THE TIME-SPACE EVOLUTION AND PROVENANCE SYSTEM OF TRIASSIC DELTAIC SYSTEM ON NORTHWEST SHELF OF AUSTRALIA

On the Northwest Shelf of Australia, there is a long-term superimposed basin. It was a continent marginal depression of a stable craton in Triassic characterized by stable structures and gentle gradient. Large deltas were developed. Using the data of drilling, seismic and palaeontology, we discussed in this paper the paleogeography, paleoclimate and sedimentation of the deposits. In a plane view, it is a wide deltaic plain with little deltaic front and no progradational reflections are observed. In the vertical sequence, however, there are thick clastic deposits intercalating thin coal seams, denoting rapid facies changes. The sediments are dominated by. unidirectional water flow. Channel-filling sand bodies with positive rhythms and deltaic sandstones are well developed and distributed. The compositional and textural maturities of the sand are rather high with poor to middle sorting and roundness. Terrigenous organic matters are rich in the mudstone. The main sand body in the North Carnarcon and Browse Basins was deposited in the Carnian period to Norrec period of Triassic, while the main sand body in the Bonaparte Basin was formed during the Ladinian Stage to Carnian period of Triassic. The Northwest Shelf of Australia widely uplifted in late Triassic, resulting in stratigraphic hiatus, shrinkage of deltas in the Browse Basin and Bonaparte Basin. However, the Triassic deltaic deposits are complete and thick in the North Carnarvon Basin, southwestern of Northwest Shelf of Auatralia. The Pilbara Block, Yilgarn Block and Kimberley Block are the three terrestrial zones which providet clastic sediments for the basin. The Pilbara and Yilgarn were mostly exposed then with Archean granitoids, but the Kimberley was exposed to air with Proterozoic metamorphic rocks and sedimentary rocks. Provenance of the deltaic deposits are studied upon mineralogy, apatite fission track and other data. The sediments of North Carnarvon came from the Pilbara and Yilgarn lands and the sediments of Browse and Bonaparte from Kimberley. The global greenhouse in Triassic and the tectonic background at the north edge of the Tethys Ocean are specially significant to the development of large deltas, in addition to adequate rainfall and rich source supply.