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.
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.
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.
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.
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.
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.