Geology of the savannas

Age of savannas

Tropical savannas are so old, hills and mountains have eroded away over vast amounts of time

The phrase "as old as the hills" is a good place to start when thinking about the geological history of northern Australia. But hills, which tend to be eroded mountains, are a pretty rare sight in the tropical savannas (let alone mountains) as even they have eroded away.

Australia is the lowest, flattest and, apart from Antarctica, the driest of the continents. Australia covers almost 7.7 million square kilometres (sq km) and compared to other countries is quite flat. Less than 1% of the country is above 1000 metres in elevation and most of the savanna region is less than 500 metres above sea-level. This is because most mountains and hills that Australia may once have had have eroded away over time.

Since the super-continent Gondwanaland broke up around 100 million years ago, there has been little geological activity like mountain building in Australia, and consequently Australian geology is relatively old and weathered. It does not feature young, high, mountain ranges.

The continent of Australia is also very old. Unlike Europe and North America, where some landscapes date back to only around 10–20,000 years ago, when great ice sheets retreated, the age of landforms in Australia is measured in many millions of years. In fact, pottering about in northern Australia you could find patches of Archean granite, which are about 2.5 billion years old. To put that into perspective, current estimates put the age of the Earth at around 4.5 billion years, although you'd be hard pressed to find rocks older than Archean except on other planets. That such rocks can be found is evidence of the incredibly long period of geological stability experienced by northern Australia.

Geography

Geographically, the mainland continent can be divided into three large areas:

the Western Plateau
the Central Lowlands
the Eastern Highlands.

The tropical savannas region overlaps with each of these regions.

The Western Plateau is relatively flat. In the tropical savannas region, numerous more rugged areas exist near the coastal boundaries of the Plateau, including the Kimberley region and Hamersley Ranges in Western Australia.

The Central Lowlands stretch from the Gulf of Carpentaria in the north of the tropical savannas through the Great Artesian Basin to the Murray-Darling Plains. Most of this area is flat and low-lying. Much of the Central Lowlands is occupied by the Great Artesian Basin, which consists of sedimentary rocks which hold water that enters in the wetter Eastern Highlands.

The Eastern Highlands stretch along most of the length of the east coast. A steep escarpment and series of high plateaus exists along the coast, which slope more gently toward inland areas. The northern section of the Eastern highlands lies within the tropical savannas region. However the highlands are less pronounce here, reaching an elevation of 1000 metres in only a few locations, including Cairns.

This escarpment is the site for some of Australia’s highest waterfalls.

Cycles of mountains and erosion

Geologists believe that there have been at least four cycles (known as 'groups' in geological terms) of mountain creation and erosion in northern Australia. Much of these cycles occurred during the Proterozoic Age? million years to 570 million years ago—when there was barely any oxygen, and simple bacteria reigned supreme. The most developed form of life then were simple anaerobic bacteria called stromatalytes.

These four cycles were the last dramatic geological events to happen in much of the savannas; since that time rock formation has by and large occurred only as a result of changes in sea level (which can result from sea level change or from the rising or falling of the land mass) and the concurrent deposition of silt.

Erosion then explains far more about the soil structure and the landscapes that characterise northern Australia than rock creation events such as volcanos. Before we get to the erosion picture though, we need to examine our fundaments: the incredibly old underbelly of the tropical savannas.

Rock creation and erosion

The rocks of northern Australia tell the dramatic story of mountain and rock creation and erosion that occurred during the Proterozoic Age. Again, for the sake of perspective, keep in mind that Gondwanaland, the great land mass which once joined Australia with India, South America, Africa and Antarctica, had not yet even formed. The tectonic plates of the world were no doubt rather different to the way we think of them today. From some rock sequences one can find in the north, it is quite evident that the top part of Australia had once been on the edge of a plate, and so was subject to collision with others. Indeed, it collided with some continent or other at least four times. Each collision resulted in the formation of mountains. In fact, it is more than likely that mountain ranges as dramatic as the Himalayas were created and then fully eroded away on a few occasions in the north of Australia.

Peneplains

Once the mountains were eroded to a peneplain (i.e. the plainest of plains), they were often submerged in sea water. Layer upon layer of stromatolyte could then accumulate (these creatures remove calcium carbonate from the water and so their presence is often evidenced by limestone deposits). As the sea water deepened over the land surface, these 'reefs' would die, leaving behind a nasty, smelly sulphorous and carbonaceos black mudstone.

These smelly episodes in geological history are very significant, especially if you're a geologist and you wish to make a living by showing mining companies where to look for mineral deposits. Oftentimes, you'll find them in these layers. To simplify somewhat, the environment of the dying stromatalytes reefs was anaerobic; minerals tend to precipitate rather more easily in such an environment, especially if there was enough sulphur or carbon to bond with.

Greywakes: the underwater avalanche

Northern Australia also has 'greywakes': chaotic rock sequences that result from dramatic underwater avalanches. Pyroclastics also tend to be present in the same sequences. These are extremely violent volcanoes that hurl rocks over great distances and often occur under water. Pyroclastics often occur in oceanic subduction zones (where one plate goes under another) as the wet rocks from the plate above meet the molten rocks of the plate below. Both of these events occur during plate tectonic collision, and represent the beginning of mountain formation. Thus the cycle continues. Note that mountains are fairly temporary affairs in geological time: their steep slopes are quickly eroded. Flat plains on the other hand are not as susceptible to the forces of erosion.

All of these events were occurring sequentially and cyclically through time. Of course this does not mean it is easy to unravel these sequences by looking at the geology of the current landscape. As mountains are pushed together, rock layers that were once horizontal become vertical and erode at different rates. What you are left with is a story that is as interesting as it is complex. Since the end of the early Proterozoic however, there has been nothing like major continental collision in the region. In other words, this part of Australia has been slumbering quietly, far from the edge of its tectonic plate, for a long, long time.

Plateau sandstones

In the mid-Proterozoic, about 1 billion years ago, huge deposits of fluviatile sediments covered much of the region and now form the relatively flat-bedded sandstone plateaus which, with their escarpments, dominate the landscapes of Kakadu, the Kimberly and Cape York. These sandstones have all the expected features of shallow water deposition: cross-bedding, ripple marks, mud cracks and rain prints. The theory it seems is that these sands were deposited in vast river deltas which over time took many different courses over northern Australia.

River deltas

Evidence of these delta areas periodically drying out enough to allow mud cracks to occur suggests that the peneplain and the sea level stayed almost at the same level, despite the fact that the land surface was rising under millennia of sediment deposition. In the end, there were several hundred metres of shallow-water deposition. This indicates a gradually subsiding land, or gradually rising sea.

These gradual movements are ascribed to isostacy, a process in which the land mass falls, or rises, very gradually because of changes in land mass weight. Land mass weight changes because of the erosion and deposition. However it can take millions of years for land masses to steady after their weights have changed.

Resistant sandstones

Sandstone escarpment in western Arnhem Land. Scattered at the base are massive chunks of earth which weathered away along vertical joint planes
Photo © Martin Armstrong PWCNT

That these landmarks are still apparent is evidence of the incredible resistance that these sandstones have to erosion—estimated at less than 500cm per million years—although only patches of it have survived. These sandstones are resistant to weathering (the chemical breakdown of rock) because they are composed mostly of quartz, and to erosion because of their horizontal bedding. That is, a flat rock will erode much more slowly than a vertical rock. However, they do have strongly developed vertical joint planes where water can enter the rock mass and begin to wear it away. They are also vulnerable at their margins, in that the rock underlying the sandstone is folded and when exposed, will fall away in great chunks. This explains the characteristic shape of the escarpments. Beneath the escarpment lies the peneplain of the wetlands, which is underlain by the ancient folded rocks that have been eroded almost to sea level.

Basalt volcanic lavas

In the Victoria River District to the west of the Darwin-Kakadu region, are extensive basalt volcanic lavas of Cambrian age, formed about 500 million years ago. These rocks were the outcome of the beginnings of the break-up of Gondwanaland. They gave rise to soils much more rich in nutrients for plants than the ancient marine sediments which cover most of the region, and this is why this region has some of the richest grazing country in the Northern Territory.

To the east of the Darwin-Kakadu region, much younger basalt lavas cover Cape York Peninsula plateau country, giving rise to the deep rich soils of that region. These were formed during the Tertiary, only tens of millions of years ago.

It is thought that these basalt lavas which exist all the way down the east coast in the form of the Great Dividing Range, were formed when the eastern side of Australia collided with another tectonic plate. Since this time however, New Zealand has broken away from the Australian mainland and in a sense acts to buffer the mainland from tectonic activity.