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Interpreting The Past
Relative Dating &Correlation
Sean Tvelia
Relative Dating
In order to interpret the past, geologists must understand the processes involved in sediment deposition:
how materials are deposited relative to one another both vertically in the geologic column and horizontally
in sedimentary facies. Along with an understanding of depositional processes, if we are to correctly
interpret the past, we must first be able to develop a timeline of geologic events.
In 1669 Nicolaus Steno developed a number of laws that explain how sedimentary rocks are deposited
relative to one another. One of these laws, the Law of Superposition, tells us that in an undeformed
sequence of sedimentary rocks (or layered igneous rocks), the oldest rocks are on the bottom. Although
this is an essential concept in relative dating, sedimentary rocks are not always undeformed; often
mountain building and volcanic episodes cause folding and uplift that can make determining the top from
the bottom more difficult.
The Principle of Original Horizontality states that layers of sediment are generally deposited
horizontally and typically follow the preexisting topography. Therefore, when confronted by rock layers
that are folded or tilted, we have to assume that the layers were originally deposited horizontally and some
event took place that later folded or caused the layers to be uplifted and deformed.
As detrital sediment, the weathered remains of other rock, is eroded from its source, it is deposited
outward in all directions until it thins and ‘pinches out’, terminates against the edges of the depositional
basin, or changes character based on the sedimentary environment. This concept is known as the Principle
of Lateral Continuity. Based on this concept we wouldn’t expect to see a layer of sediment that abruptly
stops. However, in many areas mountain building and volcanic episodes often leads to faulting and the
emplacement of igneous rock within previously deposited material leaving the appearance of abruptly
ending sedimentary layers. In these cases the Principle of Cross-cutting Relationships tells us that the
layers that have been intruded or faulted have to be older than the fault or intrusion.
Looking a little closer at sedimentary or igneous layers one might also find inclusions, pieces of one rock
that are enclosed within another rock; for example, basalt with an inclusion of underlying sandstone.
Through our understanding of how rocks form we know that these two rocks form in drastically different
environments and it is impossible to form a sedimentary rock within an igneous body. Therefore the
sedimentary rock, or inclusion, had to have been deposited first and then incorporated into a newly
forming layer. In these situations rock containing the inclusion is younger than the inclusion itself and
therefore younger than the source of the inclusion.
So far we have discussed most of the ways in which deposited material can be related to one another and
dated based on their relative positions, but deposition is only one part of the geologic cycle and if we
concentrate on only what we can see then we may miss some key points of the geologic past.
In order to recreate the geologic history of any area
accurately, we must also be aware of the material we
can’t see. Over time weathering and erosion break
down rock and transport the resulting sediment to other
locations. This process can erase millions of years of
geologic history that can never be restored. Changes in
climate and/or depositional patterns can also lead to
periods of non deposition. These breaks in the rock
record are known as unconformities. Although we
may never know exactly what material was present
before an erosive episode, being able to correctly
identify the unconformity will better our ability to
correctly interpret the geologic past.
There are three major types of unconformities: angular
unconformity, disconformity, and a nonconformity
(Figure 1). An angular unconformity occurs when
strata, tilted by orogenic episodes, are eroded.
Subsequent deposition on top of the eroded surface
Figure 1. Unconformities
then creates newer horizontal strata. A disconformity,
also known as an erosional unconformity, occurs when sequences of relatively parallel strata are separated
by a physically identifiable erosional surface (a paraconformity, exists when strata are seemingly
continuous but missing fossil assemblages demonstrate missing geologic time). Lastly, a nonconformity
occurs when sedimentary strata are in contact with either metamorphic or igneous rock. This can occur
when sediment is deposited directly over crystalline bedrock or previously metamorphosed rock or even
during the emplacement of magma.
Understanding the concepts discussed above allows the
geologist to properly order geologic events in a given area.
However, if we are to accurately interpret the past we must
be able to equate strata in local sections to strata in distant
sections.
Correlation
Correlation is the act of determining equivalency between
two or more distant lithologic units. This process can be done
either lithographically or chronostratigraphically. In
Lithographic correlation, units are equated based on
Figure 2. Simple lithographic correlation.
represented facies. This process does not account for possible
differences in the time at which the facies were deposited. Chronostratigraphic correlation involves the
identification of different facies that had been deposited at similar times. This can be done through the use
of fossil assemblages.
Correlation, either litholographic or chronostratigraphic, can be a very simple or a very complex process.
In its simplest form this process can involve simply matching up identical facies from two different
locations, as shown in Figure 2.
However, the sediments that produce sedimentary rock are often deposited parallel to each other at similar
times creating sedimentary facies. If we forget this fact and simply match similar units, we would make a
drastic error in our interpretation of the past environment. Furthermore, there are many geological
circumstances that may result in one unit being found at one locality and being completely absent at
another locality. Because of this it is often necessary for us to make interpretations with very little
evidence.
The first thing to remember when correlating local sections is
that in many examples there will be no one single right answer.
Instead, there may be three or four possible solutions all of
which need to be critically analyzed.
The principle of multiple working hypotheses states that when
confronted with a problem, it is best not to use the first solution
that comes to mind but critically examine the problem from
many perspectives. Once the problem has been analyzed, only
then can one make a list of all possible solutions regardless of
how unlikely they might seem.
Figure 3. Facies change in two local
sections due to changes in depositional
environment.
Once you have a list of possible solutions another principle
takes over. This principle is commonly known as Occam’s
Razor. This principle simply states that when confronted with a number of solutions the simplest solution
is always preferred. In science this is most often referred to as the KISS Principle (Keep It Simple, Stupid).
Simple Lithographic Correlation
This section is titled simple lithographic correlation because it is
just that, simple. In most cases facies found in one local section
are also found in other local sections. However, there are
situations where, due to geologic circumstances, rock type
changes from section to section. This is most commonly seen in
the transition from one sedimentary facies to another.
In cases where a rock unit is present in one local section but not in
another, there may be two causes. The first cause is simply a
facies change. This is caused by the different depositional
environments that exist in many areas. In these cases the rock unit
Figure 4. Depicting a pinchout
is not missing but has simply changed character as demonstrated
between local sections
in Figure 3. The only rule to the use of facies changes in
correlation is that you may not insert two facies changes between local sections.
The second cause of an absent unit may be pinchout. This is caused when a rock unit simply diminishes in
thickness across the distance between the two sections until it eventually “pinches out” before reaching the
second section (Figure 4).
Relative Dating and Simple Geologic Histories
For each of the following examples place the labeled strata in correct age order from oldest to youngest.
Label and identify any unconformities.
Example 1
Youngest _____
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Oldest
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Example 2
Youngest _____
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Oldest
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Questions:
1. In Example 1, Layer E represents a layer of the igneous rock basalt. Based on the rules of relative dating
discussed in this lab, determine whether this layer is a lava flow or a sill created by igneous intrusion.
Explain your answer.
2. Using the relative dating scheme developed in Example 1, your knowledge of sedimentary rocks, and
the key above, develop a geologic history of the area represented by Example 1
3. If Layer B in Example 2 also represents a layer of basalt, determine whether this layer is a lava flow or a
sill created by igneous intrusion. Explain your answer.
4. Using the relative dating scheme developed in Example 2, your knowledge of sedimentary rocks, and
the key above, develop a geologic history of the area represented by Example 2.
Correlation
For each example given, show two different correlation possibilities using simple lithographic correlation.
To show correlation, draw lines connecting the bases of equivalent strata. Then determine which
correlation scheme is more likely and explain why.
Example 3
Correlation 1
Correlation 2
Correlation 1
Correlation 2
Explanation:
Example 4
Explanation:
For the following example use the concept of facies changes to create two possible correlations of the local
sections. Then determine which correlation scheme is more likely and explain why. .
Example 5
Correlation 1
Correlation 2
Explanation:
Example 6
In this example use the Fossils that have been identified in the strata to correlate the two columns. Identify
any unconformities with a line to represent missing time and give a brief explanation of how the sections
could have formed.
Example 7
In the following example use the concepts of pinchout and facies change to correlate the three sections and
answer the following questions.
A. Describe what is happening to the limestone unit in the columns A, B, and C and determine a
possible reason for these observations.
B. The numbers to the left of column A represent distinct time frames. Analyze the three columns and
determine what was happening to sea level from time 1 to time 3. Use the lithographic evidence to
explain.
C. Which of the three columns was deposited closest to land? Explain your reasoning.