Australia: The Land Where Time Began
The Bonaparte Basin extends offshore from near Kununurra northwest to Ashmore Reef and north to near Flinders Shoal. It covers an area of about 270,000 km2, of which about 20,000 km2 are offshore, and is roughly of triangular shape.
In the offshore part of the basin the oldest rocks to be dated with confidence are from the Late Devonian, and in the onshore portion there are volcanic and sedimentary rocks of Cambrian age. In the Petrel Sub-basin there is an evaporite sequence that is believed to be from the Silurian. In the Bonaparte Formation of the Weaber Group, Devonian to Carboniferous rocks are from a phase of northwest rifting in the mid-Phanerozoic. The following Permo-Carboniferous sequences of the Kulshill and Kinmore Groups, from a period of reactivated rifting and sag during which the main depocentre continued to be the Petrel Sub-basin. New depocentres developed along the basin's northwest margin before the north-oriented rifting and breakup that occurred in the Jurassic. It was during this period that the deposits of the Sahul and Troughton Groups were laid down. The major grabens in the north and northwest of the basin contain the depocentres of the Flamingo and Bathurst Island Groups that were deposited following the breakup, though the Petrel Sub-basin continued to be a major deposition site until the Late Cretaceous. In the north and northwest of the basin, Carbonate shelf progradation dominated the sedimentation in the Cainozoic.
The major structural elements in the southeast of the basin trend northwest, defining a broad trough containing sediment largely from the Palaeozoic, that is believed to be up to 17 km thick. These structural elements were formed by northwest-trending rift faulting by the late Palaeozoic - Plover Shelf, Lacrosse terrace, Petrel Sub-basin and Darwin Shelf. Similar structural elements in the north and northwestern parts of the basin lie at right angles to the elements in the southern part of the basin. There are more than 10 km thick sedimentary rocks in the set of grabens in the northern part of the basin that were deposited from the Mesozoic to the Cainozoic, while in the southern part the sediments are more than 4 km thick over the adjacent highs. Northeast-trending rifting associated with the breakup of Gondwana during the Mesozoic formed this set of grabens and basement highs. Elements included are the Vulcan Sub-basin, Malita Graben, Ashmore and Sahul Platforms and Londonderry High (Fig.2, source 1).
Definition of the basin
There are few problems with the definition of the Bonaparte Basin onshore. It is flanked by ricks of the Kimberley and Sturt Blocks from the Proterozoic. The definition of the offshore section of the basin has a number of problems. The offshore portion of the Bonaparte Basin was originally extended to the edge of the Timor Trough.
The northwestern limit was defined by using the Londonderry Rise, a seafloor feature, and the shallow basement, the Sturt Block, to define the southeastern limits of the basin (e.g. Veevers, 1967; Mollan et al., 1970; Williams et al., 1973). The early definition was followed in a display of the basin outline (Playford et al., 1975, fig. 64). The northern side of the Kimberley Block is linked to the southern side of the Ashmore Platform by the southwestern basin margin. This boundary crosses the shelf adjacent to the Kimberley Block, and the Vulcan Sub-basin. The eastern margin of the Sahul Platform is linked to the western limit of the Permian and Triassic rocks of the Darwin Shelf (Fig. 2, source 1), by the eastern margin of the basin that crosses the Malita Graben. The result is that the sequence of the Browse Basin, of Triassic to Cainozoic age, and the Jurassic to Cainozoic sequence of the Money Shoals Basin are both much the same as in the Bonaparte Basin.
The Ashmore Platform and Vulcan Sub-basin were included in the Bonaparte Basin in the definition of Playford et al., (1975). This definition is followed by the report of A.J.Morey (Source 1). Other definitions have been proposed, such as one in which the Ashmore Platform and the post-Palaeozoic sequence of the Vulcan Sub-basin have been assigned to the Browse Basin (McDaniel, 1988) restricting the Bonaparte Basin to the underlying sequence of Palaeozoic age. The definition of McDaniel was based on one from Laws & Krause (1974, P.80). According to Laws and Krause, the Bonaparte and Browse Basins are separated by the Londonderry High, apparently not excluding the underlying Palaeozoic sequence from the Browse Basin. In another definition the Ashmore Platform and Vulcan Sub-basin are 'unassigned basinal elements'. In yet another definition the Bonaparte Basin was restricted to the Palaeozoic sequence south of the Malita Graben and Londonderry High.
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