Geologic Time,You must submit a photo of your annotated Figure 4 as well as the answers to each question.
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.
The Principles of Relative Time can be a very powerful tool for determining age relationships
between different rock units. Combined with absolute dating it allows 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 belwo 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)
First some simple cross sections to get you used to applying the principles
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.
C
1. Which unit is the youngest?
B
2. Which principle of relative time did you use to
arrive at this answer?
A
Figure 2
In this figure some of the strata have been tilted.
The wavy boundary represents an unconformity.
E
D
3. Were the strata tilted before unit E was deposited
or after?
C
B
A
4. Which principle of relative time did you use to
arrive at this answer?
Figure 3
This figure has some horizontal strata intruded by a
granite body.
C
5. In this figure is the intrusion D older or younger
than unit C?
B
6. Which principle of relative time did you use to
arrive at this answer?
A
D
Figure 4
This figure is a more complex cross section.
Study it carefully.
X
M
E
E
L
L
Z
Z
A
R
A
F
D
H
H
B
F
G
Granite
B
Q
There are three unconformities illustrated on the figure but I have only indicated two of them.
7. What type of unconformity does M represent?
8. What type of unconformity does D represent?
9. Indicate the location of the third unconformity on the figure with a wavy line.
What type of unconformity is it?
10. Locate the two faults illustrated on the figure (thick lines). Use arrows to indicate the
direction of movement along the faults. (look at the example on the first page)
What type of fault are they?
11. Place the rock units and structures indicated by the letters in the order in which they
occurred from oldest to youngest.
Use the table below so that you order them according to convention the oldest at the bottom.
The oldest has been done for you.
youngest
oldest
D
granite was exposed and eroded
What do I submit?
Submit the question numbers and your answers.
You must also include a photo or scan of your annotated cross section (Figure 4).