The Townsville Basin (new name) is an extensional basin which underlies the Townsville Trough, an east-west trending bathymetric feature separating the Marion and Queensland Plateaus off northeastern Australia. The present study forms part of a wider study of the regional stratigraphic and structural framework of Australia's northeastern margin undertaken as part of AGSO's Continental Margin Program. It is the first integrated study of a regional seismic grid across the Townsville Basin, and details the results of the interpretation of 5667 km of AGSO and industry seismic reflection data across the area.
With the exception of several ODP holes which intersected Late Miocene to Recent sediments, there is no direct control on the stratigraphy of the Townsville Basin. The maximum sediment thickness in the basin reaches approximately 4.5 s TWT (::::: 6.5 km). The sedimentary fill can be subdivided into two main seismic megasequences, a synrift and a sag-phase megasequence. The synrift megasequence has a maximum thickness of up to 2 s TWT (::::: 2 km) and occurs in fault-controlled depocentres. The sag-phase megasequence occurs as drape fill and reaches a thickness of up to 2.6 s TWT (::::: 3.8 km). The two megasequences have been subdivided into regionally mappable sequences. An early synrift sequence can be distinguished in deeper half-graben. This is overlain by a regionally more
extensive synrift sequence and a late-rift sequence. The early sag-phase sequences are thin or absent in the western Townsville Basin, but thicken into the central basin. The three overlying sequences consist of terrigenous and calcareous sediments of Neogene age, and are separated from the underlying section by a ?mid-Oligocene regional unconformity.
Depth to basement, total sediment thickness, synrift isopach and gravity data all indicate that the underlying rift-forming structures compartmentalise the basin into distinct sub-basins which are separated by major north-northwest to northwesttrending transverse structural zones. These transverse structures are associated with distinct changes in structural trends and are thought to represent major preexisting crustal-scale terrane boundaries or shear zones. Overall, the structural style of the Townsville Basin is characterised by a half-graben morphology with occasional, apparently asymmetric graben structures. The half-graben are bounded by major rotational normal faults and are typically composed of a number of tilt blocks. The basin boundaries are defined by switches between steep normal faults and gently dipping hinges. An important feature of the Townsville Basin is the
presence of northwest to north-northwest trending lineaments which generally offset half-graben bounding faults in a right-lateral sense along the basin margins. The rotational normal faults are compartmentalised by these lineaments which are interpreted as transfer fault zones or accommodation zones. Local thickening of late rift sediments in the opposite direction to that of the early rift sediments probably reflects at least two significant extensional structuring events during basin formation. A younger wrenching event, which occurred during early sag-phase sedimentation,
was followed by ?Late Miocene to Early Pliocene reactivation events.
The structural interpretation of the Townsville Basin confirms that it formed part of a complex rift system of probable Late Jurassic to Early Cretaceous age. This system formed through oblique extension which utilised pre-existing Palaeozoic structural trends. Comparison with interpreted structural trends of the adjacent Queensland Basin (Queensland Trough) supports the suggestion that formation of both basins was independent of the tectonism related to seafloor spreading in the Tasman and
Coral Sea Basins.
