Geology of the Gold Coast and Hinterland

 
 
The Gold Coast and hinterland has a fascinating geological history going back over 300 million years.
Geonature Walks and Tours can design a walk or tour including school excursions, short or long, that can highlite the different geological features in the area, from the early meta-sedimentary rocks of the Neranleigh-Fernvale beds that were scraped off the ocean floor (300 mya ago) to the violent and explosive Chillingham volcanic (225mya) and the more recent flows of basalt and rhyolite from the Tweed (Mt Warning) and Focal Peak volcanoes (23-24 mya).
 
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Australia is a very ancient continent with some individual grains of the mineral Zircon from the western cratons  of west Australia being dated from  4000 – 4400 million years old, nearly as old as our planet earth, believed to be around 4600 Million years old. Next to this great age,eastern Queensland is relatively young with the first rocks rising above the sea about 450 million years ago (MYA). In his book Rocks and Landscapes of the Gold Coast and Hinterland, Warwick Willmott from the Queensland Division of Geological Society of Australia, explains how around 360 MYA, the eastern  edge of southern Queensland was several hundred klms west of its present position and well west of Toowoomba. A volcanic mountain chain ran along the edge as a result of the oceanic crustal plate to the east being dragged or subducted under the continent. Shallow seas extended eastward but the Gold Coast and Brisbane region lay in a deep ocean trench further east around a 1000 m or more deep. From around 360-3oo MYA, 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. Around 300 MYA, these deep ocean sediments were compressed, crumpled and recrystallised by the action of pressure and heat as they were scraped off the ocean floor and squeezed between the crustal plates.
 
 
 
In time these beds were thrust up above sea level to form a high mountainous terrain. These beds are now known as the Neranleigh – Fernvale beds and form all the original hilly terrain of the hinterland with similar rocks being found from Ballina in the south to Yeppoon in the north. Around 250 MYA, the eastern edge of the continent stabilised until around 225 MYA, in the late Triassic Period, 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.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 MYA, 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 while they are thin behind the Gold Coast there were workable coal seams closer to Brisbane. Over the next 35 million years, the area to the west of the mountains continued to subside  to form the Morton Basin. Coarse sandstone sediments covered the coal seams and the Chillingham Volcanics, and are named the Woogaroo Subgroup. They were followed by the finer grained sandstones of the Marburg Subgroup.
 
 
As streams slowed down lakes formed and fine sediments of mudstone, siltstone and sandstone were laid down with coal forming in swampy areas. These are now the Walloon Coal Measures and contain workable coal deposits to the west. During the subsequent Cretaceous Period, these sediments were slightly folded and warped by further  crustal pressures.
 
 
Around 24 MYA, 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 seperate 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 2000 mts 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.
 
 
After the early flows of the Beechmont Basalt from the Tweed Volcano, there were smaller more violent eruptions around the flank of the volcano, possibly from subsidiary vents. These explosive eruptions show in layers of ejected ash and debris.
 
 
At Binna-Burra, carbonised branches of trees can still be seen in the cliff of tuff where the existing forest was smothered by ash over 20 MYA.
 
Carbonised tree branch in Tuff
 
 
These layers of tuff and coarse volcanic breccia can be quite thick in places but are soft and erode readily to undercut caves.
 
 

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 100 m thick.
 
 
 
As the hot flows of rhyolite made contact with the cold ground beneath, a black band of Perlite (a volcanic glass) often formed.
 
 
 
It is suspected that these flows and the ash before them may have come from subsidiary vents on the flank of the volcano and that the rhyolite plugs of Egg Rock, Charraboomba Rock and Pages Pinnacle may be filling some of these vents.
 
 
To read more about the geology of Queensland, Warwick Willmott has a number of books available in his Rocks and Landscapes series covering most of the Queensland coast and national parks. Warwick's new updated & expanded 3rd edition of Rocks Landscapes of the Gold Coast Hinterland, has recently been released. I have copies available on the Gold Coast or you can purchase a copy thru the Geological Society. 
 

Geological Society of Australia, Queensland Division

I had the pleasure of spending a bit over a week with Warwick Willmott while he researched his latest book on the Gold Coast Hinterland and I am appreciative of Geo Nature Walks and Tours acknowledgments for assisting the Geological Society with field inspections, local knowledge and photos for the book.

 
To read more about the geology of Australia you could try….
 
The Voyage of the Great Southern Ark by Reg and Maggie Morrison ……..2nd hand only  
The Geology of Australia by David Johnson …….Cambridge University press
DVDs on Geology
Nature of Australia, a Portrait of the Island Continent…….ABC shop
British Isles, a Natural History…….ABC shop
Earth the Power of the Planet……ABC shop