# ES 10 HW 4 - Stratigraphy and the ``History of Rocks'' Answer Sheet

This answer sheet is designed to give the answers to the questions in Homework 4. It is not meant as a substitute for talking to me about the homework if you have questions. Please ask me after class, or send me e-mail if you still have questions after reading this sheet. My email address is ganderson@ucsd.edu.

The maximum number of points possible on this assignment was 50.

## Stratigraphy

For this section, you had to look at Figures 1 and 2 and try to figure out the order in which rock layers were laid down. The patterns in the rectangles which represent different rock layers were significant, meaning that if you saw the same pattern in two different rectangles in two different outcrops, you were seeing the same rock layer. I built the figures that way so that you could work out the relationship in age from one layer to the next.

#### Question 1 - 4 points

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.

#### Question 2 - 6 points

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.

## Geologic Histories

These questions were a bit more complicated than the previous ones, because the rock layers were not necessarily flat, and there were faults and intrusions and other such complications to work out. You were asked to write up as complete a geologic history as possible, based on the rock units that you could see. In Question 3, this was relatively simple, because not much really has happened, but in Questions 4 and 5, there were (for example) angular unconformities with an unknown amount of missing time. Provided you made it clear that such an event happened, you did fine.

#### Question 3 - 10 points

The rocks in the figure for Question 3 were, relatively speaking, fairly undisturbed. To be sure, they had been folded and a granitic body intruded into the layers, no doubt causing some contact metamorphism (not that you could tell that from the picture, don't worry!). However, 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 was looking for.

#### Question 4 - 10 points

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.
You could argue for a last stage, where the rocks all get raised up so that our road cut or canyon or whatever exposes the rocks, but that's not necessary for this assignment.

#### Question 5 - 20 points

Oh, boy, this is the complicated case. Here there are angular unconformities, faults, and intrusive bodies. If you properly unraveled this one, you should be proud (though perhaps a bit pissed off at me). 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.