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I am quite new to geology and have a realtive dating problem which I am a bit unsure of. I know very well the principle of superposition, principle of cross-cutting, etc., but am still a little bit unsure on one part. The problem is illustrated in the following picture:

enter image description here

My question is about the intrusion (E). Obviously the intrusion must have occurred later than the deposits of layers B, C and D, but how can I tell if the intrusion occurred before or after the deposit of layer A? In my opinon, both of these scenarios are possible. The intrusion does not appear to cross through A, so I suppose it is possible that A was deposited after the intrusion. But isn't it also possible that A was deposited before the intrusion, and the intrusion simply did not make it past layer B?

If anyone can let me know if there is any certain way of reasoning whether A or E occurred first, then I would greatly appreciate it.

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    $\begingroup$ Indeed, you'd need additional information to tell which came first, A or E. $\endgroup$
    – Daniel Griscom
    Jul 16, 2016 at 20:09

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It does seem like it's impossible to know unless you have additional information. However, I think there is a hint in there. See this white halo around the intrusion? My guess (and I could be wrong here) is that it's not there for artistic reasons but rather it's there to provide a very strong hint.

My feeling that this is some kind of metamorphosed contact zone (or aureole), which transforms sedimentary rocks into their low pressure and high temperature metamorphic equivalents (shales to hornfels, limestone to marble, sandstone to quartzite, etc).

It's hard to see, but I think I can see (when zooming in ) some of the white halo extending into the bottom of layer A. That would mean A was present when E intruded, because you can't metamorphose something that's not there. Otherwise, the metamorphic aureole would be cut by layer A.

enter image description here

Again, I'm not sure because the image is low resolution. Maybe you should ask your teacher or if that's not possible try to figure out if you should already know about metamorphic contact zones in your stage of geological education.

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  • $\begingroup$ looks to me like the contact zone doesn't extend into A but you are right about asking for more information. Also what is the composition of layer A? Would you expect to see metamorphism if it was 'cooked'? $\endgroup$
    – haresfur
    Jul 16, 2016 at 23:40
  • $\begingroup$ Irregular dot pattern is usually used for mudstones, so I would expect some kind of hornfels or something similar $\endgroup$
    – Gimelist
    Jul 16, 2016 at 23:46
  • $\begingroup$ Thanks a lot for your input! Really apprecaite it! I will certainly inqurie more about this :). $\endgroup$
    – Kristian
    Jul 17, 2016 at 9:20

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