Brief outline of the Geological History of the Illawarra & Southern Highlands
By Richard Jones - April 2012
The geology in the sea cliffs around the Illawarra and the escarpment has had me curious for some time.
I have pieced together from different sources a basic geological outline of what I think possibly occurred and offered some of my own observations.
Please note that I have not taken into account folding, faulting, subsidence, uplifts or other tectonic processes.
This page is really only for interest purposes, your own investigation and research should be employed before taking the below to be factual.
Please send me an email if you strongly believe that a correction needs to be made.
Paleozoic from 450 - 340 million years ago.
Starting at the bottom or beginning of the sedimentary journey relevant to our story is the Lachlan Fold Belt. The LFB is a geological subdivision of the east part of Australia. It is a zone of folded and faulted rocks of similar age made up of Paleozoic deep and shallow marine sedimentary rocks, cherts (Chert is a fine-grained silica-rich sedimentary rock) and mafic (silicate mineral rock that is rich in magnesium and iron) volcanic rocks, it dominates New South Wales and covers an area of 200,000 km2.
At this time Australia formed the north-eastern part & coast of Gondwanaland. The warm temperatures of the past 191 million years had now changed as the planet entered a global cooling.
Dating of Carboniferous rock sequences in NSW highlights that glaciation was under way from 325-310 million years ago. Much of the Carboniferous geological record has been erased due to this glaciation and the transport of sediment materials there in. A good example of old Carboniferous conglomerate melt water river systems can be seen in Burrawang & Tallong that extended down from a mountainous region in central western NSW, flowing to what would eventually become the Sydney basin, similar conglomerate flows can also be found north of Jervis bay.
Photo taken at Gerringong NSW
Turbidite banding have their origins in underwater current deposits which are responsible for distributing vast amounts of sediment.
The idea behind Turbidite is that water must be travelling at a certain velocity in order to suspend the particle in the water and push it along. The greater the size or density of the particle relative to the fluid in which it is travelling, the higher the water velocity required to suspend it and transport it. This condition occurs in many environments including rivers, waterways, mudslides, pyroclastic flows and tidal areas aside from the deep ocean where turbidites are particularly well represented.
Permian 299-275 million years ago.
It was in the Permian as the ice thawed that sediments began to be deposited across the glaciated landscape.
Sedimentary rocks near Kiama show features of permafrost conditions (soil at or below 0 °C), this would indicate that the east coast climate was probably very similar to current day Alaska.
Evidence of fossilised Permian sea creatures can be found in the siltstone cliffs from Kiama to the south of Ulladulla. The most common of these fossils is the 'Eurydesma Fauna' ( Brachiopods, Bivalve Molluscs, Cephalopoda, Bryozoa, etc) which can be found in abundance.
The marine fauna at this time lived in shallow 'muddy', seasonally cool climate sea, basins and inlets that covered much of this coastal region. Due to the cool temperatures only one coral is known from this region and reef systems are absent. Oxygen isotopes in the calcium carbonate of the shells indicate that the shells formed in cold waters, further support of a cold-cool climate at this time.
It is suggested from research done in the north sea that some of the larger Brachiopods and bivalve molluscs may have lived to be over 150years old.
This same siltstone & mudstone can also be found in the northern suburbs along the coast cliff from Bulli to Coalcliff however the marine life is less present in these layers as Glossoptris (plants) fossils become more present.
Along the eastern Gondwana margin there was a volcanic ark that extended 1800 km from south of Sydney to Northern Queensland, this would indicate a more active tectonic region.
The volcanism lasted from around 300-265 Million years.
At Kiama during this time, lava flowed from an emergent island volcano south-east of the present coastline, this formed basaltic lava flows that overlay the red sandstone of the area. As the latite flows cooled, hexagonal basalt columns formed.
Examples of the Permian sedimentary layering.
Right: Thirroul - Glaciation and climate controlled sea levels which in turn influenced the patterns of sedimentation,
The annual layers are in couplets:
The Darker layers (siltstone) represent silt deposited during the summer thaw when the streams were flowing.
The lighter layers (mudstone) represent mud that settled during the winter when the area was frozen over.
With this knowledge, periods of extended warm or cool climate can be interpreted in the rock layers.
Left: Fossil deposits
Right: Bombo basalt latite over sandstone
Permian 275-248 million years ago.
Eventually temperatures increased and the sea became swampy delta forests that layed down over 38 million years vegetation faster then it could decompose, this 'peat' compacted down to become the coal seams (Illawarra coal measures) that fuels much of the historical coal industry of the area.
The trees of this period, mostly Glossopteris, a primitive gymnosperm (gymnosperm include conifers and have unprotected seeds) were not like the eucalyptus we see today but were seasonal and dropped their leaves in autumn, this has led to the assumption that Golossoptris may have contributed greatly to coal formations as in some locations the majority of coal is made up of Glossoptris leaves and the direct plant debris.
Observations indicate that peat is compacted by a factor of 10 in the formation of black coal. This means that a workable 1 meter seam of coal would require an original peat thickness of 10m. The accumulation of peat is a slow process in cold climates with 1mm per year being deposited, this means that it takes 1000 years to lay down 1m of peat. If we use the ratio of compaction (1:10) then 1m of coal could have taken 10,000 years to accumulate. According to Geoscience Australia the maximum thickest of the Illawarra Coal measure is 150m, such coal would have required some 1500m of peat and as you can work out would have taken 1,500,000 years to accumulate.
In 1897 a shaft was sunk in Birchgrove Sydney to reach economic coal, the seam was 864 m underground and took over 5 years to sink. This is an interesting insight into subsidence as the last visible outcrops of coal can be found around Coal Cliff, south of Sydney, with the seam being at sea level, this would indicate that north of Coal Cliff the direct Sydney basin has subsided considerably beginning in the Triassic.
If you get the chance to check out one of these coal seams, (like the ones that are visible along the Astinmer headland) you will note the cut-off at the top of the coal layer, it is at this period 80% of species on earth became extinct and was believed to have been caused by volcanic activity in Russia ('Siberian traps') + volcanic activity in China that lead to a global runaway green house effect.
The layer directly above the coal marks the end of the Permian & beginning of the Triassic or the Permian–Triassic extinction event.
At this time around Gerringong evidence of late Permian volcanic latite flows can also be found.
Left: Austinmer Right: Scarborough.
Austinmer offers one of the best views to the end of the Permian–Triassic extinction event.
Photo taken at Gerringong
It is interesting to note the differences in the strata around the end of the Permian period:
1) The coal seam of the north is not present along the coast in the south which may indicate that this region remained submerged or that the vegetation deposits may be present further inland.
2) The red soil at the top of the cliff is evidence of the volcanic activity that was present in this region at the end of the Permian.
Triassic 248-213 million years ago.
Above this coal layer we find a mixture of shale's, sandstones, claystone and conglomerates sedimentation deposit, formed from where a fast flowing stream flattens, slows, and spreads typically onto a flatter plain called an 'alluvial fan'.
Alluvial fans form around the edges of mountain ranges and are made up of sand and gravel that is shed from a rapidly eroding landscape. Fast flowing rivers transport this sediment through gorges delivering it onto fan shaped mounds.
Looking up at the escarpment of the Southern Highlands or the sandstone cliffs of Sydney & the Blue Mountains it is difficult to imagine where all of this sediment came from. To get a better idea of what occurred consider a low lying peatland basin like the ones found at the end of the Permian, as the temperature increased, melt water from the mountainous region in the now central western NSW shed sediments, as it does the fans grow, this sediment intrudes onto the peatland overlaying the peat.
Why then does the sandstone stretch from Newcastle in the north to Batemans Bay in the south, and west to the Blue Mountains towards Lithgow, and up until the edges of Mudgee in midwestern New South Wales? The answer to this is that it is typical for these fans to grow outward for a while, and then for the flow to switch to the side, forming another fan. Over time as the fan switched, in conjunction with river floodplains channels that can at any one time be 500-1000m wide, sedimentation was eventually deposited over such a large region. Old river channels are exposed in the freeway cuttings of north Sydney, around the harbour cliffs and the Three Sisters of the Blue Mountains. A modern example of this is the Brahmaputra river that drains the southern side of the Himalaya, these modern river deposits can be 100-200km across.
The beginning of the Triassic was a chaotic period for life on earth with it's impoverished biosphere and it would take well into the middle of the period for life to recover to its former diversity. Although sandstone fossils for this area are rare the ones that are discovered offer an insight into what it may have been like during this period. A few years ago a fossil collector found bone fragments amongst rocks that had been bulldozed for road works near Picton, after six years a near-complete one meter long fossil of a fresh water shark Xenacanth was reveled. Another example is of an large salamander amphibian that was discovered in a lump of sandstone that was to be used as a garden terrace in 1997. An interesting thing to keep in mind is that 240 million years ago there was no ice at the poles, warming was occurring at a time when the South Pole lay near Bourke in northern NSW, the climate had increased and the rivers, creeks, swamps and watering holes that transported vast amounts of sand & sediments offered habitats for all kinds of flora and fauna of the time. As temperature & CO2 fluctuations settled down a specialized subgroup of archosaurs (group of diapsid amniotes whose living representatives consist of modern birds and crocodilians) aka dinosaurs, first appeared in the mid-Triassic with a specialised ankle joint that enabled them to walk upright. Interesting to note that the first true mammals also evolved at this period
Left: Austinmer Right: Scarborough
The end of the Permian & beginning of the Triassic is clearly visible in the sea cliff at Austinmer.
Could the red layers be an indication of iron oxide from the volcanic activity in the south
The photo taken at Scarborough possible indicates that there was further vegetation deposits in this region in the Triassic
Jurassic 213-144 million years ago.
Around 200 million years ago in the Jurassic isolated volcanoes exploded up through the shale, coal & sandstone. Remains of around 95 volcanoes can be found in the area one of which is Mt Gibraltar, Bowral that pushed up 150 million years ago. Griffith Taylor and Douglas Mawson who mapped this area in 1903 thought the The Gib was the exposed core of a volcano. The appearance of microsyenite indicates that this intrusion took along time to cool which may indicate that it was connected to a heat source for some time. This rock was used as a building stone in Sydney, many city streets have kerbs of mircrosyenite which has resisted weathering for years.
The eastern edge of the Southern Highlands plateau uplifted along with the Blue Mountains around 70 million years ago & since eroded to its present height around 30 million years ago.
Cretaceous 144 -65 million years ago.
One of the earliest animal fossils in Asutralia is from this period. In 1984 a opalised platypus jaw was found in sediments at Lightning Ridge that were dated at 110 million years, this pushed our understanding of modern Australian fauna back further then the previous thought. It was during this time that the Australian continent finally separated from Antarctica at around 85 million years followed by the world wide extinction of dinosaurs 65 million years ago.
Volcanoes extended from the north some 32 Million years ago (Ma) to the south (0Ma), become younger as we move southward.
Orange: Lava Fields, erupted from fractures and spread over areas 100Km across.
Green: Current Hotspot Location
Cenozoic 65million years ago - Present Day
As Australia drifted north, a 'hotspot' underlying the mantle pushed up through the continental plate to form volcanoes and lava fields that extended down eastern Australia, parallelled offshore by two lines of volcanic seamounts (A seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface). As the continent drifted north the hotspot remained stationary pushing up though the crust every few million years. This volcanic chain extends from the north of Queensland to Tasmania, the volcanoes become younger as we move southward. The hotspot is currently off the north coast of Tasmania at what is know as the 'East Australian Hotspot' with it's sister parallel at the 'Tasmantid Hotspot'.
Lava flows believed to be around as thick as 60m covered most of the Southern Highlands, the result of passing over this hotspot some 10-8 Million years ago. The source of these flows is uncertain, but may have been from vents just east of The Gib or from Saddleback Mountain that is of similar age. The basalt from the flows that covers much of the highlands came from molten rock that was originally 2000 km underground. The oxidized iron in the Black Basalt weathers down to red soil, most noticeably seen in Robertson.
As Antarctica and Australia separated circumpolar currents began to cool, 30 million years ago the air and water temperatures cooled rapidly, by 15 million years ago much of Antarctica was glacial. Australia at this time began to dry out and there was an increase in plants typical of drier forests and woodlands, sea levels dropped to 100m below the present levels. At this time ice accumulated on the Southern Highlands until as recently as 20,000 years ago, leaving sculptured landscapes and deposits of till (ground rock sediments).
East-west rainfall gradient and differences in soil / erosion controlled the vegetation that ranged from rainforest's, woodlands, heaths to dryer sclerophyll forest.
The post-glacial rise in sea level between 18,000 and 6,000 years ago drowned the coastal plains and river valleys that extended to around 15km off the coast. Fronted by confined barrier systems of beaches, dunes and coastal lakes the coastal area accumulated organic matter and begin to develop coloured subsoils that can be seen today.
The local area is still evolving, the escarpment is being eroded with regular rock falls and denudation (processes that cause the wearing away of the earth's surface leading to a reduction in elevation) at about 2-5 meters per million years. Subsidence in the area occurs frequently with minor tremors being felf every 30 years or so. A magnitude 5.6 earthquake hit Robertson, Bowral, Moss Vales on the 21st of may 1961 and caused 4.1 million dollars in damage.
Tsunami are predicted to hit the eastern Australian coastline every 100 years or so due to the boundary of the Pacific Ocean, known as the Ring of Fire that experiences frequent earthquakes. Since the Pacific Tsunami Warning Centre was set up after 2004, 52 tsunami alerts have been issued that may indicate that it may be a more regular then previously thought.
That aside it is important to remember that things are changing, earth and the local geology is constantly evolving, being shaped by the various geological processes.
What will be the future of this area? As we head slowly north at a rate of 7cm a year we will eventually run into India, during our few hundred million year trip north the magnetic poles will flip from north to south numerous times, temperatures will increase which will mean at some point in the distant future this area will once again become tropical forests.
The escarpment will slowly erode to become a gentle slop down to the lower coastal edge, the seas will be higher and depending on how long temperatures continue to rise there will be drastic sea level rises, The IPCC (Intergovernmental Panel on Climate Change) Third Assessment Report (TAR) projected a sea level rise of 20 to 70 cm by 2100.
It is important with all the global warming hype to keep in mind that we are currently coming out of an ice age, and we still would have done so even if human did not exist. Current temperatures rises have been sped up by green house emission however it is important to remember that the world and earthlings before us have seen much, much worse and the current rise in temperatures is progressing slowly in comparison to the drastic changes that the earth has met with in the past. Our ice age may be ending but it will eventually come back, predicable not in the foreseeable future but at some point again earth will return to current temperatures, cool and then freeze, warm and cool again, this is as the geological record tells us the story of our planet. So enjoy the warm while it last and remember to pack your swimmers.