Christopher R. Fielding, Christopher J. Stephens & Rodney J. Holcombe

Department of Earth Sciences, University of Queensland, Qld 4072, Australia

Although the onset of thrust load-induced subsidence in ancient foreland basins is generally recorded in a deepening upward trend in resultant stratigraphic successions, detailed information on this process and its depositional products is scarce. In the eastern part of the Permo-Triassic Bowen Basin of Queensland, Australia, a transition from passive, thermal subsidence to flexural subsidence is recorded within the stratigraphy. Two coarse-grained intervals containing deposits of mass-wasting processes occur within a thick, otherwise siltstone-dominated succession (the Barfield Formation and equivalents). These intervals can be traced over at least 350 km north-south, along the present eastern edge of the basin, where they are recognisable in outcrop, drillcore and other subsurface records. Subsidence modelling of the Bowen Basin succession in nearby wells indicates that a pronounced increase in subsidence rate, attributed to the onset of foreland thrust loading, occurred during accumulation of the Barfield Formation and equivalents. One of the coarse-grained intervals is spectacularly exposed in the banks of the Fitzroy River, west of Rockhampton in coastal central Queensland. Here, a foundered zone of marine clastic sediments is overlain by a deeper water mudrock sequence which passes upward into successions of mass-flow conglomerates and diamictites, interpreted to have formed on an unstable submarine slope. The character of the mass-flow deposits, their stratigraphic position and their lateral extent, are interpreted in terms of destabilisation of a sloping marine surface by pulsed, subsurface thrust propagation.

In this paper we document the stratigraphic record of a transition from passive, thermal subsidence to flexural, thrust load-driven subsidence close to the orogenic margin of the Permo-Triassic, extensional to foreland Bowen Basin in east-central Queensland, Australia (Fig. 1). This process is recorded by the preservation of anomalously coarse-grained packages of mass flow deposits which can be traced for at least 350 km along the present eastern margin of the basin. We describe one spectacular outcrop of these coarse clastic deposits which include slump and slide masses, and a variety of sediment gravity flow deposits (Fig. 2), and interpret them in terms of destabilisation of an offshore marine environment by subsurface thrust propagation. It is hoped that these observations may assist in the search for the stratigraphic record of foreland basin establishment in other basins.

In a number of recent studies, initiation of rapid, thrust load-induced subsidence in ancient foreland basins (eg. Tankard, 1986 - Appalachian; Covey, 1986 - Taiwan; Allen et al., 1991 - Alpine; Cant & Stockmal, 1993 - Alberta) has been linked to abrupt facies changes. A deepening-upward vertical sequence trend is common, leading to the development of relatively deep marine or lacustrine environments. The early record of foreland basin subsidence is therefore often one of underfilling (but see Cant & Stockmal, 1993). The deepening trend is readily recognised on the orogenic margin of foreland basins where the basin has not been actively subsiding immediately prior to the onset of thrust loading. Recognition may be considerably more difficult, however, in situations where thrust load-induced subsidence is superimposed on a pre-existing subsidence regime. Such is the case in the Permian-Triassic Bowen Basin of eastern Queensland, Australia, a complex extensional to foreland basin (Fig. 1).

During the Permian period, eastern Australia lay at boreal latitudes within the southern hemisphere, such that low-lying areas were affected by glacial and periglacial processes. The Bowen Basin (Fig. 1) began to form in Early Permian times in response to modest crustal extension across a broad zone which included the eroded remnant of an earlier continental volcanic terrain. Thick successions of mainly alluvial and lacustrine sediments accumulated with bimodal felsic/basaltic volcanics in elongate grabens and half-grabens which developed in response to the extension. The latter stages of the extensional phase were marked by a highly diachronous marine transgression which ultimately flooded the entire basin. A brief period of middle Permian passive, thermal subsidence ensued, punctuated by an hiatus possibly related to a 260 Ma crustal contraction in the New England Fold Belt (Holcombe et al., 1993). During middle Permian times, mainly fine-grained clastic sediments accumulated as an uneven blanket across much of the basin. Modest volumes of coarse sediment were introduced to the western (cratonic) margin during this period where they accumulated in deltaic and coastal complexes.

During the Late Permian, the basin became a foreland basin in response to the propagation of thrust sheets westward into the basin from an emerging orogen to the east. From this time onward, the basin developed a pronounced cross-sectional asymmetry, with an elongate depocentre located adjacent to the eastern (orogenic) margin (Fig. 1). The initially marine basin filled rapidly with first-cycle, volcanogenic detritus derived from the volcanically resurgent, rising orogenic mountain range. Thick successions of mainly alluvial sediments continued to accumulate under the influence of cyclic thrust loading from the latest Permian until the late Middle Triassic, when the entire basin was shortened, uplifted and eroded. The protracted, contractional deformation responsible for closure of the basin has been termed the Hunter-Bowen Orogeny (Fergusson, 1991, and see Murray, 1990; Korsch et al., 1992; Fergusson et al., 1994; Fielding et al., 1995 for further information).

Adjacent to the present eastern edge of the Bowen Basin, thick successions of Permian sedimentary rocks identical to those in the basin proper are preserved within the Gogango Overfolded Zone, an area variably deformed by thrusting which lies outboard of a basement volcanic ridge, the Connors-Auburn Arch (Fig. 1). Fergusson (1991) and our own research has shown that rather than forming the eastern limit of the Permian Bowen Basin, the Connors-Auburn Arch is a thrust culmination uplifted during the later stages of the Hunter-Bowen Orogeny, and that Permian rocks in the Gogango Overfolded Zone are part of the Bowen Basin. The outcrops described later in this paper are preserved within the Gogango Overfolded Zone.

In the eastern Bowen Basin, the early Upper Permian Barfield Formation and equivalents consist of thick successions of mainly grey siltstone and interbedded siltstone/sandstone, containing a moderate diversity, locally high abundance marine fauna of bivalves, brachipods, crinoids, conulariids and corals. The Barfield Formation ranges up to about 1000 m in thickness, while to the north lateral equivalents the Moah Creek Beds are estimated to be up to 2100 m thick (Kirkegaard et al., 1970) and the Boomer Formation (probably only the lower half of the Moah Creek Beds) up to 1000 m (Malone et al., 1969). Subsidence modelling of well sections in the eastern Bowen Basin (Fig. 3A, and see also Totterdell et al., 1992) indicates a sharp increase in subsidence rate occurred during early Late Permian accumulation of the Barfield Formation. In agreement with previous workers, we attribute this effect to the onset of foreland thrust loading. At this time also, the east-west cross-sectional formation geometry changes from sheet-like to wedge-like, with a pronounced depocentre developing adjacent to the eastern structural basin margin (Figs. 1, 3B).

Within this formation, two anomalous coarse-grained intervals can be recognised and correlated from outcrops and borehole records over a north-south distance of at least 350 km (Figs. 1, 4). These two intervals, locally named the Cottenham Sandstone Member of the Barfield Formation (lower) and Acacia Formation (upper), have been recognised by previous workers, but their potential stratigraphic significance has gone unnoticed. Both intervals contain both intraformational and extraformational breccias and conglomerates, which are typically disorganised and lack internal structure: some breccias are rich in ragged, pumiceous clasts set in a tuffaceous matrix and evidently have a first-cycle volcanic provenance. We suggest that these regionally extensive packages of sediment gravity flow deposits record the onset of pulsed, thrust loading and as such the onset of foreland basin conditions in the eastern Bowen Basin.

In the western (cratonic) part of the basin, a marked deepening event occurs at a slightly higher stratigraphic position, at the top of a coastal and deltaic succession (John & Fielding, 1993; Falkner & Fielding, 1993). This flooding surface also corresponds to a marked change in sediment provenance from quartzose (craton-derived) to first-cycle volcanic lithic (orogen-derived). Regionally, the change in depositional environment (Fielding et al., 1995) and sandstone composition (Baker et al., 1993) accompany the change in cross-sectional formation geometry from sheet-like to markedly asymmetrical with a depocentre close to the eastern (orogenic) basin margin noted above (Fig. 3B). This event (which was probably diachronous east to west) is therefore interpreted to represent a change from thermal to flexural subsidence across the Bowen Basin.

Spectacular natural exposures in the banks of the Fitzroy River, west of Rockhampton in coastal central Queensland (Fig. 2), provide a superb illustration of the stratigraphic record of early foreland thrust-loading close to the original eastern margin of the Bowen Basin (Fig. 1). The rocks occur within the lower part of the Upper Permian Moah Creek Beds (a correlative of the Barfield Formation: Fig. 4), within the Gogango Overfolded Zone (Fig. 1). The exposures represent the lower of the two coarse-grained intervals noted above (Fig. 4).

The exposed succession is dominated by thick intervals of rhythmically interbedded siltstones and very fine- to medium-grained sandstones (Figs. 2A, 5). Also preserved are a unit of fine-grained siltstone in the middle of the exposed succession (Fig. 2B), and at least three discrete packages of pebble to boulder conglomerates and diamictites in the upper part (Fig. 2C-F). The succession has been subdivided into two facies associations, containing nine facies in all (Table 1). Rocks of Association A, predominantly fine-grained clastics, are present throughout the exposed interval. They enclose and are interbedded within the discrete conglomerate/breccia packages which are assigned to Association B.

Association A sediments are typical of middle Permian offshore marine shelf facies across the greater part of the Bowen Basin (cf. John & Fielding, 1993). Siltstones contain scattered lonestones, Planolites burrows and body fossils of crinoids, bivalves and brachiopods (mostly intact and articulated). Sandstones occur in sharp-based, often graded and gradationally-topped beds which display flat lamination and ripple cross-lamination only in their upper parts. Bioturbation in the tops of sandstone beds and interbedded siltstones is referrable to Rhizocorallium, Planolites, and less abundant Teichichnus, Palaeophycus and Zoophycos. Plant debris is also abundant, ranging in size from macerated fine detritus to large stalks and branches.

Association B comprises a variety of conglomerates and diamictites, with one distinctive, closely associated sandstone facies (Table 1). The coarse-grained facies, which occur mostly in non-erosively-based beds and may have either sand or silt matrix, are transitional into one another. These facies vary from clast to matrix-supported, in many instances laterally within the same bed. Two populations of clasts are evident: 1) well rounded pebbles and cobbles of basement rock types (mainly sandstone, chert, volcanics and quartz), and 2) angular to rounded pebbles, boulders and large rafts of intraformational sandstone, interlaminated sandstone/siltstone and siltstone. Little internal bedding is evident in these rocks, although a-axis and less common b-axis imbrication of coarse clasts is ubiquitous. Sandstone slabs are in places stacked in an imbricate fashion, or are rarely contorted into overturned folds. Plant debris is again abundant throughout Association B facies.

The entire succession is interpreted to have formed in a marine environment. The Cruziana ichnofacies assemblage, evident throughout the exposed succession, is typical of offshore marine shelf deposits elsewhere in the Bowen Basin (cf. Fielding, 1989; John & Fielding, 1993). A relatively quiet, offshore setting is suggested by the preservation of articulated body fossils and the complete absence of any wave- or combined-flow-generated structures within the exposed succession. An abyssal setting is considered unlikely given the above, but there is no precise constraint on formative water depth. Within the exposed succession, deepest water conditions are evidently represented by the laminated fine siltstones (Facies A1). Sharp-bounded sandstones were introduced to the offshore marine environment by tractional currents which flowed in an offshore (westward) direction (Fig. 5): the nature of these beds and their contained structures (including some sole structures) is suggestive of deposition from turbidity currents. Given the periglacial nature of the basin, it is plausible that effluent into the basin moved as turbulent underflows. The rhythmic nature of sand-silt interbedding (which is common at this stratigraphic level across the basin: cf. John & Fielding, 1993) is interpreted to arise from regular, probably seasonal discharge of water and sediment onto the shelf, and offshore movement by meteorological currents. Lonestones may have been introduced from floating ice.

Association B facies are superimposed on this background pattern of sedimentation. The disorganised, silt matrix and often matrix-rich nature of the conglomerates and diamictites indicates deposition from sediment gravity flows. This is further borne out by the sheet-like geometry of beds and the lack of basal erosion. Mass transport mechanisms ranging from plastic, laminar debris (plug) flow, through partly turbulent debris flow (cohesionless debris flow) to high-concentration turbidity flow (Nardin et al., 1979; Lowe, 1982; Nemec & Steel, 1984; Nemec, 1990) are considered responsible for these deposits (Table 1). A complete range in facies character is evident between Facies B1 and B4, suggesting a continuous spectrum of depositional process. Indeed, some beds show lateral facies change which indicates downslope transformation either from laminar debris flow into more fluidal flow or vice versa. Some of the larger sandstone slabs may have been emplaced by sliding. Association B facies were clearly deposited in an unstable, submarine slope environment.

The depositional setting responsible for all observed facies was evidently a gently sloping submarine surface which periodically became unstable, leading to the emplacement of mass-flow deposits (Fig. 6: cf. Laberg & Vorren, 1995; Edwards et al., 1995). Regional mapping and stratigraphic relationships suggest that the eastern basin margin lay some 20-25 km to the east: assuming an arbitrary formative depth of 200 m gives a paleo-shelf gradient of the order of 0.01 (c.0.6 degrees). Proximity to an exposed landmass is indicated by the abundance of large, drifted plant debris in all sediments.

Initiation of mass-flow sedimentation followed a change in the nature of the interbedded facies. This change is manifested in a soft sediment-deformed and broadly folded horizon at the top of a coarsening-upward interval (at 193 m on Fig. 5), which is abruptly overlain by fine-grained siltstone. The siltstone contains allochthonous, ovoid masses of sandstone and interlaminated siltstone/sandstone up to at least 2 m long, some of which are imbricated or show folding of internal lamination into cylinders. This horizon of foundering is interpreted to have formed by steepening of the submarine gradient and deepening of the sea floor.

Two coarsening-upward sequences overlie the foundered unit, culminating in the first conglomerate package. The mass-flow conglomerates are confined (apart from the foundered unit) to three discrete intervals: this is taken to indicate that the agents that caused shelf/slope instability were intermittent and were powerful enough to generate a rapid response. Sediments interbedded with conglomerates and enclosing conglomerate packages are typical of Association A, suggesting that the overall environment was largely unchanged through these events, other than by the movement of sediment gravity flows. The coarse-grained packages may represent a mosaic of overlapping mass flow lobes and sheets, or the distal toes of prograding deltas or submarine lobes, but the regional extent of these units suggests an overriding control operating on a basinal scale. The nature of interbedding between Association A and B and other features argue against a proglacial origin for the coarse-grained facies. We suggest that the mass flows were produced by seismic disturbances related to the initial stages of subsurface thrust propagation.

The Barfield Formation and equivalents contain a stratigraphic record of the onset of flexural subsidence during Late Permian times in the eastern Bowen Basin. The Fitzroy River section provides detailed data on the sedimentological character of sediments arising from this change. We suggest that the foundered unit described above represents disruption of a stable marine environment similar to that which characterised the middle Permian thermal subsidence phase of the basin, and that formation of the mass-flow conglomerate packages was induced by early propagation of thrust sheets in the subsurface.

The stratigraphic record of onset of flexural subsidence in foreland basins is poorly documented. This study offers insight into the nature of the process. In the case of the Barfield Formation/Moah Creek Beds, 1) stresses related to early (subsurface) thrust propagation were transmitted to the surface in a pulsed fashion, leading to periodic shelf/slope instability and emplacement of mass-flow deposits, 2) the resultant, discrete and anomalous, coarse-grained packages can be correlated with confidence over at least 350 km along the eastern edge of the Bowen Basin, and 3) accumulation of one such coarse-grained package, preceded by a deepening event and steepening of the marine surface, took place in at least three discrete episodes separated by periods of background sedimentation. This internal cyclicity may have been a result of a periodicity in seismic activity on a 10² - 10⁴ a scale.

This work has been supported by research grants from Queensland Metals Corporation to RJH, CJS and CRF, and from the Australian Research Council (Ref. A3931187) to CRF. We thank Queensland Metals Corporation for logistical assistance, and E. Burdin and J.Yago for drafting some of the diagrams.

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Figure 1 - Map showing the location and context of the Bowen Basin, of the cross-section of Figure 3B and of the stratigraphic sections in Figure 4 from Boomer Range in the north to Cockatoo Creek - 1 in the south. NEFB = New England Fold Belt.

Figure 2 - Photographs illustrating aspects of the Moah Creek Beds at the Fitzroy River locality. A) general view of thinly interbedded siltstone and fine-grained sandstone (Facies A2) typical of strata below the foundered horizon (c. 90-100 m on Fig. 3). Geologist is 1.9 m long, B) view of the foundered horizon and fine-grained siltstones (Facies A1) above that horizon (interval between 190 and 205 m on Fig. 3 illustrated). Note the blocks of sandstone-siltstone isolated within siltstone above the bedded lowermost strata, interpreted as slump masses, C) close-up showing non-erosive contact between thin-bedded strata (Facies A3) and overlying debris flow deposit showing normal grading of coarse fraction (Facies B2) within the lowermost coarse-grained package at c. 275 m on Fig. 3. Hammer is 0.27 m long, D) close-up of debris flow deposit (Facies B2) at c.255 m on Fig. 3 (lowermost coarse-grained package) showing contained blocks of siltstone and sandstone, and considerable topography on top surface of deposit (flow from right to left). Hammer is 0.27 m long, E) close-up of uppermost part of middle coarse-grained package (303-306 m on Fig. 3), showing top of debris flow deposit (Facies B2) at bottom of view, overlain by sandstone-dominated strata of Facies A3 and including a layer of dismembered and imbricately stacked sandstone slabs (flow from right to left). Hammer is 0.27 m long, F) overturned, recumbent fold in thin-bedded sandstone-siltstone mass (Facies A3) entrained within debris flow (Facies B2) at 304 m on Fig. 3. Note also imbricated sandstone slabs at the base of the flow unit (flow from right to left). Hammer is 0.27 m long.

Figure 3 - A) Decompacted Permian-Triassic subsidence curve for the basal Permian unconformity surface in UOD Cockatoo Creek-1 (Fig. 1), showing pronounced increase in subsidence rate at about 255 Ma, during accumulation of the Barfield Formation and equivalents, B) East-west cross-section across the southern Bowen Basin (see Fig. 1 for location), based on Shell regional seismic line with control from eight exploration boreholes (modified after Elliott, 1989). Dashed line indicates approximate boundary between thermal subsidence and foreland basin successions. Note also the development of Early Permian fault-bounded sub-basins in the western part of the section, which are representative of the early extensional phase of the Bowen Basin. The Jurassic - Cretaceous section of the Surat Basin unconformably overlies the Permo-Triassic of the Bowen Basin.

Figure 4 - Simplified, north-south cross-section along the eastern margin of the structural Bowen Basin (see Fig. 1 for location), illustrating stratigraphic continuity of major formations into the Gogango Overfolded Zone to the north. Note the lateral persistence of two anomalous, coarse-grained intervals in the lower and upper parts respectively of the Barfield Formation and equivalents. These intervals, which include massive, disorganised breccias and conglomerates indicative of mass flow deposition, are interpreted to have formed in response to subsurface propagation of thrusts from the east during the early Late Permian onset of foreland basin conditions in the eastern Bowen Basin. Boomer Range section based on outcrop type section of Boomer Formation (Malone et al., 1969), Fitzroy River on outcrops in the Emu-Moah Creek-Fitzroy River area (Kirkegaard et al., 1970), Banana/Moura on outcrop sections and TEP Moura-1 exploration well, Cracow on outcrop in Back Creek and GSQ Munduberra 5-8 stratigraphic drillholes, and UOD Cockatoo Creek-1 on that exploration well.

Figure 5 - Graphic log of the section exposed in the Fitzroy River locality, with detailed sample logs of the lower interbedded facies, the foundered horizon, and one mass-flow package. Note the progressive clockwise rotation of paleocurrent mean directions up-section, interpreted to reflect increasing influence of westsouthwest-directed thrust propagation.

Figure 6 - Artist’s impression of the Moah Creek Beds depositional environment, showing destabilisation of a submarine slope by subsurface thrust propagation. Diagram drawn by Joel V.R. Yago.

Table 1 - Lithofacies of the Moah Creek Beds at the Fitzroy River locality.