Tamworth Belt Research Articles

Provenance of sandstones in the Hastings Block, southern New England Orogen: implications for tectonic setting

The Devonian–Carboniferous sandstones in the Hastings Block, southern New England Orogen have a predominantly arc-like provenance based on ternary QFL or QmFLt ternary diagrams and have a lower quartz content than Permian sandstones. Stratigraphically correlated formations show similar QFL components but not the same patterns of sandstone compositional variability for formations that outcrop over a wide area such as the Mingaletta and Pappinbarra formations. It is possible to correlate most of the petrological intervals in the Tamworth Belt with the Hastings Block. The trend of petrofacies from Devonian to Permian for the southern Tamworth Belt, Tamworth Belt overall and Hastings Block, is towards sandstone with more monocrystalline quartz and less feldspar and lithics. The areal pattern of sandstone compositions within individual formations shows similarities in the percentage of quartz, feldspar or lithics. This suggests areal continuity of sandstone compositions within a formation during deposition and that the faulting did not substantially change the formations continuity at the time of deposition. The arc-like provenance of Devonian to Permian sandstones within the Hastings Block indicates that the block was likely an along-strike continuation of the Tamworth Belt. KEY POINTS The Devonian–Carboniferous sandstones have predominantly arc-like provenance on QFL or QmFLt ternary plots with a higher quartz content in younger formations. The Hastings Block and Tamworth Belt sandstones of comparable age are similar but not the same on ternary QFL and QmFLt plots. The areal pattern of the composition of sandstones within formations suggests that faulting was not active during deposition.

Paleomagnetism of the Carboniferous–Permian Myall blocks, Tamworth Belt, southern New England Orogen: Permian counterclockwise rotations and Triassic clockwise rotation

This paleomagnetic study on the Myall blocks of the southern New England Orogen complements studies on four other Tamworth Belt blocks aiming to unravel evolution of the Tamworth Belt and the Manning Orocline in particular. Focus is on ignimbritic successions that have partly interlinked stratigraphies across the five blocks and can retain primary magnetisations despite prevalent overprinting. This makes the Tamworth Belt prospective to oroclinal testing by comparing poles and pole paths for individual blocks across its curved structure. Paleomagnetic, rock magnetic and magnetic fabric results are detailed from an upper lower Carboniferous to lower Permian succession, sampled across forearc basin and arc sites (64) of the western Myall Block and forearc basin sites (16) of the eastern Myall Block. Predominantly thermal demagnetisations show a widely present low-temperature component, attributed to late Oligocene weathering, intermediate-temperature primary and overprint components confined to the Nerong Volcanics, and high-temperature primary and overprint components. Overprinting proved more prevalent in the Myall blocks than in other Tamworth Belt blocks, with five phases dated from mid-Carboniferous to mid-Triassic. Comparison of primary and overprint poles against Carboniferous, Permian and Triassic reference paths shows counterclockwise rotations ranging from 20–45° for the western Myall Block forearc basin, to 30° to 90° west to east across the southwestern Myall Block arc complex, to 85–110° for the eastern Myall Block forearc basin, occurring from the latest Carboniferous–early Permian Tablelands phase probably to the mid- to late Permian initial pulse of the Hunter-Bowen phase. The southwestern Myall Block also shows a mid-Triassic, or later, clockwise rotation ranging west to east from 50° to 110°. KEY POINTS Paleomagnetism of upper lower Carboniferous to lower Permian successions of the Myall blocks constrains evolution of the Manning Orocline. The Myall blocks are heavily affected by overprints, with five phases dated from mid-Carboniferous to mid-Triassic. Primary and overprint magnetisations show various counterclockwise and clockwise rotations of the western, eastern and southwestern parts of the Myall blocks. Counterclockwise rotations occurred from latest Carboniferous–earliest Permian probably to mid- to late Permian and clockwise rotation during or after mid-Triassic.

New Carboniferous ophiuroid from central coastal New South Wales

The ophiuroid Bronthaster retus gen. et sp. nov. is described from a single specimen preserved as external dorsal and ventral moulds, and is assigned to the Protasteridae. It occurs in the Namurian (Levipustula levis Zone) Yagon Siltstone of the Myall Lakes district of central coastal New South Wales. This rock unit forms part of the structurally designated Myall Block at the southeastern coastal termination of the Tamworth Belt. This new genus is the first stelleroid to be described from the Carboniferous in Australia. Most distinctive of the new genus is the arrangement of many small, subrhomboidal to subpentagonal, overlapping ossicles on the dorsal surface of the arms. The clearly exhibited tight relationship between successive ambulacrals in one column in ventral view suggests that some revision of previously inferred relationships within ambulacral columns in protasterids is required. The position of prominent toehooks on each ambulacral ossicle shows that they functioned to prevent lateral movement of the ambulacral column, operating in conjunction with the strong longitudinal musculature to allow arm flexure in the horizontal plane. This feature could potentially identify, within the Protasteridae, an evolutionary lineage that proceeded through the middle Palaeozoic and maintained the linear arrangement of the ambulacral columns using the toehooks, rather than fusion of the ambulacral ossicles across the arm axis. Peter A. Jell [amjell@bigpond.com] and Alex G. Cook, School of Earth Sciences, The University of Queensland, St Lucia 4072, Australia.

Paleomagnetism of the Carboniferous Gresford Block, Tamworth Belt, southern New England Orogen: minor counter-clockwise rotation of a primary arc segment

Four oroclinal structures have been identified from structural, magnetic and gravity trends across a Carboniferous continental arc, forearc basin [Tamworth Belt (TB)] and conjugate accretionary complex in the southern New England Orogen (SNEO) of eastern Australia. None of the structures has yet been confirmed conclusively by paleomagnetism as oroclinal. Ignimbrites are common within the forearc basin and have been demonstrated to retain primary magnetisations despite prevalent overprinting. They are well exposed across six major tectono-stratigraphic blocks with partly interlinked stratigraphies, making the forearc basin highly prospective to oroclinal testing by comparing pole path segments for individual blocks across curved structures. Paleomagnetic studies have shown no noticeable rotation across the western/southwestern TB (Rocky Creek, Werrie and Rouchel blocks), but documented herein is a minor counter-clockwise rotation of the Gresford Block of the southern TB. This study details paleomagnetic, rock magnetic and magnetic fabric results for 87 sites (969 samples) across the southern Gresford Block. Predominantly thermal, also alternating field and liquid nitrogen, demagnetisations show a widely present low-temperature overprint, attributed to regional late Oligocene weathering, and high-temperature primary and overprint components residing in both mainly magnetite and mainly hematite carriers. Subtle, but systematic, directional differences between magnetite and hematite subcomponents show the latter as the better cleaned, better-defined, preferred results, detailing nine primary poles of middle and late Carboniferous ages and Permian and Permo-Triassic overprints as observed elsewhere in the western/southwestern TB. The primary poles update a poorly defined mid-Carboniferous section of the SNEO pole path and demonstrate counter-clockwise rotation, quantified at about 15° ± 13° from comparison of mid-Carboniferous Martins Creek Ignimbrite Member poles for the Rouchel and Gresford blocks, that may not necessarily have been completed prior to the Hunter–Bowen phase of the Gondwanide Orogeny. This minor counter-clockwise rotation of the Gresford Block accentuates a primary curvature of the southwestern/southern TB and heralds further, more complex, rotations of the Myall Block of the southeastern TB.

Middle-Late Devonian nascent back arc formation, southern New England Orogen, NSW, Australia

Dolerite dyke swarms are common in Devonian sequences of the Tamworth Belt, southern New England Orogen but have not been studied in detail. We present new geochemical and LA-ICP-MS U-Pb zircon data obtained from dolerites in the Glenrock Station area, NSW. Our study shows that most dolerites have tholeiitic to transitional basaltic compositions, a few boninitic features. Most have chondrite normalised patterns that exhibit slight or no LREE enrichment and PM normalised patterns showing varying degrees of Nb depletion and Th enrichment.Th/Yb and Nb/Yb ratios are variable and indicate moderate to no subduction zone enrichment suggesting the magmas have been emplaced in a back arc basin (BAB) setting. N-MORB type magmas are rare leading to the conclusion that a nascent back arc had developed. This contrasts with dolerites of similar age in the Nundle area that commonly show N-MORB like characteristics suggesting that greater extension had occurred resulting in a well-developed back arc basin. U-Pb zircon dating of dolerites in the study area reveals ages varying from 383 to 386 Ma, confirming ages determined from radiolarian studies of associated sediments.Trace element geochemistry suggests the majority of the magmas have been derived from depleted lithospheric and mixed lithospheric-asthenospheric sources. Extraction of the melts responsible for the dolerites appears to have taken place at moderate depths from a primitive mantle-eclogite source. The Middle Devonian rifting event recognised in the Glenrock Station area appears to have been widespread affecting both continental and oceanic regions adjacent to and within the Australian continent. It is attributed to subduction rollback that has been impeded locally possibly as a result of subduction of an oceanic plateau and ridge. Extension was then able to take place in both oceanic and continental regions.

The Keepit arc: provenance of sedimentary rocks in the central Tablelands Complex, southern New England Orogen, Australia, as recorded by detrital zircon

ABSTRACTDetrital zircon from the Carboniferous Girrakool Beds in the central Tablelands Complex of the southern New England Orogen, Australia, is dominated by ca 350–320 Ma grains with a peak at ca 330 Ma; there are very few Proterozoic or Archean grains. A maximum deposition age for the Girrakool Beds of ca 309 Ma is identified. These data overlap the age of the Carboniferous Keepit arc, a continental volcanic arc along the western margin of the Tamworth Belt. Zircon trace-element and isotopic compositions support petrographic evidence of a volcanic arc provenance for sedimentary and metasedimentary rocks of the central Tablelands Complex. Zircon Hf isotope data for ca 350–320 Ma detrital grains become less radiogenic over the 30 million-year record. This pattern is observed with maturation of continental volcanic arcs but is opposite to the longer-term pattern documented in extensional accretionary orogens, such as the New England Orogen. Volcanic activity in the Keepit arc is inferred to decrease rapidly at ca 320 Ma, based on a major change in the detrital zircon age distribution. Although subduction continues, this decrease is inferred to coincide with the onset of trench retreat, slab rollback and the eastward migration of the magmatic arc that led to the Late Carboniferous to early Permian period of extension, S-type granite production and intrusion into the forearc basin, high-temperature–low-pressure metamorphism, and development of rift basins such as the Sydney–Gunnedah–Bowen system.

Paleomagnetism of the Carboniferous Rouchel Block, Tamworth Belt, and pole path revision for the New England Orogen, eastern Australia

ABSTRACTStructural, magnetic and gravity trends of the southern New England Orogen (SNEO) indicate four oroclinal structures, none conclusively confirmed paleomagnetically. Curved structures of the Tamworth Belt (TB)—a continental forearc exposed across six tectono-stratigraphic blocks with interlinked Carboniferous stratigraphies and extensive ignimbritic rocks known to retain primary magnetisations despite prevalent overprinting—are prospective to oroclinal testing through comparison of Carboniferous pole paths for individual blocks. Pole paths (a) have been established for the Rocky Creek and Werrie blocks (northwestern/western TB), (b) are described herein for the Rouchel Block (southwestern TB), and (c) are forthcoming for the Gresford and Myall blocks (southern/southeastern TB). The Rouchel path derives from detailed paleomagnetic, rock magnetic and magnetic fabric studies. Thermal, alternating field and liquid nitrogen demagnetisations show a low-temperature overprint, attributed to late Oligocene weathering, and high-temperature (HT) primary and overprint components in both magnetite and hematite carriers, showing slight, systematic, directional differences with hematite providing the better cleaned site poles. Seven primary mean-site poles of Tournaisian and mainly Visean age and three overprint poles show six positive fold tests, five at 95% or higher confidence levels. Two dispersed groupings of intermediate (IT) and HT overprint site poles of Permian and Permo-Triassic age are attributed to early and late phases in oroclinal evolution of the SNEO. HT and IT/HT overprint site poles of mid-Carboniferous age are attributed to Variscan Australia–Asia convergence. Individual pole paths for the Rocky Creek, Werrie and Rouchel blocks show no noticeable rotation between them, indicating primary curvature for the southwestern TB. Their integrated SNEO pole path establishes a reference frame for determining rotations of the southern and southeastern TB.

Structural inversion of the Tamworth Belt: Insights into the development of orogenic curvature in the southern New England Orogen, Australia

The middle to late Permian Hunter Bowen Event is credited with the development of orogenic curvature in the southern New England Orogen, yet contention surrounds the structural dynamics responsible for the development of this curvature. Debate is largely centred on the roles of orogen parallel strike-slip and orogen normal extension and contraction to explain the development of curvature. To evaluate the dynamic history of the Hunter Bowen Event, we present new kinematic reconstructions of the Tamworth Belt. The Tamworth Belt formed as a Carboniferous forearc basin and was subsequently inverted during the Hunter Bowen Event. Kinematic reconstructions of the Tamworth Belt are based on new maps and cross-sections built from a synthesis of best-available mapping, chronostratigraphic data and new interpretations of depth-converted seismic data. The following conclusions are made from our study: (i) the Hunter Bowen Event was dominantly driven by margin normal contraction (east–west shortening; present-day coordinates), and; (ii) variations in structural style along the strike of the Tamworth Belt can be explained by orthogonal vs. oblique inversion, which reflects the angular relationship between the principal shortening vector and continental-arc margin. Given these conclusions, we suggest that curvature around the controversial Manning Bend was influenced by the presence of primary curvature in the continental margin, and that the Hastings Block was translated along a sinistral strike-slip fault system that formed along this oblique (with respect to the regional east–west extension and convergence direction) part of the margin. Given the available temporal data, the translation of the Hastings Block took place in the Early Permian (Asselian) and therefore preceded the Hunter Bowen Event. Accordingly, we suggest that the Hunter Bowen Event was dominantly associated with enhancing curvature that was either primary in origin, or associated with fault block translation during the Early Permian. This model differs to previously proposed reconstructions where curvature largely formed by orogen parallel strike-slip transportation during the Hunter Bowen Event.

Geology and geochronology of the Emu Creek Block (northern New South Wales, Australia) and implications for oroclinal bending in the New England Orogen

The southern part of the New England Orogen exhibits a series of remarkable orogenic bends (oroclines), which include the prominent Z-shaped Texas and Coffs Harbour oroclines. The oroclines are defined by the curvature of Devonian–Carboniferous forearc basin and accretionary complex rock units. However, for much of the interpreted length of the Texas Orocline, the forearc basin is mostly concealed by younger strata, and crops out only in the Emu Creek Block in the eastern limb of the orocline. The geology of the Emu Creek Block has hitherto been relatively poorly constrained and is addressed here by presenting new data, including a revised geological map, stratigraphic sections and new detrital zircon U–Pb ages. Rocks of the Emu Creek Block include shallow-marine and deltaic sedimentary successions, corresponding to the Emu Creek and Paddys Flat formations, respectively. New detrital zircon U–Pb data indicate that these formations were deposited during the late Carboniferous and that strata were derived from a magmatic source of Devonian to Carboniferous age. The sedimentary provenance and detrital zircon age distribution suggest that the sequence was deposited in a forearc basin setting. We propose that the Emu Creek and Paddys Flat formations are arc-distal, along-strike correlatives of the northern Tamworth Belt, which is part of the forearc basin in the western limb of the Texas Orocline. These results confirm the suggestion that Devonian–Carboniferous forearc basin rocks surround the Texas Orocline and have been subjected to oroclinal bending.

Provenance comparisons between the Nambucca Block, Eastern Australia and the Torlesse Composite Terrane, New Zealand: connections and implications from detrital zircon age patterns

Detrital zircon U–Pb LAM-ICPMS age patterns for sandstones from the mid-Permian –Triassic part (Rakaia Terrane) of the accretionary wedge forming the Torlesse Composite Terrane in Otago, New Zealand, and from the early Permian Nambucca Block of the New England Orogen, eastern Australia, constrain the development of the early Gondwana margin. In Otago, the Triassic Torlesse samples have a major (64%), younger group of Permian–Early Triassic age components at ca 280, 255 and 240 Ma, and a minor (30%) older age group with a Precambrian–early Paleozoic range (ca 1000, 600 and 500 Ma). In Permian sandstones nearby, the younger, Late Permian age components are diminished (30%) with respect to the older Precambrian–early Paleozoic age group, which now also contains major (50%) and unusual Carboniferous age components at ca 350–330 Ma. Sandstones from the Nambucca Block, an early Permian extensional basin in the southern New England Orogen, follow the Torlesse pattern: the youngest. Early Permian age components are minor (<20%) and the overall age patterns are dominated (40%) by Carboniferous age components (ca 350–320 Ma). These latter zircons are inherited from either the adjacent Devonian–Carboniferous accretionary wedge (e.g. Texas-Woolomin and Coffs Harbour Blocks) or the forearc basin (Tamworth Belt) farther to the west, in which volcaniclastic-dominated sandstone units have very similar pre-Permian (principally Carboniferous) age components. This gradual variation in age patterns from Devonian–late Carboniferous time in Australia to Late Permian–mid-Cretaceous time in New Zealand suggests an evolutionary model for the Eastern Gondwanaland plate margin and the repositioning of its subduction zone. (1) A Devonian to Carboniferous accretionary wedge in the New England Orogen developing at a (present-day) Queensland position until late in the Carboniferous. (2) Early Permian outboard repositioning of the primary, magmatic arc allowing formation of extensional basins throughout the New England Orogen. (3) Early to mid-Permian translocation of the accretionary wedge and more inboard active-margin elements, southwards to their present position. This was accompanied by oroclinal bending which allowed the initiation of a new, late Permian to Early Triassic accretionary wedge (eventually the Torlesse Composite Terrane of New Zealand) in an offshore Queensland position. (4) Jurassic–Cretaceous development of this accretionary wedge offshore, in northern Zealandia, with southwards translation of the various constituent terranes of the Torlesse Composite Terrane to their present New Zealand position.

Stratigraphic relationships of Carboniferous volcanogenic successions in the Clifton–Carroll block and Werrie Syncline, northern Tamworth Belt, southern New England Orogen

Late Carboniferous volcanogenic successions in the Clifton–Carroll block, west of the Werrie Syncline, represent the southernmost extremity of the Willuri Formation, a unit which extends northwards into the Rocky Creek region in northern New South Wales. Two ignimbrites, the Iventure and Tagggarts Mountain Ignimbrite Members, previously named in the Currabubula Formation, are present in the Willuri Formation in the Clifton–Carroll block. The Taggarts Mountain Member extends farther northwards through both Gunnan and Tulcumba Ridges. The western limb of the Werrie Syncline, which is separated from the Clifton–Carroll block by the Kelvin Thrust, contains an Upper Devonian to Upper Carboniferous succession. The largely non-marine, Lower Carboniferous Merlewood Formation is typified by andesite flows, two of which are major, within an area between the Donnellys Gap Fault in the north and Piallaway in the south. The Iventure and Taggarts Mountain Ignimbrite Members in the overlying Upper Carboniferous Currabubula Formation provide a correlation with the Willuri Formation in the Clifton–Carroll block to the west, and the more widespread Taggarts Mountain Ignimbrite Member with the Willuri Formation in Gunnan and Tulcumba Ridges to the north.

Evidence for Dynamic Climate Change on Sub-106-Year Scales from the Late Paleozoic Glacial Record, Tamworth Belt, New South Wales, Australia

Abstract Models of late Paleozoic ice-age climate dynamics have remained enigmatic, in part because preservation of mid- to Late Carboniferous, ice-proximal, Gondwanan glacial deposits is limited to very few Gondwanan regions. As such, detailed sedimentologic and stratigraphic analysis of six formations comprising the well-preserved, mid-Carboniferous to Lower Permian (Namurian or Serpukhovian to late Sakmarian), terrestrial, glacially influenced record from the Tamworth Belt, New South Wales, Australia, provides a unique opportunity to test existing models of late Paleozoic climate, characterize regional glaciation, and resolve the internal pacing of climate during the late Paleozoic ice age. Facies analysis based on twelve logged vertical sections and lateral facies relationships suggests that sediment accumulated in an eastward-draining, volcaniclastically influenced alluvial fan (facies association FA1), fluvial (FA2), and lacustrine (FA3) landsystem. Evidence for proglacial outwash fan (sandar), glaciofluvial, and glaciolacustrine deposition is preserved in the form of texturally diverse diamictite, outsized clasts in fine-grained facies, and rhythmically laminated sandstone and siltstone. The association of these facies, which is termed herein as “distinctly glacial facies” and is taken as direct evidence of glaciation, does not occur through the stratigraphic extent of formations, suggesting that the Tamworth Belt of New South Wales, Australia, was subject to episodic, mountain/valley-type or outlet-type, temperate to subpolar glaciation during the Carboniferous and Early Permian. Climatic shifts and trends occurred on multiple timescales, termed herein “nested cyclicity.” Alternations between distinctly glacial facies and facies that lack evidence of glacial influence, interpreted to record alternating glacial and nonglacial conditions, occurred on timescales on the order of ~ 106 years. The cited age estimates are based on published estimates of glacial interval durations from the region, which in turn reconcile all available radiogenic isotopic data and biostratigraphic zonations. Facies trends within successions recording glacial intervals are interpreted in terms of glacial advance and retreat, which can be estimated to have occurred on timescales shorter than the duration of glacial intervals (shorter-term, sub-106-yr-order timescales) and may include Milankovitch-band cyclicity. These conclusions support the emerging view that the late Paleozoic was characterized by rapid and dynamic climatic shifts on multiple timescales, rather than as one, long protracted event as portrayed by earlier models of Gondwanan glaciation. Furthermore, this study provides an improved understanding of high-latitude climate dynamics and pacing, which may prove useful in constructing improved global models of late Paleozoic climate.

Upper crustal structure of the Tamworth Belt, New South Wales: constraints from new gravity data

New gravity data along five profiles across the western side of the southern New England Fold Belt and the adjoining Gunnedah Basin show the Namoi Gravity High over the Tamworth Belt and the Meandarra Gravity Ridge over the Gunnedah Basin. Forward modelling of gravity anomalies, combined with previous geological mapping and a seismic-reflection transect acquired by Geoscience Australia, has led to iterative testing of models of the crustal structure of the southern New England Fold Belt, which indicates that the gravity anomalies can generally be explained using the densities of the presently exposed rock units. The Namoi Gravity High over the Tamworth Belt results from the high density of the rocks of this belt that reflects the mafic volcanic source of the older sedimentary rocks in the Tamworth Belt, the burial metamorphism of the pre-Permian units and the presence of some mafic volcanic units. Modelling shows that the Woolomin Association, present immediately east of the Peel Fault and constituting the most western part of the Tablelands Complex, also has a relatively high density of 2.72 – 2.75 t/m3, and this unit also contributes to the Namoi Gravity High. The Tamworth Belt can be modelled with a configuration where the Tablelands Complex has been thrust over the Tamworth Belt along the Peel Fault that dips steeply to the east. The Tamworth Belt is thrust westward over the Sydney – Gunnedah Basin for 15 – 30 km on the Mooki Fault, which has a shallow dip (∼25°) to the east. The Meandarra Gravity Ridge in the Gunnedah Basin was modelled as a high-density volcanic rock unit with a density contrast of 0.25 t/m3 relative to the underlying rocks of the Lachlan Fold Belt. The modelled volcanic rock unit has a steep western margin, a gently tapering eastern margin and a thickness range of 4.5 – 6 km. These volcanic rocks are assumed to be Lower Permian and to be the western extension of the Permian Werrie Basalts that outcrop on the western edge of the Tamworth Belt and which have been argued to have formed in an extensional basin. Blind granitic plutons are inferred to occur near the Peel Fault along the central and the southern profiles.

Carboniferous to Lower Permian stratigraphy of the southern Tamworth Belt, southern New England Orogen, Australia: Boundary sequences of the Werrie and Rouchel blocks*

Carboniferous to Lower Permian successions along the western border of the Tamworth Belt between Wallabadah and Muswellbrook were remapped to clarify the stratigraphy and establish a boundary between the Werrie and Rouchel blocks. The boundary, located at the Waverley Fault, separates Carboniferous sequences containing different formations and volcanic members. SHRIMP AS3 dating of volcanic members indicates that successions within the Rouchel and Gresford blocks were deposited, uplifted and eroded at different times. The lacustrine Woodton Formation in the Werrie block, previously considered Carboniferous, is earliest Permian (Asselian) from palaeobotanical and SHRIMP AS3 evidence. Stratigraphic and palaeoenvironmental differences between the Werrie and Rouchel blocks suggest that they were not directly juxtaposed during the greater part of the Carboniferous, supporting palaeomagnetic evidence that blocks within the Tamworth Belt are allochthonous. Superposition of the western extremity of the Waverley Fault by Lower Permian (Sakmarian) formations and intrusion of folded and faulted Devonian to Lower Permian successions by the Barrington Tops Granodiorite (ca 280 Ma) indicate that the Werrie, Rouchel and Gresford blocks were subjected to tectonism before the Late Permian Hunter – Bowen Orogeny.

Trace-element correlation of Carboniferous ignimbrites, southern New England Orogen, Australia, by factor and discrimination analysis*

Factor and discrimination analysis of trace-element abundances in Carboniferous (Visean to possibly Westphalian) ignimbrites and minor lavas from a forearc basin succession of the southern New England Orogen, are examined to test the validity of proposed stratigraphic correlations between ignimbrites in three adjacent thrust sheets in the Rocky Creek region of the northern Tamworth Belt, northwestern New South Wales. Analyses of the ignimbrites within individual fault blocks indicate that each has a signature that can be distinguished with a high level of statistical confidence from those of the other ignimbrites. Elements that are generally the most distinctive are Nb, Y, Zr and V. A comparison of units from the Boomi and Kathrose blocks and the southern part of the Rocky Creek Syncline indicates that an ignimbrite in the lower Clifden Formation, previously informally named the ‘Eastons Arm Rhyolite Member’, belongs to the Peri Rhyolite Member. The Peri Rhyolite Member marks the base of the Kiaman reversal. In the Kathrose block, an ignimbrite, previously mapped as Peri Rhyolite Member, is removed from the latter unit and assigned to unnamed dacitic ignimbrite Cl6. In terms of trace-element content, all of the sampled ignimbrites from the Rocky Creek Conglomerate have individual signatures and are unlikely to belong to the same unit. These include: the Plagyan Ignimbrite Member from the Boomi block, Rocky Creek Syncline and southern Nandewar region, the Mt Hook Rhyolite, Darthula Rhyodacite and Pound Rock Rhyodacite Members from the Darthula block, and unnamed red ignimbrite members (including R8 and R12) from the southern part of the Maules Creek Anticline. The Wanganui Andesite Member and unnamed dacitic ignimbrite Cl13 in the lowermost Clifden Formation, and the Ermelo Pyroclastics are distinctive in terms of trace-element signatures. The Wanganui Andesite Member is the youngest, normally polarised ignimbrite beneath the base of the Kiaman reversal.

Upper Carboniferous to Lower Permian volcanic successions of the Carroll‐Nandewar region, northern Tamworth Belt, southern New England Orogen, Australia

Geological mapping in the Carroll‐Nandewar region near the northwestern margin of the Tamworth Belt has identified an Upper Carboniferous to Lower Permian succession, the Willuri Formation, containing coarse volcaniclastic sediments and interbedded dacitic to rhyolitic pyroclastics, lavas and a possible dome or coulée. The Currangandi Group (new) accommodates Upper Carboniferous Volcaniclastic formations in the northern Tamworth Belt, including the Willuri Formation. Volcanic rocks in the Willuri Formation are distributed in four stratigraphically distinct, intertonguing or overlapping packages: the Kaputar, Piney Range, Tulcumba and Gunnan packages. According to SHRIMP (AS3) dating most of the succession ranges in age from ca 320 to ca 308 Ma (Namurian‐Westphalian). Dacitic and rhyolitic lavas, apparently in sequence with the succession on the northwestern part of Tulcumba Ridge, are 297–287 Ma or younger (Early Permian). They appear to disconformably overlie the Carboniferous succession and could be either part of the Willuri Formation or related to the Boggabri Volcanics. Volcanic centres near Maules Creek (Kaputar centre) and Wean (Wean centre) appear to have been the sources for volcanics in the Kaputar package and most parts of the Rocky Creek region to the northeast, and the Tulcumba and probably the Piney Range packages, respectively. In the Gunnan package a single major ignimbrite was derived from a volcanic centre in the Werrie pyroclastic field, but the provenance of other pyroclastics is unknown. Correlations based on SHRIMP ages and the distribution of pyroclastics indicate that the Willuri Formation is equivalent to most of the Currabubula Formation in the Werrie Syncline, and to the Rocky Creek Conglomerate and Lark Hill Formation in the Rocky Creek region. Boundaries of the Werrie and Rocky Creek pyroclastic fields are adjusted to incorporate a new pyroclastic field based on the Wean volcanic centre and covering the Tulcumba and Piney Range packages of the Willuri Formation.

Stratigraphy and correlation of Carboniferous ignimbrites, Rocky Creek region, Tamworth Belt, Southern New England Orogen, New South Wales

Carboniferous (Visean to Westphalian) pyroclastics and lava flows in the Rocky Creek region, used to redefine the base of the Kiaman reversal, are formally defined or redefined and the status of the main formations clarified. These units include the Caroda Formation, containing the Kooringal Dacite, Boomi Rhyolite and Barney Springs Andesite Members; the Clifden Formation with the Wanganui Andesite, Glen Idle Rhyolite, Appleogue Dacite, Bexley Rhyolite, Pine Cliffs Rhyolite and Downs Rhyodacite Members; Rocky Creek Conglomerate with the Hazelvale Rhyodacite, Mt Hook Rhyolite, Darthula Rhyodacite and Pound Rock Rhyodacite Members; and Lark Hill Formation with the Eulowrie Pyroclastic, Tycannah Rhyodacite and The Tops Rhyolite Members; a number of informal units are also described. The restriction of most volcanic units to one of the three thrust blocks (Boomi, Kathrose and Darthula blocks) of the Rocky Creek region, suggests their current relationships reflect either shortening due to overthrusting or an original distribution affected by depositional or erosional processes. A westerly increase in the proportion of ignimbrites indicates nearness to sources in that direction. Intermediate volcanism, largely confined to southern and central parts of the Boomi block in the east, began in the Visean and ended in the early Namurian. Acid volcanism also began in the Visean in the northern Boomi block but, with the exception of the Peri Rhyolite Member of the Clifden Formation, did not become widespread until later in the Namurian and Westphalian. In contrast, only acid volcanism took place during the early Namurian to Westphalian in the Kathrose and Darthula blocks. Correlations based on AS3 and SL13 SHRIMP dates illustrate a discordance of about 3% when compared with the most likely location for the base of the Kiaman reversal. The bases of both the Rocky Creek Conglomerate and Lark Hill Formation appear to be slightly diachronous.

Carboniferous palaeomagnetism of the Werrie Block, northwestern Tamworth Belt, and the New England pole path

Palaeomagnetic, rock‐magnetic and magnetic‐fabric results are presented for Carboniferous and Lower Permian volcanic and volcaniclastic rocks (Merlewood Formation, Currabubula Formation, Werrie Basalt and Boggabri Volcanics),mainly from the Werrie Block of the northwestern Tamworth Belt, southern New England Orogen. Detailed thermal demagnetisation results (91 sites, 998 samples) show two groups of magnetic components with low (<400°C; LT) and high (500–700°C; HT) unblocking temperature ranges. Rock magnetic tests indicate the HT components to reside in magnetite, with single or pseudo‐single domain and multidomain characteristics, and in hematite. Detailed demagnetisation up to 700°C demonstrates subtle directional differences between primary components, optimally cleaned and derived from the magnetite‐hematite carrier (HT‐P2), and pseudocomponents, incompletely cleaned and derived from the mainly magnetite carrier (HT‐P1). Directional results evidence three phases of magnetic overprinting which arose from: (i) a possible Middle‐Late Cenozoic regional weathering event (LT); (ii) fluid movements associated with the Permo‐Triassic Hunter‐Bowen Orogeny (HT); and (iii) formation of the Bowen‐Gunnedah‐Sydney Basin system in the latest Carboniferous ‐ Early Permian (HT). Magnetic fabric results show the Kmax and Kmin axes of the susceptibility anisotropy ellipsoids to better concentrate in stratigraphic than in geographical (in situ) coordinates, suggesting a primary depositional pattern. The prevailing north‐south alignment of Kmax axes changes to east‐west towards the top of the Currabubula Formation. The sense of transport could not be established in absence of evidence for imbrication. Well‐defined primary magnetisation components (HT‐P2) have been determined for 39 of the 69 Currabubula Formation sites and for all sites of the Merlewood Formation (7), Werrie Basalt (9) and Boggabri Volcanics (6). Currabubula Formation site results have been combined into three mean‐site results, all with positive fold tests at the 99% confidence level. They show good agreement with correlatable results from the northward adjacent Rocky Creek Block. Integration of the palaeomagnetic poles for the Currabubula Formation and a virtual geomagnetic pole for the Merlewood Formation with earlier determined Visean to Westphalian poles for the Rocky Creek Block outlines a Visean (Tournaisian?) to Stephanian pole path for the northwestern Tamworth Belt. Comparison with a preliminary pole path for the Rouchel and western Gresford Blocks shows no evidence for significant rotational deformation between the northwestern and southwestern Tamworth Belt. Comparison with the Northern Hastings Block indicates counterclockwise rotation of the latter relative to the northwestern Tamworth Belt over about 150°, in agreement with structural estimates.

Discussion and Reply: Carboniferous palaeomagnetism of the Rocky Creek Block, northern Tamworth Belt, and the New England pole path

Discussion and Reply: Carboniferous palaeomagnetism of the Rocky Creek Block, northern Tamworth Belt, and the New England pole path

The Tamworth Belt in Southern Queensland, Australia: thrust-characterized geometry concealed by Surat Basin sediments

Abstract The subsurface geometry and tectonic development of the Devonian-Carboniferous Tamworth Belt, a fore-arc basin in the New England Orogen, Eastern Australia, has been examined using seismic reflection, aeromagnetic and gravity data. The Tamworth Belt is bounded to the west by the Moonie Fault, a thrust fault, which exhibits a fault-bend-fold geometry. Two major westward-dipping fauls form the main eastern boundary, with a series of eastward-dipping backthrusts located farther to the west. The eastern margin also coincides with a gravity and magnetic ridge, similar to the gravity and magnetic pattern of the serpentinites and iron-enriched rocks that are exposed along the Peel Fault to the south. In the investigated area, the Tamworth Belt is over 50 km wide and has been shortened by at least 10 km across strike. The sedimentary succession is at least 12 km thick and is moderately folded. Within the succession, six seismic sequences were identified, each of which is separated by a major sequence boundary.