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Geological sources of quarry materials on the Gold Coast

Rock supplies come from one important geological unit, the Neranleigh-Fernvale Beds, which backs much of the urban area between the Gold Coast and Brisbane (Figure 1). Several quarries work deposits of meta-greywacke, quartzite and greenstone from this unit. Lesser supplies come from basalt lavas of the Tweed and Focal Peak Volcanoes which back the coast in high country to the west.

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NERANLEIGH-FERNVALE BEDS
To understand the nature of these important rocks and their impact on quarry planning, we need to know something of their origins.
About 360 to 310 million years ago, the geography of the continent’s eastern side was very different. A volcanic mountain chain extended down its length, flanked by a shallow continental shelf to the east, and a deep water trench off its edge. This configuration resulted from an oceanic crustal plate to the east, colliding with and being “subducted” beneath a continental plate to the west (Figure 2). Sediments were eroded off the volcanic chain and deposited on the continental shelf. Some became unstable, and slumped down submarine canyons into the deep trench and ocean floor as dense “turbidity” currents to settle on the sea floor as beds of dirty, coarse sands and finer silts and muds. The coarser sands were deposited first near the foot of the slope, and the finer materials were carried farther to the east. Basalt lavas were also erupted onto the deep ocean floor, and in places patches of siliceous ooze built up from the accumulation of innumerable siliceous skeletons of microscopic animals called radiolaria.

About 310 to 300 million years ago, the subduction process stopped and the crustal plates squeezed together. The sediments in the trench were compressed, deformed, and pushed up above sea level to form mountainous terrain. The strata became steeply inclined and cut by numerous faults, and the sediments hardened and partially recrystallised to new minerals. The coarse dirty sands became meta-greywacke, the finer material argillite, the basalt lavas greenstone, and the siliceous ooze quartzite. These rocks are now the Neranleigh-Fernvale beds.
Bands of greywacke are the mainstay of the quarrying industry, as it is a hard, competent rock that crushes to stable, reasonably equi-dimensional particles. However, it is commonly interbedded with bands of finer argillite which need to be avoided as they produce flaky particles. To the foothills of the ranges to the west, greywacke occurs in large volumes in very thick bands with little argillite present. Several quarries work greywacke at Wolffdene, Ormeau and Staplyton, south of Beenleigh, and at Oxenford, Nerang and West Burleigh, and supply a complete range of quarry products to the Brisbane and Gold Coast markets. A quarry in the Albert Valley, south of Beenleigh, works both greenstone and greywacke. Quartzite occurs in isolated bands and lenses. It produces dusty quarry products with potential alkali reactivity problems and its use for concrete or bitumen aggregates is limited, but many bands have been worked for road pavement gravels by local governments in the past. Two commercial quarries at Stapylton still work quartzite for road pavement materials.
Investigation for quarry development must recognise that continuous beds of greywacke are difficult to trace, on account of the overlapping nature of the original sediments and their compression and deformation. Drilling to avoid unsuitable argillite is needed, and because of the steep incline of the strata, this should use angled, not vertical, holes. However, thicker bands can generally be relied upon to extend to appreciable depths.

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Other sources
Chillingham Volcanics. A band of rhyolite lavas, welded tuffs and tuffs of this unit overlie the Neranleigh-Fernvale beds to the west. They were erupted 210 million years ago and contain hard rock suitable for quarrying. However, they are in inaccessible steep terrain. Only one small road base quarry in weathered rhyolite is worked by the regional council in the Canungra Army training area.
Basalt lavas of the Focal Peak and Tweed Volcanoes. A large quantity of the Gold Coast hinterland is formed by extensive, horizontal basalt lavas that erupted from the Focal Peak Volcano to the west and the Tweed Volcano to the south 24 to 23 million years ago. Basalt is a preferred rock for quarrying because of its ease of crushing but several factors mitigate against quarry development here.
The lavas are highly variable in thickness, fracturing and degree of weathering, with solid lavas interbedded with fractured or vesicular types, and ancient incipient soil layers. It is usually difficult to establish a workable face of consistently hard basalt. The lavas are also at high elevations in scenic terrain with difficult access via minor rural roads not suitable for heavy trucks. Thicker flows were worked in one quarry in NSW at Terranora, and three medium-sized quarries operate at low elevations near Beaudesert.

SAND
Coarse sand and gravel were extracted from large alluvial deposits along the Coomera River but are now exhausted. Coarse sand is now  “manufactured” from finely crushed rock at several greywacke quarries. These supplies are supplemented from two quarries near Tamborine Village, south of Beenleigh, where weathered quartz sandstone of the Woogaroo Subgroup is scraped for sand of various gradings. This is the earliest unit of the large Moreton Basin and was deposited on braided river plains 200 million years ago.
Well graded fine sand was once produced from small creeks west of Beenleigh and along the Logan River and tributaries north of Beaudesert. These are now exhausted or off-limits for the extractive industry. The only material now produced comes from pits in the alluvium of the Logan River near Carbrook, east of Beenleigh.

Most fine sand now comes from several pits in single-sized sands of estuarine or beach ridge origin in the Woongoolba–Jacobs Well area in low country near the coast. These sands were deposited 120,000 years ago when sea level was slightly higher, and the area was a complex of estuarine and beach ridge environments. Such sands are used in mortar and with coarse sand manufactured from hard rock. Sand is also imported from northern New South Wales or sourced from building excavations along the coastal strip.

THE FUTURE
Rock supplies are adequate for the medium term, but longer term shortages loom. Sand supplies are more critical in the medium term. Formerly accessible greywacke bands of the Neranleigh-Fernvale Beds are now extensively urbanised or covered with semi-rural settlement and obtaining approval for new quarries will be difficult. To the west, the foothills of the hinterland ranges are entirely composed of greywacke but as the terrain is very steep and largely within national parks, water reserves or narrow, inaccessible valleys, establishing quarry sites would be very difficult. Only one of the eight Key Resource Areas established to protect rock resources in the region (under State Planning Policy 2/07 for the Protection of Extractive Resources) covers a new deposit not already worked, and even there planning permission is not guaranteed.

Rhyolite of the Chillingham Volcanics is within very steep terrain of the Canungra Army training area or in national parks in the Numinbah Valley. Interbedded soft tuff would also be a problem in site selection.
As described earlier, the basalt lavas from the Focal Peak and Tweed Volcanoes are problematic in that many are at high elevations in steep scenic terrain with only narrow rural road access. Interbedded horizontal lavas of varying hardness and degree of weathering also make establishing a face in consistently hard rock difficult. In the lower country to the west near Beaudesert, thick basalt resources have been located in early lavas at Bromelton (two medium sized quarries) and Cryna (a medium quarry). Further prospecting south of Beaudesert may be warranted but investigations of potential sites by the Geological Survey of Queensland have not confirmed any major deposit.
Similar constraints apply to securing additional deposits of weathered sandstone. Close semi-rural settlement has covered most of the suitable quartz sandstone of the Woogaroo Subgroup in this area. Extensive feldspar-rich and lithic-rich sandstone beds of the Marburg Subgroup to the west are unsuitable for sand extraction. However, the Woogaroo Subgroup outcrops further west in an anticline along the Beaudesert-Boonah road, and a quarry processing sandstone blocks is established there. The area is of moderate terrain and within a broad-acre rural environment, warranting further prospecting. 
A Key Resource Area has been established over several of the fine sand deposits of the Woongoolba-Jacobs Well area, but the use of land for sugar cane, and the need to address acid sulphate soil issues have prevented access to large parts of these deposits.
Under these conditions maximum use of existing quarries will be needed, with lateral and depth extensions needing to be pursued and planned for.

Warwick Willmott is a geologist who formerly worked with the Geological Survey of Queensland in a variety of fields. Malcolm Irwin is a member of the Geological Survey of Queensland and works for the Queensland Department of Employment, Economic Development and Innovation.

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