DETERMINING SEQUENCE OF EVENTS FROM A GEOLOGIC CROSS SECTION
A geologic cross section is a vertical view of strata as they would appear in a road cut or quarry face. By
interpreting the sequence of events which produced the strata and structures displayed in a cross section, it
is possible to gain an insight into the geologic history of a region.
The sequence of events portrayed in a geologic cross section can be deciphered using the following five
rules:
Rule 1. Superposition - Unless strata have been overturned by extreme folding, the oldest layer
will be on the bottom and the layers become progressively younger above.
Rule 2. Folding, Faulting and Intrusions - Strata and unconformities must have been formed
before they could be folded, faulted or cut by intrusions and are therefore older than the
period of folding, faulting or intrusion.
Rule 3. Unconformities - An erosional surface (unconformity) is always younger than the strata
upon which it is developed and the features that it truncates, but it is older than overlying
strata or features which cut across it..
Rule 4. Cross-cutting Relationships - A fault or intrusion that offsets or cuts across another fault
or intrusion is always the younger because the original fault or intrusion had to exist before it
could be offset by a fault or cut by an intrusion.
Rule 5. Folding Precedes Faulting - In general, when placed under stress, rocks fold as much as
possible before they rupture and fault.
SEQUENCE OF EVENTS IN A SECTION OF THE GRAND CANYON
H
D
K
4
O
Y
R
Unconformity Z
W
T
A
N
3
J
Unconformity S
P
E
C
2
U
X
F
1
B
Unconformity L
The above diagram represents a geologic cross section from the Grand Canyon. The following is a stepby-step explanation of how the sequence of events for this section was deciphered.
Unconformities represent breaks in the depositional history for a region. They naturally divide complex
sections into more easily understood units. One should always begin by dividing the cross section into
units bounded by unconformities and then determine the sequence of events for each unit separately
employing the rules stated above.
Using the unconformities, this cross section can be divided into four units, labeled 1-4. Based upon
superposition (Rule 1), the sequence of units from oldest to youngest is 1, 2, 3 and 4. Unit 1 is bounded
above by unconformity L and part of unconformity S. Unconformity L is truncated by unconformity S and
therefore must be older than S (Rule 3). Unit 1 contains strata X, B and part of C. Because C cuts across
unconformity L, C is younger than L (Rules 2 and 3). The remaining units, X and B, are truncated by the
unconformities and therefore must predate them. (Rule 3). B is an intrusion into X and therefore could
only form after X (Rule 2).
For unit 1 the sequence of events therefore has to be:
1. Formation of X
2. Intrusion B
3. Unconformity L
Intrusion C and unconformity S occurred after Unit 1.
Unit 2 is bounded below by unconformity L and above by unconformity S. It contains the rock layers F,
U, E and part of intrusion C. Because intrusion C extends across unconformity S, it must be younger than
the unconformity (Rules 2 and 3) Unit 2 rests directly upon unconformity L and therefore must be younger
than it (Rule 3). Based upon super position (Rule 1) layers F, U and E were deposited in that order.
Sedimentary strata are normally horizontal when deposited, but layers F, U and E are steeply inclined.
This means they were folded after they were deposited (Rule 2). Because the folds are cut by
unconformity S, which is not folded, folding occurred after deposition of F, U and E, but before
unconformity S (Rule 2).
For unit 2, the sequence of events therefore has to be:
4. Deposition of F, U and E, in that order
5. Folding of unit 1 and F, U and E
6. Unconformity S
Intrusion C occurred after Unit 2.
Unit 3 is bounded by unconformity S below and unconformity Z above. It includes layers P, J, N, A, T, W
and part of C. Layer P rests directly upon unconformity S and therefore must be younger than
unconformity S (Rule 3). Based upon superposition (Rule 1), the other layers must have been deposited in
the following order from oldest to youngest: J, N, A, T and W. Intrusion C extends into, but not beyond
layer P. C could have been intruded into the section at any time following the deposition of layer P and
before exposure of the geologic section by present-day erosion.
For unit 3, the sequence of events has to be:
7. Deposition of P. J, N, A, T and W, in that order
- any time after deposition of P, intrusion C may have occurred
8. Unconformity Z
Unit 4 is bounded by unconformity Z and the present-day erosional surface. It contains the layers Y, R, O,
K, H and D. Layer Y rests directly upon unconformity Z and therefore must be younger than unconformity
Z (Rule 3). Based upon superposition (Rule 1), the other layers must have been deposited in the following
order from oldest to youngest Y, R, O, K, H and D. Sometime after the deposition of layer D, erosion
reduced the land to its present configuration
For unit 4, the sequence of events would be:
9. Deposition of Y, R, O, K, H and D, in that order
10. Erosion to the present-day surface.
The following two page contain four sequence-of-events problems. They can be solved using the same
methods and rules employed above. For each of the problems number the listed events in the correct order
beginning with 1 as the first (oldest) event and proceed to the youngest. Answer the associated questions.
Answers are provided at the end of the exercises.
Unconformity A
Problem 1.
O
N
M
L
H
I
Fault Y
G
J
H
F
E
Z
X
Unconformity C
Problem 1.
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
Problem 2
a.
b.
c.
d.
e.
f.
g.
Deposition of L and M
Deposition of N and O
Deposition of E, F and G
Deposition of H, I and J
Intrusion of granite X
Intrusion of basalt Z
Unconformity A Type _____________
Unconformity C Type _____________
Erosion to present surface
Fault Y Type ______________
Folding of X through J Type
h. _____
J
Problem 2
_____
_____
_____
_____
_____
_____
_____
Deposition of A, E, I, O, U, and Y
Deposition of S and V
Deposition of W, Z, R, N, J and D
Unconformity L Type__________________
Erosion to present surface
Intrusion of basalt C
Folding of beds A through V.
Types ______________________
Fault P Type___________________
D
D
J
N
R
N
Z
R
Unconformity L
W
Z
Y
S
U
V
E
S
O
U
I
W
I
O
Y
O
E
C
U
A
Fault P
Unconformity O
Fault M
Problem 3
W
U
O
J
F
L
V
D
C
H
U
V
Z
K
D
A
Y
E
Y
Y
I
G
S
T
T
Problem 3
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
N
Problem 4
Deposition of K and W
Deposition of N, S T and I
Deposition of E, G, R, A H and C
Deposition of D, Z and V
Deposition of L, O, U, F and J
Folding of layers N through C
Unconformity B Type ___________________
Unconformity X Type___________________
Unconformity O Type___________________
Intrusion of Y
Fault M Type_______________________
Erosion to present surface
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
_____
_____
_____
_____
_____
_____
_____
_____
_____
_____
Deposition of A Through Y
Deposition of C. F and B
Deposition of T and I
Unconformity S Type___________________
Unconformity Q Type___________________
Intrusion of W
Fault P Type_______________________
Fault J Type_______________________
Erosion to present surface
Folding Type _______________________
Fault P
Problem 4
Unconformity Q
F
Unconformity S
Unconformity Q
I
T
F
Y
Z
C
X
G
H
E
K
D
D
M
M
A
G
X
Z
U
U
L
L
D
K
W
H
Fault P
C
B
B
F
A
Unconformity B
Z
Y
Y
R
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
Unconformity X
G
M
Fault J
Correlation
Below are three illustrated rock exposures. Correlate between them using lithostratigraphy, biostratigraphy
and chronostratigraphy.
Period
Fossils A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 1 2 3 4 5 6 7
Zone
Silurian Zone E
Zone D
Ordo.
Zone C
Zone B
Camb.
Zone A
No Fossils
No Fossils
F, G, L, N
No Fossils
No Fossils
K, N, F,
F, G, K, L, R
No Fossils
3, 2, X, R, N
K, F, N
A, C, I, 3, 4
A, 3, C, I
H, K, P, Q
G, W, 2, X
A, C, G, O,
I, L, O, S
A, C, E, O, K, M
A, D, E
H, M, G
A, C, I, K
E, Q, O
A, C, E, K, O
E, J, A, K
J, S, V, 1, 2, 3
A, J, M, B
A, M, P, Q, 7
J. M. B, P, T
B, Z, 2, 3
No Fossils
No Fossils
3, P, A, B, V
No Fossils
No Fossils
Problem 1
Problem 2
a. 09
b. 10
c. 04
d. 05
e. 01
f. 02
g. 08 - angular unconformity
h. 03 - nonfonformity
i. 11
j. 07 - reverse fault
k. 06 - anticline
a.
b.
c.
d.
e.
f.
g.
h.
Problem 3
a. 10
b. 01
c. 02
d. 05
e. 07
f. 03
g. 04 - angular unconformity
h. 06 - disconformity
i. 08 - disconformity
j. Intrusion Y occurred after the
deposition of Z (post 05)
k. 09 - normal fault
l. 11
01
02
05
04 - angular unconformtiy
08
07
03 - syncline, anticline, syncline
06 - normal fault
Problem 4
a. 01
b. 04
c. 07
d. 06 = disconformity
e. 03 - angular unconformity
f. 09 or 10
g. 05 - normal fault
h. 08 - reverse fault
i. 9 or 10
j. 02 - anticline, syncline
E
E
E
E
SILURIAN
D
E
D
D
D
C
C
B
D
C
D
ORDOVICIAN
C
C
B
B
B
A
A
A
CAMBRIAN
A