Quick query

Alvin T. Grey

Hi Guys,

can anyone confirm for me the type of rock that Benevenagh in Derry is composed of. I'm thinking schist (grey, shot full of crystal nodules, and curiously pitted and holed.) The scree and boulders are very soft an easily crumble.

Can someone narrow it down a little for me as I'm thinking of going fossil hunting in the area and it's no good if I can't identify the rock and the time period.

Thanks.

El Siglo

Alvin T. Grey wrote: »

Hi Guys,

can anyone confirm for me the type of rock that Benevenagh in Derry is composed of. I'm thinking schist (grey, shot full of crystal nodules, and curiously pitted and holed.) The scree and boulders are very soft an easily crumble.

Can someone narrow it down a little for me as I'm thinking of going fossil hunting in the area and it's no good if I can't identify the rock and the time period.

Thanks.

No, it's basalt. Schist is a sedimentary rock so it will not have crystals. Probably some dolerite dykes around there as well (dykes formed during the parting of the Atlantic about 65 ma). There should be some underlying limestone so you might be lucky and find some belemnites etc...

Alvin T. Grey

El Siglo wrote: »

No, it's basalt. Schist is a sedimentary rock so it will not have crystals. Probably some dolerite dykes around there as well (dykes formed during the parting of the Atlantic about 65 ma). There should be some underlying limestone so you might be lucky and find some belemnites etc...

That'd be nice. And if not, it's a beautiful place to spend an afternoon.
I snapped some macro pictures of the rock in question, can I post some later for you guys to confirm?

El Siglo

Alvin T. Grey wrote: »

That'd be nice. And if not, it's a beautiful place to spend an afternoon.
I snapped some macro pictures of the rock in question, can I post some later for you guys to confirm?

Yeh post away.

Alvin T. Grey

Just a quick pic

El Siglo

Alvin T. Grey wrote: »

Just a quick pic

Firstly, that's basalt as I said earlier (all of those images are basalt), my best estimation of age is 55 ma (55 million years ago) during the spreading of the Atlantic. If you can, did you take any pictures of any of the dykes or rock faces? These could be interesting for interpreting the features.

Image 1: That's amygdaloidal basalt, it's extrusive, fine grained and the crystals are anhedral, about medium to dark in colour. It's a basic volcanic rock, has a silica content about 45 - 55%, with the main minerals composed of calcium-rich plagioclase feldspar and pyroxene. You'll also find some olivine and magnetite as well. The amygdales (i.e. the white crystals) are cavities (vesicles) that were filled up with minerals, zeolites and quartz being the main ones of these. You probably found this nearer to the centre of your basalt, it's when the lava cools the air bubbles form cavities in which minerals precipitate and form the amygdales.

Image 2: That's vesicular basalt, again fine grained, extrusive, any crystals present are anhedral, euhedral. Very similar composition to most other types of basalt, your picture is text book and the mineralogy is more or less the same as amydaloidal basalt (above). The big difference is that it's covered in empty, gas bubbles known as vesicles and if these cavities were filled with minerals then the vesicular basalt would become amydaloidal. Again, cooling probably prevented this given that it looks fairly cracked and brittle looking.

Image 3: Same as above, amydaloidal basalt.

Hope that helps.

Alvin T. Grey

El Siglo wrote: »

Firstly, that's basalt as I said earlier (all of those images are basalt), my best estimation of age is 55 ma (55 million years ago) during the spreading of the Atlantic. If you can, did you take any pictures of any of the dykes or rock faces? These could be interesting for interpreting the features.

Image 1: That's amygdaloidal basalt, it's extrusive, fine grained and the crystals are anhedral, about medium to dark in colour. It's a basic volcanic rock, has a silica content about 45 - 55%, with the main minerals composed of calcium-rich plagioclase feldspar and pyroxene. You'll also find some olivine and magnetite as well. The amygdales (i.e. the white crystals) are cavities (vesicles) that were filled up with minerals, zeolites and quartz being the main ones of these. You probably found this nearer to the centre of your basalt, it's when the lava cools the air bubbles form cavities in which minerals precipitate and form the amygdales.

Image 2: That's vesicular basalt, again fine grained, extrusive, any crystals present are anhedral, euhedral. Very similar composition to most other types of basalt, your picture is text book and the mineralogy is more or less the same as amydaloidal basalt (above). The big difference is that it's covered in empty, gas bubbles known as vesicles and if these cavities were filled with minerals then the vesicular basalt would become amydaloidal. Again, cooling probably prevented this given that it looks fairly cracked and brittle looking.

Image 3: Same as above, amydaloidal basalt.

Hope that helps.

Image 2. - Yes it is very brittle, and shatters easily.
Yes it was a help. It tells me that there is bugger all chance of finding any types of fossil in that stuff. Secondly if the time line you give is correct, then ~55M years ago would have put it reletivly close to the KT border and this the likelyhood of finding flora and fauna in this rock anyway would be unlikely.

- I have another site in mind, that's fairly close. It's on the edge of the sperrins, and appears to be composed mostly of ash. I'd like to get an opinion on this also if that would be possible?

El Siglo

Alvin T. Grey wrote: »

Image 2. - Yes it is very brittle, and shatters easily.
Yes it was a help. It tells me that there is bugger all chance of finding any types of fossil in that stuff. Secondly if the time line you give is correct, then ~55M years ago would have put it reletivly close to the KT border and this the likelyhood of finding flora and fauna in this rock anyway would be unlikely.

Actually, you may find fossils in the underlying limestone, this is what occurs in places like Belshaws Quarry, Antrim (i.e. basalt lying on top of limestone). So start looking below the basalt for oolithic limestone or chalk. These are marine sedimentary rocks so they preceded the expansion of the Atlantic and will should have some belemnites and brachiopods present. The KT event killed off cretaceous creatures, but the preceding periods in particular the triassic etc... should still be preserved in the sedimentary material.

- I have another site in mind, that's fairly close. It's on the edge of the sperrins, and appears to be composed mostly of ash. I'd like to get an opinion on this also if that would be possible?

Could you give a more precise location? This book might be of use to you before you start selecting more sites. You might want to upload pictures of your site, like large scale ones where you can see the dykes and sills etc...

Alvin T. Grey

El Siglo wrote: »

Actually, you may find fossils in the underlying limestone, this is what occurs in places like Belshaws Quarry, Antrim (i.e. basalt lying on top of limestone). So start looking below the basalt for oolithic limestone or chalk. These are marine sedimentary rocks so they preceded the expansion of the Atlantic and will should have some belemnites and brachiopods present. The KT event killed off cretaceous creatures, but the preceding periods in particular the triassic etc... should still be preserved in the sedimentary material.

I just have to locate an exposed limestone rock face underneath. The problem is accessing it. Will keep searching.

Could you give a more precise location? This book might be of use to you before you start selecting more sites. You might want to upload pictures of your site, like large scale ones where you can see the dykes and sills etc...

It's the Ness river vally just outside Drumahoe. Unfortunatly all I can find geology-wise regarding this is from the lake in scotland of the same name....

BTW thank you for your patience, As you can tell I'm very very new to this stuff..

El Siglo

Just keep searching and invest in a rock hammer and rocks-minerals handbook for identification. There's a paper I've come across on the Sperrins, I can pm it to you, don't know how useful it would be but it might give you some ideas.

Alvin T. Grey

El Siglo wrote: »

Just keep searching and invest in a rock hammer and rocks-minerals handbook for identification. There's a paper I've come across on the Sperrins, I can pm it to you, don't know how useful it would be but it might give you some ideas.

Thanks, Please do.

Tom Fiat

I would be particularly interested in this paper also.
Please forward if that is possible.

El Siglo

Tom Fiat wrote: »

I would be particularly interested in this paper also.
Please forward if that is possible.

I can't without an email address, but here's the citation;

Hartley, J.J., 1937/38. The Dalradian Rocks of the Sperrin Mountains and Adjacent Areas in Northern Ireland. Proceedings of the Royal Irish Academy. Section B: Biological, Geological, and Chemical Science, Vol. 44, pp. 141-171.

Geoboy

El Siglo wrote: »

No, it's basalt. Schist is a sedimentary rock so it will not have crystals. Probably some dolerite dykes around there as well (dykes formed during the parting of the Atlantic about 65 ma). There should be some underlying limestone so you might be lucky and find some belemnites etc...

Just to correct you there Schist is not a sedimentary rock. It is a metarmorphic rock and depending on the thermobarometric (T/P) conditions it may be possible to identify certain minerals in it (mica, etc).

El Siglo

Geoboy wrote: »

Just to correct you there Schist is not a sedimentary rock. It is a metarmorphic rock and depending on the thermobarometric (T/P) conditions it may be possible to identify certain minerals in it (mica, etc).

I stand corrected! My hard rock stuff is a little rusty, but that was Gneiss of you to point it out...;)