Australia: The Land Where Time Began
The Ordovician Period - about 505-446 Ma - 69 million years
During this period Australia was still situated between 0° and 30° 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 million years ago when the sea levels reached their minimum. This glacial period has been called the "Early Palaeozoic Icehouse", and studies by a group of scientists at the University of Leicester suggests it may have resulted from CO2 from the atmosphere being locked up and buried by the dead plankton raining down to the seafloor where conditions at the time were believed to have been anoxic. The plankton are believed to have flourished in the warm, greenhouse type conditions of the earlier part of the Ordovician and when they died they accumulated in large quantities on the sea floor where the CO2 was not released by bacterial decomposition. This sequestering of the CO2 eventually lead to a depletion of the atmospheric CO2, hence the cooler conditions that led to the ice age believed to have lasted for about 30 million years.
The rich fauna of the warm, shallow seas comprised members of some the groups that were present in the Late Cambrian, with the addition of groups such as the ancestral Horseshoe Crabs and Eurypterids that first appear in the fossil record at this time. Crinoids first appear in the Early Ordovician.
The Delamerian Orogeny that began in the Late Cambrian, continued into the Ordovician, uplifting along a zone from western Tasmania to the north-northwest through western Victoria to north-central-South Australia. The high rate of erosion from this uplifted high country indicates that the climate at the time was very wet, a high rainfall being required to erode so rapidly.
The western part of the Amadeus Transverse Zone had been elevated above sea level during the Cambrian and the earliest part of the Ordovician. In the Early Ordovician, it begun a long period of subsidence that resulted in the formation of the Canning Basin. By the Middle Ordovician, the sea had covered this depression, linking up with an embayment from the eastern continental margin to form the Larapinta Sea that divided Australia into 2 separate parts separated by a shallow sea. This sea eventually joined with a narrow channel that extended along the continent's northwest margin, the first part of the Westralian Depression. To the north of the Larapinta Sea the land was low and featureless, while that to the south, made up of the southern half of Western Australia and South Australia was rugged, high country.
Late in this period the first stirrings of the Alice Springs Orogeny began, and the epicontinental seas began to retreat from the land, the Larapinta Sea shrinking as the sediments deposited in it were uplifted and eroded. At this point the Amadeus Transverse Zone was no longer a complete entity. Originally opening to the east in the Tasman Orogen but now it opened to the west in the Westralian Depression.
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.
Because of the extensive areas of shallow-water environments during the Ordovician there was a rich, diverse fauna. Filter-feeding organisms flourished. There was a rapid burst of evolution among the brachiopods, Bryozoans, Molluscs and Echinoderms, Graptolites being characteristic of the period, their rapid change make them very useful in stratigraphy. The Period has been divided into zones based on the changing types of graptolites through time, allowing geographically separated strata to be correlated.
A very large species of trilobite, a group that were very abundant at the time, has been found that grew to about 50 cm in the Stairway Sandstone that was deposited in the Larapinta Sea. The trilobite burrows, known as Cruziana, are often preserved as trace fossils. Rusophycus are the scratch-marked mud piles they produced when feeding. There were abundant predatory cephalopods, such as straight-shelled nautiloids. The warm temperature of the water is indicated by the presence of corals.
The Receptaculitids are a group with a very complex structure that arose in the Ordovician, surviving to the Devonian. They are now believed to be coralline algae, previously being classified as problematica.
The Ordovician seas that covered much of Central Australia deposited sediments containing some of the oldest fish fossils in the world. A variety of other marine fossils are preserved in them - shelled nautiloids, molluscs, trilobites.
- Ordovician brachiopods from Nelungaloo, New South Wales.
500Ma - Late Ordovician trilobite heads, New Durran, New South Wales
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|