## ES 10 HW 5 Example Greg Anderson

This is last year's problem set, which I used in the discussion on Wednesday 4 March. I've put it here to act as an example of what I'm looking for. Your problems are similar, but not the same, so you can use this as a guide. However, it will not substitute completely for talking with me if you have problems. Please send me e-mail at anderson@python.ucsd.edu if you have problems, or alternatively come talk to me at class.

### Stratigraphy

#### Question 1

You have looked at three outcrops today, and you have sketched them in your notebook (see the figure labeled ``Figures for Question 1'' below), using rectangles to represent the rock layers, with different layers having different fill patterns. You have arbitrarily labeled the rock layers A-J, carefully noting that the labels do not necessarily correspond to the correct chronological order.

You're back at camp now, and you are going to assemble the stratigraphic column. Write down the chronological order for the rock layers, with the oldest rock layer first. That is all you need to write down -- just the order of the rock layers. If you decide that some of the layers are the same rock (just with different labels), write down both labels with a slash (``/'') between them, like ``A/B''.

Your answer should look something like: I, J, G, D/E, F, A, C, B, H. Note that this order is probably not the right one.

Layers B and F are the same rock unit, as are layers A and E and layers D and J. You can use these relationships and the order of layers to work out the following chronological listing (with oldest first):

C, B/F, A/E, D/J, I, H, G. C is the oldest layer and G is the youngest layer. I know C is the oldest layer because, when I reassemble the rock column in the correct order, layer C is on the bottom. The Principle of Superposition says that older layers are below younger layers, and since there's no evidence here of folding or tilting or anything else disturbing the column, I can assume C is oldest.

#### Question 2

It has been another long day in the field, and you have been looking at more rock outcrops, and making more notebook sketches (see the figure labeled ``Figures for Question 2'' below). Again, you have labeled the rock layers arbitrarily (A-K).

You have also noted that there is an unconformity between layers ``A'' and ``B''.

It is again evening in camp, and you are trying to make sense of the order of rocks you have seen. Again, you are to write down the correct chronological order for the rock layers, just as in Question 1.

Here, things are only slightly more complicated than they are in question 1. The complication is that there is an unconformity between layers A and B (in outcrop 1). Again, the patterns are significant, and if you look at outcrop 3, you will find that layers A and G are the same, as well as layers B and J. However, there is no unconformity in outcrop 3, so you can use outcrop 3 to ``fill in the gap'' between layers A and B.

The correct chronological ordering (with oldest first) is: F, D/E, C/K, B/J, I, H, G/A. Layer F is the oldest, and layers G and A are the same youngest layer. Layers I and H are the layers missing in outcrop 1.

### Stratigraphy and the ``History of Rocks''

Here, I want you to write down the geologic history of the rocks in this area. When you are asked to write down the geologic history of a region, what does that mean? It means you need to list all the important geologic events which you can deduce from the rock section you have. Some of the things you might have to list are the order in which rock layers were laid down, when intrusions or extrusive volcanics were made relative to the other events in the history, when layers were eroded or folded or faulted, etc. Have a look at figure 7.2 in your textbook -- it does a good job of explaining how to unravel geologic histories.

#### Question 3

Today, you and the field crew have been exploring a river canyon. High up in the wall of one side of the canyon, you have seen (and sketched) what is shown below in the ``Figure for Question 3'' (surprise, surprise). Here, the big ``plus signs'' are one rock layer, the circles are another, and the filled squares are a third (you've labeled them A, B, and C). There's also a really nice granitic body.

You need to write down (in chronological order) the geologic history of the region. You don't need to be long-winded. A simple list like

1. First, this happened
2. Next, this happened
3. And then this happened
4. Finally, this happened
is fine!

The rocks in the figure for this question were, relatively speaking, fairly undisturbed. To be sure, they had been folded and a granitic body intruded into the layers, but the folding was relatively gentle, and there were no overturned folds or faults or unconformities (other than the intrusion). So the history is fairly simple. Here it is, in the form of a list:

1. First, layer C was laid down.
2. Next, layer B was laid down.
3. Layer A was laid down.
4. Layers A-C were folded into gentle anticlines and synclines.
5. Finally, a localized granitic body intruded into the layers and cooled.
A list like the one above is all I am looking for. You don't help yourself or me by being long-winded in these problems.

#### Question 4

You are again doing the same sort of work. Look at the ``Figure for Question 4'' (shown below) and write down the geologic history of the area. Again, just a simple list (as in Question 3) is all you need to do -- don't spend too much time being long-winded.

In this case, some of the rock layers were tilted and eroded after they were laid down flat, then other rock layers were deposited on top of them, making an angular unconformity. However, we have no way of knowing how much time elapsed or how many rock layers were eroded before the current structure was developed. You needed to be sure to say that. Here's my history:

1. Layers D, E, F, G, and H were laid down in that order.
2. Sometime after deposition of H, the rocks were tilted.
3. Probably at the same time, they were uplifted, and erosion kicked in.
4. After they were eroded to a flat surface, the rock layers subsided to a level where more deposition could occur.
5. After an unknown amount of time, layer C was laid down. Here there is an angular unconformity, and we have no way of knowing how much absolute time is missing.
6. Next, layer B was laid down.
7. Finally, layer A was laid down.

#### Question 5

After a lot of work, you have sketched in your notebook the ``Figure for Question 5'' shown below. Here, there are rock layers going in all sorts of directions, a fault, and some sort of granitic intrusion (which you have called layer B).

Again, you need to write down the geologic history of the region. However, you've also been asked to answer the following two questions:

1. What kind of a fault is shown in your sketch?
2. Is the granitic intrusion a dike or a sill?

Here's my history of the area:

1. Layers F, G, H, I, J, K, L, and M were laid down in that order.
2. Sometime after deposition of M, these layers were tilted back.
3. Probably at the same that they were tilted, these layers were uplifted and erosion kicked in.
4. After they were eroded to a flat surface, these layers subsided to a level where more depositon could occur.
5. After an unknown amount of time, layer E was laid down. Here there is an angular unconformity, and we have no way of knowing how much absolute time is missing.
6. After layer E was laid down, but before layer D, an active fault broke layers E, I, H, and G. We know it had to start after layer E, because the fault broke that layer, and it had to stop before layer D, because that layer is not broken by the fault. Layers G, H, and I are broken because they are older than the fault and just happen to be in the ``right'' spot.
7. Next, layer D was laid down. It is thicker on the right side of the fault because the right side of the fault moved downward, leaving a spot to be filled in by sediments.
8. Next, layers C and A were laid down in that order.
9. After layer A was laid down, a granitic body was intruded between layers A and C, doming A upward.
Complicated, yes -- but certainly not impossible.