Australia: The Land Where Time Began
Nullarbor Plain (the Bunda Plateau)
The name derives from "null (Latin for no) and arbor (Latin for tree)". By sheer coincidence, apparently, in some Aboriginal languages nulla means "none" or "not any". Maybe there is some basis to a story told in Ancient Rome by some sailors returning home with apparently wild stories about large hopping animals. Unfortunately, I can't remember where I read it.
At about 250,000 km2, the Nullarbor is the largest block of limestone, or karst (limestone characterised by caves and underground drainage) on Earth, raised from the sea 3 million years ago. Longest straight length of railway in world, 470 km, connects the east coast to the west coast across it.
There are no trees because the soil is a shallow calcium-rich loam derived mainly from sea shells. Its vegetation consists of succulents (not cactus type of succulents) such as saltbush and bluebush. The total lack of trees implied by the name refers to an area at the centre of the plain, there is in fact a cover of saltbush (Atriplex spp.) and bluebush (Maireana and Bassia spp.) with some mallee (Eucalyptus spp.) and a number of stunted eucalypts, making it different from any other arid area in Australia. In this it is more like the arid areas in other countries such as North America. Another "un-Australian" characteristic of the Nullarbor was, until the arrival of Europeans with their feral animals, a land of mammals, whereas the rest of the arid zone of Australia is more accurately described as a land of lizards.
It stretches from the arid salt lake region of Western Australia to the similarly arid plains of South Australia, and north from the Great Australian Bight to the edge of the Great Victoria Desert, though its borders are not exactly defined, it is usually said to be about 700 km east to west and about 300 km south to north. About 2/3 of its total area being in Western Australia, the remainder in South Australia.
The surface of the plain is flat, but it dips gradually from about 200-300 m above sea level in the north to between 40 and 120 m above sea level at the southern coast.
The Nullarbor Plain is composed of 3 separate parts, the Bunda Plateau, the large main section, the southern scarp which is divided into 4 sections, Wylie Scarp, Baxter Cliffs, Hampton Range and Bunda Cliffs, and 2 coastal plains, Roe Plains and Israelite Plains. Dunes partially cover the 2 coastal plains that are backed by the scarp.
The cliffs are in 2 sections, in the west the Baxter Cliffs rise to 60-100 m and in the east the Bunda Cliffs are 49-75 m high. Both sections of cliff are about 200 km long, and are coloured hues of brown in their upper portions and white lower portion. The sea pounds the cliffs relentlessly with huge waves and powerful southerly winds, but the rock strata are horizontal and vertical joints are very regular, with no weaker sections for the sea to gouge out caves or valleys. The result is that the cliffs erode by being undercut, whole slabs falling off at a time, leaving a very straight coastline, the plateau continuing to drop vertically to the sea. Behind the Israelite and Roe plains the cliffs are some distance inland from the sea, but remain vertical, having been eroded by being undercut in times of higher sea level.
The largest area of karst (cave landforms) is found in the southern part of the Bunda Plateau where massive cave systems have been weathered out of the limestone by seepage from the surface at times when the climate was wetter than the present. The caves have been divided into 2 types, shallow caves that are less than 25 m below the surface, and deep caves that can reach up to 120 m beneath the surface. The water table is reached by a lot of the larger deep caves, and where it does there are often clear lakes and complex networks of underwater passages. It is believed the water found in a number of the deeper caves may underlie much or all of the plain. The deeper caves often have very large caverns, up to hundreds of metres long. The flat floors are usually strewn with piles of fallen rocks and many have high domed ceilings. In contrast with the heat and aridity and glare of the plains, these caves are cool, dark and still.
The plain surface lacks any features that could indicate the presence of caves. Large dolines (sinkholes) often have passages leading to about 50 caves, mostly the larger ones. The gaping holes of the dolines are formed by the collapse of the roof of caves. They can be from a few metres wide to 200 m wide. The depth of these dolines can be as little as depressions or as deep as 40 m. Blowholes are the only entrance of some caves, the opening being narrow, vertical shafts up to 40 m deep. Many are called breathing holes because air can blow out of them at high speed, while at other times the air can be sucked into them. The cause of this phenomenon is not known for certain, it has been suggested that it may result from weather changes that bring changes in atmospheric pressure, the air in the subterranean tunnels being blown out when it expands on being heated and the direction of air flow reversing in cooler periods.
Fossils and Aboriginal artefacts and art have been found in many of the caves of the Nullarbor, the most famous of which is Koonalda Cave. Most other major caves of the Nullarbor are in the Western Australian section. Spelothems (cave decorations) have been found in many caves, and surrounding the water of the underground lakes there are places where delicate crystals have formed. The caves are no longer being formed or extended, as the climate is now too dry and the surface is too sparse to produce the acidic water that is required to weather the limestone, dissolving it to form the caves, stalagmites and stalactites are no longer being formed. These are known only from some shallow caves on the eastern edge of the Plain and there are also some in Lynch Cave near Loongana. Most have dried out, very little cave formation now taking place, the only changes usually being the occasional rockfall.
With an average yearly rainfall of less than 250 mm, there are no streams on the Nullarbor, the small amount of rain that falls is soon lost to evaporation or infiltrates the porous limestone, finding its way to the underground lakes and streams. Some of this ground water emerges as springs at the foot of the cliffs on the coast, but some forms soaks in the sandhills. It is often saline, though some remains fresh. The only places where there is sometimes water for a couple of weeks after rain are areas where the limestone has no cracks. In these areas the water is held for a short time after rain in small rock holes. Dongas are shallow depressions containing trees that are found in the more northern parts of the plain. They are up to 800 m in diameter and usually about 2-3 m deep with a hard clay surface, and can hold water for a short time after rain, partly because of the tree cover that keeps the temperature down, reducing the rate of evaporation.
The only parts of the Nullarbor that have more than a thin soil layer are the dongas and coastal dunes. Combined with the water shortage in the root zone, because any water there drains down to the water table, plant growth is very limited over most of the plain. Any plant growing on the plain needs to be a highly adapted desert species, such as saltbush and bluebush, that absorb atmospheric water through their leaves.
During the early part of rifting the area that was to become the Nullarbor Plain was subsiding. During this period braided river systems flowed across it to the rift valley that was forming along the southern edge of the continent. The resultant erosion deposited sediment that became conglomerate, siltstone and shale, the sediment being deposited the Early Cretaceous.
The Bight Basin covers the area of the subsiding rift valley and the onshore section, the downwarping continental margin, the part that underlies the Nullarbor. As subsidence continued the part underlying the Nullarbor and its offshore section became the Eucla Basin, the site of deposition during the Tertiary.
About 116 million years ago sea level peaked as global sea level rose dramatically. At this time, the Eromanga Sea covered much of Australia. At its highest level the Eromanga Sea connected all the flooded continental basins to the proto-Southern Ocean. This had been forming since about 132 million years ago when the Indian Ocean began to open. The Nullarbor area was connected to the flooded Officer Basin and to the west coast of Australia by the Canning Basin. The Cretaceous eustatic sea level rise was the highest in the last 400 million years. Much of the land surface around the world was flooded at this time.
The sediments accumulating in the area of the Nullarbor under the Eromanga Sea were the first marine sediments deposited in the Nullarbor part of the Bight Basin. After its peak at 116 Ma, the epicontinental sea gradually retreated off the Nullarbor region, as well as the other flooded continental basins. Most of the continent was again above sea level.
As the narrow strip of sea between the southern edge of Australia and Antarctica was growing from west to east, the Ceduna Depocentre, a major depocentre in the Bight Basin, formed. All the continental drainage from the eastern part of the continent as well as from the southern margin flowed towards this centre. At this time the Nullarbor region was again a flood plain of braided rivers.
A second phase of rifting began about 96 Ma. The sea floor began spreading in the centre of the rift, at the time of a global rearrangement of plate activities, resulting in plates moving in different directions and at different rates. At this time continental drainage was strongly from north to south, indicating that the continent sloped down from north to south. The Nullarbor remained flat, crossed by rivers. The rocks of the Nullarbor were eroded and deposited in the rift valley system.
Large, extensive river systems flowing from the Musgrave Block and the Stuart Range and Gawler Range cut deep channels across the Nullarbor. These palaeochannels can be seen on satellite images. During the Tertiary, limestone deposits were laid down to form the present-day Bunda Plateau.
About 50 Ma the Eucla Basin, the Tertiary successor of the Bight Basin, in the Nullarbor region, began another phase of downwarping and the sea returned to cover it. The lower part of Wilson's Bluff Limestone, marine sandstone and marl, was deposited at this time. The upper part was deposited in the Late Eocene under quiet water conditions as the sea level lowered.
The sea penetrated up the river valleys towards the Musgrave Range, Stuart Range and Gawler Range. Eocene sediments of the Pidinga Formation, displayed in these palaeochannels, of alternating marine and freshwater types, show how the sea level fluctuated at this time.
Lying on the north-eastern margin of the Nullarbor, the Ooldea and Barton Ranges are coastal dune ridges that mark the edge of the sea during the Eocene. The Ooldea Range is a sand range, 650 km long, standing out from the flatness of the Nullarbor Plain, 25-300 km from the present coast.
The Barton Range lies within the dunefield of the Great Victoria Desert. It was only recognised as a separate feature on aerial photographs. To the southeast of the Ooldea Range is the Paling Range, also apparently beach ridges, have been recognised from LANDSAT images. Other possible beach ridges have been found near Lake Maurice.
Following the detection of these ranges it was realised that the Eocene sea was much more extensive in this area than had previously been thought.
Strong westerly winds formed these dunes, piling up sand in the easterly quarter of the Eocene beach. At the time the sand ridges were forming it appears the climate was temperate, supporting other evidence for the climate at this time. The dunes have survived for about 37-34 million years. It is unlikely any type of climate, other than a very arid one, could have existed in the area for the remainder of the Cainozoic.
The evidence for aridity in the Nullarbor area is the earliest record of aridity in the southern part of Australia. Plant cover on the dunes and permeability of the quartz sand probably contributed to their survival for such a long time.
The Mirning people were the occupiers of the Nullarbor Plain, mostly in the south near the coast. They are not thought to have moved far into the central parts of the plain. The deeper caves were believed to be the home of Jeedara, a water serpent. According to their Dreamtime stories he was chased by Yugarilya, the Pleiades, pushing up the Bight cliffs and hiding in the caves. The air rushing in and out of blowholes was said to be his breath. They feared the places where he lived because he seized anyone he found in his territory. Evidence of their presence in some caves is seen in caves such as Koonalda Cave.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|