Six sedimentary cycles, each hundreds of metres thick, have been recognised in the Surat Basin. The Jurassic cycles (nos. 1-4) typically started with high-energy deposition of coarse sediments, and ended with labile sand, silt, and mud. The environments are thought to have been braided streams, followed by meandering streams, swamps, lakes, and deltas. After a period of non-deposition or erosion, followed by a phase of high-energy deposition, the first Cretaceous cycle (no. 5) ended with marine mud; the second (cycle 6) started with paralic silt and sand and ended with shallow marine silt. The cycles are thought to be the result of global sea-level changes, characterised by rapid falls of sea level followed by slow rises, which, respectively, lowered and raised the base of erosion in the Surat Basin. During the Jurassic, the open sea lay several hundred kilometres to the east, and the sea only occasionally entered the basin, via the Brisbane and Toowoomba Straits. In the Cretaceous, the sea level was relatively higher and eustatic falls and rises of sea level led to alternating marine regressions and transgressions. The six Surat Basin cycles correspond in time to nine global sea-level oscillations. We think that some of the latter may have been too slight to be identified in the basin. There is also evidence that local isostatic movements may have exaggerated the impact of some global cycles and obscured that of others.
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