Eastern Margin Of Australia Research Articles

Geochronology and sediment provenance of the Precipice Sandstone and Evergreen Formation in the Surat Basin, Australia: Implications for the palaeogeography of eastern Gondwana

A new view of the palaeogeography and tectonic evolution of eastern Gondwana during the late Mesozoic is emerging, driven largely by the detrital zircon record of sedimentary basins comprising the Great Australian Superbasin (GAS). The updated model suggests that a long-lived magmatic arc was present on the eastern margin of Australia, which is in stark contrast to some previous views that eastern Australia was situated well-inland of a plate boundary. However, the active magmatic arc model is derived from Late Jurassic-Cretaceous strata, with tectonic processes and sediment pathways operating in the Early Jurassic still poorly understood. The Lower Jurassic Precipice Sandstone in the Surat Basin – an important stratigraphic component of the GAS – affords an opportunity to fill such a knowledge gap. The Precipice Sandstone is also economically important due to its aquifer capacity and prospectivity for CO2 sequestration. Previous work has hypothesized that the formation contains deposits sourced from multiple terranes, yet this has not been rigorously tested. In this study, we used detrital zircon geochronology, CA-TIMS dating, petrography, and palaeocurrent analysis to characterise the sedimentary provenance of the Precipice Sandstone and the overlying Evergreen Formation. Our analysis revealed mixed provenance with multiple source terranes within the Tasmanides. The bulk of sediment was derived from the Thomson Orogen, with a lesser contribution from the New England Orogen. We found no evidence for sediment derived from the Lachlan Orogen, despite a distinct northward palaeocurrent component. Syn-depositional volcanic material within the interval corroborates the notion of continued arc magmatism along the eastern Gondwana margin, which contributed sediment to the GAS throughout the Early Jurassic. Radiometric ages place the Precipice-Evergreen succession in the Sinemurian-Toarcian which falls within the broader Hettangian-Aalenian age range suggested by recent palynostratigraphic data and suggest diachronous deposition across the basin. Despite pronounced variations in formation thickness, the Precipice Sandstone shows relatively uniform provenance across the basin, showing little evidence for multiple depocenters.

Tracing iron along the flowpath of East Australian Current using iron stable isotopes

The mechanisms delivering iron (Fe) to Southern Ocean surface waters and resulting capacity for the oceanic drawdown of CO2 are important to constrain in a changing climate. The East Australian Current (EAC) is a major western boundary current flowing south along the eastern margin of Australia, hypothesized to play an important role in entraining Fe from a variety of potential sources and supplying Fe to large annual spring phytoplankton blooms in HNLC waters. We report Fe concentration and Fe isotope (δ56Fe) profiles for both dissolved and particulate phases at stations within the EAC, in the frontal mixing zone, and in subantarctic waters along the southward flowpath of the EAC in early spring. Mixed layer dFe concentrations declined from EAC source waters (0.32–0.53 nmol kg−1) into subantarctic waters (0.27–0.42 nmol kg−1) with a concurrent deepening of the ferricline. Particulate trace metal concentrations indicate robust inputs of lithogenic particles to subtropical EAC waters, potentially impacted by advection of shelf sedimentary particles and evidence of a significant supply from local aerosol deposition. The isotopic composition of pFe in the EAC (0.3 ± 0.1‰) is similar to prior reports of aerosol δ56Fe and upper ocean δ56pFe in the western Pacific and isotopically heavier than average values for the bulk continental crust. In EAC surface waters, isotopically light δ56dFe values (0.11–0.16‰) are observed likely due to reductive release of dFe from lithogenic particles in the photic zone with a particle-dissolved phase fractionation (Δ56FedFe–pFe) of −0.1 to −0.4‰. At depth, elevated dFe concentrations and a heavy δ56dFe signature suggest a non-reductive release of dFe from lithogenic particles via desorption or dissolution processes with a Δ56FedFe–pFe of +0.3‰. During southward transit of EAC waters, mixed layer dFe pools become increasingly heavy due to biological uptake through the frontal mixing zone and into subantarctic waters. We also observe differences in particulate trace metal concentrations in intermediate waters between subantarctic stations that could result from entrainment of sedimentary particles during transit over the relatively shallow South Tasman Rise.

Continuous convergence along the paleo-Pacific margin of Australia during the Early Paleozoic: Insights from the Running River Metamorphics, NE Queensland

New geochronological findings, using zircon dating, supported with geochemical and structural data from the Running River Metamorphics (RRM), Thomson Orogen of the Tasmanides, have been used to reinvestigate the Paleo-Pacific margin of Australia during the Early Paleozoic. The RRM comprises a greenschist to amphibolite facies metamorphic succession of gneisses, amphibolite, and quartzite that is intruded by the Ordovician Falls Creek Tonalite. Protoliths of the gneisses and tonalite are interpreted to be emplaced at ~472 and ~457 Ma, respectively, whereas a maximum depositional age of the protolith of the quartzite is constrained to ~471 Ma. Geochemically, the gneisses and tonalite have calc-alkaline signature, and represent S-and I-type granitoids, respectively, that formed in a continental arc setting. The amphibolites have oceanic island basalt-like affinity, and together with the coeval quartzite are interpreted to represent a relict seamount. Five deformation events were identified in the RRM. D1 and D2, and D3 are interpreted to be subduction- and collision-related, respectively. D4 most probably accompanied the movement along the NE segment of the Clarke River Fault, and resulted in rotation of the earlier fabrics into the dominant NE orientation. The timing of tectonism can be constrained between the intrusion of the S-type gneisses at 472 Ma and the intrusion of the undeformed migmatites at ~437 Ma. The common stratigraphy, tectono-metamorphic history, geochemistry, and overlapping dates of the different provinces of the Thomson orogen suggest that following the Delamerian orogeny, the paleo-Pacific margin of Australia has been a convergent plate boundary during Middle Ordovician to Middle Silurian. Our results imply that convergence has been underway along the eastern margin of Australia during the Cambro-Silurian times and hence pushing the boundary of the Delamerian orogeny to the end of Ordovician in the northern part of the Thomson Orogen.

Diverse provenance of the Lower Cretaceous sediments of the Eromanga Basin, South Australia: constraints on basin evolution

This study presents new geochemical and mineralogical data collected on 13 drill holes across the South Australian part of the Eromanga Basin, with a focus on the Lower Cretaceous Cadna-owie Formation, Bulldog Shale, Coorikiana Sandstone and Oodnadatta Formation. Mineralogical and geochemical data are used to determine the provenance of the sediments along with the paleoenvironments at the time of deposition. The dominant clay species across the formations are kaolinite and montmorillonite, although each drill hole shows individual mineralogical assemblages. The Cadna-owie Formation is mainly composed of quartz and kaolinite; montmorillonite is the dominant clay species in the other formations. Differences in major-element chemistry are best shown on the K–Na–Ca ternary diagram and display a strong relationship with mineralogy. Trace and rare earth elements are preserved at similar average concentrations between formations and are depleted relative to the upper continental crust. The signature of the sediments is interpreted to be derived from a mix of felsic and mafic igneous rocks with almost no sedimentary recycling. Sources of the sediments are likely to be from exposed basement packages surrounding the study area such as the Gawler Range Volcanics, the Musgrave Province, the Benagerie Volcanic Suite, the Mount Painter Inlier and the Arunta Block. The variations in dominant clay species between the formations could have been driven by an increase in temperature and a decrease in rainfall during the Early Cretaceous along with seasonal variations, and/or by the input in volcanogenic sediments from the active volcanoes located along the eastern margin of Australia. KEY POINTS Lower Cretaceous formations in the South Australian Eromanga Basin show limited regional lateral and vertical geochemical and mineralogical variations. Sediments are a mix of mafic through felsic igneous rocks derived from basement terranes exposed in the Early Cretaceous. The paleoclimate during deposition evolved from cold and humid to warm and arid.

Provenance of the Langjiexue Group to the south of the Yarlung-Tsangpo Suture Zone in southeastern Tibet: Insights on the evolution of the Neo-Tethys Ocean in the Late Triassic

ABSTRACTThe Upper Triassic Langjiexue Group, which lies immediately south of the Yarlung-Tsangpo Suture Zone in the Shannan area of southeastern Tibet, represents an important part of the Tethyan Himalayan Sequence (THS). Its provenance and palaeogeography have been the subject of debate. We present new data on petrographic composition, whole-rock geochemistry, and detrital zircon U–Pb geochronology to constrain the provenance of the Langjiexue Group. The dominance of quartz grains and felsic volcanic lithic fragments suggests that the sandstones are litho-quartzose. The trace element geochemical signatures (V–Ni–Th*10, Co/Th–La/Sc, Eu/Eu*–Th/Sc) suggest derivation from felsic igneous sources. The detrital zircon age spectra display three major peaks: a Meso-to-Neoproterozoic peak (1200–900 Ma, 7–18%), a Neoproterozoic-to-Late Cambrian peak (750–500 Ma, 32–65%), and a Late Carboniferous-to-Late Triassic peak (300–200 Ma, 11–33%). The maximum depositional age of early Carnian (236–235 Ma) is obtained by calculating weighted average ages of the youngest zircons (≤250 Ma). The youngest age cluster (300–200 Ma) is incompatible with sources from neighbouring terranes, including the South Qiangtang terrane, Lhasa terrane, THS, and Higher Himalayan Crystalline. Correlations of the Permian–Triassic zircons with those of time-equivalent strata in northwest Australia, west Burma, and the Banda Arc unveil a potential connection to the Tasmanides along the convergent margin of eastern Australia. The New England Orogen (300–230 Ma) could have supplied the Langjiexue Group with magmatic materials via continent-scale drainage systems or a submarine fan complex. This scenario provides a new perspective into the transport of detritus from distal orogens to sedimentary basins thousands of kilometres away.

Late Quaternary channel and floodplain formation in a partly confined subtropical river, eastern Australia

ABSTRACTAlong the eastern margin of Australia, hydrological variability reaches a peak in the subtropics of south‐east Queensland and many rivers have entrenched characteristics. To address the nature of entrenchment and the relationship with adjacent alluvium, this paper presents the results of detailed chrono‐stratigraphic analysis of alluvial units in the partly confined mid‐reaches of Lockyer Creek, Australia. Four sites were investigated using topographic, sedimentological and chronological data. Radiocarbon and single grain optically stimulated luminescence dating indicate a large proportion of the valley fill reflects a major phase of aggradation of fine‐grained alluvium from ca. 35 ka throughout the Last Glacial Cold Period. Synchronous incision of Pleistocene alluvial fills between 11.5 and 9.3 ka suggests the current entrenched Lockyer Creek formed in response to changes in late Quaternary climate. Holocene floodplains set within the entrenched Pleistocene valley floor have basal ages of ca. 7.5 ka, but whose proximal margins are still actively accreting. This Holocene fill has accreted over the mid‐ to late Holocene but overlaps with the contemporary hydrological regime. The sedimentary nature of the Holocene fill appears to be related to persistent antecedent controls in the form of bedrock and terrace constriction.

Large igneous province or long-lived magmatic arc along the eastern margin of Australia during the Cretaceous? Insights from the sedimentary record

U-Pb geochronology and Lu-Hf isotope analysis of detrital zircon from the mid-Cretaceous Winton and Mackunda Formations in the Eromanga Basin were employed to investigate regional provenance patterns in order to better understand the tectonic setting and paleogeography of eastern Australia during the late Mesozoic. A suite of Mesozoic-aged zircon populations recovered from these formations suggests that volcanism along the eastern margin of Australia was relatively continuous from the Triassic (252 Ma) to at least the mid-Cretaceous (ca. 92 Ma). Cretaceous-age zircon populations dominate the provenance record, and a distinct upsection younging trend in Cretaceous grain ages indicates that deposition was largely synchronous with ongoing volcanism to the east. Lu-Hf isotopic data suggest that these zircon populations were sourced from igneous rocks of a mixed juvenile and crustal source, similar to Lu-Hf isotopic systematics for eastern Australian zircons from Pennsylvanian–Permian igneous assemblages (307–252 Ma), for which an active convergent margin association is well established. An extensive Cretaceous volcanic terrain, now limited to the Whitsunday Igneous Association, was once located along the northeastern margin of Australia. Results from this study support the hypothesis that the Whitsunday igneous association was the main source of Cretaceous sediment to the Eromanga Basin, and likely for sediment transported across the continent southward and into the Ceduna Delta system offshore South Australia. The Whitsunday igneous association has been interpreted as a siliceous large igneous province associated with the onset of rifting in the region and linked to opening of the Tasman and Coral Seas. Yet, in this study, we document a relatively continuous Late Triassic to Late Cretaceous (240–92 Ma) age range for detrital zircons from the Mackunda and Winton Formations, consistent with relatively uninterrupted magmatic activity along the continental margin until ca. 92 Ma (earliest Turonian). Furthermore, zircon grains across this age spectrum exhibit dominantly positive to strongly positive eHf(t) values, between +4 and +12, consistent with values known for zircon suites from older magmatic arc rocks of eastern Australia. Although these data do not support a conclusive interpretation, they are consistent with an east Australian magmatic arc related to westward subduction of paleo-Pacific oceanic crust beneath eastern Australia enduring into the Cretaceous, as distinct from extensional siliceous large igneous province magmatism unrelated to subduction and generated by rupture of continental crust.

Paleoproterozoic closure of an Australia–Laurentia seaway revealed by megaclasts of an obducted volcanic arc in Yukon, Canada

The Slab volcanics are a late Paleoproterozoic volcanic succession in the Wernecke Mountains of Yukon Territory in northwestern Canada. Fragments of the succession are preserved as megaclasts in km-scale zones of hydrothermal breccia. The largest clast is 160×380m and consists of 31 mafic lava flows and minor intercalated sandstone and tephra. Hydrothermal activity during the brecciation led to extensive metasomatic alteration, both sodic and potassic. Despite the alteration, an igneous geochemical signal is discernible. The rocks are mafic to intermediate and moderately alkalic. Their trace element profiles indicate derivation from a hydrated, enriched mantle source, consistent with an origin as a volcanic arc affected by a plume, rift or slab window. Major and trace element patterns demonstrate a cyclical magmatic evolution involving intervals of fractional crystallization punctuated by recharge with mafic magma. The Slab volcanics were previously thought to have been deposited on Laurentia but are herein regarded as part of the exotic terrane Bonnetia that was obducted onto the Laurentian continental margin in late Paleoproterozoic time. Bonnetia may have developed on or near the eastern margin of Australia as a continental or fringing arc. In that scenario, Bonnetia grew through arc magmatism as oceanic crust between Australia and Laurentia was consumed prior to terrane obduction and continental collision in the late Paleoproterozoic to early Mesoproterozoic. Sediment or tephra derived from the arc was transported westward into the interior of Australia, raising εNd values of late Paleoproterozoic sedimentary strata. Vigorous hydrothermal brecciation followed and led to fragmentation of the crust, including the obducted terrane Bonnetia. Giant blocks of the Slab volcanics and other units of Bonnetia foundered into breccia zones and moved downward for thousands of meters. Erosion removed the obducted terrane and the uppermost Laurentian crust, exposing the megaclasts.

Detrital zircon data reveal the origin of Australia’s largest delta system

The Late Cretaceous Ceduna Delta is the largest deltaic system on the Australian continent, yet its source is unknown. Apatite fission-track data reveal widespread Late Cretaceous exhumation across the southern Australian margin. New detrital zircon analysis of 786 grains from the Gnarlyknots-1 well, which penetrated the offshore delta top, show that the upper part of the delta (Santonian–Maastrichtian) was sourced largely from recycled Permian to Early Cretaceous cover and underlying basement eroded from the margin, proximal to the basin. This challenges the widely accepted model involving distal provenance of >2000 km from the eastern margin of Australia. Supplementary material The 2D seismic reflection data, results for detrital zircon LA-ICP-MS and zircon fission-track analyses, including the LA-ICP-MS method, and a list of sample intervals and ages are available at www.geolsoc.org.uk/SUP18582 .

Noble gases in spinel peridotite xenoliths from Mt Quincan, North Queensland, Australia: Undisturbed MORB-type noble gases in the subcontinental lithospheric mantle

Noble gas elemental and isotopic compositions have been determined for a suit of fresh spinel peridotite xenoliths from Mt Quincan, North Queensland (Northeast Australia). The observed helium, argon and xenon isotopic compositions are very similar to those reported for mid-ocean ridge basalts (MORB) in that they have following features: (1) 3He/ 4He ratios covering a narrow range from 8 Ra to 10 Ra, (2) 40Ar/ 36Ar (300–4000) and the 3He/ 36Ar (0.0012–0.07; corrected for some recent elemental fractionation) values that plot on the well-established MORB line, and (3) correlated excesses in 129Xe and 136Xe with regard to atmospheric xenon. MORB-like noble gas compositions suggest fluids ultimately derived from the asthenopshere are present in the subcontinental lithospheric mantle beneath northeastern Australia. The absence of any subduction-related noble gas signature precludes the preservation of fluids in the xenoliths added during Paleozoic subduction at the eastern margin of Australia. We also note that there is no mantle plume noble gas signature found in the Mt Quincan xenoliths, despite its location close to the suspected hotspot track across eastern Australia. These features are consistent with Sr and Nd isotopic characteristics of Mt Quincan xenoliths [Handler, M.R., Bennett, V.C., Carlson, R.W. 2005. Nd, Sr and Os isotope systematics in young, fertile peridotite xenoliths from northern Queensland, Australia: A unique view of depleted MORB mantle? Geochim. Cosmochim. Acta 69, 5747–5763.]. Thus, the fluid-entrapment is likely associated with subsequent rifting tectonic settings when asthenospheric fluid (and magma) could rise into the subcontinental lithospheric mantle and was able to overwrite any pre-existing signatures or even replace it with “MORB”-like mantle by physical removal of previously metasomatised lithospheric mantle. Additionally, differing from noble gas compositions previously considered typical for the subcontinental lithospheric mantle, the north Queensland data highlight the existence of noble gas isotopic heterogeneity in the subcontinental lithospheric mantle on a global scale.

Cenozoic landscape evolution in central Queensland

Central Queensland lies on the passive margin of eastern Australia and owes its landscape to processes that began following rifting and opening of the Tasman Sea. The modern landscape is the result of long-term processes of landform development, and the landforms themselves are the evidence of these processes. Hence, interpreting their significance provides an understanding of long-term landscape evolution. Along the eastern Australian coast, numerous rivers drain into the sea but among these, there are two that stand out: the neighbouring Fitzroy and Burdekin Rivers in central Queensland. These two streams have by far the largest catchments of any rivers along the eastern seaboard of Australia. The Burdekin and Fitzroy catchments contain widespread remnants of Cenozoic deposits, which accumulated predominantly in fluvial and lacustrine environments established during the Palaeogene. Alluvial sediments were supplied by erosion of nearby uplands, and accumulated in depressions and basins on a prior land surface. Volcanic activity also resulted in large lava flows in central western areas. Water was the main agent of sediment transport, distributing unconsolidated deposits along the drainage networks of the time, some of which were directed inland. It is inferred that during the Palaeogene, the divide between coastward and inland draining streams was further to the east than it is at present. Several basins were located west of the former coastal divide, and were characterised by continental environments of deposition in a generally westward drainage system. With continued accumulation of sediments, individual basins overflowed and merged to form a widespread flat-lying Palaeogene landscape that concealed an earlier land surface on which bedrock was more extensive. In the Early Cenozoic, there was a change from the depositional phase that resulted in the continental sequence, to an erosional phase that developed the modern landforms. The change from deposition to erosion probably started during the Palaeogene. Erosion continued through to the present, re-exposing parts of the basal Palaeogene sequence and earlier Mesozoic land surface. The erosional phase that shaped the landforms of the modern Burdekin and Fitzroy catchments can be explained by slowly evolving drainage basins in the interior being captured by small coastal streams—the predecessors of the Burdekin and Fitzroy Rivers. The coastal streams were short and steep in comparison with those in the interior, allowing a more active erosional environment along the coast. As the coastal streams expanded, the drainage divide moved rapidly westwards. Stream capture began a phase of regional erosion, which transported large quantities of sediments to the coast. The sediments contributed to coastal and nearshore features similar to the Holocene high sea-level examples at the mouth of the Burdekin River in the north, and the Fitzroy Delta and the Keppel Coast in the south. Large volumes of sediments were also transported beyond the present coast during low sea-levels of the Cenozoic, forming similar coastal features and contributing to a major eastward bulge on the central Queensland continental shelf. The emptying of continental basins has paralleled the development of the continental shelf bulge from the coast to the Marion Plateau.

Late Paleocene foraminiferal faunas with Chapmanina and Reticulophragmium from offshore southeastern Australia: a Tethyan influence

Samples from off southern New South Wales in approximately 1750 m water depth have yielded late Paleocene faunas of benthic and planktonic foraminifera and other invertebrates. Two faunal associations with a strong Tethyan influence, dominated respectively by Chapmanina conjuncta sp. nov. and Reticulophragmium naroomaensis sp. nov., are described; this is the first formal record of these genera from Australasia. The age of the host sediment is based on both benthic species and few planktonic foraminifera. The environment at the time was warm temperate to subtropical at the southwestern extremity of a counter-clockwise Pacific Ocean gyre. The new occurrence extends the record of Palaeogene marine sediments and palaeoenvironments along the eastern margin of Australia and confirms earlier observations that conditions in the marine environment at that time were considerably warmer along the east coast of Australia than those along the southern margin; the biogeographic association was Tethyan. The circulation model may go some way to explaining coeval warmth along the east Antarctic coast.

Paleomagnetic results indicate pre‐Permian counter‐clockwise rotation of the southern Tamworth Belt, southern New England Orogen, Australia

Visean ignimbrites from three fault‐bounded blocks within the southern Tamworth Belt, southern New England Orogen, Australia, retain a prefolding high‐temperature component of remanence, carried mainly by magnetite. Paleomagnetic data indicate that the southern Tamworth Belt occupied low to moderate paleolatitudes during the early Carboniferous. Normal and reversed polarities with low to moderate inclinations suggest that the steep, reverse polarity latest Carboniferous to earliest Permian overprint is absent from the high‐temperature component data set. The mean characteristic remanence direction of three early to middle Visean ignimbrite flows (32 sites) from the Rouchel block is: declination (dec) = 154.2° and inclination (inc) = 38.8° (α95 = 12.5°; k = 98.8), which corresponds to a paleo south pole at latitude 64.9°S, longitude 258.1°E (dp = 5.5°; dm = 9.2°, where dp is the angular length of the semiaxis of the ellipse of confidence of the calculated paleopole that lies along the pole to site great circle, and dm is the angular length of the semiaxis perpendicular to dp). Six middle to late Visean ignimbrite flows (29 sites) from the Gresford block retain a mean remanence of dec = 159.6° and inc = 54.1° (α95 = 13.7°; k = 24.7), corresponding to a paleo south pole at latitude 72.8°S, longitude 228.4°E (dp = 13.4°; dm = 19.2°). Despite widespread overprinting throughout much of the Myall block, reliable information was obtained from three late Visean ignimbrite flows (five sites), with a mean direction of dec = 118.1° and inc = 42.5° (α95 = 12.0°; k = 41.6), from which a paleo south pole of latitude 35.7°S, longitude 233.4°E (dp = 9.1°; dm = 14.7°) is calculated. The calculated paleo south poles do not fall on the early Carboniferous segment of the published apparent polar wander path for Australia and suggest that the southern Tamworth Belt has undergone significant counter‐clockwise rotation relative to cratonic Australia. Inferred rotations for the Rouchel, Gresford, and Myall blocks are approximately 80°, 80°, and 120°, respectively. Stratigraphic and geological evidence suggest rotation took place between the mid‐Namurian and latest Carboniferous and is thought to have resulted from sinistral strike‐slip during compression at the eastern margin of Australia.

Recycled noble gas and nitrogen in the subcontinental lithospheric mantle: Implications from N-He-Ar in fluid inclusions of SE Australian xenoliths.

suggests that gases in xenoliths of southeast Australia are derived from a well-mixed upper mantle reservoir. The heavier nitrogen isotopic signatures (from ~0 to +2‰) and elemental ratio of argon to nitrogen could be explained by the addition of 30% to 40% of a recycled sedimentary component. Nitrogen is indeed recycled more efficiently in the mantle than helium, preserving the trace of present or past subduction. The heavy N component has been observed in xenoliths from the eastern side of the Newer Volcanic province. Sedimentary nitrogen may result from subduction along the eastern margin of Australia, during Paleozoic time. The present nitrogen results, together with the relatively low 40 Ar/ 36 Ar ratios and apparently correlated 3 He and 36 Ar contents in those xenoliths, suggest the long-term preservation of recycled surface volatiles in the continental lithospheric mantle.

Mise en évidence de diapirs mésozoïques sur la bordure orientale de la ride de Lord Howe (Sud-Ouest Pacifique) : campagne ZoNéCo 5

Abstract The objective of the ZoNeCo 5 cruise on R/V L'Atalante was the bathymetrical and geological survey of the eastern edge of the Lord Howe Rise, a key structure between New Caledonia and the eastern margin of Australia. The acquired data better defines the relationships between the New Caledonia Basin and the Lord Howe Rise in an area which is the southern prolongation of the Fairway basin. This basin contains a thick sedimentary sequence ranging from Lower Cretaceous to the present day. This basin extends on more than 70 000 km2. The main result of the cruise was the discovery of a Pre-Cretaceous or Cretaceous series including salt or mud diapirs. At about 600 m below the sea floor and coincident with the distribution of diapirs, an extensive BSR (Bottom Simulating Reflector) suggests the existence of a large gas hydrates deposit.

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.

Late Paleozoic crustal history of central coastal Queensland interpreted from geochemistry of Mesozoic plutons: The effects of continental rifting

The eastern margin of Australia is understood to be the result of continental rifting during the Cretaceous and Tertiary. Consistent with this model, Cretaceous igneous rocks (granites to basalts) in a continental marginal setting near Bowen, Queensland are isotonically retarded, having isotopic ratios similar to those of most island arcs (Sr1 = 0.7030-0.7039, ɛNd = +6.46 to +3.00 and 206Pb/204Pb = 18.44−18.77, 207Pb/204Pb = 15.552−15.623, and 208Pb/204Pb = 37.90−38.52). These isotopic signatures are much less evolved than the Late Carboniferous-Permian batholith that many Cretaceous plutons intrude. As rocks ranging in age from about 300-100 Ma are well exposed near Bowen, we can track magma evolution through time. The significant change of magma source occurred much earlier than the Cretaceous based on the fact that Triassic granites in the same area are also isotonically primitive. We attribute the changes of magma composition to crustal rifting during the Late Permian and earliest Triassic. The Cretaceous rocks (actually latest Jurassic to Cretaceous, 145-98 Ma) themselves show compositional trends with time. Rocks of appropriate mineralogy for Al-in-hornblende geobarometry yield pressures ranging from 250 to 80 MPa for rocks ranging in age from 145 to 125 Ma, respectively. More significantly, this older group is relatively compositionally restricted, and is Sr-rich, and Y- and Zr-poor compared to 120-98 Ma rocks. This younger groups is bimodal, being comprised principally of basalts and rhyolites (granites). REE patterns for a given rock type, however, do not differ with age tribute these relatively subtle trace element differences to small differences in conditions (T, aH2O) at the site of melting. Cretaceous crustal rifting can explain the range of rock types and the spatial distribution of rocks < 120 Ma in a longitudinal strip between and overlapping with provinces of older Cretaceous intrusions. A subduction-related setting is assigned to the 145-125 Ma igneous rocks (those more than 50 Ma older than sea floor spreading).

Neocratonic magmatic‐sedimentary basins of post‐Variscan Europe and post‐Kanimblan eastern Australia generated by right‐lateral transtension of Permo‐Carboniferous Pangaea

AbstractThe stress from the initial coalescence of Pangaea during the mid‐Carboniferous (320 Ma) collision of Gondwanaland and Laurussia in the Variscan (Sudetic) magmatic fold belt was transmitted through Pangaea to generate the nappes and thrusts that dismembered the intracratonic Centralian Superbasin during the Alice Springs Orogeny and the megakinks that terminally deformed the Lachlan fold belt along the subducted Andean‐type margin of eastern Australia. Definitive collision was followed by a lacuna on the Pangaean platform that reflects uplift. The first release of heat at ≅ 300 Ma from the self‐induced Pangaean heat anomaly weakened the hypersensitive neocratonic crust of the inactive but still hot European and eastern Australian magmatic fold belts to become stress guides for right‐lateral transtension during anticlockwise rotation of Pangaea. Deep transtensional fractures provided a way into and through the neocratonic crust for magma ranging from S‐type granite to rhyodacitic ignimbrite to basalt, with alkaline undersaturated rocks in some rifts. Lagging behind the magma, the platform subsided at ≅290 Ma in basins by differential weakening of the crust during the release of Pangaean heat. The Gondwana facies accumulated in the Gondwanaland province and the Stephanian‐Rotliegend succession in Europe. The basins of Europe and eastern Australia continued to grow by transtension followed by mid‐Permian (270–265 Ma) thermal sagging and rifting. Their histories then diverged. Europe maintained its post‐orogenic course except in the Alpine region, where rifting and sagging continued to accommodate the western Tethys. The eastern fringe of Australia entered a new, Innamincka, orogenic cycle that developed in embryo at 265 Ma to a fully developed magmatic arc and yoked foreland basin at 258 Ma.

Palaeomagnetic Tests of Tectonic Models of the Tasman Fold Belt during the Neoproterozoic and Palaeozoic

Current palaeomagnetic data support a late Mesoproterozoic (1050 Ma) to mid-Neoproterozoic (725 Ma) reconstruction in which Laurentia (North America) was juxtaposed against the eastern margin of Australia and Antarctica. Neoproterozoic apparent polar wander paths (APWPs) for Laurentia and East Gondwanaland suggest that by the late Vendian (late Neoproterozoic III) the two continents were separated latitudinally by up to 50°, and by an unknown amount longitudinally. The breakup of Rodinia, arguably around 700 Ma, formed the Pacific Ocean, and implies that any ocean crust flooring the Tasman Fold Belt should date from that time. Tasmania, the only exposed Precambrian continental fragment in the Tasman Fold Belt, could have developed as part of a continental ribbon stranded in the Pacific Ocean. Most of the palaeomagnetic data from the Tasman Fold Belt comes from the Late Devonian to ?Early Carboniferous Lambian overlap assemblage, a circumstance which has helped calibrate the Australian APWP but left open questions about whether any of the older rocks are allochthonous with respect to the Gondwanan continental margin. Preliminary palaeomagnetic data from Tasmania suggest that northwestern Tasmania was in its current position with respect to the craton by the end of the Cambrian, and palaeomagnetic data from the Snowy River Volcanics in the eastern Lachlan Fold Belt are permissive of that part being adjacent to the Gondwanan continental margin by the earliest Devonian. More work is required on the Late Ordovician to Early Devonian and the mid-Carboniferous parts of the APWP of Australia and Gondwanaland, as well as in the Tasman Fold Belt, before questions about whether parts are exotic, or have rotated, can be addressed.

Significance of Devonian-Carboniferous radiolarians from accretionary terranes of the New England orogen, eastern Australia

Radiolarians provide age constraints for many previously undated terranes in the New England Orogen (NEO), a tectonic collage developed along the eastern margin of Australia. Djungati terrane, the age range of which was previously unknown, contains two distinctive siliceous sedimentary lithofacies. The oldest is a thick sequence of red, ribbon-bedded cherts which probably accumulated in a deep ocean-floor setting far from land. Middle Silurian through Late Devonian radiolarians have been recovered from these cherts. Green tuffaceous cherts which contain a latest Devonian (Famennian) radiolarian fauna depositionally overlie the lower red ribbon-bedded chert sequence. These cherts are intercalated with volcaniclastic sediments and the fauna which they contain can be used to constrain the timing of accretion of older rocks into a subduction complex. Anaiwan terrane, which was also previously undated, contains thin ribbon-bedded cherts which are depositionally overlain by tuffaceous chert, siliceous siltstones and volcaniclastic sediments. Latest Devonian (?late Famennian) and Early Carboniferous radiolarians have been recovered from these cherts and tuffaceous siltstones. Radiolarians also occur in fine-grained siliceous sediments of the Yarrimie Formation, part of the Gamilaroi terrane. These radiolarians are of Late Devonian (Frasnian) affinity and their presence indicates that blocks of limestone, which contain Givetian conodonts and corals and were previously thought to indicate the age of the Yarrimie Formation, are allochthonous.