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Geology

Geology of the Atherton Tablelands

The Atherton Tablelands extend from Kuranda to Ravenshoe, taking in Malanda & Millaa Millaa also. To the east is the Great Escarpment which descends to the coastal plain. To the west lies the Great Divide. The Tablelands increase in height southwards from 500 to 600m above sea level at Mareeba, 700-800m at Atherton and 900-1000m at Ravenshoe.

The Escarpment and the Divide extend the length of Eastern Australia. The Divide is the watershed which separates rivers that flow east to the coast, from those which flow west, and in North Queensland, northwest to the Gulf of Carpentaria. On the Tablelands, the Barron, Johnstone, Mulgrave, Russell, Tully and Herbert (Wild River) Rivers flow east. These rivers are shallow on the Tablelands, deep incises in the escarpment and then meandering across the coastal plain to the sea. The Mitchell and Walsh Rivers flow to the Gulf.

The geology of the modern landscape of the Tablelands has two main components:

1. Old rocks including granites, commonly mineralised. The first rocks formed are sedimentary rocks which were then buried and altered so that they are now folded into steeply dipping beds. These include the limestones at Chillagoe.

2. Basalts erupted upon this older bedrock. Granites intruded with eruption of rhyolitic lavas. Granite and rhyolite are derived from similar magmas, rich in quartz. The granite crystallises deep within the earth's crust, at least 5km depth, whereas the rhyolite is the same magma erupted to the surface. Granite is very coarse grained because it cools slowly, while the rhyolite is fine grained because it cools quickly.

Mineralisation fluids circulating during metamorphism, as well as through and around the granite intrusions have introduced metals into fractures and cavities. The main ores are gold, copper and zinc in the Hodgkinson rocks, and tin and wolfram in the granites.

Basalts come from the earth's mantle, about 35km down. The magma rises through the earth's crust and is erupted at a temperature around 1200-1300 degrees celcius through volcanoes. The Tablelands lavas flowed down creek and through river valleys, eventually filling them as it cooled. Most individual basalt flows are only a few metres thick so much of the Atherton Tableland basalt deposits consist of many flows. There are two quite separate styles of volcanism on the Tablelands.

VOLCANO TYPES

Shield Volcanoes - major volcanoes rising well above the surrounding landscape forming low angle slopes, extending 5 - 15kms from the vent. These volcanoes produced massive quantities of lava which spread out across the old topography, filling valleys and producing the rolling landscape of the Atherton Tablelands. Subsequent weathering has changed the dark black basalt rock to red, clay soils (marvellous agricultural soils)and erosion has cut deep gullies into the sides of the volcanoes. Examples: Bones Knob Volcano, Malanda Volcano near Meeragallen turnoff, and Glendinning (Windy Hill)

Cinder Cones and Maars - small volcanoes forming conical hills typically 40 - 80m high, although Mt Quincan is 170m, and 0.5 - 1km across. These vents produced very little lava. Instead there were ash and cinders, such as the material quarried at Quincan Quarry. The maars, such as Bromfield Swamp, are vents located where there was abundant groundwater, and were probably filled with hot mud pools, not a fiery vent. A maar is typically broader with a lower profile than the steep, conical cinder cones. These small volcanoes remain in four forms: as conical hills (The Pinnacles), as cinder cones with a significant vent (Mt Quincan), as crater lakes (Lake Barrine & Lake Eacham), or a swamp with sediment and peat (Bromfield Swamp and Lynch's Crater).  These cinder cones and maars are much younger than the shield volcanoes. They have minimal soil development with little erosion. These features indicate a much younger age, compared to the older shield volcanoes which have been deeply eroded and weathered to red clay soils.

The Mt Hypipamee Crater is a very unusual vent as it appears to be due to a massive gas explosion which blew pieces of granite throughout the forest, but very little lava was produced. However it is important because it gives a view of what must be under every volcano - somehow all the basalt had to rise from the mantle to the surface through a vertical pipe cut through the granitic country rocks.

GEOMORPHOLOGY

The heavily forested ranges north and east of the Tablelands are the rugged residuals of granite intrusions. Bartle Frere, Queenslands highest peak at 1622m, along with the Lamb and Bellenden Ker Ranges are characteristic of the resistant nature of granite. Mt Lewis is of the same origin. Around the base of the Bellenden Ker Range and along the eastern edge of the Tablelands, we see deeply eroded river valleys. These valleys flow through soft metamorphosed sediments known as the Barron River Metamorphics. Hard acid volcanics such as rhyolite have produced the southern and western hills. These rugged hills erode in a different pattern to the granites because of their being layered flows.

As the basalts are the youngest of the four main rock types they are the least eroded. The older flows south and east of Malanda being the most heavily dissected while the central and northern Tablelands retain much of the character which they possessed when vulcanism ceased. Overlying the older lava flows are thick ash layers which are responsible for the deep soil profiles.

A LOCAL PERSPECTIVE

The vegetation of north east Queensland consists of small pockets of rainforest, varying greatly in response to altitude, rainfall and soil parent material, surrounded by an expanse of open eucalypt woodland. The geographic area of the Atherton Tableland is a plateau complex bounded by the Great Dividing Range in the west and the escarpment in the east. From the northern end at 400 metres above sea level, the Tableland rises steadily to 1000m in the south. The Barron, Johnstone, Russell, Mulgrave and Herbert Rivers drain the area. The natural vegetation of the Tablelands consist mostly of rainforests. On the Tableland landforms and their associated vegetation can be rationalised by consideration of the four main rock types.

ROCK TYPES

Igneous Rock - rock which has solidified from molten or partially moten material

Plutonic - originating at great depth

Granite - a coarse grained igneous rock composed of approx 20% quartz and feldspar of which plagioclase and alkali feldspar are present in approximately equal amounts

Rhyolite - a fine grained, acidic volcanic rock compositionally equivalent to granite

Ignimbrite - a poorly sorted, pyroclastic rock of acidic origin, comprising mainly pumice and ash, possibly with broken phenocrysts and dismembered wall material of large volume

Pumice - a light coloured, highly vesiculated, acid volcanic glass with a low density

Phenocrysts - a large mineral grain within a fine grained matrix of an igneous rock, formed before the matrix has solidified

Magma - a melt, generally containing suspended crystals and dissolved gases or volatiles, formed by total or partial melting of solid crustal or mantle rocks

Lava - molten rock material on the surface and solidified magma below the surface

Country Rock (host rock) - the rock into which magma or mineralisation is intruded or emplaced

Volcano - a location where magma and volatiles issue through the crust and accumulate

Volcanic Rock - a rock formed by the solidification of lava or pyroclastic material

Pyroclastic Rock - a rock formed by the accumulation of material generated by the explosive fragmentation of magma and/or existing solid rock during a volcanic eruption

Metamorphic Rock - a rock which results from the partial or complete recrystallisation in the solid state under temperature and pressure conditions elevated with respect to the surface\

Meta-Sediment - sedimentary rocks that have undergone some metamorphism but insufficient to obliterate their sedimentary appearance

Diatreme - a vertical pipe or funnel shaped igneous intrusion, 200-2000m thick and up to 2km deep, made up of a chaotic breccia of blocks of country rock and magmatic material passing down into a dyke. A forceful intrusion of a mixture of mafic magma, volcanic gases and accidental lithic blocks and clasts

DIFFERENT TYPES OF PYROCLASTIC VOLCANOES - (pyro = of, or from fire, clastic = broken)

Maars, tuff rings, tuff cones, scoria cones (cinder), and spatter cones. They are often found along fissures or in clusters.

A Maar is a type of tuff ring in which the centre of the crater has been affected by down faulting or sagging, so that it lies below the surrounding ground surface. Depending on the initial and subsequently changing drainage factors for surface water in the crater, a lake may form, a swamp develop, or the crater is left dry. Erosion will gradually increase the craters diameter and decrease its depth.  The wall of the crater collapses, partially refilling the vent. Of the rim, only the part with the steeply outwardly dipping layers remains. The conduit filled with pyroclastic debris is called a diatreme. Magma explosively interacts with the water. The water is superheated and changes from liquid form into steam. The explosive expansion causes stress on the surrounding country rock. When steam pressure is greater than the tensile strength of the country rock, a crater is blasted out in a series of explosions. The debris of country rock is deposited as a rim around the crater. Rising magma penetrates country rock and encounters a body of water saturated rock, sediments or soil in great depth. On the Atherton Tablelands, such aquifers can be found where deeply weathered basalt from older lava flows overlies granite and metamorphic rocks.

A Tuff is a volcanic sediment

Tephra is the pyroclastic material which was explosively fragmented during a volcanic eruption, comprising ash sized glass shards, pumice (from acidic eruptions), scoria and lithic clasts and magmatic crystals. Formed by the exposive decompression of magma on reaching the surface as dissolved volcanic gases rapidly exsolve (strombolian eruption)

Tuff ring is a monogenetic volcano, of any composition but often of basalt, formed in a phreatomagmatic eruption, with a low rim and a broad, flat crater. Comprised of phreatic air fall breccias and finely bedded air fall and pyroclastic surge beds, the maximum dip of the tephra being 3 - 12 degrees.

A Tuff cone is a structure similar to a tuff ring, but generally larger and steeper sided, with a maximum thickness of 100-350 metres, and a maximum dip of 24 - 30 degrees, and with a higher proportion of mudflow deposits.

Scoria is a rusty red to black, highly vesicular, mafic (ferromagnesian) volcanic glass

Scoria (cinder) cones are accumulation of scoria close to a volcanic vent resulting from a strombolian eruption

Spatter are agglutinated masses of primary ejecta, larger than lapilli, which were erupted as fluids

Spatter cones are low mounds, small to 15m high, formed by fountains of basaltic lava. Mainly composes of spatter but also including bombs and cinder

How can the grain size or pyroclastica be distinguished?? Up to 2mm = ashes, 2 - 63mm = lapilli, greater than 63mm = bombs and blocks

Stratovolcano (composite volcano) is a volcan comprising of alternating layers from long tephra (ash eruptions) and short lava flows of dominantly andesitic composition increasing in thickness towards a central vent. There are no stratovolcanoes on the Tablelands.

FORMATIONS AND EXAMPLES

Lava columns are a result of a slow cooling process of a liquid lava body. In a 90 degree angle to the cooling surface pentagonal (five sided) shrinkage cracks develop from the cooling surfaces into the body of lava or pyroclastic material. These cooling surfaces usually are the top and the bottom of the lava flow. Both columnades can meet in the middle or a third, chaotic columnade is formed between them. Fast cooling produces a small diameter of the lava columns, slow cooling a larger diameter. Lava columns sometimes develop shrinkage cracks also horizontally. This is visible at Millstream Falls downstream and on the other side of the valley.

Vesicles in dense, heavy basaltic rocks - Any gases and internal steam within the magmatic composition tend to form bubbles and migrate to the top of the lava flow. This is why the top layers of a coherent lava flow often show a great vesiculation. If these vesicles are partly or fully filled with low temperature minerals, eg: calcite or quartz, the rock is called amygdaloidal basalt. Gas bubbles can also be found on the bottom of the lava flow, where small amounts of water from the ground surface or within the soil is trapped as steam. Where vesicles are not round but elongated, this indicates a deformation of the bubble from flow movement in the last stages of cooling.

Basalt contains a high proportion of iron, the iron particles then oxidise to rust, forming the red colour of the basalitc soil.

Examples of shield volcanoes are: Halloran Hill, Bones Knob, Lamins Hill, Malanda Volcano & Jensenvale Volcano.

Examples of Maars are: Lake Eacham, Lake Barrine, Lake Euramoo, Lynch's Crater, Strenekoff Crater, Homenko's Crater, Boots Crater, Gillies Crater and Bromfield Swamp.

Examples for volcanic cones are: Seven Sisters, Mt Quincan, Wongabel Volcanoes, Hillview Crater (Bethels Hill), Mt Putt, Tobacco Hill and Mt Weerimba.

Basaltic lavas, such as on the Tablelands, can erupt quietly as coherend flows from small boccas or openings, or from the overspill or breaching of a lava lake ponded in a crater or fire fountains of lava that reconstitute around the vent and then flow away. The products are shield volcanoes and flood basalts with long lava flows. They can also erupt explosively as pyroclastic fall deposits - the products being maars, scoria cones, tuff cones or tuff rings.