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The Drummond Basin of central Queensland preserves a large-volume succession of little studied, predominantly fluviatile, coarse-grained sedimentary rocks of mid-Mississippian age. The stratigraphy of the basin has been subdivided into three sedimentary cycles. The Cycle 1/Cycle 2 boundary records a distinct, but poorly understood change in provenance from a volcanic-dominated succession related to initial basin rifting (Cycle 1) to a quartz-rich, craton-derived succession (Cycle 2). Cycle 3 has been thought to mark a resumption of intrabasinal volcanism and related sedimentation. The purpose of this study was to enhance the understanding of the basin-wide siliciclastic sedimentation of Cycles 2 and 3, and causes for the changes in sediment provenance. This objective was achieved by constraining large-scale spatial and temporal depositional trends and investigating sediment transport pathways into and through the basin. Petrographic, QFL, paleocurrent and conglomerate clast analyses were undertaken. The observations presented here have several implications relevant to understanding the stratigraphy of the Drummond Basin and regional tectonic events at this time. Cycle 3 is revised here primarily to be a continuation of Cycle 2-style basement-derived sedimentation, rather than recording a resumption of volcanism in the area, as per prevailing models. Quartz-rich sedimentation in the Drummond Basin was, therefore, more long-lived than previously envisaged, and once established, was not significantly disrupted by volcanism. Cycle 2 formation thicknesses appear highly variable across the basin. This is unlikely to be a result of pre-existing rift-related topography as suggested in previous models. The thickness variations are more likely related to sediment bypassing and post-depositional deformation in the area. The distinctive coarse-grained, relatively quartz-rich sedimentation of Cycles 2 and 3 is unusual in its volume and extent. The sediment was transported into the basin from its southern/southwestern margin, implying long-distance transport and extrabasinal sediment supply. While the specific source terrain(s) remain unknown, one plausible tectonic driver was far-field influence of the intraplate Alice Springs Orogeny.

The Devonian subsurface Adavale Basin occupies a central position in the Paleozoic central Thomson Orogen of eastern Australia and records its tectonic setting during this time interval. Here, we have focussed on the basal volcanics of the Gumbardo Formation to clarify the tectonic setting of the basin. The approach has been to undertake stratigraphic logging, LA-ICP-MS U–Pb zircon geochronology and whole-rock geochemical analysis. The data indicate that basin initiation was rapid occurring at ca 401 Ma. The volcanic rocks are dominated by K-feldspar phyric rhyodacitic ignimbrites. The whole-rock geochemical data indicate little evidence for extensive fractional crystallisation, with the volcanic suite resembling the composition of the upper continental crust and exhibiting transitional I- to A-type tectonomagmatic affinities. One new U–Pb zircon age revealed an Early Ordovician emplacement age for a volcanic rock previously interpreted to be part of the Early Devonian Gumbardo Formation, and older basement age is consistent with seismic interpretations of uplifted basement in this region of the western Adavale Basin. Five ignimbrites dated from different stratigraphic levels within the formation yield similar emplacement ages with a pooled weighted age of 398.2 ± 1.9 Ma (mean square weighted deviation = 0.94, n = 93). Significant zircon inheritance in the volcanic rocks records reworking of Ordovician and Silurian silicic igneous basement from the Thomson Orogen and provides insight into the crustal make-up of the Thomson Orogen. Collectively, the new data presented here suggest the Adavale Basin is a cover-type basin that developed on a stabilised Thomson Orogen after the major Bindian deformation event in the late Silurian.

The origin of elevated geothermal gradients in the subsurface Thomson Orogen and the nature of the crustal basement beneath it, whether oceanic or continental, remain enigmatic. Previous studies have demonstrated that a higher crustal radiogenic input is required to explain these anomalous thermal gradients. In this study, we have investigated the nature and age of this crustal input by undertaking geochemical, geochronological and Hf and O isotope analyses of buried granitic rocks as well as evaluating the heat-producing potential of metasedimentary rocks. The mineralogy, composition and Neoproterozoic/Cambrian to Devonian age of the low to moderate heat-producing I- and S-type granitic rocks strongly contrast with the Carboniferous A-type high-heat-producing granites of the Big Lake Suite, which have been suggested to be an important contributor to the elevated geothermal gradients, near the southwest corner of the Thomson Orogen. These differences suggest the Big Lake Suite rocks do not extend into the Queensland part of the temperature anomaly. Heat production of the metasedimentary rocks is also low to moderate. Based on Hf isotope compositions of zircons characterised by mantle-like oxygen signature ( ϵ Hf(t) = –12 to +2), we propose the temperature anomaly results from the occurrence of Mesoproterozoic and/or Paleoproterozoic high-heat-producing rocks beneath the Thomson Orogen. Precambrian crust, therefore, lies well east of the Tasman line. The results do not support a Neoproterozoic to Cambrian oceanic crust, as previously suggested, but instead point to a continental substrate for the Thomson Orogen. Hf isotopes indicate an overall trend towards more isotopically juvenile compositions with a progressive reduction in the contribution of older crustal sources to granitic magmas towards the present time. Different Hf isotopic signatures for the Lachlan (ϵHf(t) = –13 to +15), Thomson (ϵHf(t) = –14 to +5) and Delamerian (ϵHf(t) = –7 to +4) orogens highlight lateral variations in the age structures of crustal basement beneath these orogens.

U–Pb dating of zircons from thin middle Permian tuffs in the Canning Basin of Western Australia by chemical abrasion-isotope dilution thermal ionisation mass spectrometry reveals a conflict with the established spore-pollen zonation. Normally, the first appearance datum of Dulhuntyispora granulata across the continent lies stratigraphically above assemblages assigned to the Microbaculispora villosa Zone. However, the youngest tuffs within non-marine facies from the M. villosa Zone in Pittston SD-1, drilled in the southwest of the Canning Basin, yielded an age of 267.04 ± 0.14 Ma, which is 1.7 million years younger than tuffs associated with the D. granulata Zone in marginal-marine facies from core holes 350–400 km to the northeast. The apparent conflict in ages is possibly due to the non-marine depositional environment having wielded a strong local influence on the palynoflora along the edge of this basin. Although the present information indicates an age 2.5 million years younger than the 266.6 Ma age previously suggested for the top of the M. villosa Zone, revisions to the ages of Roadian–Wordian spore-pollen zones are not considered justifiable without further supporting evidence. Furthermore, considerable care is needed when comparing palynological assemblages from significantly differing facies. Two basaltic sills (43.5 m and 20 m thick) immediately below the tuffaceous beds in Pittston SD-1 are coincidental, as Ar–Ar dating indicates a Late Triassic age for the intrusions.

Upper Devonian to Lower Carboniferous strata of the Campwyn Volcanics of east central Queensland preserve a substantial sequence of first-cycle volcaniclastic sedimentary and coeval volcanic rocks that record prolonged volcanic activity along the northern New England Fold Belt. The style and scale of volcanism varied with time, producing an Upper Devonian sequence of mafic volcano-sedimentary rocks overlain by a rhyolitic ignimbrite-dominated sequence that passes upward into a Lower Carboniferous limestone-bearing sedimentary sequence. We define two facies associations for the Campwyn Volcanics. A lower facies association is dominated by mafic volcanic-derived sedimentary breccias with subordinate primary mafic volcanic rocks comprising predominantly hyaloclastite and peperite. Sedimentary breccias record episodic and high energy, subaqueous depositional events with clastic material sourced from a mafic lava-dominated terrain. Some breccias contain a high proportion of attenuated dense, glassy mafic juvenile clasts, suggesting a syn-eruptive origin. The lower facies association coarsens upwards from a lithic sand-dominated sequence through a thick interval of pebble- to boulder-grade polymict volcaniclastic breccias, culminating in facies that demonstrate subaerial exposure. The silicic upper facies association marks a significant change in eruptive style, magma composition and the nature of eruptive sources, as well as the widespread development of subaerial depositional conditions. Crystal-rich, high-grade, low- to high-silica rhyolite ignimbrites dominate the base of this facies association. Biostratigraphic age controls indicate that the ignimbrite-bearing sequences are Famennian to lower-mid Tournaisian in age. The ignimbrites represent extra-caldera facies with individual units up to 40 m thick and mostly lacking coarse lithic breccias. Thick deposits of pyroclastic material interbedded with fine-grained siliceous sandstone and mudstone (locally radiolarian-bearing) were deposited from pyroclastic flows that crossed palaeoshorelines or represent syn-eruptive, resedimented pyroclastic material. Some block-bearing lithic-pumice-crystal breccias may also reflect more proximal subaqueous silicic explosive eruptions. Crystal-lithic sandstones interbedded with, and overlying the ignimbrites, contain abundant detrital volcanic quartz and feldspar derived from the pyroclastic deposits. Limestone is common in the upper part of the upper facies association, and several beds are oolitic (cf. Rockhampton Group of the Yarrol terrane). Overall, the upper facies association fines upward and is transgressive, recording a return to shallow-marine conditions. Palaeocurrent data from all stratigraphic levels in the Campwyn Volcanics indicate that the regional sediment-dispersal direction was to the northwest, and opposed to the generally accepted notion of easterly sediment dispersal from a volcanic arc source. The silicic upper facies association correlates in age and lithology to Early Carboniferous silicic volcanism in the Drummond (Cycle 1) and Burdekin Basins, Connors Arch, and in the Yarrol terranes of eastern Queensland. The widespread development of silicic volcanism in the Early Carboniferous indicates that silicic (rift-related) magmatism was not restricted to the Drummond Basin, but was part of a more substantial silicic igneous province.

The Upper Devonian to Lower Carboniferous volcanosedimentary rocks of the Yarrol terrane of the northern New England Fold Belt have previously been ascribed to a forearc basin setting. New data presented here, however, suggest that the Yarrol terrane developed as a backarc basin during the Middle to early Late Devonian. Based on field studies, we recognise four regionally applicable strati graphic units: (i) a basal, ?Middle to Upper Devonian submarine mafic volcanic suite (Monal volcanic facies association); (ii) the lower Frasnian Lochenbar beds that locally unconformably overlie the Monal volcanic facies association: (iii) the Three Moon Conglomerate (Upper Devonian - Lower Carboniferous): and (iv) the Lower Carboniferous Rockhampton Group characterised by the presence of oolitic limestone. Stratigraphic and compositional differences suggest the Monal volcanic facies association post-dates Middle Devonian silicic-dominated magmatism that was coeval with gold-copper mineralisation at Mt Morgan. The Lochenbar beds, Three Moon Conglomerate and Rockhampton Group represent a near-continuous sedimentary record of volcanism that changed in composition and style from mafic effusive (Late Devonian) to silicic explosive volcanism (Early Carboniferous). Palaeocurrent data from the Three Moon Conglomerate and Rockhampton Group indicate dispersal of sediment to the west and northwest, and are inconsistent with derivation from a volcanic-are source situated to the west (Connors-Auburn Arch). Geochemical data show that the Monal volcanic facies association ranges from tholeiitic subalkaline basalts to calc-alkaline basaltic andesite. Trace and rare-earth element abundances are distinctly MORE-like (e.g, light rare earth element depletion), with only moderate enrichment of the large-ion lithophile elements in some units, and negative Nb anomalies, suggesting a subduction-related signature. Basalts of the Monal volcanic facies association are best described as transitional between calc-alkali basalts and N-MORB. The elevated high field strength element contents (e.g. Zr, Y, Ti) are higher than modern island-are basalts, but comparable to basalts that floor modern backarc basins. This geochemical study, coupled with stratigraphic relationships, suggest that the eruption of backarc basin basalts followed widespread Middle Devonian, extension-related silicic magmatism (e.g. Retreat Batholith, Mt Morgan), and floored the Yarrol terrane. The Monal volcanic facies association thus shows similarities in its tectonic environment to the Lower Permian successions (e.g. Rookwood Volcanics) of the northern New England Fold Belt. These mafic volcanic sequences are interpreted to record two backarc basin-forming periods (Middle - Late Devonian and Late Carboniferous - Early Permian) during the Late Palaeozoic history of the New England Orogen. Silicic-dominated explosive volcanism, occurring extensively across the northern New England Fold Belt in the Early Carboniferous (Varrol terrane, Campwyn Volcanics, Drummond and Burdekin Basins), reflects another period of crustal melting and extension, most likely related to the opening of the Drummond Basin.