GEOLOGICAL BACKGROUND TO SOUTH AUSTRALIAN SCENERY

Dr Victor Gostin, Earth & Environmental Sciences, University of Adelaide

South Australian rocks record numerous episodes of uplift, erosion, deposition, folding, metamorphosis and granitic intrusion that occurred in Cambrian and Precambrian times (before 500 Ma = 500 million years).

Over 200 million years of erosion from Ordovician to latest Carboniferous (300Ma), removed many kilometres of crust, exposing rocks that had been folded and deeply buried. These now outcrop in Eyre Peninsula, Flinders Ranges and the Adelaide Hills.

Australia was then part of the Gondwana supercontinent, and it was covered by large ice caps that spread from the south, gouging out and infilling valleys with glacial and meltwater sediments. The widespread vegetated swamps produced Australia's abundant coal and gas deposits.

Another 180 million years of erosion were followed by very high Cretaceous sea levels flooding vast areas of low-lying Australia. This formed the Great Artesian Basin. Weathering and erosion of the southern edges produced the colourful breakaways of the Painted Desert.

A giant rift opened along Australia's southern edge where volcanoes erupted, the sea invaded from the west, and the final breakup of Gondwana began. Remnants of this low-relief Cretaceous topography are probably the oldest surfaces forming the highest parts of the Gawler, Flinders, and Mt Lofty Ranges.

In the Late Eocene (45Ma) the crust thinned by extension and subsided, forming the Eucla, St. Vincent, and Murray Basins. These were filled with sands, muds and marine shells that formed widespread limestones. The Mount Lofty Ranges rose gently along their eastern side.

A 650 km long coastal dune system of Eocene to Miocene age (10 Ma) is still preserved along the ancient shores of the Eucla Basin. As sea levels fell, coastal erosion resulted in the spectacular limestone cliffs of the Great Australian Bight.

In the Miocene (10-5Ma) the sea retreated from the Murray Basin. The coastal parts were eroded and covered with Pliocene coastal gravels. Oyster beds filled the long estuary of the lower Murray valley.

The last two million years were characterised by multiple sea level fluctuations (up to 140m) linked to climate changing from humid to arid. The western Mt. Lofty Ranges were elevated due to crustal compression, and rivers like the Torrens and Onkaparinga now cut steep gorges.

The Earth's crust is continuing to deform as witnessed by beach deposits 120,000 years old that are near sea level at Port Adelaide but have been uplifted to over 10m at Normanville and 16m at Mt Gambier.

We live on an ancient continent, rejuvenated in recent times.

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