Australia: The Land Where Time Began

A biography of the Australian continent 

There are 2500 m of clastic continental sedimentary rocks, mainly from the Jurassic, and marine beds, from the lower Cretaceous, that are mostly obscured by alluvium  of Cainozoic age (Exon, 1976).

There were 5 cycles of terrestrial deposition in the Jurassic, characterised by braided stream deposits of coarse sand, that grade to finer sand and silt deposited by meandering streams. The deposition cycle was completed by the deposition of labile sand, silt, mud and coal in swamps, lakes, deltas and shallow seas. Andesitic volcanic activity supplied much of the sediment during the Middle-Late Jurassic (Exon, 1976).

In the Early Cretaceous, deposition changed from terrestrial to shallow marine, the Rolling Downs Group being deposited during a series of marine transgressions and regressions that ended in the late Albion (Exon, 1976).

During the Late Cretaceous and early Tertiary a deep weathering profile developed. During the Oligocene-Miocene basic volcanic activity occurred, the Surat Basin remaining relatively stable since that time, and extensive erosion occurred around the margin of the Murray-Darling Basin (Exon, 1976).

Within the Murray-Darling Basin the physiography of the Surat Basin is comprised of extensive alluvial systems, and on the Hutton Sandstone, large areas of undulating residual sands formed, and on the Rolling Downs Group, that weather easily, undulating clayey downs country developed. Widespread flows from the mid-Tertiary have left remnants that form basalt-capped mesas in the north. Cuestas and mesas have formed in sandstone country that is dissected, where the resistant and less-resistant sandstones are interbeded. Dissected tablelands in lower areas of the basin have formed where weathering resistant ferruginised and silicified surfaces are present (Exon, 1976).

Terrestrial fluvial cycles of the Jurassic and shallow marine environments from the Early Cretaceous, characterise the geology of the Surat Basin, as evidenced in the Eromanga Basin. Among the units from the Jurassic that outcrop are the Precipice Sandstone and Evergreen Formation, the Hutton Sandstone and the Walloon Coal Measures, the Springbok Sandstone and the Westborne Formation, the Gubberamunda Sandstone and the Orallo formation. The Mooga Sandstone and Bunjil Formation were deposited in the last of the terrestrial cycles.

The end of the fluvial cycles and the beginning of deposition of the Rolling Downs Group were brought about by the marine transgression of the Early Cretaceous. Included in this group are the Doncaster member and the Coreena member of the Wollumbilla Formation, the Surat Siltstone and the Griman Creek Formation.

Sources & Further reading

1. Geology of the Murray-Darling Basin - Simplified Lithostratigraphic Groupings