TECTONIC FRAMEWORK OF AUSTRALIA'S SEDIMENTARY BASINS

Abstract

Two morphotectonic features dominated the development of Australia's Phanerozoic sedimentary basins: a rifted arch on the divergent western and southern margins, such as exists today in East Africa-Arabia, and a volcanic arc on the convergent eastern margin, such as the present Andaman-Sumatran Arc. A presumed rifted-arch system, associated on one side with the growth of the Tethyan Ocean and on the other with failed arms, developed in northwest Australia from the beginning of the Phanerozoic. A second system developed in the same area and extended southward between Greater India and Australia in the Late Carboniferous and evolved in the Late Jurassic and Early Cretaceous into the eastern Indian Ocean. Also in the Late Jurassic a rifted-arch system developed along the southern margin between Antarctica and Australia and in the Late Cretaceous evolved into the southeast Indian Ocean. On both margins the rifted-arch system was succeeded for some 30 to 40 Ma by rim basins confined between the continent and an uplifted rim alongside the continent-ocean boundary, so that restricted marine sediment was deposited over a terrain of rift-valley fluvial sediment cut into fault blocks in a configuration favourable to petroleum accumulation. In the Ceduna Plateau depocentre of the Great Australian Bight Basin a very thick rim basin was deposited in a saddle of the rifted-arch system from a copious provenance in the ancestral Eastern Highlands. On both margins the rim basins were overlain by open marine deposits, mainly carbonate, of varying thickness.

The convergent eastern margin started also at the beginning of the Phanerozoic, and developed through marginal sea opening and filling behind an island arc; widespread deformation then followed, and led in the Early Silurian to the development of a basin-and- range terrain. In the Late Devonian, a volcanic arc appeared along the continental margin, and subsequently jumped eastward twice to the mid-Cretaceous. The succession of jumps generated a vertical sequence of basins that at any one time existed side by side, so that the fore-arc basin is overlain by the foreland, in turn overlain by the pericratonic basin. The foreland and pericratonic basins, on the landward side of the volcanic arc, received volcanogenic sediment, prone to be marine, from the arc, and quartzose sediment, prone to be fluvial, from the craton; the foreland basin was subjected to shearing from transcurrent motion along the arc, so that growth structures provided receptacles for petroleum generated by heat flow from the arc. The marginal marine volcanogenic sediment was the main source and the interfingering fluvial quartz sand the reservoir.

Three tectonic regimes can be recognized, and are characterized by intervals of relatively constant latitude, climate, depositional facies, and plate configuration. A Pre-Gondwanan regime, 570 to 320 Ma ago, of low latitude, started with plate divergence on the northwest and convergence on the east, and ended with widespread deformation in the east and centre, of an age and kind similar to that affecting the ancestral Rocky Mountains of North America. A Gondwanan regime, from 320 to 95 Ma, of high latitude, with rare evaporites and carbonates, started with a rift-valley system along the western margin that developed into the Indian Ocean 160 to 125 Ma ago, and an arc jump on the east; and ended with the separation of Antarctica and Australia (by the splitting of a rifted arch) on the south, and of Australia and Lord Howe Rise (by the splitting of an arc) on the east. A Post-Gondwanan regime, 95 to 0 Ma ago, of diminishing latitude, has restored carbonates to Australia along its divergent margins, and in the north, in New Guinea, a continental margin volcanic arc, similar to the Gondwanan arcs, has led to uplift in the Neogene.