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Rocks and Landscapes of the Boonah District Warwick Willmott The Fassifern - the valley of the plugs For those who know the Fassifern Valley its scenery is unforgettable. Numerous dramatic peaks stud a broad fertile valley, with steep forested ranges to the west, south and east. How did these peaks come to be here, and what are they made of? How does their geology and that of their surroundings influence human activities today? The peaks were once plugs of hard volcanic magma that filled subsidiary vents or fractures beneath the flanks of two ancient volcanoes. The ranges to the west and south are remnants of these volcanoes. The plugs were only exposed by the later erosion of surrounding rocks. In contrast, the floor of the valley is composed of much older, easily eroded sedimentary rocks. This ancient history is best visualised by going back to the origins of the oldest rocks of the valley, and then working forward in time. 1. EARLY HISTORY – SEDIMENTS DEPOSITED ON RIVER PLAINS The oldest rocks exposed in the valley once were sediments laid down in a broad subsiding area – the Clarence-Moreton Basin - in the early Jurassic period (~205 -145 Million years ago). This was part of a much larger subsiding area, the Great Australian Superbasin, or the Great Artesian Basin. This remarkable subsidence was related to the previous history of the eastern edge of the continent. Before about 210 million years ago the eastern edge was in compression, being affected, and heated, by ‘subduction’ (or pushing) of an oceanic crustal plate to the east beneath the continent. Sinking of the Great Australian Superbasin behind the continental edge After about 210 million years ago the subduction ceased, the heavy oceanic plate slab beneath the continent sank, the continental edge relaxed, the continental crust cooled and hence contracted, and as a result the continent behind gradually sagged. A. The first sediments washed in were gravels and coarse sands, deposited by very active rivers on broad plains – these have been compacted and hardened to conglomerate and sandstone of the Woogaroo Subgroup. In this district they are exposed only along part of Boonah-Beaudesert Road where they are ‘folded’ up from beneath younger rocks. B. The next sediments were also sands, but of softer types, and they were mixed with silts and muds that were deposited on less active parts of the river plains. These are now the Marburg Subgroup, and they are well exposed in new cuttings along the Boonah-Beaudesert Road. Both sandstone units are relatively resistant to erosion, and form the hilly range defining the eastern side of valley. They give sandy, relatively infertile soils, and much of their outcrop has remained under forest cover until recently. C. Thirdly, as the rate of subsidence of the Basin slowed, streams on the river plains became sluggish and swamps developed. Impure sands, silts, muds and coal were deposited to form the Walloon Coal Measures. These rocks are soft, weather deeply, and are poorly exposed, only clearly seen in road cuttings at Maroon and west of Mount Alford. They underlie much of the valley and give subdued topography with clayey, moderately fertile soils. Some time late in the history of the basin, rocks of the Clarence-Moreton Basin were gently warped, or folded. The older sandstones were warped up beneath the range between Boonah and Beaudesert, and the Walloon Coal Measures were warped down on either side in the Fassifern and Beaudesert valleys (they were once on top of the intervening range also, but have been eroded off there). As a result the strata in the Fassifern Valley slope at gentle or moderate angles to the west. Cross- section of the sedimentary strata of the Fassifern and Beaudesert valleys 2. A LATER SHORT INTERLUDE About 60 to 50 million years ago, in the early Tertiary period, several narrow basins subsided down the length of southern Queensland. This was possibly related to tension in the crust as the Tasman and Coral Seas began to open up to the east. They were filled by eroded silts and muds, and some basalt lavas A small remnant of one basin outcrops south of Peak Crossing. Basalt lavas were erupted initially, and then a small lake flooded the basalt terrain. Impure limestone rich in magnesium (derived from weathering of basalt) was deposited, and this has been mined for agricultural dolomite (calcium magnesium carbonate). The limestone resists erosion, and forms a ridge south of Peak Crossing. 3. THE TRANSFORMING EVENT FOR THE VALLEY A. Eruption of two major volcanoes Between 34 and 6 million years ago in the later Tertiary period the eastern part of the Australian continent drifted northwards over a ‘hot-spot’ in the Earth’s mantle, deep below the crust. There basalt lavas were periodically generated, and erupted through weaknesses in the crust to build up many, sometimes overlapping, broad volcanoes. The volcanoes erupted progressively as the crust moved northwards over the stationary ‘hot-spot’, and as a result the oldest volcanoes are in the north, and the youngest in the south. Southeast Queensland moved over the hot-spot between 30 and 23 million years ago, when it appears to have been the most active. In the Fassifern district the Main Range Volcano erupted probably from a line of craters in the centre of the valley around 24 million years ago. It was built up by numerous basalt lava flows, but there were also some flows of trachyte that are more resistant to erosion and cliff-forming. The lavas originally extended right across the valley to the eastern sandstone range, where some small remnants remain. The Focal Peak Volcano erupted slightly later from a centre just to the west of Mount Barney, and also contributed lavas to the north and west (the junction between the two volcanoes is not clear). B. Last gasps of the Main Range Volcano – plugs, dykes, sills, intrusions etc Towards the end of many volcanoes, magma can be intruded within, and beneath the lava pile, in bodies of various shapes and sizes, and these were especially common beneath the Main Range Volcano. Probably most did not reach the surface, but a few may have filled subsidiary vents on the volcano’s flanks. Flinders Peak may have reached the surface, as its summit is composed of broken rock which may be the result of volcanic explosions. These intrusions are composed of a great variety of rock types, which are������������������������������������������� thought to have been ‘distilled off’ from the basalt in a deep magma chamber, although some may have come from contamination by surrounding rocks. Unfortunately deep weathering and poor ����������������� exposure of many of the less resistant rock types makes identifying and naming them in the field problematic. Most obvious are vertical plugs of hard rhyolite and trachyte, which were the end products of such ‘distillation’. They are resistant to erosion, and remain as peaks after the surrounding soft rocks are eroded away (such as the peaks around Flinders Peak). Sub-horizontal sheets or ‘sills’ of these rocks were also intruded between the strata of the Walloon Coal Measures, and these give flat resistant peaks such as Mount French. A circular fracture filled with rhyolite has formed the ‘ring-dyke’ of Minto Crags. Some bodies of trachyandesite are also semi- resistant, forming high rounded hills. Digital elevation model of three eroded ‘hot-spot’ volcanoes of southeast Queensland by Ben Cohen from NASA SRTM data More significant in volume were intrusive bodies of other rock types, such as micro-syenite, micro-diorite, basalt and dolerite. These usually are more easily eroded, outcrop poorly, and form lower, rounded hills, although they do vary in prominence. Such intrusions underlie a large part of the valley - some are large rounded bodies, whilst others are thin sills and dykes. These rocks contain many dark minerals rich in calcium, magnesium, and iron, and weather to deeper, reddish brown soils of reasonable fertility. They have been poorly recognised previously, but they contribute significantly to the fertility of the valley. C. Last gasps of the Focal Peak Volcano The history of the Focal Peak Volcano is complex and beyond the scope of this leaflet (see the book Rocks and Landscapes of the National Parks of Southern Queensland) However, to summarise, the volcano initially erupted basalt lavas from a centre over the present Focal Peak, just west of Mount Barney, and then the magma changed to rhyolite, with some rhyolite lavas erupted from Mount Gillies to the east. Towards its end, a large body of rhyolite magma was thrust upwards to cool slowly to the resistant granophyre of the present Mount Barney. Large thick ‘sills’ of rhyolite magma were also intruded in a circular pattern around the central mountain beneath Mounts Ernest, Maroon, May, Phillip and Minnages Mountain. 4. Forming THE LANDSCAPE The lavas of the two volcanoes originally extended right across the valley to the eastern sandstone ranges (where some small remnants remain), but since then large parts of the volcanoes have been eroded away. The eastern and central parts of the Main Range Volcano have been almost entirely removed, leaving only the western side, which now forms the present Main Range. Mount Walker to the north is the only vestige of the central part of this ancient mountain. This process has happened because the streams draining the eastern flanks of the volcano were steeper and more active than those on the west. Once these eroded down through the lavas to expose the soft Walloon Coal Measures beneath, they undercut the basalt lavas above by development of a steep escarpment, which retreated to the west. This escarpment can still be seen today along the Main Range, particularly in The Ramparts north of Cunninghams Gap, and its continuing retreat can be clearly seen at Teviot Falls, where the basalt lavas are being undercut and collapsing in the cliff face. Once erosion removed the basalt lavas and carved into the soft Walloon Coal Measures, the bodies of magma intruded beneath the two volcanoes were exposed. Those resistant to erosion such as rhyolite and trachyte remained standing as sharp peaks or flatter mountains (depending on their shape), while other types eroded more deeply to gently rolling terrain amongst the sedimentary rocks. Progressive erosion of the Main Range Volcano b)after erosion a) on intrusion Erosion uncovers plugs, sills & dykes intruded beneath the flank of the Main Range Volcano The escarpment on the western side of the Main Range Volcano continuing to retreat. These are The Ramparts on the northern side of Cunninghams Gap, where landslides occured in 2011 and 2013 Ben Cohen 5. INDIVIDUAL PEAKS The prominent peaks of the valley with few exceptions are composed of resistant rhyolite and trachyte. Mount Greville and Mount Moon were probably domes of rhyolite magma intruded beneath the surface rather than vent fillings. Mount Edwards was a similar body of trachyte. During its erosion it has been carved into two parts by Reynolds Creek gradully cutting down from a higher level. The group of peaks around Flinders Peak were intrusive bodies, dykes and sills of trachytes of slightly different colour, grain-size and mineral content. Flinders Peak itself is a compound intrusion of various trachytes, cut by a thick dyke on the northen side. Towards the top fragmental debris of a volcanic breccia may be the products of a volcanic explosion when the magma reached the surface. The nearby Ivory’s Rock is a small plug of rhyolite, where some outcrops of glassy obsidian represent sections that have been chilled rapidly. Minto Crags at Croftby is a circular ‘ring’ dyke of rhyolite, while the flat toped Mount French is developed on several gently inclined sills of rhyolite. dolerite weather to well developed red soils of good fertility. The basalt lavas of the range to the west weather to deep red and black fertile soils which support thick forests. In contrast rhyolite and trachyte have minerals with few nutrients and display only poor shallow soils. Mount Alford is a complex intrusion of micro-diorite and granophyre with a broken-up andesite capping, encirled by several ring-dykes of trachy-rhyolite, including Glennies Pulpit.The Sugarloaf southeast of Boonah is a plug of basalt,and the flat-topped Cunninghams Lookout west of Warrill View is formed by several flat sills of basalt and coarser dolerite. Several plugs and intrusions of basalt and dolerite have been worked for local road pavement gravels and the regional supply of crushed aggregate. One major quarry is active in a large inclined sill of basalt between Peak Crossing and Purga, and there is potential for additional quarries in the future. The limestone at Peak Crossing is worked for agricultural dolomite. 6. INFLUENCE OF THE GEOLOGY Potential for coal deposits and coal-seam gas The influence of the geology on land use in the valley is quite marked. The Walloon Coal Measures in other districts are major sources of steaming coal for electricity generation and export, and there are coal mines just to the north at Amberley, Jeebroopilly and Rosewood. There is potential for additional deposits farther south, and some small ones have been outlined in years past, but here the coal seams appear thin and discontinuous, and the very numerous intrusions within the valley have disrupted the seams to such an extent that significant workable resources will be difficult to locate. Fertility of the soils Only poor sandy soils are developed on the sandstone ranges to the east and much of this country has remained under forest or is used only for broad-scale grazing. The Walloon Coal Measures in the centre of the valley weather to heavy clay soils of moderate fertility. The extensive intrusions of micro-syenite, micro-diorite, basalt and Slope instability Extensive landslides have occurred on cleared country on Walloon Coal Measures to the east of Boonah. Removal of the forest and its transpiration has resulted in higher water pressures in the strata during high rainfall events, allowing the soft weathered rock to fail and move down hill in debris slides and earth flows. This is pronounced on slopes that fall in the same direction as the rock strata, which in this valley are mainly westerly facing. Landslides also occur on cleared hill slopes underlain by weathered and transported basalt debris at the foot of the Main Range, such as just northeast of Cunninghams Gap. Sources of quarry materials Multiple intrusive sheets or sills of rhyolite magma forming the peaks of Mount French after erosion of covering rocks Google Earth image of the ring-dyke of Minto Crags, originally a circular fracture filled by rhyolite magma. Revealed by erosion preferentially removing the surrounding softer sedimentary rocks Based on a talk to the Fassifern Field Naturalists Club June 2013 © Geological Society of Australia Qld Div 2013