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Regional Studies
The Landscape and Vineyards of the Murray Basin - Appendix 1
Wednesday, 29th July, 2015  - David Farmer

Appendix 1

The Geology and Formation of the Murray Basin

To appreciate the wine story of the Murray Basin it helps to understand how the basin formed, or its geological evolution. Please note this long section has no bearing on wine flavours. Also it is a simplification and re-arrangement of the published information.

Three issues are of special interest in considering how the Murray Basin evolved;

1) The period covers a fundamental change in climate across Australia from lush, sub-tropical and temperate greenery to a parched dry land, with the critical years being 20 million years to 5 million years ago.

2) For the last 30 million years the Earth has slowly entered an ice age and the steady cooling of Antarctica expanded some 2.5 million years across the Arctic. The accumulation of ice has lowered ocean levels, and altered climates and parts of this story are seen in the Murray Basin.

3) To complete the basin shape required a division or ridge to rise which would separate the basin from the south-western coastline; that stretch of coast between the South Australia- Victoria border to Goolwa, South Australia. For much of its formation the basin was open to the sea, and on two occasions was a shallow interior sea. The closure of this ‘ocean frontage’ to produce the shape we see today is relatively recent.

Only when the basin was cut off from the ocean could the current drainage pattern be locked into place.

A. 80 - 60 Million Years-The Beginnings of the Murray Basin

Continents are constantly adjusting to crustal strains by lifting and sinking, though the Australian continent, over the last 30 million years, has been remarkably stable when compared to the mountain building upheavals and volcanism of other continents.

For the story of the Murray Basin the major event was the tearing apart of Antarctica and Australia which began about 80 million years ago and was completed some 40 million years ago. Australia of course still continues its slow drift north. About 60 million years ago in response to this tearing or rifting, faulting and subsidence centred on the town of Renmark began in the western Murray Basin. Most likely an existing but inactive older fault line or weakness was reactivated.

The depth of this depression, as measured by the thickness of the sediments has reached 600 metres. From Renmark this subsidence slowly spread east to outline the basin shape though the thickness of sediments over the rest of the basin is about 200 metres with less at the edges.

Considering the east-west distance of 650-700 kilometres this depth of sediments records the gentlest of down warps. At the same time a similar depression was also forming to the west of Adelaide to create the gulf country.

B. 60 Million Years-Imagining the Landscape Before the Murray Basin

To understand the basin it helps to imagine what the landscape looked like prior to subsidence and the place to look for clues is the landscape that now surrounds the Murray Basin, though these thoughts are speculative.

Clockwise from the west the surrounding ranges are, the Mount Lofty Ranges, the Barrier Range extending to Broken Hill while to the north, east and to the south are the multiple ranges of the Eastern Highlands or Great Divide.

An idea of the landscape 60 million years ago can be gained by projecting the trends of ridges and valleys observed today across the basin area.

Whether the landscape relief was sharper then is speculative as there is the possibility that some of this pre-basin area was an elevated plateau.

Did the small rivers and streams at this time flow north or south and did they combine into a major river and if so where was the outlet? It is known that rivers flowed north from the Victorian highland while the climate was temperate and this vast area covered by a rain forest.

C. 60 - 30 Million Years - The Murray Basin Expands

From Renmark the subsidence gradually spread. At about 55 million years a second centre of subsidence began near the town of Hay, N.S.W.

Studies of the basin floor show it is not smooth like a bowl but is rippled with buried hills and valleys. Indeed towards the basin margins the tops of basement ridges may poke through the surface, these being parts of our imaginary landscape. By 37 million years the basin was about the current size and as the surface sank sediments filled the valleys to create a flat plain.

By this time the deepest part of the basin at Renmark contained 235 metres of sediments which over 20 million years is about a metre every 100,000 years.

With sediment transported in from the eastern and southern highlands we can imagine the slight gradient to the west being accentuated creating conditions for the rivers to flow westward and by this time we can speculate that a proto Murray River system was developing.

For the eastern half of the Murray Basin this pattern was to continue but for the western Murray Basin the history is different.

D. 30 - 28(?) Million Years - The 1st Period with a Missing Record

The layers of rock are the record of geological time though perfect uninterrupted layering is seldom found. And so it is that from about 30 million years to 28 million years, in the central and western Murray Basin, there is no record of what was happening.

This gap is marked by a weathered land surface, termed a paleo-surface. Victorian geologists have long recognised this surface and date it no older than 28 to 29 million years, and call it the Mologa Surface*.

The Mologa Surface is ferruginised and erosional and is quite widespread, meaning it is defined by an iron rich layer and this layer cuts across older rocks. Similar surfaces are seen in South Australia and NSW.

I have spent many years mapping the landscape of the Barossa Valley and the adjoining ranges and have reasoned that a trace of this surface should be preserved but have yet to find anything that could correlate to the Mologa.

This surface ends the first phase of the development of the Murray Basin.

Questions that might be asked include; had the basin stopped sinking, why did the accumulation of sediment end, and if sediments did accumulate were they later eroded way, does this surface mark a change in climate and where did the river system flow?

*I note some differences in the name/age of this surface among geologists and I have used the reference in the Geology of Victoria (2003), "Consequently the Mologa Surface in the Kerang-Cohuna area occurs in the late Early Oligocene (p 322)".

E. 28(?) - 15(?) Million Years-The 1st Sea Incursion Floods the Western Murray Basin

Around 28 million years, the western half of the Murray Basin sank below sea level and was flooded and marine sediments overlie the Mologa Surface. The sea retreated about 15 million years ago.

Whether conditions allowed the Mologa Surface to survive in the eastern Murray Basin is unknown. N.S.W. geologists refer to a surface dated at 18 million years.

As this summary deals with the major events that have formed the Murray Basin many small details are passed over. One that is worth emphasising is that the Murray Basin, from perhaps 40 million years, was very close to sea level.

Marine sediments in the south-west corner show that the sea flooded locally into the basin on a number of occasions though this was the first major ocean incursion and covered about half of the Murray Basin.

Since the basin was close to sea level quite small variations in the rate of subsidence or uplift, an increasing or decreasing influx of river borne sediments, and any sea levels changes, could shift the basin from being above or below sea level.

Interestingly the periods of ocean incursions means the famous wine growing regions of the Mount Lofty Ranges would be part of an archipelago projecting down into the southern ocean.

At this time the sea covered the area from the Grampians in Victoria, west to the Mount Lofty Ranges and was part of the Southern Ocean.

A schematic diagram of the 1st Sea flooding showing the accumulation of limestone in the Murray Basin.

F. 15(?) - 8(?) Million Years-The 2nd Period with a Missing Record.

After the sea receded until 8 million years the sedimentary record is missing and what happened during these 7 million years is not known.

Ice was building on the continent of Antarctica heralding major climate change. It is thought this was a period of mild erosion and a time of increasing aridity, and is likely the critical period for climate change across Australia which would lead to the parched inland of today.

There seems to be no name given to this interval and I am unsure whether this break in the sedimentary record is associated with a weathered land surface. The reference to this time period is the Late Miocene-Pliocene boundary.* As with the first missing record period we can only guess what the river system was doing and whether it resembled the current Murray River system.

* Geology of Victoria (2003), p 322 ' in all likelihood a younger unconformity at the Late Miocene-Pliocene boundary.'

G. 8(?)- 3.5 Million Years-The 2nd Sea Incursion and the Ice Age Looms

The 2nd sea incursion covered a larger area than the 1st sea incursion and the sediments deposited are different. The dune ridge lines were created as the ocean retreated. The ages of the retreating sea are shown.

This period of the Murray Basin is particularly interesting as the earth was slipping into the current ice age. The accumulation of ice on Antarctica probably began 35 million years ago and by this time the impact was spreading globally. The sea returned to the western Murray Basin about 8 million years ago. The sedimentary record show this flooding, or transgression, covered a larger area than the flooding of 28-15 million years extending over 50% of the western Murray Basin.

Interestingly the sediments recording this event are calcareous and siliceous sands which tell a different story to the limestone's of the first incursion which were deposited in quiet, tropical waters.

It is suggested that the second incursion was initiated by a sudden rise in sea level. The flooding was quite fast as it seems to have quickly reached a maximum, stalled and began retreating about 6.5 to 7 million years ago.

As the sea retreated, or regressed, it left behind a remarkable series of arcuate beach ridges. These beach ridges preserve the steady rhythm of the oceans halting and retreating and cover the period from about 6.5 million years to about 3.5 million years. The youngest ridges are found near Swan Reach which is just north of Murray Bridge.

It also appears that the retreating sea was gradually being funnelled into a narrower and narrower outlet similar to that of the current lower Murray River.

A great deal is known about these dune ridges as since the mid 1990s they have been extensively explored for mineral sands with some success.

The sea re-worked the sediments that flowed down the paleo river systems into the Murray Basin from the west, north east and south-east and in places concentrated the heavy minerals into layers within beach sands. These later became the target of mining companies.

H. 3.5 Million Years to 2.5 Million Years a Period of Uplift

As the sea retreated the rivers formed a new network flowing west perhaps similar to now. During wetter periods other tributaries, traces of which are still visible, were flowing.

About 3.5 million years ago, during the final stages of the retreating sea, the uplift of a narrow width of land extending from southern Victoria across South Australia to Murray Bridge became more pronounced.

It was this upheaval which completed the oval shape of the Murray Basin by closing off the south western segment from the Southern Ocean., The formation of this rise, termed the Padthaway Ridge, unfolded over a long period. The current elevation of the Padthaway ridge is 130-140 metres.

I. 2.5 Million years to 620,000 years the Great Inland Lake Bungunnia.

The extent of Lake Bungannia at 1 million years ago.

The rise of the Padthaway Ridge was now disrupting the landscape and at 2.5 million years south of Swan Reach the flow of the proto River Murray to the sea was blocked.

Perhaps this damming was caused by the uplift though the sea was retreating and receeding steps were marked with coastal dunes so a combination of coastal dunes and uplift is also possible or likely.

It is known that the area of South Australia which lies to the south west of the Padthaway Ridge is in effect re-claimed land with an approximate age of 750,000 years which is progressively younger towards the coastline and the Coorong.

It would seem then that the blockage at Swan Reach was initially not far from the sea and a modern comparison can be made with Lake Alexandrina and the Coorong without of course the closure of the Murray River.

Behind this natural barrier formed Lake Bungunnia which at its largest extent covered 50,000 square kilometres and was to last until 620,000 years ago. Rainfall calculations show that to maintain a lake of this size would have required a heavier rainfall than currently measured while fluctuations in the size of this shallow lake record minor changes in climate.

The development of the lake coincides with a major increase in the intensity of ice globally with the sudden build up of ice in the northern hemisphere which covered the Arctic beginning about 2.5 million years.

This cold period ushered in the drying of the Australian continent with the development of the huge inland area of red, arid, sandy, salty landscape that we have today.

The banks of the Murray River showing the clay deposits that accumulated in Lake Bungannia which overlie sands left by the retreating sea of the second ocean incursion.

J. 620,000 years to Recent the Drying Times

The drainage patterns of the eastern Murray Basin, which recall were always above sea level, may not have altered much for millions of years, perhaps as long as 15 million years, after the first ocean flooding.

The damming that created Lake Bungunnia was breached about 620,000 years ago. Thus with an outlet to the ocean, the drainage pattern over the western half as seen today evolved quickly.

By this time the ice age was intense and the resulting windy, drying, cold climate, produced wind-blown soils, dunal fields and an increasing arid landscape.

Ice ages are particularly interesting as they fluctuate in a predictable rhythmic way. They are made up of a great number of smaller cycles within a much larger cycle which changes from an intense cold high and slowly warms to a brief warm high. The cycle then commences again as the next cold era begins.

Much is known about the rhythmic pattern of the current ice age and since the on-set about 2.5 million years the cold to warm period has lengthened from 50,000 to 60,000 years to 100,000 to 105,000 years, with this longer pattern existing for about 700,000 years till now.

The Globes current climate, the maximum warm phase, spans about 10,000 to 14,000 years of the 100,000 to 105,000 year cycle and naturally coincides with the maximum melting of the ice sheets and in turn the highest level of the oceans.

Traces of the Murray River across the continental shelf. As the next phase of the ice age builds the ocean level falls and the new shore line forms about 80 metres lower than now.

As the cold phase returns the ice again accumulates and the oceans drop till finally a shoreline is established well out on the continental shelf at the cold cycle maximum.

Since each cycle is made up of many warmer and colder smaller cycles this means there are fluctuations of ice growing and melting. At times of ice melting in the Eastern Highlands the rivers draining into the Murray Basin would in turn become more active and some which are dry would again flow.

There are many of these dormant stream systems trending to the south west across the Murray Basin and the most spectacular of these is the Willandra Lake system.

The current ice age has not of course finished and while Antarctica is positioned over the South Pole this interesting period of geology, termed the Quaternary, will continue.

Lastly two diagrams which assist in understanding the geology.

When the ice is next at its maximum ocean levels will be much lower. Shown is the Murray Basin in relation to the exposed continental shelf. This situation will occur in 60,000 to 80,000 years.

Should the world return to a period without ice, last seen 30 million years ago, parts of Australia would flood. Note the extent of ocean incursion into the Murray Basin. This tells us that the Murray Basin has continued to subside since the second sea incursion but is above sea level because the ocean level has lowered faster. This is due to the accumulation of ice on Antarctica.

Back: The Landscape and Vineyards of the Murray Basin

References and acknowledgements:

Scheibner E. & Basden H. 1998. Geology of New South Wales, Volume 2, Department of Mineral Resources, Geological Survey of New South Wales, Memoir 13 (2).

Birch W.D. 2003. Geology of Victoria, Geological Society of Australia, Special Publication 23.

Drexel, J.F. & Preiss, W.V. 1995. The Geology of South Australia, Vol 2, South Australian Geological Survey, Bulletin 54.

Stewart, R. 1999. Murray Basin Mineral Sands Conference, Australian Institute of Geoscientists Bulletin No. 26.

Department of Earth Science, Melbourne University, particularly the web site of Professor Mike Sandiford from which I have used several striking visuals.

A general acknowledgement of all of the many science journals from which I have taken thoughts, books about landscapes and soils, and the numerous wine books and journals read over many decades and thanks to Professor Brad Pillans, Research School of Earth Science, ANU Canberra for regular stimulation.

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