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LIMESTONE CAVES
of Central Queensland
A GEO-SCIENCE FIELD TRIP
With CQUniversity Australia
Day 1 takes us to the limestone cave formations of the Rockhampton area, in Central Queensland, which we had permission to enter for this field trip. The area is well known for it's caves, which are host to a wide range of flora and fauna, a popular tourist attraction, and an important geological formation unique to the surrounding landscape.
GEOLOGICAL SETTING
The geological formation of these caves, and the surrounding area, is part of an extremely complex and ancient history that is still being researched to this day.
Geologists believe that during the late Silurian to middle Devonian ages (425-385 million years ago), coral reefs were formed around offshore volcanic islands (these are called island arcs). The limey debris from coral and marine life were intermittently covered by volcanic ashes and lavas, then regrew anew - a process that repeated itself over many cycles. This lead to the development of large limestone blocks. The remnants of marine life can be seen today in the rock record within the cave.
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Evidence of life around ancient reefs of the island arcs - a variety of sea floor life can be seen here. Can you identify them?
During the mid-Devonian period (385 million years ago), large scale geological changes caused the island arc system to collide with the continent to the west (subduction plate tectonics). This resulted in rocks being compressed, folded and uplifted as they were shunted together. Large limestone blocks originally formed around the island arcs were subsequently eroded and detached, becoming embedded in younger sediments.
Today, this limestone is present in the surrounding younger geology north of Rockhampton, and forms part of an area called the Mount Alma Formation (shown in map below).
A much more detailed geological account of this region can be found in the publication Rocks and Landscapes of the National Parks of Central Queensland.
Take a closer look
Explore the current detailed geological setting with the following map. Swipe between the Topography and the Geology layers - you can also zoom in, zoom out, pan around and click the geology map for detailed information.
Swipe between the detailed geology map and the topography of the area. You can click or tap anywhere on the geology layer to view a popup with further information (Department of Natural Resources, Mines and Energy, Queensland Government).
CAVE FORMATION
Chemical Weathering
How are the caves formed? A process known as chemical weathering is a key factor to the creation of cave systems. Chemical weathering is the process of decomposing rocks by changing their chemical composition. In caves, this process all starts with rain water, which reacts with carbon dioxide in the air to form carbonic acid.
The carbonic acid in turn reacts with the limestone (which is made of calcium carbonate - aka Calcite). This perfect combination of chemicals results in calcium and carbonate ions being stripped away from the limestone into the water as tiny amounts of the limestone is dissolved. Specifically, this is called chemical dissolution.
The compounding effect of this repeated process over a long time is two-fold. Not only is the limestone dissolved to create larger cracks and cavities within the cave (growing the cave so to speak), but also the flow of the acidic water leads to the creation of famous cave features by shifting minerals from one place to another. When the water drips into empty parts of the cave, it begins to release carbon dioxide into the air and deposit any excess calcium carbonate it carried, forming stalactites and stalagmites.
Chemical dissolution begins at the surface, as you can see by the cracks and rain pits in this photo-scan from above the cave. Click to load - 3D/VR scan of surface rocks above ground (approx 37mb)
Hydrogeology and Groundwater
When the water eventually finds it's way to the water table deep within the cave, it starts to erode the cave system horizontally, creating a series of chambers that connect to each other. As the water table falls (due to erosion and other factors external to the cave), the entire cave creation process continues deeper underground, as can be seen in the following illustration (Willmott, 2006).
Based on the illustration featured in Dissolving a subterranean world, from: Rocks and landscapes of the national parks of central Queensland by Warwick Willmott; illustrations by Lesley Blight, Tom Moore and Sybil Curtis; Geological Society Australia.
We were able to capture a range of imagery on our field trip. These included photos of cave features such as stalactites, the effect of water transport and chemical dissolution within the caves.
Click to view - Stalactite (approx 32MB)
Pan around in this 360 degree photo taken inside the Caves.
FLORA AND FAUNA
This little animal (with a body of approx. 10cm long) is related to conventional centipedes (although rather distantly) and is called a Sctellaromorph. Great for your vocabulary!
You may be tempted to think plants and animals don't have much to do with caves. This couldn't be further from the truth. Plants and animals (Flora and Fauna) are inextricably linked with the uniqueness of caves, and even their continued physical formation today.
Caves can provide a reliable habitat to both plants and animals that face increasing threat from climate change. Many green tree frogs were seen inside the cave on our field trip, as were micro bats and a variety of insects, some of which were most peculiar (see photo). Cave systems are sometimes home to unique species of animals and plants not seen anywhere else in the world. These caves are no exception, as a known habitat for less than 50 remaining specimens of the endangered and protected fern Tectaria Devexa.
Rock eating plants
Nearby trees rely on the caves as an important source of water and nutrients. Their roots dive deep underground into the cave system, often for hundreds of metres, prying open cracks and actually eating away at the rocks with acidic secretions! Shown below, a prop-root system starting from the trees above the cave progresses down into the cave.
As the larger roots reach further into the caves in search of ground water (eg, the water table as shown in the diagram far above), they also produce fibrous root systems, which get busy absorbing nutrients. To do this, the fibrous roots exude chemicals that help dissolve the rocks and extract the minerals from the water. This is called a bio-geochemical process, and in this instance has lead to the term "rock eating plants".
Rock eating plants at work. Also shown far left, a collection of moths with a colourful spiral pattern on the wings escapes the sun and heat outside (Speiredonia spectans (Family Noctuidae)).
Time Capsules of Climate History
Surprisingly, we can actually learn a great deal about the past climate in caves. Did you know that there is actually heavy water, and light water? That is, water with either 18 or 16 Oxygen isotopes. These isotopes can be used to discover information about past climates.
In cold climates, the ocean becomes slightly more concentrated in Oxygen-18 (heavy water). This happens because the precipitation of Oxygen-16 (lighter water) tends to be locked up and concentrated in polar ice, mountain glaciers and ice sheets. In times of warming, these cold regions thaw, releasing their held up water back into the ocean, which then changes the ratio between the two Oxygen isotopes. The flow on effect of this cycle is that varying ratios of Oxygen isotypes are distributed from oceans to land, and this can be preserved for later chemical study. NASA have a great article about this if you'd like to learn more.
Now remember the dripping of water through the cave, and the slow formation of stalactites and stalagmites? Imagine that stalagmite growing like the rings of a tree, but with each ring having it's own record of Oxygen isotopes. Paleo-climatologists can actually examine the ratios of Oxygen isotopes (between heavy and light) to determine a reliable history of the climate. Real climate science at work!
We hope you enjoyed this field trip of the Central Queensland cave formations, by CQUniversity .