Geology Reference
In-Depth Information
?
An excellent example is the history of the Colorado Plateau
(
Figure 17.14). This region provides a record of events
occurring over approximately 2 billion years. Because of the
forces of erosion, the entire record is not preserved at any
single location. Within the walls of the Grand Canyon are
rocks of the Precambrian and Paleozoic eras, whereas Paleo-
zoic and Mesozoic Era rocks are found in Zion National Park,
and Mesozoic and Cenozoic Era rocks are exposed in Bryce
Canyon (Figure 17.14). By correlating the uppermost rocks at
one location with the lowermost equivalent rocks of another
area, geologists can decipher the history of the entire region.
Although geologists match up rocks on the basis of simi-
lar rock type and superposition, correlation of this type can be
done only in a limited area where beds can be traced from one
site to another. To correlate rock units over a large area or to
correlate age-equivalent units of different composition, fossils
and the principle of fossil succession must be used.
Fossils are useful as relative time indicators because they
are the remains of organisms that lived for a certain length
of time during the geologic past. Fossils that are easily identi-
fi ed, are geographically widespread, and existed for a rather
short interval of geologic time are particularly useful. Such
fossils are
guide fossils
or
index fossils
(
◗
What Would You Do
You are a member of a regional planning commission that is
considering a plan for constructing what is said to be a much-
needed river dam that will create a recreational lake. Oppo-
nents of the dam project have come to you with a geologic
report and map showing that a fault underlies the area of the
proposed dam, and the fault trace can be clearly seen at the
surface. The opponents say that the fault may be active, and
thus someday it will move suddenly, bursting the dam and
sending a wall of water downstream. You seek the advice of a
local geologist who has worked in the dam area. She tells you
that she found a lava fl ow covering the fault, less than a kilo-
meter from the proposed dam project site. Can you use this
information, along with a radiometric date from the lava fl ow,
to help convince the opponents that the fault has not moved
in any direction (vertically or laterally) in the recent past? How
would you do this, and what type of reasoning would you use?
Because most fossils have fairly long geologic ranges,
geologists construct
concurrent range zones
to determine the
age of the sedimentary rocks containing the fossils. Concur-
rent range zones are established by plotting the overlapping
geologic ranges of two or more fossils that have different geo-
logic ranges (
Figure 17.15). The
trilobite
Paradoxides
and the brachiopod
Atrypa
meet these
criteria and are therefore good guide fossils. In contrast, the
brachiopod
Lingula
is easily identifi ed and widespread, but
its long geologic range of Ordovician to Recent makes it of
little use in correlation.
◗
Figure 17.16). The fi rst and last occurrences
of fossils are used to determine zone boundaries. Correlat-
ing concurrent range zones is probably the most accurate
method of determining time equivalence.
◗
T
Q
P
T
L
S
K
Q
J
P
T
Sedimentary
Rocks
Lava flow S
G
L
K
Q
N
F
J
O
N
P
N
E
L
L
G
I
D
K
K
I
J
I
J
C
F
R
D
G
G
C
B
F
F
H
A
E
B
E
M
D
S
E
Igneous
Rocks
A
R
D
C
M
A
◗
Figure 17.11
Block Diagram of a Hypothetical Area A block diagram of a hypothetical area in which the various relative
dating principles can be applied to determine its geologic history. See Figure 17.12 to see how the geologic history was
determined using relative dating principles.
Search WWH ::
Custom Search