Making Australia : landscape, tectonics and climate

1. Ancient Australia  (by way of introduction)

 
We live on an ancient continent, containing some of the most ancient geological features on the planet.

Amongst this ancient legacy includes the oldest minerals; the 4.4 billion year old zircons from jack Hills in WA, which tell of an infant planet, less than 100 million years old.

 
Australia has some of the oldest evidence for life in the 3.5 billion year old stromatilites from the Pilbara, the modern analogues of which are no only found in a few places including Shark Bay in WA.

Australia has few of the obvious manifestations of the Earth’s dynamism that so endanger life in many other parts of the planet.

We have no active volcanoes, although the youngest known volcano is only 8000 years old  (check).

Earthquakes are relatively infrequent.

We have no great Mountain ranges, or rift valleys.

As a consequence, we view our landscapes as a relics of times long past.

In many parts of the continent, e can indeed literally walk landscapes little changed from the time of dinosaurs over 100 million years ago.

It is a matter of debate just how old our oldest landscapes are.

Certainly our of the most dramatic of our oldest forms are the intricate, now desiccated,  palaeo-valleys that   lace the arid interior of WA.

These valleys have there origins some 300 million years ago , dating back to a time when much of the continent was covered in a vast ice sheet.

At this time, there was no such thing as Australia. Rather we formed part of a very much larger continent of Gondwana, the last great supercontinent.

Australia s a continent only became recognisable some 50 million years ago, in the final act of a 100 million year long, rather tangled, plate tectonic separation that spilt Gondwana into the southern continents.

Australia, now moving north at 6-7 cm/year (the speed at which fingernails grow) is running the fastest of all continents, so as not to miss the party that is building the next great supercontinent of “Panthalasia”.

Throughout geologic time, the dance of the continents, with the various tenuous geopolitical allegiances, has played a crucial role in the evolution of the climate.

As Australia has moved north it ahs dried out, and global climates have deteriorated to allow the growth of the polar ice caps.

The links between tectonics and climate change are having a profound influence in Earth Science in part because they are intimately connected the emergence of our own species. 

Our own bipedal gait owes its origins to the drying of Africa and opening of its savannahs.

More recently, the building of ice sheets has impacted on Climate is not solely controlled by tectonics.

For example, the coming and going of ice sheets, that has so influenced the way our ancestors dispersed around the globe,  is due to variations in the solar insolation due to wobbles in our orbit around the sun.

Now our species is acting a geological agent on a scale that has rarely if ever occurred before, we are rightly concerned with our impact on the earthy and the consequence it will have for future generations.

(sustainability – a la george fischer)

Earth Science provides a reference point, and the context,  for understanding global change – the lesson of geology being that change is the only constant.

Our view that Australia is ancient land has meant that we have overlooked the record of our landscapes in contributing to the debate on global change.

This is unfortunate because, although much of landscapes are indeed ancient, there is also a profound imprint in our landscapes of restlessness reflecting both climatic and tectonic change.

Understanding this is important, for Australia’s northward motion is playing a crucial role in global climate change in a way we are only now beginning to understand.

In order to understand the drivers of global change, ad assess our own impacts, we need to understand the vital Australian record.

As Australia has moved northwards, it has opened seaways and closed others fundamentally changing the pattern of ocean circulation.

As part of the greater Indo-Australian plate, its northward motion has contributed to the building of the great Himalayan mountain range, and the Tibetan plateau, as well as smaller, but nevertheless spectacular rages of New Guinea and New Zealand.

The Tibetan plateau is so vast that it has dramatically affected atmospheric circulation,  impeding the way that the atmosphere transfers tropical heat to the poles.

In this talk we will show how the recent global change has left its imprint on our landscapes, particularly in southeast Australia, around the Murray basin.

Indeed, we hope to show that Australia contains an extraordinary archive of this change; an archive that, on the one hand, records a tectonic restlessness of the continent, and, on the other, records the deterioration of the global climate. 

 In order to reveal this extraordinary archive we will track the northward drift of the continent across the mid latitudes into the sub-tropical high-pressure belt.

We will see how during this northward motion the continent has begun to tilt and bend, with the forces that driving this motion giving rise to the earthquakes that occasionally rock our continent, such as the Newcastle quake of 1989 that killed 13 people (check).

We will see how these forces have shaped the Murray basin, damming and diverting a river, the history of which reveals an extraordinary struggle to balance its form with water-flow.

We will see how the rise and fall of seas as ice caps have grown and shrunk dramatically imprinted themselves on the landscapes of the Murray Basin,

We will debate the ideas that link Australians northward motion to global climate change that, ultimately, led to the emergence of our species.

We will track the nature of the environments that accompanied the arrival of the first Australians, some 50,00 years ago; 

These environments were dramatically different from today, and remind us of the great lesson of geology, that the only “constant is change”, a lesson we are obliged to reveal as our species emerges as a new geological force.


2. From Gondwana to Goondiwindi (Australia the fastest continent)

 

The fragmentation of Gondwana into the southern continents was a complex process that lasted over 100 million years (check).

Australia, the last of the Gondwana fragments to severe ties with Antarctica, began to drift slowly north around 100 million years ago.

The birth of the continent occurred at around 45 million years ago, when Australia’s northward drift rapidly accelerated from less than 1 cm/year to around 6-7 cm/year, the rate at which fingernails grow).

Since then Australia has traversed some 3000 km on a trajectory slightly east of north,

Its northward drift accompanied by India which, as part of the giant  Indo-Australian  tectonic plate, has ploughed its way some 2500 kms into Asia to raise the Himalayan mountains.

This northward motion is driven by, on the one hand, the pull of old ocean crust that is being subducted deep within the earth beneath south-east Asia to the northwest of Australia and,  on the other, the push from the submarine mountain ranges that lie along the southern boundary of the plate, beneath the Indian and Southern oceans.

The northward motion of Australia is reflected in two ways in its landscapes.

Firstly, in the tilting of the continent, north-side down, south side up, producing such things as the Nullabor plain, an emergent sea bed built between 40 and 20 million years ago. when se-levels were about 100 metres higher than today.

The interior of the plain is defined by beach deposits that rise almost 300 metres above sea level, indicating several hundred metres of uplift.

Equivalent seashores in the northern part of the continent now lie implying a north-side down tilting of more than 100 metres.

The tilting, is part real and part apparent.

The down-flow in the deep mantle to the north of Australia is dragging down the northern part of the continent.

Simultaneously, the accumulation of dense cold material deep within the earth is causing the sea to bulge outwards above this region, by about 70 metres, with the combined effects of north-side down tilting and sea bulging producing the apparent flooding of the northern margin.

While we tend to think of Australia as an ancient, tectonically inactive continent, we do experience surprisingly large earthquakes.

The Newcastle earthquake of 1989 was magnitude 5.6, quite moderate in comparison to the magnitude 7+ earthquakes that frequently rock more active parts of the world, with energy releases up to 100 times that of the Newcastle quake.

The largest earthquake record in south-east Australia occurred in 1893 near Robe in SA and is estimated at magnitude 6.5, around 20 times as powerful as the Newcastle quake.

The largest Earthquake recorded in Australia, at Meberrie some 1000 km northwest of Perth in 194? , at magnitude 6.8 released more than 40 times the energy of Newcastle.

Our landscapes reveal a surprising amount of prehistoric seismic activity.

For example, near Echuca, the Murray river was diverted by movement on the Cadell fault, up thrown by some x metres to the west and so by forming the Barmah Forrest.

The main faulting which must have been a succession of very large quakes, occurred around 50,000 years ago as the first Australian began to occupy the land.

The impact of repeated earthquake activity, extending back about 5 million years is most dramatically imprinted on the landscapes of the Flinders and Mount Lofty ranges near Adelaide.

Here we can see the ranges bounded by active faults that have slipped at up to 50 metres per million years. For the last 5 million years or so, consistent with the occurrence of a magnitude 6+ event every 100 years or so.

Similar faulting has double the elevation of southern Uplands in Victoria and added several hundred metres to the elevation of the eastern highlands south of Canberra.

The earthquakes and faults relate to the building of stress within in the Indo-Australian plate as the forces that drive the plate northwards are resisted by the mountains built as the plate collides with its northern and eastern neighbours.

Indeed these stresses have reached the point that plate is now beginning to break up as revealed by a succession of large earthquakes rocking the old ocean crust of the central Indian ocean, the only part of the old ocean floor anywhere on Earth that shows such seismicity.

As revealed by the Barmah forest, his young faulting forms part of the extraordinary archive of Murray basin landscapes.

But these landscapes also tell another story of global climate change.

To unravel the tectonic signals form the climates we need understand both.

 

3. The story of the Murray River

Understanding the landscape takes us back to environments of Miocene time.

At 20 million years, Australia was located some 1200 km further south, some 10 degrees higher latitude in what is now a cold part of the Southern Ocean.

Great paradox here, in that instead of being cold and stormy (as in the Southern Ocean there today) Miocene climatic signatures reflect warm wet conditions, conditions which we relate to sub-tropical environments more characteristic of northern Australia today.

Reconstructions of marine environments with Indonesian type foraminifera, low energy wave regimes, stable sea level.

As Australia moved north, major changes occurred to produce the cold dry climates of Quaternary Ice Age times, a climatic regime which continues today.

Instead of Australia drifting into the dry, subtropical pressure belt characteristic of today's climate, the sequence of climatic changes actually overtook the drifting plate in its equator-ward traverse.

Evidence from the Murray Basin exemplifies the sequence and nature of the changes which accompanied this globally important transition.

The Miocene marine invasion of the southern Australian coastline was massive, depositing perhaps the world's largest limestone deposits. That signature is clearly represented in the geological record of the MB.

Following the long stable period of carbonate deposition, a major drop in sea level occurred in mid- to late-Miocene. From near 10 to at least 6.5million years ago retreat of the sea resulting in major erosion of older calcareous sediments with deposition of fluvial sand and gravels as streams advanced across the exposed marine planation surface.

This marine regression, correlating in time with the drying of the Mediterranean sea, seems suspiciously to reflect a major growth of ice in East Antarctica. That event however was short-lived.

Near 6.5 million, the sea advanced once more, extending inland to near its previous shoreline, transgressing across the fluvial sands and gravels of the regressive phase.

This phase of marine advance sets the scene for the crucial stage of climatic transition, global sea level dropped, a major change in wind wave regimes occurred with the onset of cold dry conditions of Ice Age times.

PLIOCENE RETREAT : Retreat of the sea from the Murray Basin involved  a sequence of cyclic oscillations with small (?40 metre) regression followed by transgression on a pattern of overall falling sea levels (increasing ice volumes…Antarctica). The resulting coastline sand barrier systems, deposited at or near ancient shores, provides a remarkable contour map of a Pliocene landscape.

The sand ridge system is notable for both it internally parallel ridges and, unlike modern coastline equivalents, the absence of wind generated parabolic blow-outs.

LAKE BUNGUNNIA

Tectonics: In the advanced stage of marine retreat, tectonic movement blocked the rivers (Murray and Darling) draining to the sea. The result was the formation of a vast inland lake, Lake Bungunnia.

Covering more than 40,000km2 in area, the lake deposited a thick sequence of mainly acidic clays (Blanchetown Clay) passing upwards into a calcareous sequence of thin limestones, oolitic and microbial stromatilites. These have provided materials suitable for paleomagnetic dating.

The formation of Lake Bungunnia occurred near 3.5 million years ago. After depositing a blanket of clays some 10-20 metres thick across the basin, the lake drained somewhere before 800,000 years ago, an event dating to near 1 million years.

To maintain a lake of the size of Bungunnia, the combined inflow of the Murray-Darling system would have to be more than twice its present discharge. Indeed the remarkable channel that cut the gorge tract of the Murray, immediately after draining of the lake, posseses channel dimensions only produced by rivers with bank-full discharge 5-8 times that of the present Murray system. By any estimate, the discharge of the basin at that time (about 1 million years ago) was an order of magnitude higher than in the same region today.

By the time the lake was drained, probably during a period of abnormally high discharge (?high rainfall), the coastline had retreated to near Naracoorte, a period of critical transition in the geochemistry of sediments and soils together with  changes in wind wave regimes heralding the onset of arid conditions on mainland Australia after some 20million years of warm wet environments.

Sediments and soils changed from acidic, ferricrete/silcrete profiles to alkaline, calcrete signatures.

Shoreline sediments changed from low energy beach /barrier sands with few shells  to high energy, coarse, often cobble beaches with abundant shelly materials.

Coastline wind erosion for the first time produced extensive dunes, with the generation of inland arid landforms of type and scale never previously present throughout the MB and large tracts of inland Australia.

This period forms a critical phase in the evolution of Australian environments. Moreover, the  changes registered here are a reflection of changes in global marine and terrestrial environments on a scale not previously registered through the previous 250 million years. Ice Age aridity had arrived.

What caused these momentous changes?

Discussion/presentation of insolation cyclic changes.

4. Tectonics and climate change

 

5. The coming of man : the new geological agent

The last million years has witnessed the successive or cyclic alternation between cold, dry ice ages (low sea levels) and warm high sea level interglacials. To obtain a picture of impact on the Oz landscape, our best evidence comes from considerations of the last glacial cycle, the story of the last 120,000 years.

Within that time, some of the best evidence comes from the dunes and lakes of the Murray Basin.

Dry fossil lakes of western NSW preserve the legacy of times when the climate was both wetter and drier than today.

Lunettes, fossil lakeshore dunes,  across southern Australia act as critical indicators of hydrologic and climatic change. They reflect times when watertables were high with major dune formation during transitional periods from high to low lake levels, from wet to dry regimes.

Applying these criteria to lakes in the Willandra (Lake Mungo)region, with detailed accurate dating, we establish a pattern of lake level and dune building phases for the past 60,000 years.

A long wet phase from 60 to 40,000 gave way to drying and dune building leading to the prevalence of major cold dry conditions of the last glacial maximum near 20,000 years ago.

Temperatures at this time were some 6 to 10 degrees below present. Low evaporation from colder ocean surfaces resulted in reduced precipitation.

The cold dry conditions provided extremely harsh conditions for the Australian flora and fauna, the former poorly adapted to intense frost conditions.

 

If LGM environments were harsh on plants and animals, their impacts on human occupants can only be imagined.

The arrival of human in Australia, certainly by 50,000 years ago, set the scene for a whole new dimension of changes, challenging those produced for millions of years earlier by natural planetary dynamics. With the evolution and arrival of humans in  Australia, the landscape was subjected to the influence of un-natural fire and perhaps decimation of its larger animals by hunting to extinction. The pristine vegetation was certainly changed.

The story encapsulated in the life and times of Mungo Man, although some 40,000 years remote from us today, transmits many lessons. We would have great trouble recognising the landscape and climate of his times. We may summarise the effects of some of those major changes on the climatic evolution of south-eastern Australia.

60,000 year landscape

LGM (20,000) year landscape.

6. Sustainability and the role of the Earth Sciences

Although the impact on the land of original occupants may have been substantial, especially in its use of fire, its totality over the 50,000 years pales by comparison with that of our industrial culture over the past 230 years.

Humans have now replaced volcanoes, earthquakes and other natural disasters as the major modifying power on the entire planet. Here in Australia, we become almost numbed to the daily litany of environmental problems, responsibility for which lies with ourselves. And therein lies part of the problem.

As geologists, we become almost inured to the constancy of change. Take for example, the Murray River. A comparison of its channels at 1 million, 60,000 and present day illustrate the huge variation in form and function over time. Today, the influence of humans has reduced this once majestic stream in places to little more than a salty trickle.

Salinity, soil erosion, atmospheric pollution, loss of biodiversity, death of our streams, the litany of human influence is almost endless. In all of these, there is probably no single issue, more problematical in its analysis but more important in a global context, than the major changes on atmospheric gases consequential on our burning of fossil fuels, items of special importance and therefore responsibility of the geological community.

While we may well debate the balance of human v natural variability in the severe droughts, global increase in both maximum and minimum temperatures (especially night-time temperatures), the reality of changing climate cannot be denied. Diminishing water supplies of almost every major city act demand our increasingly urgent attention. While changes in climatic systems due to human agency may well be debated, the risk factor in the huge geological experiment we are currently conducting would be denied only by the ignorant or the partisan.

Australia is unique in possessing some of the best indicators to help discriminate between natural climatic variability and the influence in the past 200 years of human modification.

Our responsibility to alert the community, to carry them with us in this journey of education into the wondrous stories of the planet's evolutionary history in general, of this land in particular.