terrane models

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Northern Bowen Basin Project
Regional Framework

BASIN EVOLUTION

The Bowen Basin evolved in three phases above a basement of Early Palaeozoic metamorphic and sedimentary rocks of the Drummond Basin and Anakie Block:

• It initiated with rifting in the early Permian resulting in a series of isolated fault-bounded basins filled with volcanics and sediments (eg. Lizzie Creek Volcanics, Reids Dome Beds).

• Once rifting ceased by the Middle Permian, thermal relaxation took over as the mechanism for subsidence, and marine conditions dominated the Basin until the Late Permian (eg. Back Creek Group, Tiverton Formation).

• The progressive filling of the basin and deposition of widespread thick coal measures (eg. Moranbah-German Creek CM and Rangal CM) mark the onset of foreland loading during the Latest Permian, which controlled subsidence in the basin until its closure during the Middle Triassic.

DEFORMATION EVENTS

A series of younger deformation events added structural complexity to the coal measures. These include:

• propagation of thrust faults into the western basin during the Middle Triassic;

• intrusion of sills and dykes in the Mid-Cretaceous;

• uplift and joint formation in the Late Cretaceous; and

• major volcanic activity during the Tertiary.

REFERENCE
Fielding, C.R., Sliwa, R., Holcombe, R.J. and Kassan, J., 2000. A new palaeogeographic synthesis of the Bowen Basin of central Queensland, Bowen Basin Symposium 2000. In J.W. Beeston, (ed), Bowen Basin Symposium 2000-the New Millenium-Geology, Geological Society of Australia Inc., October 2000, 287-302.

REGIONAL STRUCTURE

The nature and age of the crust beneath the Bowen Basin can only be inferred from exposed rocks outside the basin area and a deep seismic survey acquired by Geoscience Australia, as exploration drilling to date has not penetrated the basin succession away from its immediate margins.

The oldest dated rocks within the region are within the Neoproterozoic Anakie inlier (age of metamorphism) and the Marlborough ophiolite, suggesting that the crust underlying the Bowen Basin may be at least of this age. Other units that appear to continue underneath the basin are the Carboniferous intrusive rocks of the Urannah-Connors Arch to the east and the Drummond Basin succession to the west.

The Geoscience Australia gravity image clearly highlights the main basement compartments of the Bowen Basin. The Comet Ridge, Taroom Trough and Collinsville Shelf are particularly recognizable as basement highs or lows. At higher resolution, the distribution of high and low density areas beneath the western Bowen Basin does not correlate well with the outcrop pattern or thickness distributions of the Permian sediments, which suggests that the gravity signature is derived from a combination of the Bowen and Drummond Basin successions over a heterogenous metamorphic/igneous basement.

Outside the exposed basin area, the dominant grain of the image consists of northwest trending “ridges” and “valleys”. The ridges generally correlate with large near surface or outcropping intrusions, while some of the valleys are fault-bounded basins (eg. Duaringa Basin). This northwest trending fabric was enhanced during the Late Permian to Middle Triassic Hunter-Bowen compression that is responsible for the thrust faulting.

The northeasterly trends are subtler. They have long been recognised as a series of corridors partitioning compartments with consistent geology and structural style (Hammond and Mallett, 1987). The criteria developed as indicators of corridors included closures of map scale folds or lateral shifts in their axes, zones of increased fault disruption, steps in the boundaries of basin elements or structural zones (e.g. Comet Ridge) and lines of intrusive bodies along their length.

While the presence of these structures is indisputable, their main periods of activity were during the early parts of Basin evolution (shape of the basin), during the Hunter-Bowen compression (compartmentalization of folding and faulting) and during the Cretaceous and Tertiary intrusive events. There is little evidence for any major activity on these structures during the deposition of the Bowen Basin sediments.

North-south trending lineaments are shorter than the northeast trending ones, and are confined to eastern Australia. They are only weakly defined in the regional gravity data, but stand out in the outcropping geology. They are defined either by the edges of major geological units such as the northwestern and southeastern margins of the exposed Bowen Basin, or major basin and basement structures including the southern Denison Trough, the Comet Ridge, the eastern bounding fault of the Styx Basin and sections of the Jellinbah and Moonie Faults.

REFERENCES
Esterle, J.S. and Sliwa, R., 2002. Bowen Basin Supermodel 2000. ACARP Report # C9021, 200pp.

Hammond, R. and Mallett, C.W., 1987. A tectonic framework for coal measure deformation in the southern Bowen Basin. Advances in the Study of the Sydney Basin. Proceedings of the Symposium, 21, 193-196.