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 email@example.com.
The maximum number of points possible on this assignment was 50.
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.
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.
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.
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.
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:
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:
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:
You were asked to answer two further questions in this problem. My answers are:
You could argue that the granitic body is actually a laccolith, which is a sort of mushroom-shaped body of igneous rock, which usually domes up the layer(s) above it (as with layer A). I wouldn't take points off for such an answer -- in fact, I'd be impressed.