Australia: The Land Where Time Began
The Great Journey North
along the Way
65 million years ago (Ma), at the end of the Cretaceous-the start of the Tertiary, Australia was still a wide peninsula connected to Antarctica along the line of what is now the southern coast of the continent, jutting out from the of Gondwana and lay near the South Pole. The seas surrounding the continent were about 10-15 C, substantially higher than the seas at those latitudes now.
Most of Australia was covered by rainforests, except for patches where the soil type was unsuitable, and areas of exposed granite, and steep-sided ridges of reworked rocks and infertile areas, that are known to have been present in parts of Australia since the beginning of geological history of the continent.
It can be assumed that aridity had occurred, at least in some places and at some time in the past, because some plants had already evolved the adaptations that would allow them to flourish in the increasingly arid continent in the future. When the first angiosperms that evolved about 120 Ma they had small leaves, which is one of the constellation of features associated with xeromorphic (drought adapted) plants at the present. They are believed to have lived in the harsh environments of seasonal rivers. Some think the first angiosperms were possibly weeds that lived in the dry disturbed environments.
The fact that 1/3 of the 32 families of angiosperms that had evolved by the beginning of the Tertiary have modern descendents that are adapted to arid environments suggests that angiosperms may have evolved under regimes of at least some, possibly seasonal, aridity. Extreme aridity in Australia has been connected with cold times, as in ice ages. Among the plants that have these xeromorphic-type adaptations are the saltbushes and bluebushes, members of the Chenopodiaceae, that thrive in ecosystems that are characteristically low, open and dry. It seems that at the start of the Tertiary there may have been communities of xeromorphic plants, possibly growing in open habitats on impoverished soils, but surrounded by rainforest.
At about 51 Ma the sea temperature dropped sharply, in fact it was the sharpest drop throughout the Tertiary. The ocean temperatures around Australia dropped to glacial levels for the first time in 200 million years. The sudden drop in oceanic temperatures was the result of the strengthening of the Antarctic Circumpolar Current as the other southern continents comprising Gondwana finally broke from Antarctica allowing currents to form unimpeded by land. As Australia moved away from Antarctica the Antarctic Circumpolar Current deflected the subtropical weather systems northwards. The Transantarctic Mountains rose to 5 km high at this time, which added to the cooling of the continent. Australia is believed to have been covered by a deeply weathered mantle of up to 100 m deep that had formed under a high rainfall regime. A number of large rivers wound their way across the continent. It was during this period that extensive sedimentary deposits formed in low-lying inland areas, such as the Canning Basin and Officer Basin in the west and the Great Artesian Basin in the east. When both temperatures and rainfall were high silica moved through the soil in solution from the edges of the basins to the lower lying area where it accumulated in the deeply weathered and possibly waterlogged material. The precipitate that came out of solution was in the form of a cement of quartz or opal. This precipitated silica forms the resistant capping layers on the tops of flat-topped hills or mesas so characteristic of modern deserts.
The formation of silcrete from this time indicates that the climate was changing from one of consistently wet conditions to a more seasonal climate where it was hot and dry for at least part of the year. Pollen recovered from deposits from this time indicates that the vegetation of the continent was becoming a more complicated mosaic of vegetation in central Australia in response to both soils and a seasonally dry climate. The pollen indicates that the vegetation graded from rainforests of the type now found in the tropics, but at higher altitude, to sclerophyll forest vegetation, mostly of Casuarina, and some grasslands.
The sclerophyll leaf type is not restricted to Australia, but it is more dominant and widespread in Australia that elsewhere. Casuarina, with its thin hard leaves, shows that sclerophylly was well established by this time. It is basically an adaptation to poor soils rather than to low rainfall, and poor soils are found throughout Australia, though it also functions as an adaptation to water stress. It is a preadaptation that evolution used to adapt the plants to the increasingly arid conditions.
By the Early Oligocene, about 38 Ma, Antarctica was covered by ice. By about 30 Ma, there were periods when Antarctic glaciers were more extensive than now. It was at this time that the final connection between Australia and Antarctica was broken. The Antarctic Circumpolar Current deflected the warm currents coming south along Australia's east coast and the Antarctic atmospheric circulation, called the Antarctic Polar Vortex blocked the warmer air from the north from reaching the Antarctic continent. There are no known fossils from the Oligocene of central Australia, indicating there may have been such severe aridity that fossils were unable to form at that time. During the following Miocene the rainforests again spread across large parts of Australia as warm temperatures and high summer rainfall prevailed. There was no warming trend in Antarctica. In Antarctica spasmodic alternations of glacier advance and retreat continued from the Late Oligocene.
The Antarctic experienced a sudden drop in temperature with rapid expansion of the ice sheets about 14 million years ago. At this time in the Arctic temperatures also dropped and the was a sudden expansion of ice sheets there as well. There was so much expansion of ice sheets that it may have been the start of the present 'ice age', rather than the usually accepted 2 million years ago. The present ice age is characterised by regular glacials with short interglacials. The present is one of these interglacials.
The vast rivers that had crossed the continent dried up and temperatures dropped, so that the atmosphere could hold less moisture, hence less rain. In Australia the old rainforests were fragmenting and being broken up by Casuarina and Acacia forests, with some eucalypts and grasslands and herbs such as wildflowers. Fossil fruit have been found near Woomera. These fruit resemble fruit from several xeromorphic taxa such as Eucaluyptus, Leptospermum, Melaleuca-Calistemon, Grivillea and Santalum and some leaves similar to Banksia. These plants were growing among plants with affinities to modern New Guinea rainforest plants.
About 15 mya the continent ploughed into islands to the north. Beginning about 14 million years ago, Australia was connected by land to New Guinea a number of times as sea levels fell during glacial periods. At these times fauna, and to a lesser extent flora, were exchanged between Australia and New Guinea, Cape York Peninsula being the part of Australia involved in these exchanges.
The northern margin shows evidence of strong growth, as indicated by its morphology, volcanicity, seismicity and rapid vertical movements. Wegener stated "the great Australian block, with its anvil-shaped, thickened, anterior end, formed by New Guinea being formed into a high and recent mountain chain, forced itself from the southeast between the previously closed chain of the more southerly Sunda Islands and of the Bismark Archipelago" (Wegener, 1924, p.67).
The evidence for the interaction between the oceanic lithosphere and
the continental lithosphere of the
Indo-Australian Plate, is seen in a
number of features such as a 7,000 km long orogen, through the
archipelago of the troughs and high islands of the Banda Arc and the
The boundary between the continental lithosphere and the Cainozoic oceanic lithosphere lying along then axis of the North New Guinea Basin inboard and the , 100-200 km oceanward of the crest of the orogen.
The Northern Ranges Provinces of central New Guinea.
Volcanic arcs, the Inner Banda Arc to the west and Eastern Papuan Province, Bismark Volcanic Arc, to the east.
The New Guinea terranes to the north docked with the Australian craton by 25 Ma, 15 Ma and 10 Ma (Pigram & Davies, 1987).
3 islands of the non-volcanic outer Banda arc, Tanimbar, Timor, and Sumba, rose above sealevel by 2 Ma (Veevers et al., 1978; Pigram & Davies, 1987). Balanced on the other side of New Guinea by the Bismarck arc are the islands of the volcanic inner Banda Arc.
By the time Australia had moved far enough north into the equatorial region it began to progressively block the atmospheric and oceanic currents, changing the circulation patterns that had previously prevailed. Currents turned north and south from the equatorial latitudes were cooled and returned towards the equator along the western edges of the southern continents. This is the explanation of the deserts being situated along the west coast of the continents, including Australia.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|