Australia Drifting - Changes on the
continent, a timeline
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 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
High ground was mostly restricted to the southern parts of the Northern
Territory and the
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
Victoria Desert in Western Australia, volcanic activity occurred
that spread vast sheets of basalt across a very large area. The
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.
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
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
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.
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.
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
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
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
Baragwanathia Flora of Victoria is found in parts of these sequences
deposited during the Late Silurian and Early 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
The climates of the world were warm to hot, and there were no
polar ice caps. Worldwide, many evaporites were formed, indicating
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,
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
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
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
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
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.
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
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
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
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.
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
Canning Basins in Western Australia, and the
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
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
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.
All the continental blocks of the world were united in
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
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
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
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.
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
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
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 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
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
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
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
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.
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
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
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
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
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
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.
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.
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.
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.
Globally, the water temperature of the oceans were warmer
than in the
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
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
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
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
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 sea
again rose, Characterising the modern coastline.
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
- White, Mary E., 1994, After the Greening, The Browning of
Australia, Kangaroo Press.
- White, Mary E., 1993, The Nature of Hidden Worlds,