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Grand High Tops from Whitegum Lookout, Warrumbungle National Park

The effect of the mantle hotspot on Eastern Australia
Belougery Spire. Photo Susan Wright, Destination NSW
Grand High Tops. Photo Susan Wright, Destination NSW
Crater Bluff a 'classic' volcanic plug (or neck) of hard trachyte
The Breadknife. Photo Susan Wright, Destination NSW
Warrumbungle National Park

Warrumbungle National Park

Latitude -31.286731, Longitude 148.996909

Located in the North West Region of NSW, nearest large town Coonabarabran

Source: www.environment.nsw.gov.au/NationalParks/parkGeology.aspx?id=N0035

Source: www.environment.gov.au/cgi-bin/ahdb/search.pl?mode=place_detail;place_id=105853

Source: Wikipedia

Source: www.resources.nsw.gov.au/geological/heritage-maps

Link to Detailed Map

Aboriginal heritage

Archaeological evidence indicates that Indigenous people have occupied the Warrumbungles for at least the last 5000 years. The name 'Warrumbungle' comes from the Kamilaroi language, and is believed to mean 'crooked mountains'. This is an appropriate name describing the distinctive jagged skyline of the Warrumbungles, with its domes and spires separated by forested ridges and deep gorges and surrounded by plains and tablelands.


Warrumbungle National Park formation identification

Geology

The Warrumbungle National Park contains the best representation of exposed volcanic features like spires, plugs, domes, dykes, sills, lava-flows, tuff layers, and horizontal and vertical columns within the main north-south volcanic line in eastern Australia.

How the landscape was formed

The story of today's landscape began 180 million years ago when the region was covered by large shallow lakes. At the bottom of these lakes, sediment laid down during the Jurassic period (the age of the dinosaurs) was slowly compressed over millions of years to form sandstone. This bedrock is now known as 'Pilliga Sandstone' and it forms the base of the region today. Before the volcano the area was probably not unlike the Pilliga to the north - wooded, undulating to flat sandstone country cut by shallow valleys and creeks.

Although no volcanoes are currently active in Eastern Australia, a broad strip parallel to the coast from North Queensland to Tasmania, including the Warrumbungles, has been affected by volcanic activity over the past 70 million years. (see "The effect of the mantle hotspot on Eastern Australia" on this website under "General Geological Information"\).

The central volcanoes at Cape Hillsborough and the Glasshouse Mountains in Queensland, the Nandewar Ranges (Mt Kaputar National Park), the Warrumbungles and Mt Canobolas in New South Wales and Mt Macedon in Victoria get progressively younger as you travel south. This indicates that as the crustal plate carrying Australia travelled north under the influence of continental drift, it passed over a fixed hotspot in the earth's mantle, which periodically erupted forming mountains.

Initially, about 17 million years ago, thick trachyte lava welled up from a number of different vents over a wide area, which subsequently became blocked as the trachyte solidified. About 16 million years ago as the volcano aged, more fluid basalt lava flowed from new vents, alternating with ash and scoria explosions. The later flows filled the spaces between the earlier trachyte domes, and built a large cone rising approximately 1000 metres, with a diameter of nearly 50 kilometres. As time progressed the magma became less and less viscous, spreading further and further from the vents and forming thick trachyte flows such as Mt Exmouth and Siding Spring Mountain. In the final stages from 14 million years ago, the flows became thinner and longer and more basaltic.

Since the end of volcanism about 13 million years ago, erosion has cut through 90% of the volcanic pile, removing most of the later deposits and the softer rocks to expose the products of the early phases of the volcano. These are hard trachyte features, high in silica, and resistant to weathering and include domes, plugs and dykes some reaching heights of 700 metres. The Grand High Tops is a section of the range where volcanic remnants are especially clustered and pyroclastic rock is also found in this area.

Lava domes where molten rock has bubbled to the surface and clogged up the source vent can be seen at Belougery Split Rock and Crater Bluff. A spectacular trachyte dyke can be seen at The Breadknife and the largest lava dome of the Warrumbungles at Bluff Mountain which has a near-vertical 250 metre high face. There are also old lava flows at Mt Exmouth (highest point in Warrumbungles at 1,206 metres above sea level) and Siding Spring Mountain, just outside the park boundary.

Crater Bluff is a 'classic' volcanic plug (or neck) of hard trachyte. The plug formed below the surface from cooling magma which blocked the vent; pressure from below being insufficient to clear the solidifying mass. The resistant plug is prominent now as adjacent softer materials (tuff and breccias) have been removed by erosion, leaving the plug surrounded by a bowl-shaped depression. The rock making up Crater Bluff has been shown to be similar in composition to that of The Breadknife; the Crater Bluff magma mass thus most likely to have also formed The Breadknife.

Bluff Mountain is a lava dome, an igneous extrusive landform, built from trachyte lava which was viscous enough to pile up around the vent rather than flowing away. It is a single mass of trachyte, with somewhat coarser crystals than other nearby trachyte features - probably due to slower cooling. During formation the lava dome swelled and shed blocks of trachyte breccia from its outer cooling and expanding surface. This material piled up around the dome base giving scree slopes that formed a cover over much of the dome. Shrinkage joints are visible on parts of the mountain. Bluff Mountain offers marvellous views in all directions, including the many peaks of the park, the surrounding plains and distant ranges. Rock climbing is popular on the steep north face.

The Breadknife, a 90m high narrow rock wall, is a trachyte dyke. It was formed when magma was forced through a crack in the earth's surface and intruded vertically into volcanic tuff and breccia, then cooled and set into a hard, narrow mass with numerous contraction joints. Because the rock surrounding The Breadknife was softer then the solidified magma, erosion has left a long sliver of stone exposed which is hundreds of metres long and only several metres wide. The Breadknife and Grand High Tops hiking track offers excellent views, as well as the opportunity to study the contact surface with surrounding pyroclastic rocks. National Park rules prohibit climbing The Breadknife to protect both the landform and visitors.

Four types of trachytes with varying chemical compositions can be distinguished in the area. They can be distinguished by colour differences, which range from green to green-blue, to blue, to white. Later the chemical composition changed, resulting in eruption of hawaiites and trachyandesites, which are today found south-east of the park and surrounding Tonduron Mountain, which is just outside the boundary of the National Park.

The local topographic relief is considerable. The many peaks standing in excess of 1,000 metres above sea level contrast with the nearby plains to the west (maximum of 300 metres above sea level) and the hilly country to the east (about 600 metres above sea level). The rivers and creeks, which drain the Warrumbungle volcanic pile, form a radial pattern common to many volcanic landscapes. Quaternary deposits of unconsolidated sand and silt occur locally along most watercourses. The underlying Pilliga Sandstone is one of the main intake aquifers for the Great Australian (Artesian) Basin and marks the eastern margin of the Basin.

Fossils have been found in the area. Originally, lakes formed within the Warrumbungle volcano supported a high diversity of diatoms, which were then deposited in the lake floors. Animal and leaf remains have been found to the north-east of the park boundary at Bugaldie (Sutherland 1995).

Towards the southeast, a broad belt of basalt outcrops extends towards the Liverpool Range. Near Chalk Mountain are outcrops of diatomite.

Chalk Mountain Diatomite Mine

Diatomite (Diatomaceous Earth) is composed essentially of the siliceous cases or "frustules" of minute plants known as diatoms which inhabit fresh and salt water. It is a very light, porous substance, somewhat friable. Although it may look like chalk it is not, pure diatomite contains silica and water only but in most deposits it includes other impurities. Diatomite is essentially a siliceous substance in which the silica is of organic origin as distinct from "siliceous earths" in which the silica may be either detrital or precipitated from chemical solution.

Many deposits are lacustrine (deposited in a lake) and are associated closely with volcanic rocks which probably furnished silica to the water from which it was extracted by the diatoms and used to build their siliceous envelopes.

The Bugaldie Chalk Mine is one of the few mineral extractive industries located within the Warrumbungle Shire; the mine was also unique within New South Wales. The former workings of the Chalk Mine are situated towards the crown of a volcanic plug.

Mining of diatomite was established in the 1920s by Davis Gelatine. This company reclaimed diatomaceous earth from a site at the top of Rundle Mountain (Chalk Mountain) near Bugaldie. Diatomaceous earth is used in the chemical industry and in swimming pool filters. Access to the remains of the mine and quarries are on private land and cannot be visited by the public but you can see Mt Rundle about 4.75km directly west of Bugaldie which is located on the Barradine Road 25km north of Coonabarabran.

Getting There

The Warrumbungle National Park is approximately 500 kilometres north-west of Sydney and can be reached from the east via Coonabarabran, from the west via Coonamble and from the south through Gilgandra and Tooraweenah. The area is best accessed from the Newell Highway. Entry to the park is gained by travelling from Coonabarabran along Timor Road (entry fees apply). Look for the sign leading off Timor Road to the National Parks & Wildlife information centre where you can book camping sites and get information and maps for the nine walking tracks across the central peaks. There is plenty of accommodation available in Coonabarabran and its surrounds, but book early as sometimes there are events which use up much of the available accommodation.


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