Abstract submitted to the Australian geological Convention Townsville, July 1998:

Lorraine M. Campbell, Rodney J. Holcombe & Christopher R. Fielding

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

The Esk "Trough" is a NNW-SSE trending structural entity in southeast Queensland which extends over a distance of some 260 km, has a maximum width of 35 km (typically about 25 km) and is approximately 6500 km² in area. This feature is defined by the surface exposure of the Toogoolawah Group of Early to Middle Triassic continental sediments and intermediate volcanics (predominantly volcanic sediments). The outcrop of these Triassic rocks is largely confined between the Devonian-Carboniferous North D’Aguilar and Yarraman Blocks on the eastern and western sides respectively. At its southwestern extent the "Trough" is unconformably overlain by Late Triassic-Early Jurassic Moreton Basin sediments. At its southern end it is bounded on both sides by sequences of Permian marine sediments and intermediate and felsic volcanics - the Cressbrook Creek Group to the west and the Northbrook Beds to the east. The relationship of these Permian units to the "Trough" is faulted and unconformable and they are faulted against the Palaeozoic basement blocks.

The Toogoolawah Group, in the southern and central part of the "Trough", and in ascending stratigraphic order, is traditionally defined as consisting of the Bryden Formation, the Neara Volcanics and the Esk Formation which are exposed respectively from east to west. New data suggest that the stratigraphic order of the Toogoolawah Group is incorrect, with the Neara Volcanics being the youngest unit. The Bryden Formation, found as isolated outcrop areas along the south-eastern side of the "Trough", and the Esk Formation, found continuously exposed along most of the western side of the "Trough", are very similar in character and composition and structurally appear to be lateral equivalents. The Toogoolawah Group reaches a maximum of around 5000 m thick within the central part of the "Trough" with the basal sedimentary package (Bryden Formation and Esk Formation) having a maximum thickness of about 2000 m and no stratigraphic top of the Neara Volcanics is exposed. Similarities in fossil fauna and petrology between the Permian Cressbrook Creek Group and the Northbrook Beds suggest that they may be one rock unit which is laterally continuous beneath the Toogoolawah Group.

The Toogoolawah Group has most commonly been equated in the literature with the Esk Trough, a graben or a rift valley. On both margins of the "graben" the unconformable base of the Toogoolawah Group is exposed and in neither case corresponds to, or sedimentologically is close to, a basinal fault. On the eastern side there is an angular unconformity with the Early Permian Marumba Beds and a nonconformity with the Late Carboniferous Claddagh Granodiorite. On the western side there is an unconformity with an altered pillowed basalt unit of possible Permian age and an angular unconformity with the Maronghi Creek Beds of the Yarraman Block (E. Willey, pers. comm.).

The steeply dipping marine Marumba Beds, exposed beneath the eastern unconformity, were heavily oxidised by subaerial exposure prior to accumulation of the non-marine Bryden Formation. The implication of this is that Hunter-Bowen thrusting, which initiated in the Late Permian, had, by the Early Triassic, produced an exposed fold-thrust highland which provided the source area for the basal Bryden Formation conglomerates which are a base of slope deposit.

The Toogoolawah Group was deposited in a more extensive basin than its current "Esk Trough" exposure indicates. There are sub-horizontally dipping outliers of Neara Volcanics overlying deformed basement rocks in the North D’Aguilar Block to the east. At the unconformity with the Claddagh Granodiorite, the Neara Volcanics consist of fine grained to medium grained sandstones, mudstones and pebble conglomerates, typical of fluvial deposits (higher in the sequence a greater volume of, and larger clast conglomerates occur).

The depositional basin of the Toogoolawah Group was more regionally extensive than the "Esk Trough" but overlies a deformed, Early Permian rift sequence (Marumba Beds). Faults related to this Early Permian rift likely persisted and controlled Late Permian thrusting as well as later deformation in the Middle Triassic and post-Triassic times. Thus the Esk Trough, as a graben structure, refers to an Early Permian feature, not a Triassic structure. The Toogoolawah Group Triassic sequence has a similar geometry and is stratigraphically comparable to the Triassic succession on the eastern side of the Bowen Basin and thus may be part of the Hunter-Bowen foreland basin system.