GeologyRocks

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.

Thank you very much Jon.

Jules

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. 

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

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.

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.

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.

HI JULES ,  I TOO AM STUDYING A SIMILAR QUESTION , AND AM HAVING DIFFICULIES GETTING MY HEAD AROUND IT, I WAS WONDERING IF YOU HAVE IMPROVED OR MODIFIED YOUR ORIGINAL RESPONSE TO YOUR OWN QUESTION , WITH THE POSSIBILITY THAT HOOGLEBOOGLE'S INTERPRETATION OF EVENTS (THE SANDSTONE IS UNDERNEATH THE GABBRO IS "CORRECT") IS THE "TRUTH" ? PLEASE GET IN TOUCH AS ANY HELP ON THIS OR ANY OTHER QUESTION FOR THAT MATTER WOULD BE VERY MUCH APPRECIATED. EVERYONE . THANKYOU.

Hi

The Gabbro is cutting through both the rocks. So the schist is the oldest, with the sandstone in second place and finally the gabbro intrudes both as the youngest.

First - there's no need to shout. Talking in all CAPS is "shouting". Plus it makes your post really difficult to read I know you sent a PM too on this, but I'll answer here. The wording in the first post is ambiguous. It is difficult to tell if the sequence is:

1. regional metamorphism of existing rock
2. gabbro intrusion
3. sandstone layers

OR

1. regional metamorphism of existing rock
2. sandstone layers
3. gabbro intrusion

If the wording from the question is exactly as in the first post, then I would be inclined to go with the second option as, to me, "cuts across a layer of schist and a layer of sandstone above" says the intrusion cuts both the schist and the sandstone. The "above" I take to mean that the sandstone is above the schist.

Remember, when carrying out these interpretation to include any tectonics, such as dipping strat, faults, folds, etc. Also any unconformatites should be labelled as such (there is one in the above sequence). Good luck!