Australia: The Land Where Time Began

A biography of the Australian continent 

The Murray Basin

One of the events associated with the movement of Australia away from Antarctica was the formation of basins such as the Murray Basin. Tectonic movements resulted in the formation of low mountain ranges by the uplift of rocks, of Proterozoic and Palaeozoic age, between which was a saucer shaped structure. Thin horizontal sedimentary strata were deposited as the basin, about 320,000 km2 in area, that began to subside about 60 Ma. Slow rates of subsidence and low rates of sediment supply are indicated by the Cainozoic history (Brown & Stephenson, 1989). In the eastern portion of the basin the area has not been affected by marine incursions, whereas the western portion has undergone repeated marine incursions, the most recent ended between 3 and 2 Ma in the Pliocene.

The present-day basin is divided into the Mallee and the Riverine Plain, the result of the different geological histories of the 2 sections of the basin, the western section being covered by marine environments for millions of years over a number of episodes, producing alternating marine sedimentary layers, the eastern portion had no such marine incursions so had only freshwater sedimentary deposits.

During the Pleistocene, when the dry, windy climate resulted in the Mallee, being the western portion of the basin, was subjected to the usual ice age conditions when sand and salt are scattered far and wide from the dry salt lake beds by the strong winds that occur in these times in Australia. This increased the differences between the 2 portions of the basin. A large part of the basin was a salt desert during the last glacial maximum, between about 22,000 and 18,000 years ago. For at least the last few million years, the Basin has been characterised by natural phenomena such as salt lakes, groundwater discharge zones and salty groundwater that enters rivers. Periodically, adjacent areas have been contaminated during climatically related events.

Beneath the upper layers of the surface sediments of the basin the groundwater holds the salt accumulated over millions of years of marine inundation as well as that added by blowing salt during the glacial phases of the ice age. At places where groundwater reaches the surface, salt lakes or groups of salt lakes, form. On a regional scale these zones form 'boinkas', plains of salt and gypsum, such as the Raak Plain in the northern part of the northern Wimmera. Before interference by human activity, a balance had been reached, as occurs in natural systems, the biota coevolving with the environment over millions of years.

The low relief of the landscape, and the resulting low hydraulic gradients, have made the region sensitive to changes in the water budget, making the balance of the whole system very fragile, reacting to changes on much shorter timescales than would normally be expected.  This fragility has become very apparent with the changes brought about by changing land use since European colonisation. Since colonisation, farming practices have greatly changed the water budget of the area, with pressurisation of underlying aquifers resulting from such changes as the addition of irrigation water, changing the boundaries of groundwater discharge. The rising groundwater has reached, or in some places, is about to reach the root zone, salinisation becoming a huge problem over a vast area, that is still increasing.

In the Tertiary 3 major sequences of deposition correlate with the rise and fall of the oceans. In the Palaeocene a depositional sequence began, continuing through the Eocene to the Lower Oligocene. The Warina Sand of the Renmark Group was laid down in the central western depocentre during the Palaeocene to Eocene. The Olney Formation of the Renmark Group was deposited in fluvio-lacustrine environments in the Eocene as carbonaceous silt, sand and clay (Brown & Stephenson, 1989).

The second sequence was deposited from the Oligocene to the Middle Miocene, as a result of a major marine transgression that covered the western area of the Murray Basin. The marl and limestone of the Murray Group was deposited in the epicontinental sea, and in the shallow marine marginal environments clay was deposited. The Olney Formation, Geera Clay and the Winnambool Formation prograded locally back over the Murray Group limestones when the sea level dropped in the Middle Miocene. A short period of erosion followed this decline of sea level, and according to most authors the sea retreated entirely from the Murray Basin at this time (Brown & Stephenson, 1989).

The third depositional sequence occurred during the Upper Miocene to the Pliocene, involving a series of marine transgressions and regressions. Clay and marl were deposited in a shallow marine environment during the initial transgression, and the Calivil Formation, comprised of coarse-grained sand, was deposited in fluvial and fluvio-lacustrine environments in the eastern half of the Murray Basin. The Loxton-Parilla Sands were deposited in the Early Pliocene as fluvial and sandplain deposits during a marine regression (Brown & Stephenson, 1989).

During the Early Pleistocene the western margin of the Murray Basin was uplifted leading to tectonic damming of the Murray River and the formation of Lake Bungunnia in which the Blanchetown Clay was deposited. Uplift increased the erosion rate along the northwest margin, the resulting erosion material forming the colluvial Pooraka Formation. Fluvial sedimentation continued to the east and north, leading to the deposition of the Shepparton Formation, as well as the formation of the Riverine Plain (Brown & Stephenson, 1989).

Increasing aridity is indicated for palaeoclimates of the late Cainozoic, with the development of Murray Basin landforms being strongly related to variations in climate, and interactions between fluctuating saline groundwater and processes of the surface geomorphology. Processes of aeolian dunefields, groundwater discharge lakes, lunettes of gypsum and clay, and bed-load channel sands were activated by glacial aridity. The distribution of dunefields has been controlled mainly by the availability of sand, dunefields being initiated and reactivated by climatic conditions. The vegetation in the interglacials, that are wetter, stabilise land forms, suspended load channel sediments and lunettes dominated by sand (Brown & Stephenson, 1989).

Sources & Further reading

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

 

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Last updated 30/04/2010 

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading