Geology Reference
In-Depth Information
A
B
C
◗
Figure 17.6
The Principle of Fossil Succession This generalized diagram shows how geologists use
the principle of fossil succession to identify strata of the same age in different areas. The rocks in the
three sections encompassed by the dashed lines contain similar fossils and are therefore the same age.
Note that the youngest rocks in this region are in section B, whereas the oldest rocks are in section C.
MYA
Relative Dating
We can decipher the geologic history of
the area represented by the block diagram
in
0
0
1
1
2
2
3
Figure 17.11 by applying the various
relative-dating principles just discussed.
The methods and logic used in this ex-
ample are the same as those applied by
19th-century geologists in constructing
the geologic time scale.
According to the principles of
superposition and original horizon-
tality, beds A-G were deposited hori-
zontally; then either they were tilted,
faulted (H), and eroded, or after de-
position, they were faulted (H), tilted,
and then eroded (
◗
3
Unconformity
6
7
4
Amount
of rock
removed
by erosion
Hiatus
8
5
9
6
10
7
11
8
12
9
d
10
Figure 17.12a-c).
Because the fault cuts beds A-G, it must
be younger than the beds according to the
principle of cross-cutting relationships.
Beds J-L were then deposited hor-
izontally over this erosional surface,
producing an angular unconformity
(I) (Figure 17.12d). Following depo-
sition of these three beds, the entire
sequence was intruded by a dike (M),
which, according to the principle of
◗
11
12
a
b
c
◗
Figure 17.7
The Development of a Hiatus and an Unconformity
a
Deposition
began 12 million years ago (mya) and continued more or less uninterrupted until 4 mya.
b
Between 3 and 4 mya, an episode of erosion occurred. During that time, some of the
strata deposited earlier was eroded.
c
A hiatus of 3 million years thus exists between
the older strata and the strata that formed during a renewed episode of deposition
that began 3 mya.
d
The actual stratigraphic record as seen in an outcrop today. The
unconformity is the surface separating the strata and represents a major break in our
record of geologic time.
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