Australia: The Land Where Time Began

A biography of the Australian continent 

Australia Drifting - Changes on the continent, a timeline

4500-540 Ma Precambrian

 

540-490   Ma Cambrian

A high proportion of the world's land surface was situated in low and middle latitudes, indicating that the climate would probably have been hot on most landmasses, with possible areas of aridity. It has been suggested there could possibly have been a large temperature gradient at least for some parts of the Cambrian, based on some evidence of glaciation in North America. This also suggests the seas may have been cool. As life was still confined to the water at this time, the water temperature would have a big effect on it. Because the only evidence for glaciation over such a long period is fragmentary, some believe there were possibly more glacial periods, the evidence for which has yet to be found.

Australia

Australia was in the Northern Hemisphere during the Cambrian, with part of the West Australian coastline being on the Equator, the reminder of the continent extending between 0o and 30o north. Sea levels were high globally. An embayment into central Australia increased the area of shallow marine environments late in the the Period.

High ground was mostly restricted to the southern parts of the Northern Territory and the Kimberleys of Western Australia. Locally, some areas were affected by aridity, with some embayments of the epicontinental sea sometimes being cut off from the main part of the sea, which resulted in evaporation of the water and the deposition of evaporites such as phosphorites, gypsum, as well as other evaporite deposits formed.

In the north of the Northern Territory, and in the region of the Great Victoria Desert in Western Australia, volcanic activity occurred that spread vast sheets of basalt across a very large area. The Deccan Traps in modern India are a good example of the basaltic flows that occurred in Cambrian Australia, and are of a similar scale. In New South Wales, Victoria and Tasmania, volcanism on a smaller scale occurred.

The sea over the area around the Gulf of Carpentaria and the central areas of the continent retreated as a result of folding and uplift of the zone through Adelaide and northward, the Delamerian Orogeny, that occurred during the Late Cambrian.

 

490-446   Ma Ordovician

During this period Australia was still situated between 00 and 300 N. High global sea levels at this time saw large areas of epicontinental, shallow, warm seas populated with rich assemblages of fauna, with abundant corals. At the close of the period the global climate cooled, ushering in an ice age that persisted until the Silurian. As more water was locked up in the vast glaciers the sea levels fell around the world. Australia was not affected very badly by the glaciation as it was near the equator at the time.

Australia was part of Gondwana, and there were a number of comparatively small blocks in the Northern Hemisphere that were evolving into the landmasses. During the Ordovician, Gondwana swung away from the equator, and by the end of the Period, the northern part of South America was situated further into polar regions, but the position of Australia was not changed much from that in the Cambrian, being on the opposite side of Gondwana to South America, the South American end having swung around much more than the Australian end.

The climate appears to have been hot in the Early Ordovician, cooler in the Middle Ordovician, hot again in the Late Ordovician, but cool again near the end of the Period, when polar ice caps were present that lasted into the earliest Silurian. The glacial period appears to have reached its maximum at about 440 Ma when the sea levels reached their minimum.

Australia

The Delamerian Orogeny that began in the Late Cambrian, continued into the Ordovician, uplifting crust along a zone from western Tasmania to the north-northwest through western Victoria to north-central-South Australia. The high rate of erosion from this uplifted high country indicates that the climate at the time was very wet, a high rainfall being required to erode so rapidly.

The western part of the Amadeus Transverse Zone had been elevated above sea level during the Cambrian and the earliest part of the Ordovician. In the Early Ordovician, it begun a long period of subsidence that resulted in the formation of the Canning Basin. By the Middle Ordovician, the sea had covered this depression, linking up with an embayment from the eastern continental margin to form the Larapinta Sea that divided Australia into 2 separate parts separated by a shallow sea. This sea eventually joined with a narrow channel that extended along the continent's northwest margin, the first part of the Westralian Depression. To the north of the Larapinta sea, the land was low and featureless, while that to the south, made up of the southern half of Western Australia and South Australia was rugged high country.

Along the eastern margin, deep water sedimentation continued. In the area of present-day eastern Australia, volcanic activity was increasing and becoming more widespread. Volcanic rocks were accumulating episodically, the Lachlan Orogeny.

 

446-416   Ma Silurian

Australia was straddling the equator.

The Australian part of Gondwana was still on the equator, with the rest of Gondwana in the Southern Hemisphere, the South Pole being situated on North Africa. North America, Scandinavia, and some of Europe were combined and straddled the equator. In the Northern Hemisphere there were a number of blocks that had yet to collide to form Asia.

Following the ice age on the boundary between the Ordovician and Silurian, sea levels rose globally. Most of the glaciation had occurred in north Africa, and when the glaciers retreated they were replaced by a shallow epicontinental sea. The ice age sediments were overlain with organically rich sediments.

Australia

During this period Australia was still situated on the equator, so the climate would have been hot.

During the earliest part of the Period the glacial phase was at its most intense, and as the continent was still on the equator, the climate was probably cool, but not glaciated. Sea levels reached their lowest level at the start of the Period, rising again after the ice melted. In the Middle Silurian, the global climate was warm to hot, and there were no polar ice caps. Evidence from all the margins of Australia supports the suggestion that the oceans were warm at this time.

Late in the Ordovician, the Alice Springs Orogeny had begun a round of mountain-building. Mountain-building occurred along the eastern seaboard, as the Lachlan Orogeny continued to uplift the land. Among the strata uplifted during this orogeny were marine limestones formed by the reefs that flourished in the shallow seas of the marine incursions, soon to be eroded during the tectonic phase. Stromatoporoids, not true corals, formed some of the the reefs. Remnants of these extensive limestones can be seen in the Bungonia and Jenolan Caves in New South Wales and caves at Chillagoe in Queensland. The caves being formed by the subsequent weathering, with stalactites and stalagmites formed by the dripping water.

Along the southeast margin of the continent, a complex fracture system developed leading to subsidence of the Darling and Adavale Basins. A series of depressions and troughs developed on the northeastern margin. In the southeastern sector of the continent, a complex pattern of troughs, shelves and elongated islands formed offshore from a fairly narrow shelf, their positions often changing over time. This was followed by a period of volcanic activity in which volcanic rocks were intruded in this area. Sometimes, in shallow water, they formed islands, sometimes cooling below the surface to form granite basoliths. In the deeper troughs sedimentation continued. Sandstones, shales and limestones were deposited in shelf areas. Trilobites, Molluscs and rugose and tabulate corals are found in the limestones.

Sedimentary deposits, such as carbonates, were formed in the new marine environments as the seas encroached on the western margin of the continent during the Late Silurian. That there were periods of aridity at this time is shown by the presence of evaporites, that formed when embayments dried out. The position of Australia between the equator and 30o N indicates it had a hot climate. The presence of coral reefs shows that the oceans were warm. The formation of elongated islands and troughs along the eastern seaboard that began in the parts of the Silurian continued. The sediments that formed between the mainland and these islands formed a continuous rock from the Silurian to the Devonian. The Baragwanathia Flora of Victoria is found in parts of these sequences deposited during the Late Silurian and Early Devonian.

416-359   Ma Devonian

In the Northern Hemisphere, the landmasses were a number of separate blocks. Gondwana was in the Southern Hemisphere, the only part sill on the equator was Australia, on an outer edge as it always was throughout it association with Gondwana, like a peninsula. The climates of the world were warm to hot, and there were no polar ice caps. Worldwide, many evaporites were formed, indicating widespread aridity.

Australia lay between 15o N and 15o S, making it a tropical country during the Early to Middle Devonian. By the Late Devonian it had moved to be between 30o and 45o S, and its orientation had changed slightly, at which point the climate would have cooled.

Australia was apparently warm and tropical during the Devonian, as evidenced by the fish and invertebrate diversity, large areas of carbonate deposits in the Lachlan Fold Belt of south-eastern Australia and north Queensland and the Canning Basin in Western Australia, oolitic limestones in the Wee Jasper area of New South Wales.

Volcanic activity continued along the eastern margin of the continent, adding new, stabilised land to the eastern edge, and by the end of the Devonian the coastline was substantially as it is at present. Sediments collected on a large delta in the Adelaide Basin during the Early Devonian that formed the reservoir rocks of the Gilmour Natural Gas Field, the oldest known hydrocarbons found in eastern Australia. At this time, near Melbourne and Bathurst, deep ocean troughs were accumulating sediments, in eastern Victoria and around Canberra there were shallow seas with reefs and a rich, diverse shallow water fauna  populating a shelf that extended north through the New England area all the way to North Queensland.

The eastern and western coasts were again linked by a broad band of epicontinental sea, the successor to the Larapinta Sea, during the Early Devonian, that had an abundance of shallow water environments. In the western part of this sea was a major incursion into the Canning Basin. An orogeny began to affect the eastern half of Australia during the Middle Devonian that caused the seas in the eastern part of the epicontinental sea to retreat to the east, exposing more land on the eastern edge of the continent. The westward retreat of the remainder of the epicontinental sea was caused by concurrent elevation of central Australia.  A small incursion in the western Canning Basin was all that remained by the Late Devonian. From the Middle Devonian the west coast basins underwent regional subsidence that resulted in marine transgressions into the Carnarvon and Bonaparte Basins.

As the raised central rocks eroded, the sediments banked up against the emerging mountains along the eastern margin where the north-south trending mountainous region was being created by the Tabberabberan Orogeny.  The high country that resulted was a watershed for the east flowing river systems that carried large amounts of sediments to the floodplains on the coast. These rivers teemed with freshwater fish.

By the Late Devonian, aridification was widespread throughout the world, as it was in Australia. After the marine incursions retreated from the western part of the Canning Basin in Western Australia the area became desert. The widespread aridification in Australia is indicated by the presence of wind-blown sediments over large areas of the continent. Towards the end of the Devonian, the climate again became hot and humid.

 

359-299   Ma Carboniferous

The Carboniferous Period saw great changes in the arrangement of landmasses on the Earth. During this period Australia was still on the outer margin of Gondwana. When the Tethys Sea opened up, the part of Gondwana occupied by Australia was swung down south into the Antarctic latitudes. By the time it reached its southernmost point half the Australian landmass, still attached to Gondwana, was covered by continental ice sheets. As the climate cooled, the flourishing life in the wetter areas around the water bodies began to decline, becoming impoverished, as did the sea bordering Australia. On land and in the sea a typical polar pattern of low diversity of life evolved. On the land the vegetation around the margins of the ice sheets was of a low-growing tundra type.

The South Pole was situated on Gondwana, and glaciation, and the formation of permanent ice caps, had profound effects on the southernmost parts of Gondwana. Climatic fluctuations are believed to have characterised the Carboniferous with the glacial and interglacial phases alternating.

Australia

As the Carboniferous opened Australia hadn't changed position much from its location at the close of the Devonian, but it had been rotated slightly, and had started to move south. Over the next 74 million years of the Carboniferous it travelled from the equatorial region into high latitudes. The north-south axis of the continent was at 90o to that of the present continent by the Late Carboniferous, being affected by winter darkness that lasted unbroken for months and becoming intensely cold. Glaciers began to form on the high country and glaciers are believed to have spread until a permanent ice cap had formed that, at its peak, seems to have covered more than half of Gondwana.

In the Yarrol and New England provinces major tectonic activity resulted from the interaction of the stabilised land of the continent and that of the proto-Pacific Plate. Increased volcanic activity, deformation  of rock strata , igneous rock intrusion, combined with uplift of the area to increase the area of stabilised land. The Sydney, Galilee and Bowen Basins resulted from all this activity, the depressions being originally complexes of trough-like rifts that were created by the tearing strains of interplate movements, volcanoes being associated with some of the rifts. The troughs were flooded by marine incursions and most of them were filled by the sediments derived from eroding land and the volcanic material, the basins being formed by those that weren't filled by the sediments and where subsidence continued.

In an area of northwestern New South Wales and southwestern Queensland, a zone of crustal weakness developed in a previously stabilised part of the crust, forming a depression that became the Cooper Basin that was occupied by swamps in the Permian, the algae deposited kerogen from which the natural gas, light oils and condensates of the area were produced.

In central Australia the Alice Springs Orogeny continued the uplift of the mountainous country in the area of the MacDonnell Ranges.

 

299-251   Ma Permian

In the earliest part of the Permian the glacial phase continued from the Carboniferous. It is not certain whether the ice sheets were continuous, as they are in Antarctica at the present, or whether there were a number of centres of glaciation, which are thought to have possibly changed over time, with unglaciated land between the centres. The evidence from South Africa and South America suggesting that the glaciers in those continents are older than those in Australia, is thought to support the notion that there were a number of centres of glaciation, not a single solid mass of  ice. Whichever is the case, it seems likely that there would have been refugia in which at least some of the flora and fauna could survive the worst of the climatic conditions. At the present time it is believed that ice caps have covered Antarctica for about 15 million years, but fossil wood has been found in the rocks of the Serious Formation dated to 5 Ma, indicating that forests could survive the polar conditions, presumably at times when the polar climate ameliorated for long enough for forests to become established. It is believed that this probably also occurred during the glacial period in the Late Carboniferous and Early Permian.

Evidence has been found of aridification in parts of the Euramerican landmasses that were in the warm to hot zone on the equator at this time. Australia has always had periods of increased aridity during ice ages in the Northern Hemisphere.

Australia

After the end of the glacial phase the climate took some time to warm, remaining cool to cold throughout the Early Permian. Glaciers persisted on the high country in the east of the continent. As the climate warmed a rich, diverse flora evolved in the coal swamps, that persisted in many areas until late in the Permian, though the conditions are believed to have been probably warm-temperate. Throughout the Permian the climate was very seasonal. This strong seasonality is indicated by the pronounced annual rings found in the petrified wood from the time.

Epicontinental seas covered large parts of South Australia and Victoria, with a branch on the join between Antarctica and Australia separating the mainland from Tasmania, early in the Permian. In Western Australia the sea covered part of the coast and the Canning Basin. The distribution of sediments accumulating in the basins during the Permian were largely controlled by major sea level changes and tectonic activity.

There are believed to have been 2 major sealevel rises during the Permian. The first was caused by the melting of the ice cap, resulting in the flooding of the Carnarvon and Canning Basins in Western Australia, and the Sydney and Bowen Basins in the east of in the continent. The sealevel rise in the Middle Permian, when the Sydney and Bowen Basins were again flooded, the last major marine incursion on the east coast, was caused by local subsidence. In the Late Permian there was a major global sealevel rise. The east coast basins were not subjected to marine incursions at this time, the main effect being the impounding of run-off water in the major basins forming vast floodplains. The Cooper, Galilee, Bowen and Sydney Basins were all influenced by inward-draining, silting regimes.

Sediments were accumulating in the central zone of the Westralian Depression and the Canning Basin in Western Australia. Deltas in the Bonaparte Gulf supplied the sediments from which the reservoir rocks of the natural gas accumulations formed. Volcanic activity occurred from the Sydney basin north to Townsville in Queensland. Tectonics in the New England area folded rock strata, intruded granites and uplifted the area as a result of tectonic plate interactions in the Yarrol-New England provinces. The activity in these areas was the last of the phase of activity of the Tasman Orogeny. By this time most of the crust of the Australian landmass had been stabilised. With the increased weight of the accumulating sediments the basins of eastern Australia continued to subside.

At this time, the Westralian Depression became increasingly pronounced, assuming a dog-leg shape. In the Mesozoic the Westralian Depression became a rift zone as Gondwana began to split up.

On the eastern margin of Gondwana adjacent to Australia, a sequence of sediments of marine and volcanic origin were deposited in a deep, subsiding trough. Subsequent uplift of these rocks were to become part of the New Zealand landmass. At a thickness of 20,000 m, they are the most complete sequence of Permian rocks known in the world.

 

251-199   Ma Triassic

All the continental blocks of the world were united in Pangaea during the Triassic. Globally, heat and aridity characterise the climate of the Triassic, with the arid belts indicated by evaporites extending up to 50 deg either side of the Equator, being more widespread and abundant than at any other time in the history of the earth. The evaporite deposits of the world were most widespread during the Middle Triassic, indicating it was the most intensely arid part of the most intensely arid Period. The Early and Late Triassic were apparently as hot, but more humid than the Middle Tertiary. Red-beds that are widespread in deposits dating from the Early and Late Triassic are believed to indicate alternating arid and humid conditions. It seems for once Australia "struck it lucky", being down at the south pole, well away from the arid belt in the equatorial regions.

The Triassic climate was characterised by temperatures higher than the present. There doesn't seem to have been any polar ice caps and the climates in most parts of the world were probably more uniform than at present. There is not a lot of climate evidence from Australia, but the formation of Middle and Late Triassic coal deposits at Leigh Creek in South Australia, Tasmania and coastal areas in New South Wales and Queensland indicate a wetter climate than in earlier times. Early Triassic gypsum deposits in some of these locales give a different impression of the climate in Australia, indicating at least pockets of arid conditions. In other parts of the world there were massive evaporite deposits, much larger than in Australia at that time, indicating intensely arid conditions in many parts of the world. They were experiencing a monsoonal regime.

The distribution of reefs at this time is very similar to that in the Devonian, suggesting a similar temperature range, with maxima probably higher than the present and with warm seas. At the high latitudes where Australia was at the time, the climate being less extreme, probably warm-temperate and more humid, conditions which are thought to have possibly extended as far as the South Pole. For part of the Triassic the South Pole was situated near Bourke in New South Wales.

The Triassic Period began after the end of the Late Carboniferous to Early Permian glaciation, and after the marine incursions retreated around the coast of the continent. Sediments deposited during this period consist almost exclusively of river and lake deposits, with only a minor component of marine rocks forming. At this time Australia was situated in fairly low latitudes and the climate is believed to have been humid and temperate, with marked seasonal changes in rainfall.

During the Triassic, fresh-water sedimentation was a feature of the basins of the eastern part of the continent. Vast systems of lakes were gradually silting up in Queensland,  river erosion of catchments being the main contributors. In the Bowen Basin, the sediments were coming from the basin margins. What is thought to have been one of the last remaining floodplains like those found in the Permian, in the Sydney Basin, was receiving vast amounts of sediment from rivers eroding the New England highlands. The sedimentary rocks that formed from these sediments are the Narrabeen Group. The massive Hawkesbury Sandstone and strata formed in the Late Triassic, that were comprised of coarser sands, derived from areas to the south. This suggests that there was high country in the southeast, or even as far south as Antarctica. By the Triassic the only remaining coal swamps were small areas at Leigh Creek in South Australia, Tasmania and northeastern New South Wales and nearby southeastern Queensland. At this time volcanoes were active along the eastern coast, on the continental shelf and to the east of the major basins.

There was a marine incursion down the Westralian Depression onto the North West Shelf in the Early Triassic, and there were 2 incursions further inland on the coast of Western Australia, on the northern margin of the Canning Basin and into the Bonaparte Basin. By the Middle Triassic these incursions had drained back to the sea, the whole continent then being dry land.

In the Early Triassic a river system with deltas formed near the Exmouth Plateau after the North West Shelf marine transgression retreated. Later in the Period there was another marine transgression in this area. The sediments formed sandstone during the Triassic that  were some of the main natural gas reservoirs, here as well other deposits from other parts of Australia. Unlike other parts of the world, where the deposits are mainly of marine origin, the Australian deposits of oil and gas that formed after the Devonian are mostly of terrestrial origin.

By late in the Triassic marine sediments were being deposited only in the northwest sector of the Westralian Depression because it had been filled-in elsewhere. In the rifts that form when landmasses are separating a typical series of events occurs as seen in the Westralian Depression, where there was an alternating cycle of marine transgression and retreat, with the deposition of sediment in the depression.

 

199-145   Ma Jurassic

The Jurassic opened with the landmasses still forming combined as Pangaea. With the exception of part of western North America, that remained covered by shallow seas, continental crust of Pangaea was above sealevel. On the edges of this submerged area the Sierra Nevada Mountains were being uplifted during the Jurassic. As the Jurassic progressed the first rumblings of the breakup of Pangaea began to gather strength as the tectonic plates were reactivated and began the process of separation. Rift lines had formed along the margins of the rifts where they were to separate, large quantities of basalts and dolerites poured out along these lines. The rocks that formed at this time are found in all the former members of Gondwana. In Tasmania they can be seen at the mouth of the Derwent River.

The beginnings of the separation of Gondwana from Pangaea was soon followed by rifts between the southern continents that comprised Gondwana. It was at this time that the Australian continent was delineated. By the end of the Jurassic the rocks of the ancestral New Zealand landmass had been raised above sealevel on the eastern margin of Gondwana.

During the Jurassic Gondwana began to break up with the major continents beginning to drift away. Africa and South America began the breakup, then India rifted away from Antarctica. By the close of the Jurassic Australia was the only continent still joined to Antarctica. The first to arrive, the last to leave. The changing arrangement of the landmasses produced major changes in climate and ocean circulation, greatly affecting the global climate.

Australia

Australia was still at high latitudes during the Jurassic, the main driving force for adaptation being winter nights that were months long. Dolerites and basalts being erupted along the rift lines were the first indications of the changes to come as Australia began to separate from Antarctica. These erosion-resistant rocks are seen as some dramatic features, especially in Tasmania.

The first step in the process of separation was that fragments of crust began moving away from the coast of Western Australia. It is thought possible that prior to this separation the western part of Thailand may have been situated beside the northwestern sector of the Westralian Depression where the rifting started, though proof of this is lacking.

Late in the Period there was a marine transgression on the northwestern margin of the continent. Rifting and faulting had been occurring for millions of years before the seafloor began spreading. These rifts involved a tearing and shearing component that resulted in parallel ridges and troughs at an angle to the continental margins, not in a single straight line. These basins filled with sediment and became traps for hydrocarbons. In the Perth Basin, activity along the major Darling Fault led to the accumulation of sediments in a deep trough. In the Middle Jurassic there was a brief marine incursion into this trough.

From the Middle Jurassic there was faulting and rifting along the southern margin of Australia in preparation for the separation of Australia from Antarctica. This was the last link severed when Gondwana finally broke up. Late in the Jurassic there were active volcanic vents (diatremes) near the eastern margin of the Sydney Basin that is thought to have been connected with the impending opening of the Tasman Sea that separated the outer margin of the supercontinent from the main landmass. During the Jurassic the Tasman Orogen, having been stabilised, was being eroded by major river systems that drained mainly east to the sea north of Brisbane. There was a broad series of interconnected depressions, each of which had its own river, lake and swamp systems. Combined they covered a vast expanse of country. Between the early part of the Middle Jurassic and the earliest Cretaceous, over 5 tectonic cycles that were associated with the break-up of eastern Gondwana, all these systems were converted to the Tasman Depression, a major structural unit. High ground was raised to the northwest and the south in pulses of activity as the depression subsided.

Four of these cycles occurred in the Jurassic, as catchments were raised rejuvenated rivers eroded the high ground, carrying large quantities of coarse sediments to the depression. The gradients of the rivers were reduced as erosion lowered the catchments, silting up the lakes and swamps, after which fine sediments accumulated. A rapid phase of erosion began following another uplift. At the end of the Jurassic, lagoons and deltas formed along the northeast margin of the depression during the 4th cycle. Global sealevel rise affected the drainage patterns leading to the development of these features. In the following earliest Cretaceous, the 5th cycle was truncated when the whole depression was flooded by a global sealevel rise.

To the east of Gondwana a volcanic arc was present on an active zone of interaction between the proto-Pacific plate and the Gondwanan plate. During the tectonic events associated with the uplift of the ancestral New Zealand landmass above sealevel from the Middle Jurassic the outer margin of Gondwana was bent into an S-shape (orocline) in the Tasman-Antarctica region. Volcanic ash from these volcanoes contributed to the sediments being deposited in the Tasman Depression.

The sediments of the Tasman Depression contain the main aquifers of the Great Artesian Basin, and the reservoir rocks where the accumulation of oil and gas in the Roma, Moonie and Jackson fields.

In northeast New South Wales and southeast Queensland coal swamps developed along the lower reaches of the east-draining rivers in the Middle to Late Jurassic. North of Perth coal swamps developed.

The Amazon is the closest modern analogue to the environment of the Tasman Depression in the Jurassic and the Early Cretaceous. It covered a vast area of low relief with large, slow-flowing rivers, lakes and swamps. It was covered with luxuriant vegetation as a result of the warm and humid climate. Unlike the present-day Amazon, there were no Angiosperms, the forests being composed of conifers, cycadophytes, ginkgophytes and ferns. The Tasman Depression in the Jurassic was unique, having luxuriant vegetation like the Amazon area but with summers during which  the sun never set and winters with months of continuous darkness.

During the Jurassic, freshwater sediments, with shallow marine incursions, accumulated near Geraldton and the Kimberley district, along the western margin of Australia. In central Queensland and northern New South Wales a large inland river system developed. The sediments deposited in the freshwater rivers and lakes of this drainage basin contained fish fossils at several localities, but amphibian and reptile bones are rare.

Based on oxygen isotope studies of the sediments from this time it is believed the climate of Western Australia was warm. Queensland seems to have been cooler and wetter, coal swamps formed in southern Queensland.

 

145-66     Ma Cretaceous

The Cretaceous has been called  a time of change, because the changes that took place during this period on a global scale were enormous. Australia was still attached to Antarctica and ancestral New Zealand was still attached to Marie Byrd Land in Antarctica. The southern continents were beginning to break from Gondwana, and the effects on the world were enormous. As the continents moved from Gondwana, the new arrangement of the land masses led to new circulation patterns in the oceans and the atmosphere. The result was a huge effect on the climate of the world.

In the Early Cetaceous there was a global sea level rise as the continents separated. Around the world vast areas of land were flooded. At 110 Ma the sea level reached it maximum height, after which it rapidly dropped, and by the Late Cretaceous most continental areas were again dry land. Resulting from the plate movements, mountain building surged in many places. It was at this time that the Rockies of North America, the Andes of South America and the Alps of Europe were uplifted.

There was a sudden cooling phase at the start of the Cretaceous, the severity or length of which is unknown. For the rest of the period the temperatures continued to rise so that most of the Period was characterised by a warm to hot climate, the mean annual temperatures being about 10-15 C warmer than at present. The temperature gradient between the tropics and the poles was about half the present gradient. It is believed tropical and subtropical conditions extended much further south and north than now, possibly up to 70o at its greatest extent, and the poles had a warm temperate climate. It is even estimated that the abyssal water was about 15oC compared with 2o C at the present. Ocean circulation was sluggish and had almost no vertical zonation.  During the Period there appear to have been times of great aridity around the world, as evidenced by widespread evaporite deposits, that have been found in a band either side of the Equator, that are believed to have extended to possibly 45odeg on either side of the Equator. The Late Cretaceous was the warmest time in the Phanerozoic Era. At the end of the Period there was a sudden cooling that ushered in a regime of fluctuation of climatic patterns that continued up to the most recent ice age.

Tectonic movements were raising and lowering blocks of crust at the time of the rising sealevel, leading to flooding locally, where there were areas of  raising and lowering crust. As a result, new environments were becoming available for colonisation even before the epicontinental seas began to retreat. Following the retreat of the seas, the areas exposed contained salt marshes, swamps, dunes, and areas of sand that had been the sea bed.

Australia

Now it was Australia's turn to break from Antarctica, bringing an end to Gondwana. The rifting took a long time to complete, not being completely separated until the Early Tertiary. Sea levels rose and central Australia was submerged by a last inland sea. At this time southern and central Queensland were a series of islands in this inland sea. The dinosaur fossils that are known are mostly in the sediments deposited by this inland sea over much of Queensland, New South Wales and South Australia. It is believed that the fossils found were dead dinosaurs washed into the shallow seas during floods.

During the Cretaceous Australia was still attached to Gondwana, but as the process of separation got under way, rift valleys formed a complex system along the lines of separation of the 2 continents. It had been thought that at the time Australia was between 40odeg and 75o S, later evidence suggested that it was actually between 50o and 85o S. For the first 50 million years of the Cretaceous the sea level had been rising and this caused the flooding of the Tasman Depression as it continued its subsidence. Low-lying central and southern basins adjacent to the Depression were also flooded at this time, as were West Australian basins where the Amadeus Transverse Zone had previously been. The continent had been separated into 4 blocks of land by the vast expanse of the Eromanga Sea.

The marine deposits that were laid down in this epicontinental sea now outcrop, or are present in the subsurface, over about 1/3 of the continent. Deposits of terrestrial origin that were formed at the start and towards the end of the period are not common.

The Maryburian Orogeny, a major phase of mountain building that occurred in coastal Queensland, was the final act of the Tasman Orogen.

Along the southern margin, basins were formed by the rifting that was under way, were accumulating large amounts of sediment throughout the Cretaceous. The accumulations of Bass Strait contained coal deposits. In the Gippsland Basin, Otway Basin and Bass Basin the rifts were slowly evolving, and continued to accumulate sediments and volcanic detritus. The sea inundated the rift zone progressively from west to east, and by late in the Period Tasmania was the last connection to Antarctica.

Throughout the Cretaceous the break-up of Gondwana picked up speed. The western margin of Australia was becoming progressively delineated as sea floor spreading began near Carnarvon and Perth on the edge of the Westralian Depression. India began breaking away from the western arm of the Depression, its northward movement beginning at about 125 Ma. At the same time, Africa broke from Antarctica and began rotating away from India.

By the time the Tasman Sea began opening, about 80 Ma, the southeast margin of the continent was established. The whole of the outer edge of Gondwana, comprising the Lord Howe Rise and the Ancestral New Zealand landmass, extending from the Campbell Plateau to New Caledonia, began moving away from the east coast of Australia. The movement ceased about 60 Ma and the distance separating the 2 has remained constant ever since. The tectonic events involved in the opening of the Tasman Sea led to the tilting of the eastern margin of Australia and the uplift of the Great Divide. Antarctica was at the South Pole during the Cretaceous and has remained there as the other continents moved north, Australia being the last block to remain attached  after the others had separated, though the beginnings of separation had started.

 

66-55       Ma Palaeocene

At this time Australia had started to separate from Antarctica, but the eastern and western margins were the only parts that were free. The climate was warm and wet, and most of the continent was covered by rainforest.

55-34       Ma Eocene

Throughout the Eocene, the early Southern Ocean was widening, the last connection being flooded about 45 Ma, though it continued as a shallow shoaling area until the end of the Epoch, after which it was deep enough to be classed as a seaway. The forerunner of the Circum Polar Current con then begin to form.

 

34-23       Ma Oligocene

The temperature of the surface water on the Campbell Plateau adjacent to Antarctica had been dropping at the start of the Oligocene. It is thought possible that the west Antarctic ice sheet may have begun to form, leading to the drop in sealevel as it expanded. The significant cooling at the poles increased the temperature gradient from Pole to Equator, water temperature dropping 6-7o C.

About 30 Ma, in the Early Oligocene, the Drake Passage between South America and Antarctica opened, removing the last impediment to the formation of the circum-polar current resulting in Antarctica becoming more isolated and leading to more intense cooling. It is believed the rain-bearing westerly wind systems would probably have moved further north as a result of the cooling. These winds would then have brought large amounts of rain to the parts of Australia that were between the Pole and 40 S. If this was the case, the areas in the north and northwest of the continent would have been much more arid, a condition which could have began preparing the fauna and flora to increased seasonality and eventually to the vast areas of extreme aridity that were to come.

 

23-5         Ma Miocene

Globally, the water temperature of the oceans were warmer than in the Oligocene in the first half of the Miocene. This was the only length of time during the Cainozoic that the cooling trend was reversed. In the Tasman Sea the temperatures of the surface waters were a few degrees above the present level. On the southern margin of the continent, the shallow shelf waters were very warm, between 15-20o C, at similar levels to water in the subtropical and tropical areas at the present. It is believed the currents in the oceans were probably sluggish, as they usually are when temperature gradients between the Equator and the Poles were small. Sea levels were rising during the Miocene. It is believed this was mainly because of melting ice, but may also have resulted from the continental collisions in the western Pacific and Asia.

Extensive limestones were formed in the Eucla Basin, Murray Basin, Otway Basin and Gippsland Basin from carbonate oozes that had formed beneath shallow marine transgressions. Throughout the Early Miocene the southern part of the continent had a wet climate. Sealevels began dropping as the ice caps increased. In the Middle Miocene, Bass Strait opened, only to close and open many times as the sealevel fluctuated, leading up to and during the latest ice age of the Pleistocene.

When the permanent ice cap formed on Antarctica about 15 Ma, temperatures dropped and an increasingly dry anticyclonic circulation began the process of aridification of northern and central Australia. In the arid parts of Western Australia, the river systems have been reduced to a series of salt lakes, having been inactive since the Middle Miocene.

 

5-2.5        Ma Pliocene

After the sudden cooling phase at the close of the Miocene, The Miocene Terminal Event, there was a warming phase between 5 and 3 Ma. It is believed that part of the West Antarctic ice sheet may have melted during this warm phase. The cooling trend then resumed, the Arctic ice cap starting to form about 3 Ma. This was the first time the Arctic Ocean froze.

The fluctuations of wet-dry, cold-warm phases as the ice caps expanded and contracted began to come in rapid pulses. The largest extent of the Arctic ice cap at this time was only half of that it achieved during the Pleistocene. 

In the Pliocene, the Panama Isthmus rose, the blockage of the flow of the equatorial currents in the sea that resulted contributed to the increased severity of the cooling phase. The regions to the north of Australia were being deformed as Australia moved north, more land being elevated above sea level and making migration from Asia easier, especially during glacial phases when the continental shelves were exposed. About 5 Ma rodents arrived in Australia. Prior to their arrival, the only placental mammals were bats, which the Riversleigh deposits show arrived in the Miocene or Late Oligocene.

The drying trend that began during the Middle Miocene became more rapid after the warm phase at the start of the Pliocene, at which time very different sub-regions developed and the central deserts spread. When the northern ice cap formed about 3 Ma, ice regimes became firmly established.

Anticyclonic high pressure zones moved north bringing winter rainfall to the southern and southwestern margins of Australia. The establishment of a Mediterranean type of climate led to the evolution of a highly individual flora, with 80 % of species being endemic, in southern Western Australia. A similar climate developed in the Southwestern Cape of South Africa developed at the same time, with parallel development of the flora, both evolving from a similar Gondwanan stock, resulting in a unique flora on both continents.

At this time plate movements brought the northern part of the continent into the hot, humid climatic zone, allowing the northern areas to avoid the aridification trend continuing over much of the continent. Refuge areas were provided by the high country along the east coast and by the elevation of the New Guinea mountains. Rainfall and conditions remaining suitable for the survival of Closed Forest and Rainforest species, as well as marsupials and birds and other forest animals.

As tectonic processes go, the Kosciusko Uplift that raised the eastern highlands was a gentle process. The rejuvenated coast-ward flowing river systems cut the deep river valleys that later became the "drowned valley" systems when the sealevel again rose, characterising the modern coastline.

The aridity of Australia exceeded that of the present, but it was about 200,000 years ago that actual desert conditions were reached. It is believed there were probably virtual deserts in pockets throughout the Neogene, that resulted in a highly specialised desert flora and fauna. When the desert areas expanded these desert-adapted forms were ready to colonise the desert areas as they expanded.

 

   
   
   
   
   

Sources & Further reading

  1. White, Mary E., 1994, After the Greening, The Browning of Australia,  Kangaroo Press.
  2. White, Mary E., 1993, The Nature of Hidden Worlds, Reed.

 

 

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Last Updated 11/11/2010 

 

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