Tectonostratigraphic evolution of the Townsville Basin, Townsville Trough, offshore northeastern Australia

Abstract

The Townsville Basin is an extensional sedimentary basin underlying the Townsville Trough, an east‐west‐trending bathymetric feature separating the Marion and Queensland Plateaus off northeastern Australia. With the exception of several Ocean Drilling Program holes which intersected Late Miocene to Holocene sediments, there is no direct control on the stratigraphy of the Townsville Basin. Thus, the timing of basin formation and the age of the sedimentary sequences can only be interpreted within a regional tectonostratigraphic context. The maximum sediment thickness in the basin is approximately 6.5 km [4.5 s two‐way time (twt)]. This fill can be subdivided into two main seismic megasequences of synrift and sag‐phase affinities. The synrift megasequence is of probable Cretaceous age and is restricted to fault‐controlled depocentres; it has a maximum thickness of 4 km (2 s twt). The sag‐phase megasequence is Tertiary in age and occurs as drape fill; it reaches a maximum thickness of 3.8 km (2.6 s twt). The two megasequences have been subdivided into regionally mappable sequences. The structural style of the Townsville Basin is characterised by a half‐graben morphology. The half‐grabens are bounded by major normal faults and typically contain a number of rotational blocks. Depth to basement, total sediment thickness, synrift isopach and gravity data all indicate that the basin is compartmentalised into distinct sub‐basins by major north‐northwest‐ to northwest‐trending transverse structural zones. These transverse structures are associated with distinct changes in structural trends and may represent major pre‐existing crustal‐scale basement structures. Local thickening of late synrift sediments in the opposite direction to that of early synrift sediments probably reflects at least two discrete extensional events during basin formation. A younger structuring event, which occurred during early sag‐phase sedimentation, was followed by multiple reactivation in the ?Late Miocene to Early Pliocene. The structural interpretation of the Townsville Basin indicates that it formed part of a complex rift system of probable Late Jurassic to Early Cretaceous age. This system formed as a result of oblique extension that utilised pre‐existing Palaeozoic structural trends. Comparison with interpreted structural trends of the adjacent Queensland Basin (Queensland Trough) supports the suggestion that the formation of both basins was independent of the tectonism related to sea‐floor spreading in the Tasman and Coral Sea Basins.