Australia: The Land Where Time Began
Yilgarn Craton, Western Australia see Earthquakes
The Yilgarn Block in Western Australia is one of the most ancient landscapes preserved anywhere on Earth.
The Yilgarn Plateau is a craton, one of the original masses of rock that rose out of the sea in the Late Archaean, about 2700 ma to form the original landmass of Australia. At this time the eastern part of Australia had yet to form. Yilgarnia is the name given to the land surface of the Yilgarn craton. The 65,000 km2 of the Yilgarn Craton lies in the southern part of Western Australia, from Meekatharra and Wiluna in the north to the south coast and from Yamarna and Balladonia in the east to just short of the west coast. The Darling Scarp forms a clear-cut line that separates it from the much younger Swan Coastal Plain along its western edge. The Darling Scarp is the edge of the rift formed at the beginning of the separation of India from Australia, which preceded the separation of Australia from the present continent of Antarctica about 45 million years ago during the break up of Gondwana. The Yilgarn Block has not been submerged since it rose out of the sea almost 3000 million years ago. It was one of the blocks of crust, cratons, which were later joined together to form the present continent of Australia, at a much later date
It has been accepted, based on dates of meteorites that were believed to have formed from the same dust and gas cloud that the Earth formed from, that the age of the earth was 4550 million years. It was assumed that the bombardment of the Earth by meteorites that were swept up as the Earth moved along its orbit would have been sufficiently intense to keep the Earth molten until 4030 million years ago, which is the age of the oldest known rocks from Canada. Then detrital zircon crystals were found in metamorphosed rock at Mt. Narryer and Jack Hill on the Yilgarn Carton in Western Australia that were 4276 million years old. Now, more recently, new detrital zircons were found in the same area with an age of 4404 +/- 8 million years ago, which pushed back the age of the oldest known continental crust by about 130 million years. Nature, Vol.409, pp 175 & 178..
The latest find of zircon crystals also show oxygen isotope ratios that indicate that they formed in the presence of liquid water, and microinclusions of SiO2 indicate that they formed in molten granite, and hence continental crust. The granite would have been a batholith that was heaved up in the birth of the Yilgarn Craton. The result of this is that there was both continental crust and liquid water only about 140 million after the generally accepted age of the formation of the Earth. The Earth was previously thought to have been too hot to have liquid water so close to its formation from the dust cloud.
There are a large number of granite outcrops on the surface of the Yilgarn craton, many with undercut sides, the largest and best known being Wave Rock, east of Hyden, Western Australia. The granite of these outcrops is very weather-resistant, and hence erosion-resistant, both because the rock is very hard and because many of the outcrops are solid masses with few or no cracks or faults that the forces of erosion can work on. Because of this the erosion that does occur on them is mainly when they are buried and humic acid in the soil can eat at their sides and cause the undercut formations, as with Wave Rock, that only becomes visible when the soil level surrounding the rock has been lowered by erosion.
Glaciers ground their way across the Yilgarn Plateau from about the Late Carboniferous to the Early Permian. Immediately after the glaciers there was no organised drainage system, any there before the glaciation were ground out of existence by the glaciers. The drainage systems that are present now developed after the glaciers had gone, a watershed runs about north-south, approximately down the centre of the plateau. This watershed reaches about 600 m in the north and to 300 m in the south. The rivers on the western slopes still flow - the Swan River, Murchison River, Gascoyne River, Ashburton River, and Fortescue River - getting sufficient rain under the present climatic conditions. As with other north-south ranges in Australia, the main weather systems move across the continent from west to east, dropping their rain mostly on the western slopes.
The rivers in the eastward-flowing drainage systems, which drained into the Canning Basin, Officer Basin and Eucla Basins, stopped flowing, at least on a regular basis, once the continental drying took hold. These palaeochannels are now represented by chains of salt lakes. No rivers flow to the south as a result of tilting of the southern margin along a line called the Jarrawood Axis, aka the Ravensthorpe Ramp. This tilting occurred about 30 MA, and the new drainage system is still developing. This event divided the drainage of the area affected into a coastal region, where the rivers still flow to the sea, and the Plateau region.
Uplift occurred at about the end of the Cretaceous, 65-60 Ma, in the Canning and Officer Basins. They each developed their own drainage systems, the Canning draining northwest and the Officer south. The Eucla Basin was uplifted after the retreat of the Miocene marine incursion, about 30 Ma. The Eucla Basin became the Nullarbor Plain, aka Bunda Plateau.
Much of the Western Australia landscape has remained largely unchanged since the Late Cretaceous or Early Tertiary, 65-60 Ma, with its palaeodrainages established. Sometime before the Mid Miocene, the rivers making up these drainage systems haven't flowed on a regular basis since about 15 Ma.
The Yilgarn Plateau has 7 major Palaeodrainage provinces.
Valley-fill deposits make possible the dating of these palaeodrainages. Late Eocene, 45-34 Ma lignitic deposits formed the valley-fills in the Kalgoorlie region. The Rollo's Bore beds are up to 120 m thick. The same beds under Lake Lefroy and Lake Cowan, these beds are overlain by the 30 m thick Late Eocene Norseman Limestone marine sediments. This shows that the palaeodrainages were formed earlier than Late Eocene.
On the western margin, the Beaufort and Darkan palaeochannels are the first fairly complete palaeochannels found to be flowing east to west in the south-western sector of the Yilgarn Craton. 60 km of the Beaufort palaeochannel has been traced from Quongering Pool, 25 km north-north-east of Kojonup, to Haddleton Springs. It is thought to have discharged into the Perth Basin across the Darling Fault. It may have been the ancestor of the Preston or Collie River, or maybe of both,
There are up to 65 m of sediment, sand, silt and clay, in the palaeochannel. They record the history of a period in the Eocene from 45-35 Ma. The Beaufort started out as a meandering river system, then as the Yilgarn Block began to tilt to the south the gradient of the palaeoriver was reduced. Flow slowed and lakes began to form, as evidenced by the lacustrine sediments that were deposited in the valley. As a result of the tilting, the Blackwood River captured the headwaters of the ancestral Beaufort. After cutting down through the bedrock south of Lake Towerrinning it beheaded the Beaufort. Darlngup and Haddleton Creeks, 2 south-flowing tributaries of the Blackwood River, cut the palaeochannel west of Towerrinning. They trunk valleys of the palaeodrainages often have calcrete layers in their sediments, forming freshwater aquifers under land where all surface water is saline.
As with playas elsewhere in Australia, those of the Yilgarn Block are usually dry salt pans, extremely hot on summer days, extremely cold on cold winter nights, and receive water only when a cyclone crosses the coast and becomes a rain depression that dumps huge amounts of rain across the landscape. These events are up to 10 years apart, yet the seeds of life, in the form of brine shrimp eggs, hatch and within days the brine shrimps (Paratremia) are abundant in the waters of the new lake.
Within days of the filling of the lake thousands of banded stilts, some from as much as 1000 km away, arrive and begin to breed. These are endemic to Australia, and like many other Australian animals and plants, have a number of unique features. They form colonies of 20,000 nests by 2 weeks after the end of the rain. It is still unknown what triggers the move from the coastal wetlands to congregate on the playas, just in time for the brine shrimp hatching. The result is that that they breed only every 5-10 years when the flood events occur.
Their eggs hatch after 3 weeks and the chicks feed on the brine shrimp along the water edge. The begin moving off from the islands where they nested, in small groups, paddling downwind towards the other end of the lake, where they gather in the shallows. This can be 50 km from the nesting islands.
Unlike any other wading birds in the world, they nest communally, the nests being separated by 30 cm. They lay white eggs with a few black streaks and blotches. The chicks are white and downy, and gather in crèches. They have a very unusual habitat requirement - they only breed on newly filled playas. The habitat requirement is because the chicks feed exclusively on brine shrimps which hatch in the playa lakes shortly after the playas are flooded from eggs that have been dormant for the years between floods, surviving blistering summer heat, freezing cold winter nights, extreme dryness and salinity.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|