Lab9_Correlation
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California University of Pennsylvania *
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Course
4200
Subject
Geology
Date
Dec 6, 2023
Type
Pages
11
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GEOL 4200: Sedimentology and Stratigraphy
Lab 9: Stratigraphic Correlation
Introduction:
You are aware that geologists can determine the
relative
ages of rock layers in an outcrop
or well log.
But how do they determine whether rocks or geologic events occurring at one location are
of the same age as those in another location?
The process of determining rocks or events occurring at
different locations at the same time is called correlation.
Geologists have developed a system for correlating rocks by looking for similarities in composition and
rock layer sequences at different locations.
Certain fossils, called “index” fossils, existed for short
periods of time and were distributed over large geographic areas.
They can be especially useful in
correlating rock layers.
Additionally, “marker beds,” unique layers representing relatively discrete
events in time distributed over a large area are also useful.
For example, a single, large volcanic
eruption can deposit an ash blanket over thousands of square miles, representing the immediate
aftermath and those days to weeks as the ash falls from the atmosphere.
Objective
: At the end of today’s lab, you sh
ould be able to:
1.
Recognize conventional geologic symbols for rock types and analyze their stratigraphic
relationships to determine progradational vs. retrogradational sequences.
2.
Construct a geologic history of a region by observing rock layers in different localities.
Vocabulary you should be comfortable with…
Absolute age, Relative age, Index Fossil, Correlation, Marker bed, Unconformity, Superposition
Example of Parsimony
- correlation diagram showing four possible correlations.
Using these two columns (at left) to represent locations of
outcrops that are stratigraphically related.
Consider the four
diagrams below as possible options for correlating them.
Note the rock types indicated by the symbols.
What two rock types are shown?
a.
____________________________
b.
____________________________
Example of a Pinch-out/Interfingering
–
Outcrops A and B are shown with layers 4 and 7 NOT correlating
(different rocks or rock types).
This can be dealt with in three different ways!
See Below.
Working out
the “right answer” to this type of issue could result in a “non
-
unique” solution that needs rigorous
supporting evidence.
Part 1: The four diagrams below represent four outcrops at different locations.
1.
(Next page) Reconstruct the complete sequence of events.
Assume that the oldest rocks are at
the bottom and the youngest are on top.
NOTE: the letters are at the tops of the columns.
a.
Draw lines between matching rock units.
b.
Draw in unconformities (darken the lines of contact) where a rock unit is missing.
2.
Draw in the layers of the complete sequence on the appropriate column on the following page.
3.
Label the rock units on the stratigraphic column based on the symbols as indicated.
Include the
unconformities.
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Part 2: Cross sections below represent four different locations across New York State.
1.
Reconstruct the complete sequence of events on the strat column on the next page.
Assume
that the oldest rocks are on the bottom (letters are at the bottom, right side of page) and the
youngest are on top.
NOTE: Each rock is only used once.
2.
Include the rock types and any perceived unconformities adjacent to the stratigraphic column.
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Part 3: Here are three different outcrops.
1.
Use the rock type of the strata to correlate these columns.
2.
Draw lines on this page, representing equivalent boundaries between rock layers from one
column to the next.
3.
Reconstruct the complete column on the next page.
Assume that the oldest rocks are on the
bottom and the youngest are on top (numbers are at the top of each column).
4.
Include the rock types and any perceived unconformities (thicken lines on the column) adjacent
to the stratigraphic column.
Part 4: Fossils for Correlation
1.
The sketches below are of index fossils from specific geologic time periods.
2.
The stratigraphic columns indicate the fossils present in these rock units.
3.
Using the age of the fossils and the rock types to reconstruct the complete sequence of events
and draw in a stratigraphic column.
NOTE: Some rock types may be used more than once.
Stratigraphic Columns for Part 4
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Answer the following questions:
1.
What is a possible explanation for why strata may be absent from some outcrops?
2.
List two characteristics of a fossil that would make it a good index fossil.
3.
How can an index fossil be found in more than one type of rock?
4.
What is the minimum age of the lowest strata in Part 4?
5.
What does a field geologist look for in rock outcrops to help identify different rock layers?
6.
Explain how it is possible that a given type of fossil may be found in a rock stratum at one
outcrop, but missing from that same layer at another outcrop.
7.
In Part 3, what is a reason the limestone in column 1 is thinner than the limestone in column 2?
8.
According to the appearance of the right side of each column in Part 3, which rock type appears
to be the LEAST resistant to weathering and erosion?
CONCLUSION: Why is it necessary to observe the rock layers at several different localities to obtain a
complete sequence of events?