QUESTION: Hello, I would like to ask about limestone. Why there is a variety in color of limestone in one outcrop. For example, in my area of study, in one outcrop (see image attached), there is a mixture of grey, orange and pink in colour of the limestone. they re not seperated by any bedding to differentiate the color. When thin section made out of the sample based on color, it show varies in texture. Some are crystalline and some are grainstone, some are mudstone. Thank you for helping me.
ANSWER: Hi Norliana,
Nice picture. You don't mention where this outcrop is but I will assume Malaysia since it is associated with your name in the question. Most times, this kind of color is due to iron content in the rock. The yellow and reds are where the iron has come in contact with oxygen in the atmosphere and has oxidized; in effect it has rusted. There will almost always be some small quantity of iron, even if it's only molecular, in limestone. The reason the rest of the outcrop is not iron stained is because, for some reason, oxygen hasn't reached the iron molecules yet. It could be the stone is tighter there or not enough groundwater has moved through it, or that simply not enough time has elapsed for the iron to oxidize.
Here's a pretty good image of iron staining in an outcrop;
Hope this helps.
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QUESTION: Hi, thank you for your answer. This outcrop located at Perlis, Malaysia. Sorry to not mentioned it before. Your answer really help me a lot. But, why it is differ when it comes to texture? in one outcrop, without seperation by the bedding, the texture is mix, consist of grainstone, crystalline and mudstone.why is it so? thank you again.
Hi again Norliana,
Your second question is a lot more difficult to answer. Normally, different texture (grain size and such) is due to the distance from the source rock. If there are larger grains in a rock it implies a closer source because they haven't undergone the erosion that transport causes. If the grains are smaller it usually means they've travelled further.
The differing grain sizes might mean different porosity and permeability, which, in turn, would allow solutions to move through the rock, or not. As I mentioned in the first answer, oxygen interaction with iron causes the staining. If water can't flow through the stone there is no interaction and no staining.
But, in this case, the outcrop is limestone and that presents a problem. It is a sedimentary rock and can be made up of grains that are mostly organic remains such as shells, and other small fragments like clays. But a lot of limestone is a precipitate so there are no grains.
It can be more complex. But, like a clastic sedimentary rock, precipitates can have finer or coarser texture. The texture may impede groundwater flow, restricting access to oxygen. In that case there would be no iron stain.
The difficulty is, that requires study of the specific outcrop. The answers I gave are very general, the true nature of the intermittent staining might only be determined through experimentation. In that case no one, who hasn't studied that outcrop or those similar, can give you an exact answer. I suspect those studies have been done by geologists familiar with the geology in Malaysia. Your best bet now would be to contact the Geology Department of a local university. I'll bet they have a more definitive answer to your question.
Hope this helps.