Variably deformed Permian sedimentary rocks are extensively preserved in the northernmost New England Fold Belt of coastal central Queensland. Publications arising from joint BMR/GSQ mapping of the area suggested that the Bowen Basin originally covered much of what is now referred to as the northernmost New England Fold Belt, with Permian sediment accumulating in a series of discrete but interconnected, north to northnorthwest-elongate depocentres (Malone, 1964; Malone et al., 1969; Dickins & Malone, 1973). Subsequent work (Kirkegaard et al., 1970; Day et al., 1978; Day et al., 1983), however, developed a model in which Permian sediments of the Gogango Overfolded Zone (within the New England Fold Belt) accumulated in a deep marine basin (the Grantleigh Trough) that was separated from the Bowen Basin to the west by a volcanically active, physiographic barrier along the line of the Connors and Auburn Arches. According to this model, the Connors-Auburn Arch was the site of a continental volcanic arc (the Camboon Volcanic Arc) which was active during Early Permian, basin-forming times. Stratigraphic and sedimentological data presented herein suggest that the Connors-Auburn Arch did not form a basin-marginal physiographic feature during accumulation of the Bowen Basin succession, and that Permian strata now incorporated in the New England Fold Belt were continuous with the Bowen Basin (consistent with the earlier work of Malone, Dickins and others). Within correlative formations, facies assemblages are identical on either side of the Connors-Auburn Arch, and no evidence of basin-marginal facies is found adjacent to that basement feature. Palaeocurrent data collected from a variety of stratigraphic levels and geographic locations indicate sediment dispersal consistently westward from a source at least as far east as the present coastline, across the Connors-Auburn Arch and into the Bowen Basin. Therefore, the deep marine Grantleigh Trough does not exist, and it is proposed that use of this term be discontinued.

Permian, Stratigraphy, New England Fold Belt, Bowen Basin, central Queensland

Permian stratigraphy, northern New England Fold Belt

Introduction

Historically, the interpretation of variably deformed Permian and Triassic rocks preserved within the New England Fold Belt of coastal central Queensland (Figure 1) has been controversial, and several critical issues remain unresolved by previous work. The area, which lies to the east of the structural Bowen Basin (Folded Zone and Nebo Synclinorium), includes north-northwest-elongate outcrop belts of 1) inferred Devonian to Lower Permian volcanic and intrusive rocks (Connors and Auburn Arch: Malone et al. 1966; Dear 1994), 2) Permian sedimentary rocks (Gogango Overfolded Zone: Malone et al. 1969; Dickins & Malone 1973), 3) Devonian to Permian sedimentary and volcanic rocks (Yarrol and Calliope Blocks: Kirkegaard et al. 1970), 4) ophiolitic and metamorphic rocks of uncertain age (Marlborough Block: Kirkegaard et al. 1970; Murray 1974) and 5) Devonian to Carboniferous metasedimentary rocks (Wandilla or Coastal Block: Fergusson et al. 1993). Most of these domains are bounded by faults that separate rocks of disparate age and character developed during the Permo-Triassic contractional, Hunter-Bowen event (Holcombe et al., this volume, b).

Contrasting interpretations of the Connors Arch basement ridge and adjacent Gogango Overfolded Zone, and their relationship to the Bowen Basin to the west (Figure 1), have been published. In contrast to earlier work by Malone, Dickins and others, Day et al. (1978) proposed that the Connors Arch was the site of a continental volcanic arc that formed the eastern margin of the Bowen Basin during Early Permian times, and that sedimentary rocks of the Gogango Overfolded Zone formed in a separate, deep-water basin, the "Grantleigh Trough" (Kirkegaard et al. 1970). As will be discussed in more detail below, this view remains popular despite significant contradictory evidence.

In this paper, we report new stratigraphic and sedimentological data which complement the structural analysis summarised in two companion papers by Holcombe et al. (this volume, a & b). These data indicate that the Connors Arch was not a physiographic barrier during accumulation of the Bowen Basin succession, that the rocks of the Gogango Overfolded Zone accumulated within a contiguous Bowen Basin, and that the entire area achieved its present configuration following crustal shortening in the latest Permian to early Late Triassic (the Hunter-Bowen Event sensu Holcombe et al. this volume, b). We suggest that there is no evidence to support the notion of a Permian volcanic arc along the Connors Arch, active during formation of the Bowen Basin, nor of the existence of a deep marine trough in the position of the so-called Grantleigh Trough. We propose that use of the term "Grantleigh Trough" be discontinued.

Early investigations in the Bowen Basin (e.g. in the Collinsville area: Reid 1929, 1930) identified that, at least locally, the eastern margin of the Basin is a structural margin coincident with the western edge of an uplifted igneous complex (the Connors Arch of Malone et al. 1966; Connors-Auburn Arch of Day et al. 1978). The Geological Map of Queensland (1953) applied the earlier terminology of Lower Bowen Volcanics used by Jack and Etheridge (1892) to all of the volcano-sedimentary sequences exposed at the base of the Permian succession within the Basin and extending across the Connors Arch, although most of the Permian rocks within the New England Fold Belt (NEFB), or Eungella-Cracow Mobile Belt as it was then known, were identified as undifferentiated Upper Palaeozoic.

The early history of the Bowen Basin was summarised by Malone (1964) and Dickins and Malone (1973), drawing on the results of 1:250 000 scale mapping (Malone et al. 1964, Malone et al. 1966; Jensen et al. 1966; Malone et al. 1969). They showed a series of palaeogeographic maps, based principally on a stratigraphic analysis, in which the basin developed in terms of an early Lower Permian series of volcanic provinces and volcano-sedimentary sub-basins (the Reids Dome Beds in the southwest, the Lizzie Creek Volcanics and Carmila Beds in the north, and the Camboon Andesite in the southeast) overlain by marine sedimentary rocks (Back Creek Group) that were deposited across the southern Connors Arch and farther east than the present coastline. This interpretation recognised that the original area of sedimentation extended outside the present structure that is the Bowen Basin.

Dickins and Malone (1973), nonetheless, expressed difficulty in resolving the stratigraphic association of formations that overlay the Connors-Auburn Arch due to the effects of deformation in the Gogango Overfolded Zone, and gave a confused assessment of all elements in the southeastern part of the basin. It appears that they regarded the Rannes Beds as a composite stratigraphic package that could contain rocks ranging in age from Siluro-Devonian through to pre-Upper Permian, broadly characterising them as "Lower Permian and older". They described the Rannes Beds as partly equivalent to, but predominantly younger than, the Camboon Volcanics, a relationship supported by Kirkegaard et al. (1970), and figured them as unconformably overlain by both marine and non-marine Permian sediments (their Figure 7). Similarities in lithology to sediments of the Back Creek Group and Boomer Formation were noted, and it appears that they regarded local in-faulting as a possible cause for the inclusion of later Permian strata within the Rannes Beds.

Uncertainty as to the stratigraphic significance of the Rannes Beds also led Dickins and Malone (1973) to difficulties in the interpretation of sequences preserved east of the Bowen Basin in the New England Fold Belt. Correlation of the Youlambie Conglomerate with the Carmila Beds or lower Back Creek Group was suggested on fossil evidence, although they were unable to determine whether the Youlambie Conglomerate formed within an eastern basin that was continuous with, or separated from, the Carmila Beds or Back Creek Group. In their summary, they did not develop a model that allowed for deposition outside the preserved areas of exposure of all of the stratigraphic units.

Kirkegaard et al. (1970) regarded the Rannes Beds as a discrete formation deposited within the Grantleigh Trough, a downwarp that included the terrestrial Camboon Volcanics in the west, and marine rocks of the Rannes Beds and Rookwood Volcanics. In a general sense, Dickins & Malone (1973) followed this interpretation with their early sub-basin model. Dickins and Malone suggested that this region may have been deformed prior to the deposition of Late Permian marine sediments (upper Back Creek Group), whilst Kirkegaard et al. suggested uplift possibly accompanied by deformation prior to accumulation of the Moah Creek Beds, which they equated "at least in part" with the Boomer Formation.

Day et al. (1978), in developing a model for the tectonic evolution of the Tasman Fold Belt, showed an initial downwarp developing over the Bowen Basin during the Late Carboniferous to Early Permian (the Combarngo Volcanics and associated continental sediments: Day et al. 1983) followed by Early to mid-Permian development of the Bowen Basin "to the west of the Camboon Volcanic Arc". The Camboon Volcanic Arc was interpreted to have developed along the site of the present Connors-Auburn Arch. They specified the volcanics along the western side of the Connors Arch as forming the basal sequence of the Bowen Basin, and the Grantleigh Trough as a deep trough east of the arc.

The model of Day et al. (1978) marks a significant change from the earlier history outlined by Dickins & Malone (1973) in that:

• volcanism during the early history of the basin is sourced from along the Connors-Auburn Arch, rather than from within the troughs as envisaged by Dickins and Malone, implying the presence of a significant constructive edifice that should reasonably have supplied abundant primary and reworked volcanic detritus to the basin. The concept of the arc was also developed as a feature related to westward-directed subduction, a concept not embodied in any of the previous literature.
• volcanics cropping out along the western flanks of the Connors and Auburn Arches were described as the "basal sequence of the Bowen Basin" whilst volcanic and volcaniclastic rocks east of the Connors Arch were not related to the basin, leaving the resultant impression that the eastern margin of the basin was coincident with the western margin of the Connors and Auburn Arches.
• the concept of the Grantleigh Trough as a separate elongate basin formed east of, and coevally with, the arc was further developed.

The impression as to the limited extent of the basin was reinforced by the absence of any further discussion of basin development. Day et al. (1983) later stated that the main marine depositional trough of the basin was along the presently-defined northeastern structural margin, associating the sediment accumulation with local formation names (Gebbie and Blenheim sub-groups). They also emphasised the distinction of the Grantleigh Trough as a deep-water trough containing "flysch-type sediments" and spilitic pillow basalts "with gross features typical of ocean floor basalts".

Subsequent interpretations of the geological evolution of the eastern Bowen Basin and northern New England Fold Belt (Henderson 1980; Harrington & Korsch 1985; Murray et al. 1987; Hammond 1987; Fergusson & Leitch 1993) have not significantly challenged the palaeogeographic implications of the Day et al. model for the Permian.

Fergusson (1991) developed the first well-constrained model for the crustal architecture beneath the southern Connors Arch as a structural culmination developed during the latest Permian to Middle Triassic contractional deformation that closed the basin (the Hunter-Bowen Orogeny). Our research suggests, in addition, that the north-northwest trend of Connors Arch north of Rockhampton may be a consequence of west-southwest-directed thrusting which, as well as producing the intense deformation within the Gogango Overfolded Zone, may have imposed a regional structural fabric that does not reflect the original orientation of the basin margin.

Sedimentological data presented here indicate that, at least until the latest Permian, the present eastern limit of exposed basin rocks should not be interpreted in terms of a basin margin (Fielding et al. 1994; in general agreement with Malone 1964; Dickins and Malone et al. 1973). Stratigraphic units can be correlated over at least 350 km south to north from the southeast Bowen Basin into the Gogango Overfolded Zone, and from west to east across and around the basement inliers of the Connors and Auburn Arches. Facies within stratigraphically equivalent formations on either side of the Connors Arch, particularly in the middle Permian marine units, are identical and do not indicate a basin-marginal location. Palaeocurrent data gathered from these units indicate similar (westward) sediment dispersal directions on both sides of the Arch.

On this basis we support the earlier conclusion of Reid, Malone, Dickins and others that during the Permian period, the Bowen Basin extended for some distance to the east of the present position of the Connors Arch. Given the intensity and nature of deformation associated with closure of the basin during the Hunter-Bowen Orogeny, the original margin may be only locally preserved within onshore Queensland and its position remains speculative.

The major Permian stratigraphic units encountered in the Strathmuir Synclinorium and northern Gogango Overfolded Zone can be correlated with lithologically similar units of equivalent age in the eastern Bowen Basin (Figure 2). North-south correlations along the present eastern edge of the Bowen Basin and into the Gogango Overfolded Zone are illustrated in Figure 3. Detailed logs of measured sections through parts of these formations are shown in Figure 4 as representative illustrations of facies.

In an earlier paper (Fielding et al. 1995), the Bowen Basin stratigraphy was divided into three gross packages, interpreted in terms of Early Permian crustal extension, mid Permian passive thermal subsidence, and Late Permian to Middle Triassic, foreland thrust load-induced subsidence. Each phase of basin-forming activity has left a distinctive record in terms of gross cross-sectional stratal geometry, sediment composition (petrofacies), lithofacies assemblage and interpreted sediment dispersal patterns. The Permian stratigraphy of the northernmost NEFB can also be reconciled to this model.

The characteristics of the various formations are discussed in more detail below, in ascending stratigraphic order. Throughout this paper, correlations used are consistent with those of Draper et al. (1990), with reference to additional biostratigraphic data of Dickins & Malone (1973) and Briggs (1993). The absolute timeframe is that of Jones (1996).

Over much of the exposed eastern margin of the structural Bowen Basin, late Early Permian sedimentary strata unconformably or disconformably overlie volcanic, volcaniclastic and minor sedimentary rocks of mid-Carboniferous to Early Permian age, referred to the Camboon, Connors and Lizzie Creek Volcanics (Dickins & Malone 1973; Dear 1994) and considered by these authors as components of the Connors-Auburn Arch. Similar relationships have been noted on the eastern side of the Connors Arch, where Permian sediments of the Carmila Beds and Calen Coal Measures overlie Connors and Lizzie Creek Volcanics. We have found volcanic rocks of similar compositional variety and mode of origin within the Lizzie Creek and Connors Volcanics on both sides of the Connors-Auburn Arch, within the Camboon Volcanics on the western side and within the lower part of the Carmila Beds on the eastern side. Accordingly, given the limited available geochronological data (see Dear 1994; Holcombe et al. this volume, a, for more details), we regard these volcanic successions as broadly coeval. In each case, the top of the volcanic succession is marked by an abrupt contact with overlying, mainly fine-grained, Early Permian sedimentary rocks (e.g. Figure 4A). Locally, the Permian sedimentary section contains volcanic and intrusive rocks of predominantly basaltic composition, with minor felsic ignimbrites (e.g. Figure 4B). Such relationships have been observed throughout the NNEFB in the study area, although in some eastern parts (e.g. east of the Rookwood Thrust: Fig. 2), sedimentary rocks of the Yarrol Block form basement to Permian strata.

The Connors Volcanics and equivalents are a composite succession of lavas, intrusives and volcaniclastics of widely varying composition. In the southern Connors Arch, Dear (1994) recognised four discrete and correlatable volcanic units separated by extensive conglomerate bodies. Although these units cannot as yet be correlated regionally, a similar, composite architecture has been noted within the Camboon Volcanics at Cracow (Jones 1994) and the Connors Volcanics near Nebo (McPhie & Fielding, unpublished data). The context of these volcanic rocks is as yet uncertain (see discussion in Holcombe et al. this volume, a), and there is no clear indication as to whether a significant constructive edifice existed during the eruption of these volcanics (along the present Connors Arch). The consistent eastward palaeocurrent direction evident from sedimentary structures in volcanic clast conglomerates within and immediately overlying the volcanics on both sides of the Connors Arch may, however, indicate the existence of a north-south extensive, high-profile volcanic terrain during Late Carboniferous to earliest Permian time (Figure 5A). A marine environment to the east of the volcanic system is indicated locally by the occurrence of Early Permian marine invertebrate fossils (Fauna 1 of Dickins et al. 1964; see also Briggs 1993) within the basal conglomerates (e.g. at Charon Point peninsula, BMR locality DU179: Figure 4A). The context of other similar fossil occurrences within the Lizzie Creek Volcanics of the Connors Arch (e.g. BMR localities SL59, SL60: Malone et al. 1969) is as yet unclear.

The contact between volcanic basement and Permian sedimentary strata may be diachronous. Radiometric dates (summarised in Webb & McDougall 1968; Holcombe et al. this volume, a; Allen et al. in prep.) indicate that magmatism in the Connors-Auburn Arch extended into the Permian, whereas sedimentary rocks equated with the overlying Bowen Basin succession are known to have formed as far back as the basal Permian (e.g. SHRIMP date of 293 Ma from zircons in felsic ignimbrite within sediments of Carmila Beds at Dumbleton Rocks near Mackay: Figure 4B, Allen et al. in prep.). From these relationships, it is suggested that the extensional event that formed the Bowen Basin may have begun in the earliest Permian or even Late Carboniferous, and may have been continuous through the magmatic episode that produced the Connors-Camboon Volcanics into basin formation (Holcombe et al. this volume, a).

Thick (up to 3000 m: Kirkegaard et al. 1970), laterally variable successions of mainly non-marine, Early Permian clastic sedimentary rocks overlie volcanic or sedimentary basement over large parts of the eastern Bowen Basin and adjacent NEFB. West and south of Rockhampton, these strata form the Youlambie Conglomerate and locally the uppermost parts of the Camboon Volcanics, whereas to the north they form the upper parts of the Carmila Beds and Lizzie Creek Volcanics, and lower part of the Calen Coal Measures.

Complete sections through this thick succession (including enclosing units) have been logged at Scrub Creek west of Rockhampton (8951-990158 to 957127: Figure 6), and Hazelwood Creek southwest of Eungella (8555-537499 to 476492: Fielding, Stephens, McPhie, unpublished data). In both cases, the largely non-marine Permian sedimentary strata overlie volcanic and sedimentary basement of uncertain age. Outcrop patterns in the Scrub Creek area demonstrate interfingering between the Youlambie Conglomerate and Rookwood Volcanics (which have been found to contain Artinskian foraminifera: O’Connell 1995), and marine invertebrate macrofossils referrable to the Artinskian E. warwicki or lower E. preovalis biozones of Briggs (1993; personal communication 1995) have been found at a horizon within the Youlambie Conglomerate below the Rookwoods (Figure 6). In general, and in both the sections noted above, the Early Permian rocks are overlain by sandstones and siltstones containing middle Permian marine faunas.

Conglomerates dominated by subangular to well-rounded clasts of volcanic and intrusive rocks with a volcanic lithic matrix are commonly, but not ubiquitously, developed at the base of the succession. At a few localities near the present coast, the conglomerates are interbedded with bioclastic (and siliciclastic debris-rich) limestones which contain Sakmarian fossils (noted above: Figure 4A). Elsewhere (e.g. Hazelwood Creek to Nebo), the basal part of the unit is dominated by basaltic sandstones rich in fresh, first-cycle volcanic detritus.

Where present, the basal, coarse clastic rocks typically pass upward into thick successions of interbedded claystones and siltstones, thin-bedded siltstone-fine sandstone, discrete sandstone bodies, minor coal and locally thick conglomerates with well-rounded clasts of granite and volcanic lithologies (e.g. Figure 4B). In some areas (e.g. along the western edge of the northern Connors Arch between Nebo and Blenheim station), the succession contains numerous intraformational breccias with clasts of sandstone and siltstone up to several metres in diameter. Direct evidence of contemporaneous basaltic and felsic volcanic activity is locally preserved in the form of basalt lavas and high-level, pillowed intrusives, plus minor dacitic to rhyolitic ignimbrites and airfall tuffs, while indirect evidence abounds in the abundance of first-cycle volcanic clasts and the typically first-cycle volcanic provenance of sandstones. A feature of these rocks is the preservation of intra-formational unconformities (e.g. Figure 4B).

These facies are interpreted as the products of lacustrine and fluvial environments in rapidly subsiding, tectonically and volcanically active basins, and are similar in most respects to the stratigraphically equivalent Reid’s Dome beds in the Denison Trough to the west (Draper & Beeston 1985). Basement clast conglomerates are interpreted as alluvial fan and plain deposits shed from elevated terrains composed of older volcanic and intrusive rocks. Local occurrences of fossiliferous, bioclastic limestones indicate occasional marine transgressions to at least the easternmost part of the study area. Intraformational breccias, which are particularly characteristic along the western side of the northern Connors Arch, are interpreted as subaqueous slide and mass flow deposits down a tectonically steepened, subaqueous surface near an active fault or faults. Palaeocurrent data from these rocks typically indicate complex patterns of sediment dispersal, in some cases centripetal into areas of known outcrop (Figure 5B).

Dickins & Malone (1973) suggested that these sediments accumulated in discrete sub-basins, a view we support. We further suggest that these sub-basins were formed by limited continental extension in the Early Permian, as part of a suite of such basins that includes the Denison, Arbroath and Bogong Troughs further to the west and southwest. Remnants of two such (possibly half-graben) sub-basins can be defined along the western side and eastern sides of the Connors Arch, possibly originally separated by a basement ridge that supplied some sediment (Figure 2), and further basin complexes occupied parts of the Gogango Overfolded Zone and the area around Biloela in the southeast (see palaeogeographic maps of Fielding et al. 1995). This succession of predominantly fine-grained sedimentary rocks forms the earliest, clearly distinguishable record of Bowen Basin sediment accumulation.

In the Gogango Overfolded Zone west of Rockhampton, the upper part of the extensional phase succession contains a thick pile of basaltic intrusive and subaqueous eruptive rocks with minor sediments (the Rookwood Volcanics: Kirkegaard et al. 1970; O’Connell 1995). Our mapping has shown that this unit overlies, but also passes eastward into, the Youlambie Conglomerate (Figure 6), and is overlain by a fine-grained marine sedimentary sequence. The whole package is carried on the Rookwood Thrust which surfaces to the west (Figure 2 of Holcombe et al. this volume, b). Facies relationships (O’Connell 1995) suggest sediment and lava accumulation on an uneven, tectonically active submarine surface of uncertain depth.

Marine fossil-bearing strata are scattered throughout the Early Permian extensional package, but are particularly abundant in the uppermost part. These clastic and carbonate sedimentary rocks, biostratigraphically dated as late Artinskian (Briggs 1993, and see Figure 2), include the Fairyland and Buffel Formations of the Cracow area, the Tiverton Formation further north and unnamed equivalents. Parfrey (1986) interpreted a fauna collected from volcanogenic sandstones, mapped as part of the Camboon Volcanics near Biloela, as correlative with the Buffel and Tiverton Formations. Lateral facies and thickness changes are characteristic of these formations, and Draper (1988) has demonstrated their confinement within extensional sub-basins in the Cracow area. Locally, well-sorted bioclastic limestones of this age occur on both the western (e.g. Yatton Limestone) and eastern (e.g. in the headwaters of Leura Creek: Figure 4D) sides of the Connors Arch (Malone et al. 1969), and of the Auburn Arch farther south (Dear et al. 1971).

Overlying the Lower Permian units described above, and locally directly overlying basement (eg., Figure 4C), are laterally extensive, sheet-like accumulations of marine strata (Figure 2). In the southeast, these rocks are referred to the Oxtrack and Barfield Formations (Dear et al. 1971), which have been mapped both to the west and east of the Auburn Arch between Cracow and Biloela. Farther north, identical facies of equivalent age have been mapped as Gebbie sub-Group and undifferentiated Back Creek Group on the western side of the Connors Arch, and as undifferentiated Back Creek Group and Moah Creek Beds to the east (Malone et al. 1969; Kirkegaard et al. 1970). Elsewhere across the Gogango Overfolded Zone, more strongly deformed but otherwise identical strata have been mapped as Rannes Beds and Boomer Formation (Figure 2, Malone et al. 1969; Kirkegaard et al. 1970; and see Holcombe et al. this volume, b for discussion).

The base of this marine succession is an hiatal surface across much of the Bowen Basin (Fielding et al. 1995). Overlying this surface in some areas, notably on the flanks of basement blocks, are thin bioclastic limestone or calcareous sandstone units. In the south, the carbonates are referred to the Oxtrack Formation (Kungurian to Ufimian), and have again been recognised both to the west and east of the Auburn Arch. These limestones contain a distinctly different fauna to that of the underlying Buffel Formation and equivalents (Fauna IV: Dickins & Malone 1973; Echinolosia n. sp. B/C biozones of Briggs 1993).

The carbonates are overlain, or in places the basement is directly overlain, by thick, monotonous successions of fossiliferous, thin-bedded sandstone-siltstone strata (Figure 3) containing Ufimian to Kazanian faunas (Fauna IV, Echinolosia n. sp. D to Pseudostrophalosia n. sp. Zones). Interbedding is rhythmic (Figure 4C-G), with varying proportions of the two principal lithologies. Sandstone beds vary from c. 0.1 m to 1.0 m in thickness overall, but individual exposures typically show relative consistency of bed thickness. Sedimentary structures are mainly ripple cross-lamination with some cross-bedding, flat lamination and soft-sediment structures, and rare flute and other sole marks. Palaeocurrent data collected from such structures consistently indicate westward sediment dispersal (Figure 5C). A variety of marine body and trace fossils has been recorded from these rocks, along with scattered wood debris. Locally (e.g. in road cuttings near the crest of the Gogango Range: 8950-994736), hummocky cross-stratification was noted but this structure is uncommon.

The carbonates are interpreted as coastal and shallow marine deposits, formed during the initial stages of passive, slow subsidence (cf. Draper 1988). The overlying clastics are interpreted as deposits of mainly offshore marine shelf or ramp environments, where coarse sediment was introduced by offshore-directed (westward), meteorological currents, possibly as turbulent underflows. Hummocky cross-stratified units may reflect intermittent shallower water depths in some areas.

Notwithstanding the variation in structural style, the middle Permian marine strata are palaeontologically similar (Dickins & Malone 1973), and are here regarded as part of the same succession (Figure 2). Graphic logs from sections exposed in units mapped as "undifferentiated Back Creek Group", Boomer Formation and Moah Creek Beds on both western and eastern sides of the Connors Arch (Figure 4) indicate no significant differences in lithofacies assemblage. Furthermore, the eastern part of the Duaringa 1:250,000 geological map shows along-strike contacts between rocks mapped as Rannes Beds and Boomer Formation. The Rannes Beds are distinguished only by their more intense structural deformation, and are therefore included in this association. None of the facies recognised in this suite of rocks is considered indicative of basin marginal situations: there are virtually no basement clast conglomerates, and no facies transitions that might indicate shoaling onto an emergent Connors-Auburn Arch. Furthermore, palaeocurrent data collected from widely distributed localities (Figure 5C) indicate consistent westward sediment dispersal on both sides of the Arch, and are interpreted to reflect sediment derivation from a source terrain to the east of the present outcrop belt (and by inference, east of the present coastline). The succession accumulated during a period of regional, passive subsidence, interpreted to reflect thermal relaxation of the earlier extensional terrain.

Punctuating this monotonous succession of thin-bedded strata are at least two discrete intervals dominated by chaotic breccias, diamictites and conglomerates. These intervals have been recognised north-south for at least 350 km (Figure 3), and occur both on the west and east sides of the Connors-Auburn Arch. The coarse-grained rocks contain both rounded basement clasts, and blocks and slabs of intraformational sandstone and siltstone, typically suspended within a poorly sorted, fine-grained matrix which is also rich in detrital wood. They are interbedded with, and enclosed by, interbedded sandstone-siltstone strata identical to those noted above and which contain marine trace and body fossils. Fielding et al. (in press) interpret these rocks as the products of tectonic destabilisation of a submarine surface, and suggest that they may indicate the onset of foreland thrust-induced subsidence in the Bowen Basin. The middle Permian marine succession may therefore be said to contain a record of the transition from passive, thermal subsidence to thrust load-induced, foreland basin-style subsidence.

The middle Permian marine strata coarsen upward into coarse-grained clastic facies that are devoid of marine biota. In the south, this occurs in the lower part of the Gyranda Formation (Banana Member), which is only recognised on the western side of the Auburn Arch. Further north, the same transition occurs at the boundary between the Moah Creek Beds and Dinner Creek Conglomerate (e.g. Figure 4H), but has to date been recognised only east of the Connors Arch within the Gogango Overfolded Zone. This most likely reflects the poor quality of exposure in the Bowen Basin proper and the masking effect of the Tertiary Duaringa Basin, rather than its explicit non-occurrence. In its most easterly exposure, the Dinner Creek Conglomerate lies unconformably on Carboniferous sedimentary rocks of the Yarrol Block (Kirkegaard et al. 1970).

The lower Gyranda Formation (Banana Member) coarsens up from body and trace fossil-bearing, tuffaceous siltstones into sandstone-dominated strata containing a variety of current-generated structures. The upper half of the unit is predominantly interbedded tuffaceous sandstones and conglomerates which contain well-rounded clasts of mainly volcanic lithologies. Palaeocurrent data indicate sediment transport to the west and south (Figure 5D, and see Miller 1992). The Moah Creek Beds-Dinner Creek Conglomerate transition is similar (Figure 4H), and coarsens upward into well rounded-clast conglomerates dominated by metasedimentary quartzite clasts. Tuffaceous siltstones and fine sandstones that are interbedded with conglomerate bodies are rich in plant fossil debris, including well-preserved Glossopteris leaves. Palaeocurrent data indicate sediment transport towards the west-southwest (Figure 5D).

The coarsening-upward sequences described above are interpreted to record the final infilling of the marine Bowen Basin. The thickness of these coarsening-upward intervals gives an approximation of water depth of 20-100 m during this sedimentation. The abrupt transition between offshore marine silts and non-marine coarse clastics illustrated in Figure 4H, on the other hand, is suggestive of a relative base-level drop causing incision of the depositional surface. The notion of a drop in relative base-level preceding accumulation of the conglomerates is also supported by the direct unconformity between the Dinner Creek Conglomerate and Carboniferous basement at Stanwell. The conglomerates are interpreted as the products of mainly high-energy fluvial processes, which delivered sediment into the basin from a source area to the east. Contemporaneous volcanic activity is indicated by the overwhelmingly volcanic lithic and tuffaceous composition of silts and sands. The source and context of this volcanic activity are at present uncertain.

In the south, the Gyranda Formation is overlain by the uppermost Permian Baralaba Coal Measures and equivalents, which are extensive across the entire Bowen Basin (Fielding et al. 1993). A conglomerate-dominated succession close to the present structural Bowen Basin margin (from Theodore southward) passes westward towards Moura and upward stratigraphically into more sandstone-dominated and coal-rich facies. Palaeocurrent data indicate westward sediment dispersal (Figure 5D). These patterns can be traced northward as far as Baralaba, but are then obscured by Tertiary and Quaternary strata of the Duaringa Basin. No latest Permian coal measures are known from within or east of the Gogango Overfolded Zone - this may be due to non-deposition, subsequent erosion, or may indicate that the Dinner Creek Conglomerate is in part equivalent to the Baralaba Coal Measures.

The latest Permian coal measures are interpreted as the products of high-energy alluvial environments close to the basin margin, which passed downdip (westward) into deposits of large, meandering rivers and extensive floodbasin and wetland environments (see Fielding et al. 1993, 1996 for more details). As such, the coarse alluvial facies may indicate the early stages of westward migration of the eastern basin margin during the latest Permian.

The similarity in character of the Permian succession across the Bowen Basin and Gogango Overfolded Zone suggests that it accumulated in a single sedimentary basin. Stratigraphic thicknesses, sequence architecture, lithofacies assemblages and palaeocurrent distributions are remarkably similar. On this basis, we propose that the Permian succession of the NNEFB described herein formed within a single depositional entity of which the structurally-defined Bowen Basin is only a part.

The succession began to accumulate during the earliest Permian, at least locally, in response to a long-lived episode of modest crustal extension. This extension pulled apart a basement terrain which, in the study area, was a complex of volcanic, intrusive, sedimentary and metasedimentary rocks of Devonian to earliest Permian age. A series of discrete, probably fault-bounded sub-basins formed, some of which were initially sites of basaltic (and occasionally felsic) magmatic activity. A basin-and-range topography caused complex patterns of sediment dispersal into the early sub-basins. Some coarse sediment was supplied from emergent basement highs (including, we suggest, a ridge or ridges along parts of the present Connors-Auburn Arch: Figure 2), while much of the material was probably derived from contemporaneous silicic and/or basaltic volcanism.

During the latter stages of this extensional phase, particularly in the east, marine incursions became increasingly frequent. Individual sub-basins, now parts of the allochthonous Gogango Overfolded Zone, became the sites of voluminous, subaqueous basaltic magmatic activity (represented by the Rookwood Volcanics; see Fielding et al. 1995 for a palaeogeographic map). Lithostratigraphic and palaeontological data (O’Connell 1995) argue strongly for a shallow marine environment of formation for the Rookwood Volcanics, in contrast to the deep marine, "flysch trough" envisaged by Day et al. (1983). Importantly, no evidence exists for a volcanic "arc" active within the region during this part of the Permian period.

This period of extension was followed by an hiatus (Figure 2), which may have been related to upwarp of the thinned, extended crust as has been described from other extensional basins (e.g. Busby & Ingersoll 1995). Fielding et al. (1995) suggested that this hiatus might correlate with a 260 Ma contractional event recognised in southeast Queensland, but recent revision of the absolute timescale suggests that this hiatus occurred c. 275-272 Ma (Jones 1996, and see Figure 2). The largely infilled extensional topography subsided slowly and evenly across the entire area, giving rise to a basin-wide transgression and the accumulation of successions of relatively fine-grained, shallow marine strata. Facies and palaeocurrent data demonstrate that, after the initial stages of the transgression when basement highs may have remained as relative highs (e.g. giving rise to the carbonate deposits of the Oxtrack Formation, etc.), none of the original basement terrains that separated extensional sub-basins remained emergent. We confirm the conclusion of some earlier workers (summarised by Dickins & Malone 1973) that marine sediments must have covered the entire area now forming the Connors and Auburn Arch during the middle part of the Permian, and suggest that these sediments were sourced from an emergent terrain some distance to the east of the present limit of exposure.

The foregoing argues that there is no basis for the notion of a deep marine, "flysch trough" in what is now the Gogango Overfolded Zone. Palaeocurrent data indicate sediment dispersal across the entire area towards the main part of the Bowen Basin, rather than focusing into a discrete basin (Figure 5). No facies that might be indicative of a deep, tectonically active basin are preserved, and the thickness estimates provided by previous workers are exaggerated by structural repetition (e.g. the Rannes Beds; Holcombe et al. this volume a, b). We therefore propose that use of the term "Grantleigh Trough" be discontinued.

The period of passive thermal subsidence (~272-267 Ma: Figure 2) was terminated by the onset of thrust load-induced subsidence, as recorded by the disorganised submarine conglomerates in the Barfield Formation and Moah Creek Beds. The succeeding stratigraphic units are interpreted to record gradual, probably pulsed encroachment of thrust fronts from the east (Holcombe et al. this volume, b), leading to occlusion of the marine basin and establishment of alluvial plain conditions across the entire region. As for the thermal sag-phase sediments, facies and palaeocurrent patterns indicate that sediments accumulated continuously, and without interruption, across present basement ridges.

Sediment accumulation in alluvial and lacustrine environments continued in the Bowen Basin proper until latest Middle Triassic times (Fielding et al. 1995). Thin-skinned thrust deformation of the entire basin occurred at around 235-230 Ma, terminating sediment accumulation. Little record of the Triassic is preserved in the northernmost New England Fold Belt. One of few possible Triassic units is the Native Cat Andesite, which crops out in the area around Stanwell west of Rockhampton. Fergusson et al. (1994) used the apparently flat-lying aspect of this unit as evidence that it postdates the main Triassic thrusting event, although there are no geochronological data and field relationships are equivocal. Based on cross-cutting relationships in the Bowen Basin, Korsch et al. (1992, 1995) interpreted the basin-wide thrusting event of the Hunter-Bowen Orogeny as Late Triassic in age, which is in accord with our data from the Capricorn Region (Holcombe et al. this volume, b). We agree with the structural interpretation of Fergusson (1991 et seq.) that the Connors Arch became an elevated basement high during this thrusting episode.

Permian sedimentary successions are preserved within a fold-thrust belt of Late Triassic age in the northernmost New England Fold Belt of coastal central Queensland. The nature of these strata, in terms of lithostratigraphic architecture, lithofacies assemblages and palaeocurrent distributions suggests that they formed as part of the Bowen Basin. This interpretation is in contrast with some previous interpretations which regard the Bowen Basin as separated from a deep marine basin to the east (the Grantleigh Trough) by volcanism along a basement ridge (the Camboon Volcanic Arc along the Connors Arch). We find no evidence to support the notion of an upstanding Permian volcanic arc along the Connors-Auburn Arch during sediment accumulation in the Bowen Basin, and no evidence that Permian rocks of the Gogango Overfolded Zone were separate from the basin except during Early Permian extension when the area was divided into discrete sub-basins.

In accord with Malone, Dickins and co-workers, we suggest that the original Bowen Basin extended at least as far east as the present coastline during the Permian, and that source areas for the preserved Permian strata subsequently have been rifted offshore during Cretaceous continental breakup (Falvey and Mutter 1981; Ewart et al. 1992). We suspect that the basin margin advanced progressively westward in the Early and Middle Triassic in response to a mountain building episode associated with the Hunter-Bowen Orogeny. We support the view put by Fergusson (1991) that the Connors Arch is an allochthonous basement block emplaced during Hunter-Bowen thrusting in the Late Triassic, when much, if not all, of the coastal central Queensland area was transported westward as thrust sheets during contraction (Holcombe et al. this volume, b).

The data presented here, and new dating by us and other workers, requires significant revision of current concepts of Bowen Basin history and definition of the Bowen Basin. The three-phase evolution for the basin development, and our stratigraphic analysis, clearly defines two distinct tectonic events that gave rise to sediment accumulation in the basin, an early extensional environment and a later contractional environment. These events need not have formed a related tectonic cycle. In contrast with common usage and the ideas presented in many publications, the early basin was a much more extensive structure that subsequently developed its present structural eastern boundary in response to the Hunter-Bowen contraction. The original geometry of the eastern margin was significantly modified, in position and probably in orientation, during this event.

In addition, the definition of the base of the sedimentary sequence that filled the basin is unclear. Our interpretations suggest that age-dated sequences with sedimentological characteristics of half-graben development, and hence characteristics of extension basin formation, began forming while relatively productive magmatism was still occurring, as evidenced by Early Permian ages for volcanism in the Connors-Camboon Province and intrusions of the Urannah Suite. The magmatic rocks also show evidence of an extensional tectonic setting (Holcombe et al., this volume a). The character and distribution of the early sedimentary sequences are not well constrained and require considerable new field work. However, we recognise a change, typically across a disconformity or unconformity, upward to a dominantly fine-grained, still predominantly graben-confined, sedimentary sequence associated explicitly with mafic-dominated volcanism, in which there is little or limited input from a silicic basement or volcanic terrain. This sequence is overlain conformably by fine grained sediments of the thermal sag phase which define the first continuous sheet-like sediments in the basin.

Whether the base of the basin is regarded as the base of the earliest extensional fill, the base of the transition to fine-grained rocks associated with mafic volcanism, or the arrival of the first sheet-like deposits is likely to engender considerable debate depending on the preference of the researcher. We do not explicitly recommend a convention here as considerable new work is yet required. It is, nonetheless, critical that the concept of the original basin be divorced from the present structural feature in any discussions of basin evolution, such that the great body of information preserved in rocks residing outside the "Bowen Basin" not be excluded.

This research was supported by an ARC Large Grant to A. Ewart and J. McPhie (A39232338), and by a research grant to the authors from Queensland Metals Corporation. Both sources of funding are gratefully acknowledged. David Briggs and Vince Palmieri provided useful advice on biostratigraphic issues, and reviews by C.L. Fergusson, R.A. Henderson and C.G. Murray improved the manuscript.

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Figure 1 - Map showing major structural elements within the eastern Bowen Basin and adjacent New England Fold Belt of coastal central Queensland. The "Grantleigh Trough" of Kirkegaard et al. (1970) covers the area shown as Gogango Overfolded Zone.

Figure 2 - East-west time-space diagram illustrating the time equivalence of Permian stratigraphic units from the eastern Bowen Basin across the Strathmuir Synclinorium, Gogango Overfolded Zone and Craigilee Block. Absolute time scale is that of Jones (1996). Notation adjacent to time scale is as follows: 1 - Early Permian extension; 2 - mid-Permian passive thermal subsidence; 3 - Late Permian to Middle Triassic foreland thrust loading. Diagonal ruling indicates major gaps in the stratigraphic record. All formations are constrained by biostratigraphic data except where indicated by "?". Stratigraphic names are conventional, but interpretations of their ranges are ours (see text for details).

Figure 3 - Interpreted north-south stratigraphic cross-section along the eastern Bowen Basin illustrating continuity of the major middle and Late Permian formations over at least 350 km from the northern Gogango Overfolded Zone in the north to the southeast Bowen Basin in the south (see Figure 1 for location of sections). Dominant sediment dispersal direction in these formations was east to west (Fig. 5). YC - Youlambie Conglomerate, BU/OX - Buffel/Oxtrack Formations, BAR - Barfield Formation, MCB - Moah Creek Beds, BF - Boomer Formation, FT - Flat Top Formation, GY - Gyranda Formation, BCM - Baralaba Coal Measures, DCC - Dinner Creek Conglomerate. Dotted lines indicate correlations. The packages of interbedded mass flow conglomerates noted in the text are shown by shading.

Figure 4 - Selected logged sections through parts of Permian formations in the eastern Bowen Basin and adjacent New England Fold Belt discussed in the text. A) Carmila Beds near Charon’s Point within the northern Gogango Overfolded Zone (8852-936984), showing volcanic rocks of interpreted Connors Volcanics overlain by coarse basal conglomerates and interbedded marine bioclastic limestones, in turn fining upward into interbedded siltstone-sandstone strata typical of the upper Carmila Beds. B) Carmila Beds at Dumbleton Rocks, near Mackay (8755-156606), showing facies typical of the Early Permian extensional package. Note the intraformational unconformity at 150-152 m and the presence of two silicic ignimbrites, zircons from the lower of which have yielded a SHRIMP age of 293 Ma (Allen et al., this volume). C) Undifferentiated Back Creek Group overlying Connors Volcanics near Yatton Creek (8752-406795) within the Nebo Synclinorium of the Bowen Basin, showing marine strata characteristic of the thermal subsidence package. D) Undifferentiated Back Creek Group overlying Connors Volcanics in the headwaters of Leura Creek within the southern Strathmuir Synclinorium (8851-732393), showing basal volcanic lithic sandstones containing marine fossils in their upper part reminiscent of the Fairyland Formation, overlain by basement-clast-rich bioclastic limestones containing Fauna II fossils, passing in turn upward into bioturbated siltstone-sandstone strata typical of the Barfield Formation and equivalents. E) Undifferentiated Back Creek Group at Apis Creek within the Strathmuir Synclinorium (8852-595677), showing typical middle Permian shallow marine facies. F) Boomer Formation in the lower Leura Creek in the Nebo Synclinorium (8851-630346), comprising similar strata to E. G) Boomer Formation exposed in a road cutting in the Broadsound Range (northern Gogango Overfolded Zone: 8852-754656), of similar origin to E and F. H) uppermost Moah Creek Beds and overlying Dinner Creek Conglomerate in a road cutting in the Native Cat Range within the Gogango Overfolded Zone(8951-068974), showing abrupt transition from marine strata of the Moah Creek Beds into non-marine facies of the Dinner Creek Conglomerate. I) Key to symbols used.

Figure 5 - Summary of palaeocurrent data collected from Permian strata in the eastern Bowen Basin and adjacent New England Fold Belt. a) Connors Volcanics and overlying coarse basal conglomerates, b) Carmila Beds (upper sedimentary package)/Youlambie Conglomerate and Lizzie Creek Volcanics (upper sedimentary package), and equivalents - products of Early Permian extension, c) Oxtrack/Barfield Formations and equivalents - products of mid-Permian, passive thermal subsidence, d) Gyranda Formation, Dinner Creek Conglomerate, Baralaba Coal Measures and equivalents - products of Late Permian thrust loading.

Figure 6 - Geological map of the Aeroview-Scrub Creek area within the central Gogango Overfolded Zone, showing the stratigraphic equivalence of the Rookwood Volcanics with the uppermost Youlambie Conglomerate. Note the two marine fossil localities which also constrain the unit biostratigraphically.