How Old is the Blue Mountains

 

How Old Are They?

The story of the Blue Mountains begins some 300 million years ago.  This ancient rocks under the mountains are around  470 million years old.  The Sydney Basin  started to feel the layers and started to sink. The seas  then inundated the depression. 

Sand collected in the basin, which continued to subside. As the deeper beds were buried, they were forged into hard rocks by heat and pressure.

Above them, the first layers of sand formed sandstones about 200 m thick. (called  the Narrabeen Sandstone) Then the  sands that followed formed the Hawkesbury sandstones. They were about 300 m thick.

came into existence as a result of earth movements during the mid-Permian (270 million years ago) and for the next 70 million years, (Permian – Triassic) was subject to periodic episodes of marine transgressions and regressions which alternately inundated and exposed the developing basin. As a result the basin received large amounts of sediment from both land-based, (glacial, river and lake) and marine (near shore and deep sea) sources. Sedimentation was continuous throughout this time. Great depths of sand, mud and coarse pebbles, up to a maximum of 5000 metres, accumulated in some places and this is now seen in the depth of rock so well exposed along the coast and in cliffs of the Blue Mountains.

250 million years ago large rivers began dumping vast quantities of sand on top of the shales, burying them. Throughout this burial process, the weight of the accumulating sediments caused the layers to sink, creating a basin.  The process in the rock cycle which forms shale is compaction.

Sedimentation appears to have largely ceased by the end of the Triassic, 205 million years ago and since then episodes of volcanism, weathering and soil formation, uplift and valley enhancement have resulted in the topography we see today. As the current surface of the basin is an erosional one, this would indicate a long period of stable conditions have existed for some time.

The mountains were built by rivers. A movement in the earth meant that the landscape was flooded by a shallow sea from the east. Back in Gondwana times there once was a great inland sea.

Streams flowing into this sea carried huge amounts of sediment, which were deposited in horizontal layers. Later, these layers formed rock beds of shales, siltstones and mudstones. In swampy areas around the margins of the sea, piles of dead vegetation were buried under the sediment. They would eventually become seams of coal. All in all, about 500 m of marine sediments were laid down at this time - between 250 and 280 million years ago.

After the subsidence of the continental shelf, as a result of tectonic activity during the mid-Permian, 270mya  the developing Sydney Basin received massive amounts of sediment from the Lachlan Fold Belt to the west,  along with  conglomerates and ash from waning volcanism in the south.

Marine sediments, including thick sequences of sands and muds were deposited in a transgressive sequence from the south east as the sea inundated the slowly subsiding basin. These sequences now comprise the Shoalhaven Group which are particularly well exposed along the headlands and rock platforms of southern beaches such as Wasp Head, Pebbly Beach and Berrara Beach. The marine sequence is not extensive in the north as it took up to 20 million years for the sea to flood across the basin from the south east.

About 170 million years ago, the sands stopped being deposited. Forces in the earth started pushing the rock strata upwards. The hard rock layers on the bottom bent and flexed, but the sandstone above them fractured into a series of vertical cracks called joints.

Eventually, the rock layers rose into a broad plateau (and they may still be rising). The plateau was highest on its western edge, and sloped down to an abrupt downturn at its eastern edge. You can see this today, in the low escarpment just west of Penrith and the Nepean River.

By 225MYA ( late Permian) large rivers flowing out of the rising New England Fold Belt to the north and the Lachlan Fold Belt to the west, deposited vast amounts of sediment in the northern and western edges of the basin.

These rivers built up extensive flood plains and deltas, particularly in the north, in which the coal forming plant material was being laid down. Eventually the build up of sediment pushed the sea out of the basin during a gradual marine regression and vast amounts of terrestrial sediment were laid down.

The trees that had flourished in the warm climate only lived 30-40 years and when they died, along with the other vegetation  formed the layers of coal

Fern fossil in coal.

Coal typically forms on land from vegetation in lowland, swampy, mire environments. Stagnant waterlogged soil prevents the accumulated plant debris from breaking down. The recognisable remains of plants are often visible within coals and associated shales, confirming their plant-origin. The picture above shows a piece of coal containing a network of fossilised fern leaves - clear evidence that it was formed from vegetable remains.

The accumulated plant debris initially forms a material known as peat. The geological processes of burial beneath later sediment and alteration by heat and pressure convert the peat to coal; a process known as coalification.

For the peat to become coal, it must be buried by sediment. Burial compacts the peat and, consequently, much water is squeezed out during the first stages of burial. Continued burial and the addition of heat and time cause the complex hydrocarbon compounds in the peat to break down and alter in a variety of ways. The gaseous alteration products (methane is one) are typically expelled from the deposit, and the deposit becomes more and more carbon-rich as the other elements disperse. The stages of this trend proceed from plant debris through peat, lignite, sub-bituminous coal, bituminous coal, anthracite coal to graphite (a pure carbon mineral). Source: www.uky.edu   .http://www.discoveringfossils.co.uk/fossilfuels.htm

 

 These sediments are now seen as the Coal Measures which comprise great depths of sandstone, mudstone, shale and the economically important coal seams which are mined in such places as Lithgow, Newcastle and Wollongong.

As the flanking mountains were worn down by erosion the amount of sediment entering the basin declined and several periods of localised marine transgressions and regressions occurred resulting in a complicated mix of terrestrial and marine sediments.

The uplift wasn't necessarily a calm, gradual affair. It featured some dramatic volcanic activity, probably starting around 150 million years ago. A number of volcanic necks, called diatremes, flowed up through the cracks in the sandstone and shale. Then, more recently, basalt lava poured from vents and spread over the landscape.

By analysing the radioactive minerals in this basalt rock, geologists have found that some of these flows are around 17 million years old.

When you look at the Blue Mountains, you're looking at the plateau created by the uplift 170 million years ago. The reason they look like mountains is that the plateau has been dissected. Deep valleys and gorges have been cut into it, leaving stubborn peaks behind.

So what did the dissecting?

Weather was partly responsible - the effects of wind, rain, and heating and cooling. But gravity takes the most credit for this spectacular landscape. Water must always travel downhill, forming rivers as it does so. These rivers, which look absurdly small from the cliff-top lookouts, have carved out the magnificent valleys they meander through.

But this hasn't simply been a case of water gradually wearing its way down through the layers of rock - otherwise the mountains would be rounded, not chiselled.

Sandstone is relatively resistant to erosion, but the shales and coals underneath it are much softer. These lower layers wear back relatively easily, undermining the sandstone.

Weakened by vertical joints and the softer layers of shale within them, massive blocks of sandstone break off and topple down the slopes. Sheer cliffs are left behind, gradually retreating with each dislodged block.

Most of our canyons are very recent additions to the landscape. In most cases, the creation of canyons coincides with the departure of glaciers from their valleys.

As the ice began to melt, it not only released ancestral river beds from their icy prison, but it also released immense amounts of water.

So much water was released that the rivers were provided with an incredible erosional potential. This extra water supply allowed them to rapidly alter their channels, and the down-cutting may have begun almost instantly.

As the glaciers largely, eventually the initial deluge subsided, and the rate of erosion slowed accordingly, but didn’t stop.

Most of our canyons continue to deepen even today.

While you stand at the top of  of the escarpment of the Blue Mountains  the valley floor is slowly being etched and scoured. The rate may have slowed, but the process is unrelenting.

THE GREAT DIVIDING RANGE

Australia is a very flat continent where the average elevation is just 330 metres, the lowest in the world. What Australia lacks in height is more that made up for in the variety, geological age and unique appearance of its mountains and rocky outcrops - some of the oldest and most interesting exposed rocks in the world.

The Great Dividing Range has it origins many millions of years ago when the continents of earth were fused together as the Gondwana land mass. A huge uplift in the earth's crust occurred over millions of years, (between 5.4 million to 10,000 years ago) during the Pliocene and the Pleistocene Epochs. This was just after the extinction of the dinosaurs and during the time that modern humans first appeared.

The Great Dividing Range, or the Eastern Highlands, is Australia's most substantial mountain range and the third longest in the world.[2] The range stretches more than 3,500 kilometres (2,175 mi) from Dauan Island off the northeastern tip of Queensland, running the entire length of the eastern coastline through New South Wales, then into Victoria and turning west, before finally fading into the central plain at the Grampians in western Victoria. The width of the range varies from about 160 km (100 mi) to over 300 km (190 mi).[3]

The Dividing Range does not consist of a single mountain range. It consists of a complex of mountain ranges, plateaus, upland areas and escarpments with an ancient and complex geological history.

The highest  mountain in the range is Mount Kosciuszko  at 2,228 metres above sea level.

The crest of the range is defined by the watershed or boundary between the drainage basins of rivers which drain directly eastward into the Pacific Ocean

The Great Dividing Range was formed during the Carboniferous period—some 300 million years ago—when Australia collided with what is now parts of South America and New Zealand. The range has experienced significant erosion since.

Prior to white settlement the ranges were home to Aboriginal tribes. Evidence remains in some places of their occupation by decorated caves, campsites and trails used to travel between the coastal and inland regions.

 

 
 
 Source:
he fine particles that compose shale can remain suspended in water long after the larger and denser particles of sand have deposited. Shales are typically deposited in very slow moving water and are often found in lakes and lagoonal deposits, in river deltas, on floodplains and offshore from beach sands. They can also be deposited on the continental shelf, in relatively deep, quiet water. This process could have taken millions of years to complete.
http://www.uprct.nsw.gov.au/HTML/Info%20Sheets/Catchment/C3%20-%20Geology%20Overview.htm

 

The Sydney Basin came into existence as a result of earth movements during the mid-Permian (270 million years ago) and for the next 70 million years, (Permian – Triassic) was subject to periodic episodes of marine transgressions and regressions which alternately inundated and exposed the developing basin. As a result the basin received large amounts of sediment from both land-based, (glacial, river and lake) and marine (near shore and deep sea) sources. Sedimentation was continuous throughout this time. Great depths of sand, mud and coarse pebbles, up to a maximum of 5000 metres, accumulated in some places and this is now seen in the depth of rock so well exposed along the coast and in cliffs of the Blue Mountains.

Sedimentation appears to have largely ceased by the end of the Triassic, 205 million years ago and since then episodes of volcanism, weathering and soil formation, uplift and valley enhancement have resulted in the topography we see today. As the current surface of the basin is an erosional one, this would indicate a long period of stable conditions have existed for some time.

Depositional History of the Sydney Basin.

After the subsidence of the continental shelf, as a result of tectonic activity during the mid-Permian, the developing Sydney Basin received massive amounts of sediment from the Lachlan Fold Belt to the west, including thick fluvio-glacial conglomerates plus ash from waning volcanism in the south. Marine sediments, including thick sequences of sands and muds were deposited in a transgressive sequence from the south east as the sea inundated the slowly subsiding basin. These sequences now comprise the Shoalhaven Group which are particularly well exposed along the headlands and rock platforms of southern beaches such as Wasp Head, Pebbly Beach and Berrara Beach. The marine sequence is not extensive in the north as it took up to 20 million years for the sea to flood across the basin from the south east.

By the late Permian, (225Ma) large rivers flowing out of the rising New England Fold Belt to the north and the Lachlan Fold Belt to the west, deposited vast amounts of sediment in the northern and western edges of the basin. These rivers built up extensive flood plains and deltas, particularly in the north, in which the coal forming plant material was being laid down. Eventually the build up of sediment pushed the sea out of the basin during a gradual marine regression and vast amounts of terrestrial sediment were laid down. These sediments are now seen as the Coal Measures which comprise great depths of sandstone, mudstone, shale and the economically important coal seams which are mined in such places as Lithgow, Newcastle and Wollongong. As the flanking mountains were worn down by erosion the amount of sediment entering the basin declined and several periods of localised marine transgressions and regressions occurred resulting in a complicated mix of terrestrial and marine sediments.

Similarly the Triassic sedimentary episode is quite complex as a variety of lithological and palaeoenvironmental facies developed. Renewed uplift in the New England Fold Belt meant that large streams again flowed from the north, covering the basin in a complex mix of alluvial deposits – sand, mud, pebbles and gravel – which now form the Narrabeen Sandstones. Outcrops can be seen at Narrabeen, Long Reef and in the cliffs of the Blue Mountains. These sediments are mainly of fluvial derivation; deposits laid down in meandering streams, flood plains, deltas and swamps. Soil profiles developed in the Triassic suggest that the area was exposed to the atmosphere for long periods of time.

A minor tectonic episode, resulting in the uplift of the generally cratonised Lachlan Fold Belt in the south west lead to a major shift in the palaeocurrent direction in the mid Triassic, (225Ma). Coarse quartz sediment derived from the Lachlan Fold Belt was carried north eastwards across the basin in energetic, braided streams. This formed a sand sheet many metres thick which is now exposed as the Hawkesbury Sandstones at North and South Head and in cuttings along the Sydney-Newcastle freeway, particularly near the Hawkesbury River and Mooni Mooni Bridges.

The last depositional stage in the history of the basin occurred during the mid-late Triassic, (210 Ma) as meandering streams flowed once again from the north-west, depositing muds and fine sands in a single major marine regression. Lagoon, levee, peat marsh and flood plain deposits were common throughout this period. These now form the residual Wianamatta Shales exposed in isolated pockets in the centre of the basin, such as on parts of the Cumberland Plain in the Upper Parramatta River Catchment. The majority of these deposits have been removed by erosion in the 200 million years between deposition and the present.

Post-depositional History of the Sydney Basin.

During the Jurassic explosive volcanic events occurred which resulted in the formation of diatremes or breccia pipes such as those at Hornsby, Wallgrove and Prospect.

Other unrelated episodes of igneous activity resulted in the basalt cappings of some heights in the Blue Mountains, eg Mt. Tomah and the intrusion of dolerite dykes into the Narrabeen and Hawkesbury Sandstones now exposed along the coast. These intrusions were controlled by the very dominant jointing in the rocks and generally trend NW-SE.

The Blue Mountains, Hornsby and Woronora Plateaux developed during the Kosciusko Uplift maybe 60 MA, with warping along the Lapstone Monocline. This tectonic activity could well be related to the opening of the Tasman Sea and associated basaltic activity 60 – 80 Ma.

As the plateaux rose, streams such as the prior-Nepean and Cox’s and Parramatta Rivers became deeply entrenched in their valleys as down-cutting kept pace with uplift. The plateaux are now deeply dissected by these streams.

Coastal geomorphology is dominated by beaches, steep cliffs and drowned valleys, (rias) which have been influenced by changes in sea level throughout the last million years. The lowering of sea level resulted in the deposition of thick layers of sand which now form our beaches and sand dunes. The rise in sea level in more recent times has produced characteristic drowned river valleys such as Broken Bay and Sydney Harbour, enhanced the development of coastal lagoons such as Dee Why, Queenscliffe and Narrabeen and rock platforms such as Long Reef.

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