15 questions about geology

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Geol 1202 Planet Earth
Assignment: Geologic Cross Sections
The rocks that form the crust of the Earth are often exposed as complicated sequences of
layered, folded and faulted strata. A geologist has to be able to look at these rocks and be able to
unravel the sequence of events to reconstruct the geological history for that area. There are
several tools or methods used to illustrate what the geology of an area looks like.
A geologic map shows a bird’s eye view of the rocks exposed
at the earth’s surface and also provides information about rocks
that are not exposed as well.
a simple map view
A geologic cross section shows the surface of the earth and a
vertical slice of the rocks beneath the surface. Geological data
such as faults or contacts are projected from the surface into the
cross section to predict what might be down there.
Data obtained from drilling or seismic surveys are used to
construct cross sections.
a cross section
Block diagrams are a combination of geologic maps
and cross sections. They are three-dimensional
representations of a block of the earth that can aid
in the interpretation of geological features.
a block diagram
In these simple illustrations faults are marked by a thick straight black line. The direction of
movement along the fault is indicated by the arrows.
Unconformities are marked by the wavy line.
fault
unconformity
The Principles of Relative Time can be a very powerful tool for determining age relationships
between different rock units. Combined with absolute dating they allow us to reconstruct the
structural history of a valley, a coast, or even an entire continent.
In this assignment we are going to apply the principles of relative time to some cross sections.
But first an example.
Take few minutes and look at the figure below – try to think about what must have happened for the
rock units to end up in this specific configuration.
G
F
D
C
B
E
A
10 Ma
20 Ma
In this simple example we can quickly determine the sequence of events and the relative ages of the
rocks given the isotopic ages of the two intrusions. Notice that the events are listed below with the
oldest at the bottom and youngest at the top.
Youngest
G – finally the rocks of unit G were deposited (and have to be younger than 10Ma)
F – there was another period of erosion and deposition less than 10 million years ago (can you see the
unconformity that truncates the middle layer of sedimentary rocks (D) and the 10 Ma intrusion?)
E – the rocks were intruded again 10 million years ago (isotopic date of 10 Ma)
D – there was a period of deposition (it has to be younger than 20 Ma because it is above the
unconformity
C – there was a period of erosion between 20 and 10 million years ago. (can you see the unconformity
that truncates the 20 Ma intrusion?)
B – the rocks were intruded 20 million years ago. (isotopic date of 20 Ma)
A – the lowest sequence of sedimentary rocks were deposited and then deformed more than 20
million years ago. (the 20 Ma intrusion cuts across the deformed layers)
Oldest
NAME:
First some simple cross sections to get you used to applying the principles
STUDENT #
The letters indicate rock strata or structural features.
Assume that all rock units are sedimentary unless labelled otherwise.
Granite is an igneous rock – usually intrusive.
Dikes are igneous intrusions.
Unconformities are indicated by wavy lines
Faults are straight thick black lines.
Questions
Figure 1
This figure is of a basic sequence of flat lying strata.
G
1. Which is the oldest?
E
2. Which is the youngest (or most recent)?
F
3. Which principle of relative time did you use to arrive
at this answer?
Figure 2
In this figure some of the strata have been tilted. The
wavy boundary U represents an unconformity.
4. Were the strata tilted before unit
or after?
E
E
was deposited
5. Which principle of relative time did you use to arrive
at this answer?
U
D
C
F
B
A
6. Refer back to Lecture Three and identify the type of
unconformity represented by U
Figure 3
This figure is of a basic sequence of flat lying strata that
have been cut by a fault.
A
C
C
7. Is the fault older or younger than the strata?
8. Which principle of relative time did you use to arrive at
this answer?
E
D
E
D
Figure 4
This figure has some horizontal strata intruded by a
granite body.
9. In this figure is the intrusion
than unit C ?
C
older or younger
I
B
10. Which principle of relative time did you use to
arrive at this answer?
I
A
11. List the age of the strata in the correct order
starting with the oldest at the bottom and the youngest
at the top.
Isotopic Dating:
When used in conjunction with the principles of relative time isotopic (radiometric) dates help refine
the interpretation of the geological history of an area and constrain the age of strata or events. For
example, suppose a friend told you that their older brother was 28 and their younger sister was 24.
With this information you could confidently constrain the age of your friend to be older than 24 but
younger than 28.
Figure 5
This figure has some horizontal strata intruded by a
granite body. The intrusion I and dyke D
have been dated with radioactive minerals of known
half-lives.
D
is 300 Ma
I
is 645 Ma
12. What is the age of strata
M
D
L
L
?
I
What do I submit?
You can just write the question numbers and give your answers for the first twelve questions. You will
need to print the last page, then answer the questions in the space provided, and annotate the figure
as instructed. Then submit a photo of the completed page with the rest of your answers. You may
insert the image into a Word document with your other answers if you wish.
Don’t forget to put your name ans student number on the sheet.
NAME:
STUDENT #
Figure 6
E
H
I
H
B
I
I
D
A
A
J
F
F
G
F
G
G
C
C
13. Place the geologic features (formations, faults, unconformities) labelled A to I
in the proper sequence from oldest to youngest in the space below.
youngest
oldest
_________
_________
_________
_________
_________
_________
_________
_________
_________
_________
14. Place arrows on each side of the fault
15. The dyke
granite
G
D
J
indicating the relative sense of movement.
has been radiometrically dated as 37 Ma, the diorite
C
as 48Ma, and the
as 66Ma. Using these ages constrain the age of the following features.
E
J
G

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