Download Geologic block diagrams and relative age (Based on an exercise in

Geologic block diagrams and relative age
(Based on an exercise in Tarbuck, Lutgens, Pinzke. 2006. Applications and
Investigations in Earth Science,
5th edition. Prentice Hall, Upper Saddle River, NJ.)
In your reading assignments and in class, you’ve learned about the principles and
techniques geologists use to decipher the Earth’s history by placing geological events in
their proper sequence and establishing how long ago the event occurred. The exercises
below ask you to use the information you’ve learned to interpret hypothetical portions of
the geologic column.
For the 2nd lab exam you should be able to
1. List and explain some of the principles that are used to determine relative ages of
geological events.
2. Determine the sequence of geological events that have occurred in an area by
applying the techniques and procedures for relative dating.
Terms you should be able to define and concepts you should understand:
Relative dating (see lecture notes)
Uniformitarianism (see lecture notes)
Principle of Horizontality (see lecture notes)
Principle of Superposition (see lecture notes)
Inclusion (see below)
Unconformity (see text, section 4-4)
Cross-cutting (see below)
Figure one illustrates a geological cross section, a side view, of the rocks beneath the
surface of a hypothetical region. Use this figure to answer the two questions below the
figure.
Figure 1. Geological block diagram of a hypothetical region showing igneous intrusive features
andsedimentary rocks.
1. Of the two sequences of rocks, A-D and E-G, which were disturbed by crustal
movement after its deposition, A-D or E-G? What principal did you apply to
arrive at your conclusion?
2. Apply the principle of superposition to determine the relative ages of the
undisturbed sequence of sedimentary rocks. List the oldest rock first.
Oldest _________________________________________Youngest
Inclusions
Inclusions are pieces of one rock unit that are contained within another unit (Figure 2).
The rock mass adjacent to the one containing the inclusions must have been there first in
order to provide the rock garments. Therefore, the rock containing the inclusions is the
younger of the two.
Refer to Figure 2, below, to answer the next two questions.
3. Identify and label the inclusions in the figure
4. Of the two rocks, B and C, which rock is older?
Unconformities
As long as continuous sedimentation occurs at a particular place, there will be an
uninterrupted record of the material and fossils. However, if the sedimentation process is
suspended by an emergence of the area from below sea level, then no sediment will be
deposited and an erosion surface will develop. The result is that no rock record will exist
for a part of geological time. Such a gap in the rock record is termed an unconformity. An
unconformity is typically shown on a cross-sectional diagram by a wavy line. Several
types of unconformities are illustrated in your Labaroatory Manual.
5. Identify and label an example of an angular unconformity in Figure 1. (go back to Fig. 1)
Principle of Cross-Cutting Relationships
Whenever a fault or intrusive igneous rock cuts through an existing feature, it is younger
than the structure it cuts. For example, if a granite dike cuts through a sandstone layer,
the sandstone had to be there first and, therefore, is older than the dike. Figure 3 (below)
illustrates a geological cross section showing sedimentary rocks (A,B,D,E,F, and
G), an igneous intrusion feature called a dike (C), and a fault (H). Use Figure 3 to
answer the following 4 questions.
6. Is the igneous intrusion, C, younger or older than sedimentary rocks B and D?
7. Is fault H younger or older than the sedimentary beds A-E?
8. Is the relative age of fault H younger or older than the sedimentary layer F?
9. Did the fault occur before or after the igneous intrusion? Explain how you arrived
at your answer.
Figure 3. Geological block diagram of a hypothetical area showing an igneous intrusion (C), a
fault (H),and sedimentary rocks.
Refer back too Figure 1.
10. Is the igneous intrusion H younger or older than rock layer E and younger or older
than stratum D?
11. What evidence supports the conclusion that the igneous intrusive feature called a
sill, C, is more recent than both of the rock layers B and D and older than the
igneous intrusion H?
Applying Relative Dating Techniques
Geologists often apply several of the techniques of relative dating when investigating the
geologic history of an area. Use Figure 4 to complete the questions below.
12. Which of the structures or layers are obviously igneous rocks?
13. Which structure is a fault?
14. Identify the unconformities in the figure.
15. Is rock layer I older or younger than layer J? What principle did you apply to
determine your answer?
16. Is the fault older or younger than rock layer I? What principle did you apply to
determine your answer?
17. Is the stratum K older or younger than layers A and B. What two principles did
you apply to determine your answer?
18. Is the age of intrusion L older or younger than layers J, I, H, G, and F?
19. List the entire sequence of events, in order from oldest to youngest, by writing the
appropriate letter in the space provided on the figure.
20. Explain why it was difficult to place the fault in a specific position among the
sequence of events.