Geology Reference
In-Depth Information
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whereas a sill, resulting from later igneous intrusion, is younger
than all of the beds below it and younger than the immediately
overlying bed as well.
To resolve such relative-age problems as these, geologists
look to see whether the sedimentary rocks in contact with
the igneous rocks show signs of baking or alteration by heat
(see the section on contact metamorphism in Chapter 7).
A sedimentary rock that shows such effects must be older
than the igneous rock with which it is in contact. In Figure
17.4, for example, a sill produces a zone of baking immedi-
ately above and below it because it intruded into previously
existing sedimentary rocks. A lava flow, in contrast, bakes
only those rocks below it.
Another way to determine relative ages is by using the
principle of inclusions
. This principle holds that inclu-
sions, or fragments of one rock contained within a layer of
another, are older than the rock layer itself. The batholith
shown in
What Would You Do
You have been chosen to be part of the fi rst astronaut crew to
land on Mars. You were selected because you are a geologist,
and therefore your primary responsibility is to map the geol-
ogy of the landing site area. An important goal of the mission
is to work out the geologic history of the area. How will you go
about this? Will you be able to use the principles of relative
dating? How will you correlate the various rock units? Will you
be able to determine absolute ages? How will you do this?
Accordingly, the geologic record is incomplete wherever an
unconformity is present, just as a book with missing pages
is incomplete, and the interval of geologic time not repre-
sented by strata is called a
hiatus
(
Figure 17.7).
The general term
unconformity
encompasses three specifi c
types of surfaces. First, a
disconformity
is a surface of erosion
or nondeposition separating younger from older rocks, both
of which are parallel with one another (
◗
Figure 17.5a contains sandstone inclusions, and
the sandstone unit shows the effects of baking. Accordingly,
we conclude that the sandstone is older than the batholith.
In Figure 17.5b, however, the sandstone contains granite
rock pieces, indicating that the batholith was the source
rock for the inclusions and is therefore older than the
sandstone.
Fossils have been known for centuries (see Chapter 18),
yet their utility in relative dating and geologic mapping was
not fully appreciated until the early 19th century. William
Smith (1769-1839), an English civil engineer involved in
surveying and building canals in southern England, indepen-
dently recognized the principle of superposition by reasoning
that the fossils at the bottom of a sequence of strata are older
than those at the top of the sequence. This recognition served
as the basis for the
principle of fossil succession
or the
prin-
ciple of faunal and fl oral succession
, as it is sometimes called
(
◗
Figure 17.8). Unless
the erosional surface separating the older from the younger
parallel beds is well defi ned or distinct, the disconformity fre-
quently resembles an ordinary bedding plane. Hence, many
disconformities are diffi cult to recognize and must be identi-
fi ed on the basis of fossil assemblages.
Second, an
angular unconformity
is an erosional sur-
face on tilted or folded strata over which younger strata were
deposited (
◗
Figure 17.9). The strata below the unconform-
able surface generally dip more steeply than those above,
producing an angular relationship.
The angular unconformity illustrated in Figure 17.9b is
probably the most famous in the world. It was here at Sic-
car Point, Scotland, that James Hutton realized that severe
upheavals had tilted the lower rocks and formed mountains
that were then worn away and covered by younger, fl at-lying
rocks. The erosional surface between the older tilted rocks
and the younger fl at-lying strata meant that a signifi cant gap
existed in the geologic record. Although Hutton did not use
the term
unconformity
, he was the first to understand and
explain the signifi cance of such discontinuities in the geo-
logic record.
A
nonconformity
is the third type of unconformity.
Here, an erosional surface cut into metamorphic or igne-
ous rocks is covered by sedimentary rocks (
◗
Figure 17.6).
According to this principle, fossil assemblages succeed
one another through time in a regular and predictable order.
The validity and successful use of this principle depend on
three points: (1) Life has varied through time, (2) fossil as-
semblages are recognizably different from one another, and
(3) the relative ages of the fossil assemblages can be deter-
mined. Observations of fossils in older versus younger strata
clearly demonstrate that life-forms have changed. Because
this is true, fossil assemblages (point 2) are recognizably dif-
ferent. Furthermore, superposition can be used to demon-
strate the relative ages of the fossil assemblages.
Our discussion so far has been concerned with vertical re-
lationships among conformable strata—that is, sequences
of rock in which deposition was more or less continuous.
A bedding plane between strata may represent a depositional
break of anywhere from minutes to tens of years, but it is in-
consequential in the context of geologic time. However, in
some sequences of strata, surfaces known as
unconformities
may be present, representing times of nondeposition, ero-
sion, or both. Unconformities encompass long periods of
geologic time, perhaps millions or tens of millions of years.
◗
Figure 17.10).
This type of unconformity closely resembles an intrusive
igneous contact with sedimentary rocks. The principle of
inclusions (Figure 17.5) is helpful in determining whether
the relationship between the underlying igneous rocks and
the overlying sedimentary rocks is the result of an intrusion
or erosion. A nonconformity is also marked in many places
by an ancient zone of weathering, or even a reddened,
brick-like soil horizon, or paleosol. In the case of an intru-
sion, the igneous rocks are younger, whereas in the case of
erosion, the sedimentary rocks are younger. Being able to
distinguish between a nonconformity and an intrusive con-
tact is important because they represent different sequences
of events.
◗
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