Eastern Australian Basin Development

Australia: The Land Where Time Began

Eastern Australian Basin Development

Beginning on a limited scale in the latest Triassic (Rhaetian) the deposition of sediment in the broad intracratonic basins of the eastern Australian region, though a widespread unconformity formed at about the start of the Jurassic as a result of the final compressional pulse of the Permo-Triassic tectonic regime. The unconformity of the end-Triassic embraces the Hettangian locally along the northwestern margin of Australia, though the authors suggest sedimentation may have continued without interruption, at least in some areas, across the Triassic-Jurassic boundary (Bradshaw & Young, 1990, 1992; Exon & Colwell, 1994). The start of rifting has been suggested as the main cause of the break in deposition in northwestern Australia, rift volcanics dated to the latest Triassic-earliest Jurassic indicate volcanism in the area of the future breakup. The unconformity of the Hettangian-earliest Sinemurian, is present in the western Clarence-Moreton Basin, and basins further to the west, though it appears sedimentation continued across the period boundary in the eastern Clarence-Moreton Basin, and possibly also in the Nambour Basin (McKellar, in press). Deposition resumed elsewhere in the early Sinemurian and continued to increase greatly throughout the Early Jurassic. Sediments ultimately accumulated in the western Clarence-Moreton, Surat, Mulgildie, and Eromanga Basins as sedimentation expanded to the west (McKellar, in press).

The Alisporites/Falcisporites Microflora, representing the temperate Dicroidium Flora, changes to palynofloras of Jurassic aspect dominated by cheirolepidiacean pollen (Classopollis), is delimited where such a hiatus is present in eastern Australia, has been suggested to reflect the shift that has been assumed to lower palaeolatitudes that have warmer climates (e.g. Grant-Mackie et al., 2000). In the eastern Clarence-Moreton Basin and probably the Nambour Basin, where the hiatus appears to be absent, a transpositional palynoflora is present in the Hettangian and earliest Sinemurian, though according to the authors it is far more closely allied to the Alisporites Microflora, based on data from N. de Jersey & J. McKellar.

In the Jurassic-Cretaceous of the Surat Basin 6 major sedimentation cycles have been recognised that are represented to a greater of lesser degree in the adjoining basins, depending on the extent of the preservation of the succession (Exon, 1976, 1980; Exon & Burger, 1981). Sandy deposits of high-energy braided river systems began each cycle and deposits dominated by mudrock of low-energy, meandering rivers, lakes and mires terminated each cycle. The Walloon Coal Measures of the present is the result of the formation of extensive forest-mire systems from the Middle Jurassic at the end of one of these cycles, that resulted from ponding in the Clarence-Moreton, Surat and Mulgildie Basins. Episodic flexure along the eastern margin of the continent, as a result of thermal doming and incipient rifting, is suggested by the authors¹ to have probably caused the development of these large-scale sedimentary cycles, some more than 10 Ma long; and within the Clarence-Moreton Basin, extension/incipient rifting. The authors¹ suggest the Surat, Clarence-Moreton and northern Nambour ("Maryborough") Basin had previously been incorrectly interpreted in terms of a foreland-basin system, the cycles of sedimentation being attributed to the periodic tectonic uplift in the orogen and that they suggested there was a correlation between the cycles and changes in the sea levels globally to such an extent that it suggested there were global links between tectonism and eustasy (Jones & Veevers, 1983, 1984). It had been suggested by others (Exon & Burger, 1981; Burger, 1986, 1989) that changing sedimentation patterns were related directly to movements of sea levels, based on the most likely ages associated with palynofloras, to correlate with the global sea-level curve (Vail et al., 1977).

According to the authors¹ other curves (Vail & Todd, 1981; Haq et al., 1987) were used to make slight modifications to these relationships that were subsequently adopted (Burger, 1994a, 1994b; Burger and Shafik, 1996). In the Surat Basin ages assigned to Jurassic formations that had been interpreted from sea-level curve correlations differed from ages based on correlations of their attendant palynofloras with those that had been dated accurately by the associated marine faunas in New Zealand (de Jersey & Raine, 2002; McKellar, in press). This suggests to the authors¹ that the sedimentation patterns of eastern Australia may not fully reflect "global" sea-level curves that have been published, though the oolitic ironstone deposition that is geographically extensive in southeastern Queensland, in the Surat, Mulgildie, Clarence-Moreton, and Nambour Basins (e.g. Grant-Mackie et al., 2000) appears to correlate with the global sea level peak in the early Toarcian (Mackellar, in press). The authors suggest such deposits appear to have formed in climates that were warm and humid and generally associated with conditions of rising base level (Hallman, 1975; Frakes et al., 1992, p. 63).

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