Australia: The Land Where Time Began

A biography of the Australian continent 

Eastern Australia

In the Early and Middle Cretaceous, 100-96 Ma, there were high mountains along the eastern edge of the continent. The 'rift' mountains were the result of Chilean-type volcanic activity. Since then, the volcanic areas moved away from the east coast and the mountains were eroded, lowering the gradient of the slopes and reducing the erosion rate.

At their peak these mountains were high, much higher than any in Australia now, and the climate was much wetter, and warmer, and the continent was covered with forests. Under these conditions rivers formed that had the size and power of the present-day Congo. The river systems along the eastern coast had sufficient volume and power to move sediment all the way to deposit in the Ceduna depocentre in the rift valley that was forming between Australia and Antarctica. This sediment dump is 9 km thick.

Mesozoic to Tertiary

The basement rocks of the region are Palaeozoic, having been eroded to a palaeoplain. On this are 2 sedimentary basins separated by a system of divides. The Great Artesian Basin, sinking and having sediment deposited into it in the Jurassic and Cretaceous, 170-65 Ma. The other is the Cainozoic Murray Basin. It started to subside about 60 Ma in the Early Tertiary. South of this is the Cretaceous-Early Tertiary Gippsland-Otway Basin.

In the Jurassic prior to the break-up of Gondwana, Australia was connected to Antarctica in the south and along what is now the east coast, the New Zealand subcontinent, aka Pacifica, was connected across the border of the yet to be separated Australia and Antarctica. At this time the Great Artesian Basin  (GAB) received sediments from the New Zealand subcontinent and from Antarctica. The GAB has existed since before the separation from Antarctica, and also predates the formation of the eastern highlands and the Murray Basin. Gravel-filled channels from these old river connections are still present, crossing the Victoria, Great Divide, and Canobolas Divide.

During the Cretaceous, the southern section of the palaeoplain tilted, creating the Victoria Divide. This occurred in association with the formation of the rift valley between Australia and Antarctica, and the beginnings, progressing form west to east, of the Southern Ocean. This watershed was situated about where the Victorian Highlands are at present. It separated the GAB from the Gippsland-Otway Basin.

The Otway Basin originated as a result of the rifting that was opening to form the Southern Ocean. The Gippsland Basin was a result of incomplete rifting of an arm of the Tasman Sea. The remains of the rivers that originally flowed from the Antarctic highlands to the Australian lowlands can been seen in the palaeodrainages near Esperance in Western Australia. The Great Divide was a result of rifting that formed the Tasman Sea as the New Zealand subcontinent moved away from the present Australian east coast. Some reversed rivers are present on the eastern side of the Victorian and Great Divides.

Drainage in the south-east continued to the GAB in the north from the Victoria Divide and from the inland side of the newly formed Great Divide. The at about 60 Ma the Murray Basin began to subside. At this time a new warp axis was formed, the Canobolas Divide, separating the 2 basins. This divide formed as a result of the downwarping of the Murray Basin, not uplift of the divide. When the Murray Basin formed, the drainage was mostly westward, resulting in the end of movement of sediment to the GAB.

The lines of divides between the basins are lines of crustal weakness, compared to surrounding areas. Volcanic activity has occurred along these lines of weakness. Throughout the Cainozoic volcanoes erupted along these weakened lines in the crust. Along the Great Divide some of these were the Ebor Volcano, Monaro Volcano, Barrington Volcano and Warning Volcano. Also, Mt Canobolas on the Canobolas Divide. In the Oligocene and Miocene, until some time in the Late Miocene, lava flows were widespread along the Great Divide and the eastern Victoria Divide.

The Tertiary

In the early Cainozoic there were widely distributed basalt flows in eastern New South Wales. some of the places they occurred are the Hunter Valley and surrounding areas, the Shoalhaven region and the Monaro. As a result of recent studies and dating of basalt flows, a reassessment of the geological history of eastern Australia is presented here.

• Prior to the basaltic flows the topography of the Monaro was similar to the present time, drainage systems that had developed prior to the Early Eocene to Oligocene eruptions were similar to those of the present.
• By the start of the Miocene, the headwaters of the Lachlan River and Wollondilly River were close to their present beds.
• Locally, volcanoes disrupted drainage patterns, the pattern around the volcanoes becoming radial around the volcanic areas.
• Apart from the Late Tertiary and Quaternary lava flows in western Victoria, not much volcanic activity occurs in basins.
• In the Miocene, blocks of fault-bounded terrain were elevated, resulting in a major uplift of the Gippsland-Snowy Mountains block.
• The Gippsland and Kosciusko areas were affected by several major faults, having throws of several hundreds of metres to a kilometre, in some areas.
• Lake George was created by a localised tilt block movement on a fault on the southern highlands. The northern tributaries of the local river system were diverted.

The lower reaches of south-eastern Australian rivers have been affected by sea level changes. A eustatic sea level fall in the Late Miocene led to valley downcutting, such as in Gippsland. In the Early Pliocene sea level rose and the Murray Basin was inundated, the tidal conditions being felt far upstream.

Sources & Further reading

Mary E White, Running Down, Water in a Changing Land, Kangaroo Press, 2000