave promise of a substantial light-liquids accumulation in Goodwyn field. Geophysical coverage was higher in 1985 than in 1984, substantially so in the offshore basins. The Mereenie-Alice Springs oil pipeline came on-stream in 1985, and construction began on the Amadeus basin-Darwin gas pipeline. Higher production rates brought Australia to a level of self-sufficiency for the first time, with hydrocarbon exports of 145,000 b/d and imports of 140,000 b/d. Gippsland basin fields reached a new 1-day peak (538,680 bbl) and averaged 474,000 BOPD for the year, contributing 90% of the nation's production. Thus, a greater proportion came from other fields than previously, with Queensland production surpassing Western Australia's for the first time since Barrow Island field came on-stream in 1967.

Geochemistry of high-temperature coal ashes and the sedimentary environment of the New South Wales coals, Australia

Chemical analyses of high-temperature coal ashes were used to establish the distribution, association and relationship between major inorganic elements such as Si, Al, Ti, Fe, Mn, Mg, Ca, Na, K, P, S and CO 2 in a number of New South Wales economic coal seams and to study the composition and character of mineral matter in these coals. The methods used for the evaluation of the data were statistical analysis (univariate and bivariate), ratios, normative mineral composition and variation diagrams. The distribution of major and minor inorganic elements in coal appears to be related to the amount of mineral matter occurring in coal (determined as ash yield) and its mineralogical composition. The quantitative variations in levels of these elements can be classified as in-seam and inter-seam variations. In-seam variations are largely ash yield dependent, i.e. the levels of an element (wt.%) in coal increase along with the increase of its ash content (wt.%). The inter-seam variations are more complex and are related to both ash yield and to the mineralogical composition of mineral matter. The principal components of New South Wales coal ashes are silicon and aluminium. Silicon may be present as silica or combined with aluminium in different proportions to form clay minerals, such as kaolinite, illite, mixed-layer clay minerals, and smectite. Thus, the concentration levels of aluminium in relation to silicon in coal may give an indication about the character of clay minerals present in coal. Ratios and correlation coefficients of element pairs such as Al and Ti, Na and K, and Na and Al were used to determine differences in the chemical composition of high-temperature coal ashes of seams from various stratigraphic positions and provinces. In some seams the nature of associations of these elements is more significant than in others. This is interpreted as being a product of specific environmental conditions controlling the deposition of these seams. The nature of clay mineral content in coal is believed to be a major reason for chemical dissimilarities found between seams of various stratigraphic levels and geographic areas. For example, in some seams kaolinite, in others expandable clay minerals are dominant. The vertical distribution of these minerals has a stratigraphic significance. Within the Upper Permian Newcastle Coal Measures a trend from kaolinite-rich through to expandable minerals-rich and to kaolinite-rich assemblages can be observed from the bottom to the top. These changes are noticeably gradual. All significant variations in the clay mineral assemblages could relate to the long-term changes in the provenance of sedimentary material, weathering conditions in the source area and the rate of subsidence in the place of deposition. These changes are associated with major tectonic events controlling the history of sedimentation within the paralic Sydney and Gunnedah Basins during the Permian.

Deltaic depositional systems, coal distribution and quality, and petroleum potential, Permian Gunnedah basin, N.S.W., Australia

Upper Permian sediments of the lower Black Jack Formation were deposited as a major delta system in the Gunnedah Basin, a foredeep of the rising New England Fold Belt. Two principal depositional units are recognized in the lower Black Jack Formation: the lower delta-plain facies and the shallow-marine facies. A third but minor depositional unit, the western bed-load fluvial facies, is correlative with the upper part of the shallow-marine facies. Geometry of the framework sandstones indicates that the lower delta-plain facies was deposited in a combination of fluvial-dominated and wave-modified delta systems and that the shallow-marine facies was deposited in a tide-dominated system. Sediment source was predominantly from the northeast except for the western bed-load fluvial facies which was derived from the Lachlan Fold Belt region to the west. The lower delta-plain facies was deposited during a major episode of delta construction. Principal components recognized in drillcore are distributary-channel, crevasse-splay and splay-subdelta, interdistributary-bay, lagoon and marsh deposits. Transgression of the delta, possibly initiated by a major (?) eustatic sea-level rise, led to deposition of the shallow-marine facies. This facies consists chiefly of beach, barrier-beach, tidal sand ridge, and tidal channel-fill deposits. Subsequent lowering of sea level was accompanied by deposition of the western bed-load fluvial facies. This facies consists predominantly of upward-fining fluvial channels which display considerable marine reworking toward the basin centre. A vast coal swamp was established rapidly after the marine conditions regressed and is represented by the Hoskissons Coal Member. Coals of commercial interest are developed within the lower delta-plain facies and above the shallow-marine facies. The lobate geometry of the coal deposits in the lower delta-plain facies mirrors the geometry of the framework sandstones and indicates a deltaic origin for coal formation. Two mechanisms for coal formation are recognized in the lower delta-plain facies. Thick, laterally extensive coals, such as the Melvilles Coal Member, are thought to have originated as blanket peats following delta abandonment. Thin, discontinuous coals formed in interdistributary depressions during active deltaic progradation. Coal above the shallow-marine facies is contained in one major seam, the Hoskissons Coal Member. This seam probably formed mostly as a back-barrier or lagoonal peat. Depositional setting strongly influenced coal quality. Isoash trends for the Melvilles Coal Member suggest a marginal-marine influence during peat accumulation. Ash content of the Hoskissons Coal Member has a similar distribution to the percentage of sandstone in the underlying shallow-marine facies. Potential exists for petroleum accumulation in the lower Black Jack Formation. Suitable reservoir rocks are present in the lower delta-plain and shallow-marine facies. Underlying prodelta facies are potential source rocks.

Late Permian to Cainozoic tectonics of the New England Orogen

Abstract The development of the New England Orogen, until the middle Permian, was divided into three tectonic periods by Harrington and Korsch (Aust. J. Earth Sci. 32, 163–179,1985). These tectonic periods were dominated firstly by W‐facing volcanic arc systems which in tectonic period (TP) 3 were replaced by the E‐facing Gympie arc system. In TP4 the upper marine beds and coal measures of the Sydney, Gunnedah and Bowen basins were deposited and were covered by Early Triassic strata. In the W there was little structural disturbance in TP4 except in the Gunnedah Basin near the Mooki Fault where the Lower Triassic is unconformable on the Permian coal measures, but in the E there was a fundamental change with the formation of the major Late Permian and Early Triassic TP4 magmatic arc in Zone B and in the Yarrol Orogen. During TP5, in the Middle and Late Triassic, the Stanthorpe Plutonic Suite was intruded and the Ipswich Basin was formed and filled with volcanics and coal measures. A deformation in the Late Triassic at the beginning of TP6 was significant throughout the orogen, causing the deformation of the Gunnedah, Bowen and Ipswich basins which are overlain unconformably by the Surat and Moreton basins. In TP7 in the Late Jurassic and Early Cretaceous, there was volcanism and deformation in the Maryborough Basin and granitoids were intruded. It is inferred that there was major deformation and plutonism in Zone C because the major Rangitata Orogeny occurred in New Zealand. In TP8 the Whitsunday belt of mid‐Cretaceous silicic volcanics and high‐level plutons was probably a major feature because volcanic debris is abundant in sediments of the same age in the Surat and Eromanga basins. In TP9, in the Late Cretaceous and Cainozoic, Zone C was removed from Australia when the Tasman and Coral seas opened, the eastern part of Australia developed into a cymatogen by vertical movements, and there were extensive eruptions of basaltic volcanoes and lava fields in a set of arcs. Seven sets of granitoid rocks occur in Zone B and the Yarrol Orogen and it is thought that they formed by the subduction of continental crust (C‐subduction).

Permo‐carboniferous silicic volcanism and palaeogeography on the western edge of the New England Orogen, north‐eastern New South Wales

Deposition of Late Carboniferous and Early Permian continental conglomeratic strata along the western edge of the New England Orogen was continuously accompanied by silicic volcanism. Disconformable relationships exist locally between the Early Permian Glossopteris‐bearing rocks and the Late Carboniferous Rhacopteris‐bearing rocks but have negligible stratigraphic significance since erosional surfaces are common within the older sequence and probably also occur in the younger sequence. The western volcanic terrain which had supplied ignimbrites, airfall deposits and volcanogenic detritus eastward during the Late Carboniferous still had some active centres in the Early Permian, though eruptions waned in scale and changed in style to include effusion of lavas. Proximal Early Permian volcanics comprised of lithic‐rich ignimbrites and lavas at Boggabri are the remnants of one such centre. After eruptions had ceased, denudation of this and other remaining centres contributed volcanogenic detritus, including ‘flint clay’ clasts, to coal measure sediments (Greta equivalent) accumulating in the flanking low‐lying areas of the incipient Gunnedah Basin. There is no evidence in support of easterly provenance for these Early Permian coal measures nor for movement on the Mooki Thrust at this time.