SEQUENCES OF EVENTS For your sequences of events assignment, we are going to loo
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SEQUENCES OF EVENTS For your sequences of events assignment, we are going to look at four different sequences. You will identify the order of events using the OTY principle for each. 1. Look at this simple geological cross section and work out its sequence of geological events. Figure 1. Geological cross section showing several geological formations. The ground surface is the top of the cross section profile. Standard geological symbols are in use here dots (sandstone); circles (conglomerate); dashes (shale); and bricks (limestone). a. List the sequence of events starting with the oldest at the bottom. 9. Erosion of Q and R . . 5. Erosion of D .1. Deposition of A b. What type of unconformity is present in this geological cross section?Explanation / Answer
Figure 1. a)
9. Erosion of Q and R.
8. Deposition of R.
7. Deposition of Q.
6. Deposition of P.
5. Erosion of D.
4. Deposition of D.
3. Deposition of C.
2. Deposition of B.
1. Deposition of A.
This is rather a simple succession, nothing much to explain.
b) A disconformity is present between D and P, indicated by a time gap or surface of erosion between two parallel layers of sedimentary rock.
Figure 2. a)
12. Deposition of siltstone Y.
11. Deposition of sandstone X
10. Deposition of conglomerate
9. Erosion of U’’.
8. Intrusion of granite
7. Tilting of all the beds.
6. Deposition of limestone.
5. Deposition of shale.
4. Deposition of siltstone Q.
3. Deposition sandstone Y.
2. Erosion of U’.
1. Deposition of schist.
It can be seen that the beds below U’’ are tilted and are older than the intrusion, as it crosscuts the formations. An angular unconformity exists in U’’. Above the U’’, the beds are younger and parallel to each other, so the relative age determination becomes easy.
Figure 3.
11. Deposition of tuff (tf).
10. Erosion of U’.
9. Granite intrusion
8. Faulting
7. Deposition of the conglomerate.
6. Deposition of shale.
5. Deposition of sandstone.
4. Deposition of conglomerate (cg).
3. Erosion of U’’
2. Intrusion of rhyolite
1. Deposition of schist.
This was a bit complex one. I could not go with the blanks given and prepared a new succession. I feel it is correct. Schist bed is deposited first, then rhyolite is intruded into schist, followed by erosion of U’’. Then conglomerate, shale and sandstone and again conglomerate is deposited. Then the entire sequence is faulted including the rhyolite intrusion. As the granite intrusion is not affected by faulting and crosscuts all the other beds deposited below U’, it is younger than all of them. After erosion of U’, a tuff bed is deposited which is unaffected by faulting or intrusion.
Figure 4.
11. Deposition of B
10. Deposition of C.
9. Erosion of U’
8. Intrusion of A
7. Faulting along XY.
6. Folding of layers
5. Deposition of D
4. Deposition of E
3. Deposition of F
2. Deposition of G
1. Deposition of H.
This is rather a simple geologic succession, where successive beds H, G, F, E, and D have deposited, and then they are folded. After that, the folded beds are faulted along XY fault, can be clearly seen in the figure. Although the beds are faulted, the intrusion A is not affected by the faulting, that is why it can be interpreted that this intrusion is younger than the faulting. The next succession is easy to establish.
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