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Australia is a very ancient continent and there are some individual zircon grains discovered in the west of Australia to be between 4400-4000million years old. Queensland however is much younger with the first rocks rising above the sea around 450million years ago.
Around 360million years ago, the eastern edge of southern Queensland was several hundred km west of its present position. Shallow seas extended eastward but the Gold Coast and Brisbane region lay in a deep ocean trench further east around a 1000m or more deep. From around 360-300million years ago, large quantities of eroded sediments from the volcanic chain were deposited onto the Continental shelf from where some slid off into the deep ocean trench to form thick overlapping beds of sands, silts, and mud’s. Some flows of basalt and beds of silica were also added and continued to build up in great depths.
At 290-240 MYA, the ocean plate subducted under the continental plate and with immense heat and pressure, the sediments were scraped off the ocean floor, crumpled and folded to form steeply inclined strata of metamorphosed rocks. Eventually these beds were thrust up above sea level to form a high mountainous terrain. These form what is known as the Neranleigh-Fernvale beds and form all the original terrain of the hinterland with similar rocks being found from Ballina to Yeppoon. From then, the Eastern edge of the continent stabilised until the late Triassic Period, about 225million years ago, when there were violent volcanic eruptions along its eastern edge.
In the Gold Coast hinterland, these explosive eruptions produced Rhyolite lavas and fragmental rocks known as the Chillingham Volcanics. Unlike the fluid flows of basalt lava, the Rhyolite flows were viscous, meaning sticky and they covered much smaller areas than the basalts. Some flows were up to 100m thick. These were covered by later sandstone beds and flows from the Tweed Volcano but due to erosion can still be seen in some areas between Chillingham in NSW to as far north as Mt Tamborine. During the late Triassic to Jurassic Period 213 – 144 million years ago, erosion of these high mountains began and before long small rivers and streams developed, with siltstones, shales and sandstone being deposited in low areas. These are known as the Ipswich coal measures and although these are quite thin behind the gold coast, they become more substantial closer to Brisbane. During the next 35million years, the area west continued to subside to form the Morton Basin. Coarse sandstone sediments covered the coal seams and the Chillingham Volcanics and subsequently covered again by finer grained sandstones.
During the subsequent Cretaceous Period, these sediments were slightly folded and warped by further crustal pressures. Around 24million years ago, the Focal Peak Volcano west of Mt Barney erupted with flows of basalt reaching as far east as Tamborine. This was soon followed by numerous flows of basalt from the separate Tweed Volcano and due to their highly fluid nature, they were able to flow long distances. Over several million years this ‘shield volcano’ reached a height of around 2000m above sea level with the early flows covering the existing valleys and hills as they went. The basalt flows reached as far south as Lismore, to Tamborine in the north and to Mt Lindesay in the west. By 23million years ago, a number of deep valleys had worn their way down thru the meta-sediments to the coastline and the first flows of basalt from the Tweed Volcano filled the ancient valleys up. Now that the valleys were full of basalt, rainwater could not flow down them anymore and the older Neranleigh-Fernvale beds either side of the valley started to erode the hills either side of the flow.
Like the other ridges running into the coastline along the Gold Coast, the top of Currumbin Ridge was once the bottom of an ancient river valley that was filled in by the early flows of basalt from the Tweed Volcano around 20 - 23 million years ago, and after 23million years most of the original hills have disappeared leaving the old valley floor sitting on top of the ridge. There are still some large basalt boulders sitting on the ridge above Woodgie Street overlooking the Currumbin and Elephant Rocks.
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The Currumbin, Elephant and Flat rocks are made up of steeply inclined well-bedded argillite and fine-grained greywacke of the Neranleigh-Fernvale beds and are older than the basalt boulders found on the Currumbin headland and the boulders of basalt brought in and placed around the car park. However, this is a description of what you see in the rocks here and how they have formed. The feature you see here is a wave-cut platform.
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Wave-cut platforms form where the sea erodes into the land. Because surf erosion cannot dig down below the base of the surf zone, a fairly level surface results, the wave-cut platform, made up of the wave-cut bench at the cliff base and the abrasion platform farther offshore. Remnant rock knobs are called chimneys. Chimneys are smaller than stacks, which have a shape more like a mesa. Chimneys are taller than skerries, which are low-standing rocks that can be covered in high water. Currumbin Rock, Elephant Rock and Flat Rock are good examples of wave-cut platforms in this area. These platforms were developed by wave erosion undermining the cliffs and have now been isolated but not yet completely worn away. These rocks will eventually erode into the sea leaving the platforms, as they are not as resilient as the headlands along the coast, as these are all basalt from the lava flows.
To claim your Earthcache,
1. Tell us what kind of weathering can be found on the chimney.(Reference point 1)
2. How many years ago the rocks were deposited as sediment.
3. At GZ if you leave the sand, how wide is the platform?
4. At GZ, take your picture with the chimney in the background (optional)
