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While looking for a place to hide a physical geocache, I stopped at these strange rocks. I recognised them fairly easily, but was struck by a couple of things.
How did they get here? Why are they this colour, and what specific rock type are they?
Let's start with the specific rock type.
Upon observation, what one can see is a sediment with a mixture of pebble to cobble-sized fragments (geological term: clasts).
They appear to be fairly well-rounded to sub-rounded clasts.
The sediment contains a mix of many different particle sizes, but one or two sizes generally prevail. It indicates the transporting agent (most likely here, a previous river) had fluctuating energy levels or didn't sort the grains thoroughly before dropping them. I have learnt that this is called "poor to moderate sorting".
Obviously, these clasts are somehow being held together to make these larger rocks. Clasts are supported by a finer sandy-like material or sediment. This material is called the matrix. Within the matrix is the "glue" - the cementing agent that holds it together. That could be silica, calcite or iron oxides. Hmmm. That latter sounds interesting. 😊
So, technically, this outcrop is an example of a conglomerate, a coarse-grained sedimentary rock composed of rounded clasts, cemented together within a finer-grained matrix.
The deposits here also represent a "high-energy depositional environment", where fast-moving water was capable of carrying somewhat larger gravel and cobble-sized material. When river energy decreased, the heavier clasts were deposited, forming gravel bars and channel deposits. Over time, additional sediment buried the deposit, and mineral-rich groundwater cemented the material into solid rock.
But why up here?
My geology friend suggested that while this elevated conglomerate outcrop represents the remains of an ancient high-energy river system, the softer surrounding sediments were gradually eroded, and the well-cemented gravel deposit resisted weathering and remained as a topographic high, preserving evidence of past river environments in the modern (contemporary) landscape. A topographic high is any raised area of land that stands at a higher elevation than the surrounding terrain.
So there you have it: several features visible at the outcrop support its identification as a conglomerate:
Rounded to subrounded clasts
Pebble- to cobble-sized fragments
Poor to moderate sorting
Clasts supported by a finer sediment matrix
Evidence of water transport and deposition
Finally, the colour.
The final observation I made was that the conglomerate rocks here stand out because they are a very dark colour. Look around - you should see several more half-buried in the ground.
At first, I thought it could be lichen, moss, or similar. But the clasts themselves remain natural. So it is in the matrix 'glues'? These are generally silica, calcite or iron oxides.
Well, silica is colourless or white. Rule that one out.
Iron oxides are primarily red, with yellow, brown, and black hues. The black is either magnetite (nothing magnetic here) or wustite, which is much rarer and often found in meteorites. So that brings us to calcites.
Calcite is basically colourless or white, but can have variants in shades of blue, green, yellow, orange, pink, red, brown, grey, lavender, and black. Black? It could be the result of dense carbon or bituminous impurities. Could be.
But after a little research, I was told:
Given the Harlin/Colinton area geology** in Queensland, iron-rich weathering and manganese staining are very plausible explanations. If the rock becomes lighter when freshly broken, that usually means the dark colour is mainly surface weathering. If it stays black all the way through, the colouring is probably part of the original sediment or cement.
[ ** If you would like some heavy reading, go to this UQ document]
YOUR TASK.
1. Examine the clasts within the outcrop. Are they mostly rounded or angular?
2. Compare the largest and smallest clasts visible in the outcrop. What geological description describes the variation in size?
3. Break a small section of the surface of the rock, and decide the cause of the black colouring.
4. Include the obligatory identity photograph of yourself or something identifying you as attending this location.
First, log your Earthcache Find, then MESSAGE me your answers and photo. You may include the photo in your Log IF it does not identify the rock in question. The nearby Telecom posts might be useful here.
Thank you for visiting this earthcache, and I trust you also found some of the other cache types close by.