Baṉambarrŋa (Rainbow Cliff) is a sheer section of coastline where the exposed and weathered rock profile produces a striking colourful effect on the cliff face as it plunges over 40 metres onto a sandy beach and into the sea.
This earthscience lesson is about how and why the colours occur in the cliff face at Baṉambarrŋa.
To log this earthcache…
To log this earthcache you will need to visit GZ after the cache publication date and send me a message or email addressing each of the following earthscience tasks based on your observations and the information supplied:
1. Note the colours you can see in the cliff face. Would you say they are uniformly distributed, patchy, layered horizontally, blended or something else? Is there any sort of pattern visible in the colouration?
2. How do you think the minerals that cause the colours found their way into the cliff face (eg - were they in the sediments when they were deposited or did they seep down from the surface)?
3. One colour can be seen exuding/oozing from the base of the cliff onto the beach below. What colour is it and why do you think is this happening?
4. Take a photo of yourself or something with your team name on it showing the cliffs and either post it with your log or send it to me with your answers.
You may log your visit once you have submitted your answers to me. I will respond confirming permission to log, or seeking clarification if I feel that you have not properly answered the questions. Logs with no answers sent or photo included will be deleted.
Please do not post your answers in your log.
What is happening here?
The cliffs along the coastline here consist of a layer of hard laterite underlain by finer sandstones, siltstones and softer clays. The laterite is a result of silica and other soluble materials being leached from rocks during tropical weathering processes, leaving behind residual concentrations of iron and aluminium oxides.
The colours in the cliff face are caused by mineral contamination. The various minerals were either:
- present in the sandstone, siltstone and claystone sediments when they were deposited;
- leeched into the rocks from adjacent rocks later on; or
- resulted from a combination of both.
Different combinations of minerals produce different pigments. For example.:
- White – indicates no contamination.
- Yellow – is due to the presence of limonite (FeO(OH)·nH2O), a hydrated iron hydroxide.
- Brown – is due to goethite (FeO(OH)), a partly hydrated iron oxide.
- Red – is caused by hematite (Fe2O3), an anhydrous iron oxide.
- Purple – similar to red chemically, but of a different hue caused by different light diffraction properties associated with a greater average particle size.
- Sienna – caused by a combination of limonite and a small amount of manganese oxide, which makes it a darker colour than the yellow associated with limonite on its own.
- Umber – is similar to sienna but with a larger proportion of manganese, which makes a darker brown.
Pigmentaion in the cliffs here is an ongoing process as erosion, weathering and chemical reactions within the rocks continues to occur.
Getting there
A Dhimurru permit is required to visit this area.
Baṉambarrŋa is a 15-20 minute drive from Nhulunbuy. Access is via a formed gravel road to the eastern side where you will see a spectacular view of the cliffs from the laterite shelf. It is then a 250 metre walk from the car parking area to the base of the cliff that will require some rock-hopping and may only be possible when the tide is not high.
Baṉambarrŋa is a very sacred and special place for Dhalwaŋu, the Aboriginal custodians of the area. Visitors are permitted but access to the top of the cliff is not allowed. The coloured clay cannot be touched and nothing may be removed from the cliff area.
Enjoy your visit, stay safe and be crocodile aware.