I wonder if someone can confirm that my thinking is correct on a question:
A gabbro intrusion cuts across a layer of schist and a layer of sandstone above. I have to comment on the geological processes that probably took place, in the correct order, based on my geology knowledge. So, here goes:
The schist would have originally been deposited first as a layer of mudstone, which through regional metamorphism formed into the layer of schist. An unknown layer (or layers) of rock above the schist would have been present for the gabbro to then intrude into (since it could not have formed by rapid cooling at the surface). Then through erosion/ weathering etc, the layers of rock above the schist would have eventually exposed the upper part of the gabbro intrusion. Finally, the layer of sandstone would have been deposited and lithified (through compaction and cementation) to form the sandstone. I assume the sandstone wasn't present when the intrusion took place because it hasn't undergone contact metamorphism. I also assume that the upper surface of the schist is therefore an unconformity.
Is this about right??
I am wondering what happens to schist when it undergoes contact metamorphism? I assume that as it is baked, the mineral structure recrystalizes and becomes finer?
Sounds just about right. There would have been more than just the one layer of sandstone, etc on top - you need subsequent erosion, etc to expose the sandstone too (remember to include any dip on the sandstone, if some exists).
The schist->sandstone contant is an unconformity.
Contact metamorphism on a schist? Interesting question. My vague memories of metamorphic petrology say that a schuist occurs at moderate temperatures and pressures - a few hundred degrees C. The gabbro is likely to be hotter than that, so around the intrusion you might see the schist with altered mineralogy. I doubt you can assume that the grains would be finer: in fact the reverse could be true.
The question states that the gabbro cuts accross the schist and sandstone. Doesn't that mean that the sandstone must be there before the gabbro intrusion?
I read it as the intrusion cuts the schist, but the there is a layer of sandstone above. However, if the gabbro cuts the schist and the sandstone, you are correct, hoogleboogle (great name )
I am obviously studying the same thing as jules who posted the original question. The question states that the gabbro cuts accross both the other rocks (schist and sandstone), and the schist is the oldest.
I am struggling with this question just because of a lack of extra information. There is no mention of contact metamorphism, or fossils or anything, so writing a geological history of the region seems very random (at least to me).
I want to say that the region is under the ocean, but in my textbook it says that schist is formed under mountains, and the oceanic crust is mainly basaltic. I also want to say that the gabbro was formed at an oceanic arc - but there is no other evidence of that - i.e. pillow lava, basalt etc. Yet my resources tell me that if this rock was at a mid-ocean ridge it would likely have been subducted and so not be available for me to write about.
I keep writing my answer, contadicting myself, and then starting again. Any suggestions on how to make assumptions would be really appreciated.
There's only a few assumptions you can make, depending on the level of information you have.
So, some assumptions you can make:
- Use cross-cutting to obtain relative placements of rock units
- Most sedimentary rocks form under marine conditions (not necessarily ocean, it could be a continental sea, like the North Sea)
- Schist is caused by regional metamorphism such as that observed during orogenesis
Don't try and over analyse the info you have. From what I remember at A-level, these kinds of exercises did not require a lot of detail (such as the origin of the gabbro, the orogeny associated with the schist, etc, etc), just the order in which events happened.
I now think the point of the question is straight forward. The schist is the oldest, then the gabbro and finally the sandstone which was deposited after the intrusion took place and after the original layers of rock above were pushed up and eroded away. This meant that the schist and gabbro sticking out were on the surface where new deposits were then layed on top.
I think this is the most plausible answer based on the lack of information.
Interesting to read the problems at hand. As Jon says keep the answers simple and only talk about those things which you can definately see. One of the commonest mistakes I observe through my students is over complication of the material. In this situation things to avoid are absolutes, such as specific age (you can't possibly know the age of the exposure without the background information), tectonic setting, (unless certain sequences are present).In the case of the problem at hand you can surmise that both contact and regional metamorphism has taken place but you can't say where this occurred. I know that the changing composition of intrusions points to certain settings but without other information you can't be sure.
The skills that are being looked for in a problem such as this are the individuals ability to apply geological laws, in this case these would be the law of cross-cutting relationships and steno's laws of superposition, (oldest bed at bottom, original horizontality and lateral continuity). there are some important variations to take into account but when dealing with a problem like this those can be discarded. Always stick to the simple explanation of those things that you see and you will do well.
Rank: Topaz
Joined: 18/12/2006
Points: 3126
Sounds just about right. There would have been more than just the one layer of sandstone, etc on top - you need subsequent erosion, etc to expose the sandstone too (remember to include any dip on the sandstone, if some exists).
The schist->sandstone contant is an unconformity.
Contact metamorphism on a schist? Interesting question. My vague memories of metamorphic petrology say that a schuist occurs at moderate temperatures and pressures - a few hundred degrees C. The gabbro is likely to be hotter than that, so around the intrusion you might see the schist with altered mineralogy. I doubt you can assume that the grains would be finer: in fact the reverse could be true.
Hope that helps.
Geologists are gneiss!!
Rank: Talc
Joined: 29/03/2007
Points: 3
Thank you very much Jon.
Jules
Rank: Talc
Joined: 10/04/2007
Points: 3
The question states that the gabbro cuts accross the schist and sandstone. Doesn't that mean that the sandstone must be there before the gabbro intrusion?
Rank: Topaz
Joined: 18/12/2006
Points: 3126
I read it as the intrusion cuts the schist, but the there is a layer of sandstone above. However, if the gabbro cuts the schist and the sandstone, you are correct, hoogleboogle (great name
)
Geologists are gneiss!!
Rank: Talc
Joined: 10/04/2007
Points: 3
I am obviously studying the same thing as jules who posted the original question. The question states that the gabbro cuts accross both the other rocks (schist and sandstone), and the schist is the oldest.
I am struggling with this question just because of a lack of extra information. There is no mention of contact metamorphism, or fossils or anything, so writing a geological history of the region seems very random (at least to me).
I want to say that the region is under the ocean, but in my textbook it says that schist is formed under mountains, and the oceanic crust is mainly basaltic. I also want to say that the gabbro was formed at an oceanic arc - but there is no other evidence of that - i.e. pillow lava, basalt etc. Yet my resources tell me that if this rock was at a mid-ocean ridge it would likely have been subducted and so not be available for me to write about.
I keep writing my answer, contadicting myself, and then starting again. Any suggestions on how to make assumptions would be really appreciated.
Rank: Topaz
Joined: 18/12/2006
Points: 3126
Is this A-Level (or 1st year undergrad) work?
If so, this is the kind of thing that I was taught at A-Level:
/tutorials/geotime_cross_cutting
There's only a few assumptions you can make, depending on the level of information you have.
So, some assumptions you can make:
- Use cross-cutting to obtain relative placements of rock units
- Most sedimentary rocks form under marine conditions (not necessarily ocean, it could be a continental sea, like the North Sea)
- Schist is caused by regional metamorphism such as that observed during orogenesis
Don't try and over analyse the info you have. From what I remember at A-level, these kinds of exercises did not require a lot of detail (such as the origin of the gabbro, the orogeny associated with the schist, etc, etc), just the order in which events happened.
Hope that lot helps
Geologists are gneiss!!
Rank: Talc
Joined: 10/04/2007
Points: 3
Thats great. I started this question trying to give it specific ages, like 443 Ma. I find the simple questions the most difficult.
The link above is very helpful - I might sketch myself something similar.
Rank: Talc
Joined: 29/03/2007
Points: 3
Hello both,
I now think the point of the question is straight forward. The schist is the oldest, then the gabbro and finally the sandstone which was deposited after the intrusion took place and after the original layers of rock above were pushed up and eroded away. This meant that the schist and gabbro sticking out were on the surface where new deposits were then layed on top.
I think this is the most plausible answer based on the lack of information.
Jules.
Rank: Talc
Joined: 24/10/2006
Points: 41
Cum hoc ergo propter hoc
Hi Everyone.
Interesting to read the problems at hand. As Jon says keep the answers simple and only talk about those things which you can definately see. One of the commonest mistakes I observe through my students is over complication of the material. In this situation things to avoid are absolutes, such as specific age (you can't possibly know the age of the exposure without the background information), tectonic setting, (unless certain sequences are present).In the case of the problem at hand you can surmise that both contact and regional metamorphism has taken place but you can't say where this occurred. I know that the changing composition of intrusions points to certain settings but without other information you can't be sure.
The skills that are being looked for in a problem such as this are the individuals ability to apply geological laws, in this case these would be the law of cross-cutting relationships and steno's laws of superposition, (oldest bed at bottom, original horizontality and lateral continuity). there are some important variations to take into account but when dealing with a problem like this those can be discarded. Always stick to the simple explanation of those things that you see and you will do well.
Cum hoc ergo propter hoc