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Thread: Geology of Corsica - Some Questions

  1. #1 Geology of Corsica - Some Questions 
    exchemist
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    I've just got back from visiting the area around Ajaccio. Most of the rocks are clearly granite, but I was intrigued to find a bay, a few miles to the South, (just South of Capu di Muru, to be exact) where there seemed to be a mixture of clearly layered, softer light coloured rocks, intermingled with what looked like clinker, resting above the granite, the top of which was only a few feet above sea level at that point, allowing the sea to have eroded chunks of the weaker material above. I took them to be extrusive volcanic deposits (ash layers, tuff, or something related) but I can't find on the web any mention of extrusive volcanism on Corsica. Am I barking up the wrong tree?


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    This is going to be a rambling response. First, I'll address the issue of volcanic rocks on Corsica, then try to identify what you found.

    So, are there any volcanic rocks on Corsica?

    There are ophiolites on Corsica and as former oceanic crust they might be expected to include submarine lavas. There also appears to have been some Carboniferous-Permian volcanic activity throughout the Western Mediterranean that included Corsica. This is related to the Hercynian orogeny.

    Is it possible that the layers you saw were deeply weathered granite? This can leave a soft matrix of clay, from weathered feldspar and mica, with remnant quartz grains. IT doesn't sound like what you saw, but verbal descriptions can be misleading. I'd like to eliminate that possibilty. I'm guessing you don't have a photo.


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    Quote Originally Posted by John Galt View Post
    This is going to be a rambling response. First, I'll address the issue of volcanic rocks on Corsica, then try to identify what you found.

    So, are there any volcanic rocks on Corsica?

    There are ophiolites on Corsica and as former oceanic crust they might be expected to include submarine lavas. There also appears to have been some Carboniferous-Permian volcanic activity throughout the Western Mediterranean that included Corsica. This is related to the Hercynian orogeny.

    Is it possible that the layers you saw were deeply weathered granite? This can leave a soft matrix of clay, from weathered feldspar and mica, with remnant quartz grains. IT doesn't sound like what you saw, but verbal descriptions can be misleading. I'd like to eliminate that possibilty. I'm guessing you don't have a photo.
    Thanks for the reply, John. Sadly I indeed did not take my camera on the beach, being nervous about salt and sand. It was only once there that I thought the rocks were strangely dissimilar from the granite I had seen everywhere else. I did see a lot of weathered granite up in the hills, which appears just as you say, with light coloured earthy matrix - complete with a few tree roots! - but containing grains of quartz, some up to 5mm across. This was different because it was a sandwich, with granite at the bottom, then this light coloured softer layer, with clear, almost horizontal stratification, and then capped by this clinker-like dark rock with very fine grains. It may be that the top layer only appeared like clinker due to the way the sea had eroded it. I would not swear it contained vesicles within the rock itself.

    Next time I clearly need to take (a) a camera or at least my mobile and (b) a hammer .

    But thanks anyway for the comments about volcanism. I have read in the Michelin Guide that the granite is Hercynian (as I think is the granite in Brittany?), and that there was a later (Alpine) overthrust in the NE part of the island which is schist and so forth. But I was in the SW so that can't be it. Michelin mentioned ignimbrites but was irritatingly unclear about where they were to be found. (Still, at least the French are educated enough to realise that geology helps explain a lot about a place - the Cadogan guide has not a word on the subject. But then the French are usually interested in how the terroir for wine will be, for which a bit of geology is indispensable.)

    What we also saw on another beach was pink/white granite with enormous pockets of pure quartz and feldspar, about 5cm diameter. I've never seen such big lumps. Presumably this must have been the product of spectacularly slow crystallisation.
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    Quote Originally Posted by exchemist View Post
    [What we also saw on another beach was pink/white granite with enormous pockets of pure quartz and feldspar, about 5cm diameter. I've never seen such big lumps. Presumably this must have been the product of spectacularly slow crystallisation.
    These could have been the remanants of the last stages of migmatisation. Do any of the images in the second link look like what you saw?
    Migmatite - Wikipedia, the free encyclopedia
    migmatite - Bing Images

    Migmatisation would be interesting to you as a chemist, I think, since of all the rock forming processes this is the one that is dominated by chemistry and much less so by physics.
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    Quote Originally Posted by John Galt View Post
    Quote Originally Posted by exchemist View Post
    [What we also saw on another beach was pink/white granite with enormous pockets of pure quartz and feldspar, about 5cm diameter. I've never seen such big lumps. Presumably this must have been the product of spectacularly slow crystallisation.
    These could have been the remanants of the last stages of migmatisation. Do any of the images in the second link look like what you saw?
    Migmatite - Wikipedia, the free encyclopedia
    migmatite - Bing Images


    Migmatisation would be interesting to you as a chemist, I think, since of all the rock forming processes this is the one that is dominated by chemistry and much less so by physics.
    No don't think it was that John. The centre layer, which is the one that baffled me most, actually resembled this, which I've just found on the web and think may be from Sardinia:

    Corsica capo di muro - Bing Images

    Although at Cala D'Orzo the layers are nearly horizontal.
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    Quote Originally Posted by John Galt View Post
    Quote Originally Posted by exchemist View Post
    [What we also saw on another beach was pink/white granite with enormous pockets of pure quartz and feldspar, about 5cm diameter. I've never seen such big lumps. Presumably this must have been the product of spectacularly slow crystallisation.
    These could have been the remanants of the last stages of migmatisation. Do any of the images in the second link look like what you saw?
    Migmatite - Wikipedia, the free encyclopedia
    migmatite - Bing Images

    Migmatisation would be interesting to you as a chemist, I think, since of all the rock forming processes this is the one that is dominated by chemistry and much less so by physics.
    Or this, also from Cala Spinosa in Sardinia, seems to show the same juxtaposition of Granite with this softer, layered material:

    Panoramio - Photo of Cala Spinosa

    B
    y the way, this Cala Spinosa is on the NW tip of Sardinia, not that far from the location on Corsica I visited....

    I think it could well be the same formation.
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    Universal Mind John Galt's Avatar
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    So, here the granite is almost certainly intrusive and younger than the sediments. The sediments are clastic (i.e. composed of fragments) and appear to range from silts to medium grained (possibly coarse) sandstones. There is a lot of interbedding and some cross-bedding. They appear to be delta, or certainly river, deposits. The colouration suggests possible desert conditions. If so I would suspect Permian age.
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    Quote Originally Posted by John Galt View Post
    Quote Originally Posted by exchemist View Post
    [What we also saw on another beach was pink/white granite with enormous pockets of pure quartz and feldspar, about 5cm diameter. I've never seen such big lumps. Presumably this must have been the product of spectacularly slow crystallisation.
    These could have been the remanants of the last stages of migmatisation. Do any of the images in the second link look like what you saw?
    Migmatite - Wikipedia, the free encyclopedia
    migmatite - Bing Images

    Migmatisation would be interesting to you as a chemist, I think, since of all the rock forming processes this is the one that is dominated by chemistry and much less so by physics.
    Sorry just realised the migmatite suggestion was apropos the pink/white granite, not my mystery formation at Cala D'Orzo. Apologies for the confusion.

    The quartz lumps were on another beach (Isolella). Migmatites I see involve curved bands: what I saw was just lumps of quartz, about 5 cm across, in a matrix of rusty orange/pink, which I took to be feldspar. Sort like granite on a giant scale.
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    Quote Originally Posted by John Galt View Post
    So, here the granite is almost certainly intrusive and younger than the sediments. The sediments are clastic (i.e. composed of fragments) and appear to range from silts to medium grained (possibly coarse) sandstones. There is a lot of interbedding and some cross-bedding. They appear to be delta, or certainly river, deposits. The colouration suggests possible desert conditions. If so I would suspect Permian age.
    How interesting. So the sediments predate the granite. I had not thought of that. I had thought generally granite intrusions were enormous batholiths that would cook the rocks around them and this create a metamorphic layer. Can granite intrude more suddenly and in smaller bodies that do not do this, then?
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    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by John Galt View Post
    So, here the granite is almost certainly intrusive and younger than the sediments. The sediments are clastic (i.e. composed of fragments) and appear to range from silts to medium grained (possibly coarse) sandstones. There is a lot of interbedding and some cross-bedding. They appear to be delta, or certainly river, deposits. The colouration suggests possible desert conditions. If so I would suspect Permian age.
    How interesting. So the sediments predate the granite. I had not thought of that. I had thought generally granite intrusions were enormous batholiths that would cook the rocks around them and this create a metamorphic layer. Can granite intrude more suddenly and in smaller bodies that do not do this, then?
    If you look at the picture you provided, with the granite on the left and the sediments on the right, you can see there is sharp, vertical boundary between the two. That could arise in one of two ways. It could be a faulted contact - in which case the relative ages could not be determined without a considerable amount of field and lab work; or, it is an intrusive contact, in which case the granite is younger.

    You are correct that intrusive granites will tend to thermally metamorphose the rocks around them. However, granitic magma is the lowest temperature of the common magmas and it is often intruded as slush, with many much of the melt already crystallised, so that the heat content of the intrusion is lower than one might expect. Even so I might have expected to see some visible changes adjacent to the granite - I saw none. That points to a faulted contact. What swayed me in the direction of intrusion is the smaller granite mass some feet to the left of the main body. It does not appear to be fault bounded and therefore I went for the intrusion option. Frankly, it's difficult without actually being there so see the contacts and rocks close up - and keep in mind I am not a practising geologist and my field work is several decades in the past.
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  12. #11  
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    Quote Originally Posted by John Galt View Post
    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by John Galt View Post
    So, here the granite is almost certainly intrusive and younger than the sediments. The sediments are clastic (i.e. composed of fragments) and appear to range from silts to medium grained (possibly coarse) sandstones. There is a lot of interbedding and some cross-bedding. They appear to be delta, or certainly river, deposits. The colouration suggests possible desert conditions. If so I would suspect Permian age.
    How interesting. So the sediments predate the granite. I had not thought of that. I had thought generally granite intrusions were enormous batholiths that would cook the rocks around them and this create a metamorphic layer. Can granite intrude more suddenly and in smaller bodies that do not do this, then?
    If you look at the picture you provided, with the granite on the left and the sediments on the right, you can see there is sharp, vertical boundary between the two. That could arise in one of two ways. It could be a faulted contact - in which case the relative ages could not be determined without a considerable amount of field and lab work; or, it is an intrusive contact, in which case the granite is younger.

    You are correct that intrusive granites will tend to thermally metamorphose the rocks around them. However, granitic magma is the lowest temperature of the common magmas and it is often intruded as slush, with many much of the melt already crystallised, so that the heat content of the intrusion is lower than one might expect. Even so I might have expected to see some visible changes adjacent to the granite - I saw none. That points to a faulted contact. What swayed me in the direction of intrusion is the smaller granite mass some feet to the left of the main body. It does not appear to be fault bounded and therefore I went for the intrusion option. Frankly, it's difficult without actually being there so see the contacts and rocks close up - and keep in mind I am not a practising geologist and my field work is several decades in the past.
    Many thanks, very clear. The picture I sent was from NW Sardinia, where the discontinuity is oblique and not far from vertical, whereas in SW Corsica where I saw something very like it, it was practically horizontal. So I'd have thought that if it is indeed the same formation, then it would not be due to a fault. So the intrusion explanation appears the more likely. I didn't know about granite being intruded as slush, which obviously would give off a lot less latent heat than a fully liquid magma - interesting.
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