Australia: The Land Where Time Began
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.
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 Ediacaran 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, Zambia, 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 author3 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.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|