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Australia: The Land Where Time Began |
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Pangaea Late in the Palaeozoic North America and Europe collided to form a single landmass, the line of the collision is the same line along which they later separated when the Atlantic Ocean began forming as the spreading seabed pushed them apart along the rift line. At the time of their merging, Europe extended east as far as the present Ural Mountains, where Europe later became joined to the Asian landmass. The sediments that had been deposited to the east of Europe were raised to form the Urals as Asia ploughed into the eastern edge of Europe in the Permian, as Pangaea was formed, remaining intact from 300-200 Ma.
At the start of the
Palaeozoic Era,
Gondwana,
which at the time had all the southern continents on a single plate, was
situated on the Equator. During the
Carboniferous
the Tethys Sea opened up and Gondwana was swung southwards. At this time
Asia was made up of a number of blocks, China, Siberia and parts of
Southeast Asia, the blocks slowly moving towards each other, and all of
them collided with Euramerica, and Gondwana joined them all in one
massive supercontinent, Pangaea. This merging of all the major
landmasses of the Earth occurred in the Late
Permian.
By the start of the
Mesozoic,
Pangaea was an established, stable, single landmass. Large amounts of
differential uplift and subsidence occurred in the interior of Pangaea
throughout the Palaeozoic. Deep sequences of sediments accumulated in
the major basins that were created. At the time the lower boundary of the Palaeozoic was set the
n deposit
hadn't been discovered. It was the time when the first animals with
easily fossilised parts, such as shells and exoskeletons of marine
invertebrates, appeared in the fossil record. From the start of the
Cambrian,
throughout the
Ordovician
and part way through the
Silurian,
the fossil record was restricted to aquatic organisms, mostly marine. It
was a time of rapid diversification, most animal phyla appearing at this
time. Until sometime in the Late Silurian plant life was restricted to
algae, from microscopic unicellular planktonic forms to red, green,
brown and coralline seaweeds that exist at the present, land plants
first appearing in the Late
Silurian.
In the Late Cambrian and Early
Ordovician the opening
if the Rheic Ocean between the mainland of
Gondwana and the
offshore terranes resulted from the rifting of the Avalonia terranes
from Gondwana. The Rheic Ocean, of an uncertain width, was still present
between Gondwana and Laurentia after the accretion of Avalonia and the
closing of Iapetus.
A new series of arc terranes rifted from Gondwana, also at this time,
leading to the opening of the Palaeotethys Ocean. The
accretion of these Gondwanan terranes onto Laurentia, followed by the
collision of Gondwana and Laurentia, resulted from the opening of the
Palaeotethys and the closing of the Rheic Ocean. This collision led to
the Alleghenian and Variscan orogenies
of North America, Africa and southwest Europe of the Permo-Carboniferous.
The final assembly of Pangaea resulted from the suturing about 280 Ma of
Baltica and Siberia, among the collisions in Asia. At about 250 Ma the
supercontinent of Pangaea had reached its greatest extent.
The fragmentation of Pangaea has been described as heterogeneous. The
breaks first began in the Mid-Jurassic when the central Atlantic opened
a bit after about 180 Ma (Lawver et al., 2003), and the rifting
of Lhasa and West Burma from Gondwana. It is indicated by magnetic
anomalies that the southern Atlantic Ocean had started to open by about
135 Ma. Sometime between 140-120 Ma North America and Europe began to
rift apart. By about 150 Ma Africa began to separate from Antarctica,
and by about 95 Ma Australia had begun separating from Antarctica, and
India also separated from Antarctica at about this time.
Based on these dates it seems most of Pangaea broke up between 150-95
Ma. The rifting of continental fragments, such as Baja California and
Arabia are still rifting from the remnants of Pangaea. Closing oceans
accompanied the breakup of Pangaea, as is believed to have occurred
during the breakup of earlier supercontinents, such as the Palaeotethys
Ocean and the Neotethys Ocean, as well as collisions, such as those that
are presently proceeding in southern Asia, southern Europe and
Indonesia.
According to Murphy, Nance and Cawood:
'The mechanisms responsible for the formation of Pangaea are
enigmatic. To a first order, we know where and when, but not why. At the
heart of this debate is a lack of understanding of the forces that
initiate the subduction process. Likewise, the documented evolution of
Pangaea highlights fundamental gaps in our understanding of the
processes responsible for its amalgamation. To understand the processes
leading to the formation of Pangaea, we need to investigate the
geodynamic linkages between the evolution of the interior Rheic Ocean
and the penecontemporaneous evolution of the exterior Palaeopacific
Ocean.'2.
According to the author3 Pangaea took about 50 My to assemble
all its constituent parts into a single massive landmass that stretched
from pole to pole, leaving the scars of its fusion that can still be
seen at the present, in the form of mountain ranges that formed in
orogenies associated with the collisions of the various continents as
they were fused into Pangaea. Amalgamation of continents is a very slow
process that operates intermittently, and in Southern Hemisphere
Gondwana,
that single vast landmass that was comprised of all the southern
continents, had existed as a single entity for a large part of the
Palaeozoic.
The author3 says the mountain ranges that were squeezed up as
Gondwana collided with the northern continents have resulted from a
number of orogenies that formed part of the massive mountain range, the
Central Pangaean Mountains, and combining the various orogenies, the
Hercynian, Acadian, Appalachian, Alleghenian, Ouachitan Orogenies into
the Great Pangaean Orogeny. He says the collision began in the east, in
part of what is Africa at the present, then extended west to what is now
the southwestern states of North America, about 7,000 km long,
stretching from coast to coast of the newly formed supercontinent. It
has been suggested that parts of this mountain range probably reached as
much as 1000 km wide. It has been estimated that if erosion hadn't taken
place once they were raised they may have reached as high as 50 km, but
erosion would have begun as soon as they were raised above the previous
surface, increasing in intensity as they gained height, and once they
were high enough to be snow-capped the rate of erosion would have been
greater than the rate of growth. The mountains of the present are being
eroded at different rates, the high Himalayas at about 15 m/1000 years
and the Andes about 1-3 m/1000years. The height a mountain can reach is
also limited by the process of subsidence that results from the enormous
mass of the mountains, the weight causing the curst to subside.
The same process is involved in the loading of thick ice sheets. In this
the loading that results from the formation of these very thick ice
sheets is reversed when they thaw. This process can be seen in Sweden at
the present where the land surface is still rising as a result of the
unloading as the vast sheets of ice that formed during the last glacial
period melted. Long after the glaciers have gone the land it still
rising.
The author3 suggests that it is unlikely the Central Pangaean
Ranges could have grown higher than about 10,000 m, as a result of the
controlling factors of erosion and subsidence. Some have contended that
Pangaea was more a close association of continents rather than a single
supercontinent. The author3 disagrees with them, claiming the
evidence of the suturing of Gondwana and Laurasia into a single landmass
is there to be seen in various places around the world. He also says
that the Ural Mountains resulted from the fusion of Kazakhstan and
Siberia to Laurasia.
At depths below 25 km rocks become molten and it is in huge magma
chambers where it fractionates, the heavier minerals sinking and the
lighter minerals rising, then eventually cooling and crystallising. The
granite that results has 3 main components, silica that is glassy
quartz, feldspar, that is pink and mica, that forms as small shiny
plates. According to the author3 just such granite that
formed at the core of the Central Pangaean Ranges outcrops in the Middle
Atlas, the westernmost of 3 Atlas Mountain chains.
Evidence of the first erosion phase can be seen in deposits in Morocco
as reddish sandstone that has cross-lamination, pebbly sandstones and
conglomerates, sediments that were even coarser grained, and that
contained pebbles and boulders. The presence of powerful rivers, flash
floods, debris flows and avalanches is indicated by sedimentary
structures such as ripples, sand-waves and chaotica. Fossil shells are
extremely scarce in these sandstones, what fossils there were are
occasional branch imprints, and tree trunks that had long before
decayed, as well as some plant fragments that were unidentifiable that
had become coal. Together with the red colour of the soils in the
surrounding area, these are indicative of continental deposits, wherever
they are found in the world, of rivers and deserts and alluvial fans and
scree slopes that are normally found around the fringes of mountain
ranges that were being eroded. These are called the red-bed association
that are typically found in the fossil record immediately after a bout
of mountain building.
The New Red Sandstone is the name given to the beds derived from the
erosion of the Central Pangaean Mountains, as well as from other ranges
formed as the continents sutured together at the formation of Pangaea.
The Old Red Sandstone is an analogous series of red beds from the
Devonian, older than the New Red Sandstone, that is found at scattered
sites around the Northern Hemisphere lands that were once part of
Laurasia. The age of the New Red Sandstone covers a range of dates from
about 280-220 Ma, a period during which there were intense changes to
life. The continental sediments from this time are comparatively barren
of fossils, as is usual for continental sediments, so they are not the
best deposits to search for signs of changes in life forms.
These continental sediments are not without value to those seeking to
understand the past. There is evidence of the intense degree of erosion
that resulted from the raising of rocks to high altitudes in the
formation of mountain ranges, when kilometres of rocks were stripped
from the newly raised mountains. Several agents or erosion, seasonal
rain, snow and ice, as well as powerful winds, combined to extremely
effectively reduce the height and mass of these mountain chains. To the
north and south of the Central Pangaean Mountains vast quantities of
sediment was removed and deposited in mountain fans, powerful rivers
carrying sediment that was deposited as alluvial and fluvial sediments.
These successions display uniformity wherever they are found, along the
Atlantic coast of the US and Canada, a swathe from Devon through central
and northern England, in the UK, and parts of Russia, China and South
Korea.
The huge power of these rivers flowing across the supercontinent can be
gauged by the very large size of some of the boulders that are present
in the conglomerates, as well as the scale of the deposits. The author3
suggests there is no doubt the central regions of this huge continent
were dry. He says there would have been large inland drainage basins
with ephemeral lakes that would have been much like
Lake Eyre
in Australia of the present. As occurs in the
inland drainage system
in Australia, rivers that were intermittently torrential carried water
to these lakes, though it is not possible to determine if the deposits
were laid down by annual meltwater streams or events such as 100-year
flash floods, the author3 suggesting that flash
floods are the more likely of the 2 possibilities.
Rivers would have carried at least some of the sediment to the ocean. To
the north of the Central Pangaean Mountains it flowed to the west where
the rivers emptied into the main Panthalassa-sic Ocean and to the south
the rivers flowed to east, emptying into the Tethys Ocean. The flow
direction of palaeorivers can be determined by measuring the dip
direction of fine laminations, and by examining fossil ripples, where
dunes or ripples are deposited by rivers have a unidirectional flow the
dunes or ripples have an asymmetric form, the gentle slope facing
upstream, and the steeper, avalanche slope, facing downstream.
The other environmental condition that can be determined from the
Pangaean New Red Sandstone has a direct relationship with the vast
expanse of the supercontinent, resulting in the interior being thousands
of kilometres from the climatically ameliorating effect of the coast.
The interior parts of Pangaea that were at low latitudes, it has been
shown by computer models that 2 mm per year would have been the maximum
amount of rainfall per year, translating to no rain at all for years at
a time, the drought being broken by occasional torrential deluges. The
temperatures in summer would have been above 50o C. According
to the author3 this was extreme desert that was probably
larger, hotter and drier than the Sahara, Gobi and
Simpson Desert
or any other modern desert.
The sand grains found in the sandstone of New Red Sandstone have the
form of desert aeolian sand, not sand formed by water. The sand grains
in aeolian sand are polished and perfectly rounded, similar to frosted
glass, from the grain-to-grain collisions that occur when they are
airborne. With sand from rivers and seas the collisions are muted by the
ambient water so they are not polished to the same degree. As the sand
grains are blown into dunes and ripples they are size sorted, and the
avalanche of these bedforms are preserved as cross-laminations.
The whirling sand grains carried by the wind abrade the larger rocks and
pebbles that are scattered across a desert in a similar way. The
windward side eventually becomes flattened to some degree, and when they
are flipped over by an unusually strong wind gust a new face becomes
flattened. These stones eventually have 3 faces, such stones being known
as a dreikanter. Many dreikanters dating to the Permian have been found
in drill cores from the North Sea, some of which were tarnished by a
coating of black manganese dioxide after being exposed for long periods
in an extreme climate.
There is very little evidence of life in the scorching interior of
Pangaea as the extreme conditions have made the fossilisation of any
animals that lived there almost impossible. Some fossils have been found
in the finer-grained sediments of inland lakes and shallow seas where
the mud from the ephemeral rivers was carried in times of flood. As a
result of earthquakes and earth movements leading to tectonic
readjustments that were significant, allowed the marine water to
periodically inundate the inland seas. At these times fingers of the
Tethys Ocean penetrated far inland, a prime example being the Zechstein
Sea of northern Europe.
Large areas of north-eastern Pangaea, that included much of central
England, the North Sea, the Netherlands, Belgium and northern Germany
were once flooded by the sea. The Zechstein Sea was filled at different
times by an arm of the Tethys Ocean from the south, or from the
Panthalassa Ocean by a similar narrow seaway, that covered the sand of
the former desert with a vast expanse of shallow, salty water. From the
Tethys a flora and fauna that was fully marine followed the expansion of
this marine water. There are many very well preserved fossils that
provide a unique window to the Tethys Ocean of the Permian, in copper
rich shales (kupferschiefer) of southern Germany. This is one of
the few places that something can be learned about the fish of the
Tethys of this time, among them long extinct unusual species. Among this
fauna there are also shelled bivalves and brachiopods that had adapted
to cope with the battering of waves along the shore, as well as gardens
of delicate fan-shaped bryozoans, porous sponges and sea lilies
(crinoids) with multiple arms.
These connections with the Tethys Ocean were only temporary or partial,
and when the connection was again closed off evaporation in the very hot
conditions soon dried out the water bodies, replacing the water with
hundreds of metres of salt, that mostly comprised gypsum (calcium
sulphate) and halite (sodium chloride), forming blistering white
deposits over the sands of the desert, that are now salt mines from
Cheshire in central England to Stassfurt in Germany. It also meant that
the fossils of Tethyan life disappeared from continental Pangaea.
According to the author3 the Ural Mountains in Russia that
form a straight line nearly 2000 km long, stretching across the western
Russian plains, fading away just before reaching the Arctic Ocean in the
north and in the south, fading away beneath the lowlands of Kazakhstan,
were uplifted in the final stage of construction of Pangaea. After 260
My of erosion they are now no more than 2,000 m high.
Broad shelf seas covered much of the Middle East, and on the southern
margin of Tethys, flooded across India and Pakistan, as well as to the
northern shores of Australia. The climate of these parts of Pangaea
would have been generally wetter and more temperate than in the interior
of Pangaea, as they had more chance of receiving coastal rains.
Swamp-like conditions were widespread across the deltas and coastal
plains, as they do at the present. Coal of the same age occurs in
Australia, India, South America and South Africa. The plants of the
Permian that formed the coal were different from those of the present.
The Great Parana Basin, at mid-latitudes of the Southern Hemisphere,
also an inland drainage system, and its namesake in Brazil and the
Kalahari Basin of southern Africa, the 2 areas having been joined
together, covering a combined area of 2.5 million km2 about
the combined area of the Simpson Desert and the
Lake Eyre Basin,
that cover much of the eastern part of Australia at the present. The
fossil record of this region of Pangaea recorded 3 important stages of
reptilian evolution, stages that were to shape both the Age of Dinosaurs
and the Age of Mammals, and that eventually led to the evolution of
humans.
In this region fossils of
archosaurs
have been found, the root group that gave rise to the marine reptiles
and on land the dinosaurs, some group of which led to the evolution of
birds. The primitive archosaurs first had to survive the greatest mass
extinction event known to occur in the history of the Earth, the event
at the close of the Permian.
Cynodont fossils were also found in this region, a different reptilian
lineage from that of the dinosaurs. The
dicynodonts
were synapsids and the
archosaurs (and the dinosaurs) were diapsids. The last of the synapsids
to evolve were cynodonts, one of the most successful, around the time of
the beginning of the formation of the Tethys Ocean. Beginning as
wolf-like carnivores, they adapted rapidly, spreading to all types of
habitat worldwide, and after surviving the Permian extinction event,
persisted for a further 70 My. Their descendants went on to evolve into
mammals, and eventually humans. The first published description of a
dicynodont came from the Luangwa Valley, Zambia3, in the opposite end of
the Great Parana Basin from Brazil. A species of cynodont found in
Brazil has also been found in Luangwa.
The Irati Formation, Partecal, Brazil, was deposited in part of a vast
inland sea that 250 Ma would have been the deep interior of Pangaea, a
time when the climate at this location would have been hot and arid.
Similar to the Zechstein Sea, though on a much larger scale, this was a
marine incursion that reached the heart of Pangaea. The marine water was
either an arm of the Tethys Ocean or the main Panthalassa Ocean that
reached far inland, bringing with it many of the plants and animals that
populated those oceans 250 Ma.
Rocks have been found covered with thousands of small ostracod fossils.
The water appears to have been extremely salty or brackish, or possibly
fresh, as there were large numbers of animals but low diversity
indicated unusual salinity. The sediments producing these animals were
interleaved with layers in which the deposit of black organic-rich
shales is indicative of anoxic conditions. The remains of mesosaurs, in
the form of scattered ribs and vertebrae, as well as some that were
still articulated, and almost completely intact have been found. This
was evidence of the first marine aquatic reptiles that evolved from
animals that were entirely land-based. They were similar to small
alligators in appearance, and have been suggested by some to have fed on
plankton blooms by filtering the water through their teeth as occurs
through the baleen of baleen whales. Another suggestion is that they fed
on small fish and crustaceans, possibly ostracods. The debate as to
whether they were actually marine or lived in large inland seas has
still to be settled.
When the assembly of Pangaea was complete about 260 Ma Tethys became a
fully demarkated ocean, its eastern margin being an arc of scattered
islands that separated it from the Panthalassa Ocean. Periodically, arms
of the Tethys Ocean extended deep into the interior of Pangaea, at times
of high sea level or tectonic activity that lowered continental plates,
flooding parts of the supercontinent with shallow seas. These marine
transgressions brought life to the arid interior of Pangaea, though
these seas often evaporated after only a short period, leaving in their
place the dissolved salts they had carried, such as sodium chloride,
calcium sulphate, etc., that formed thick deposits, known as evaporites.
The Stow3 suggests, based on the available evidence that
the supercontinent of Pangaea may have remained as an intact single
landmass for as little as a few 10s of millions of years before the slow
breakup began. One of the least understood events that occurred in
Pangaea was the almost instantaneously, geologically speaking, cracks
throughout the entire supercontinent. Basaltic lavas exuded from these
cracks in huge volumes, the relics of these massive eruptions being
present today in various places, especially around the margins of the
Atlantic in the form of ancient flows and intrusive dykes.
One suggestion for the mechanism that led to the breakup of Pangaea is
that it moved across a mantle hotspot, or possibly several hot spots. It
is now believed that hot spots are induced to form beneath large
continental plates. According to either hypothesis, when a hot spot
forms beneath a continental plate the plate first bulges above the hot
spot until a radial pattern of fractures forms, typically with 3
principal fractures known as a triple junction. Rift valleys form when
some or all of these lines of weakened crust founder. The valleys are
widened still further by these, the
lava being forced up by the pressure below. The ancestral Tethys Ocean
flooded these rifts, and this time the flooding was deep and permanent
in some of the arms, oceanic crust forming the floors of deep, narrow
ocean basins.
The nature and effects on the surface crust resulting from having hot
spots beneath the crust are visible on the horn of Africa. Centred on
Addis Ababa, the Abyssinian Highlands have reached a height of more than
4000 m, due to long term domal uplift of the crust above the Ethiopian
hotspot. The resulting triple junction has 3 arms that have led to the
formation of the African Rift Valley, the Gulf of Aden and the Red Sea.
The East African Rift System has the potential of opening up as a new
ocean that would split Africa apart. Rifts began forming in Pangaea.
When the rifts widened and deepened, the Tethys and Panthalassa Oceans
were ready to pour in to them as soon as they opened.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |