Lab 7: Correlation Background and Example Problems
Stratigraphic columns
Diagrams showing the sequence of sedimentary rocks at an outcrop are columns
drawn to scale. In addition, standardised patterns and symbols are used to
represent different lithologies, textures, sedimentary structures, fossils, etc. Two
kinds of graphic stratigraphic column are illustrated in Figure 1. It is usual to
show the "erosional profile" perspective given at the right, which extends coarser,
more massive, and usually more resistant layers to the right and indents the less
resistant layers to the left.
Figure 1: Model Stratigraphic Column
Physical Correlation
Sedimentary rock formations can be locally correlated with some degree of
confidence by comparing their physical and chemical characteristics even when
there are no fossils. As distances between outcrops increase correlations
become increasingly inaccurate and require as much evidence as possible (e.g.,
fossils). Physical correlation can be done by a) "walking out" the outcrop, b)
lithologic similarity, c) position in a sequence, d) relationships to unconformities
or structures, and e) geophysical logs for wells.
"Walking Out" an Outcrop
If a rock unit is well exposed, it may be possible to correlate the rock layers from
one area to another simply by "walking" along the outcrop, or from outcrop to
outcrop.
Lithologic Similarity
Correlation can also be made by looking for similar compositions, similar
variations in composition, color, grain sizes (largest, smallest, average, sorting,
etc.), grain characteristics (roundness, sphericity, frosting, etc.), and/or
sedimentary structures. Some layers may be thin, sharply defined, laterally
persistent, and of markedly different composition. For example, local storm
deposits or volcanic ash deposits may stand out and are excellent time markers.
Such units are called key beds or marker beds.
Example 1: Correlate the units in the two columns in Figure 2 on the basis of
lithologic similarity (standard symbols are used). Draw straight lines (in pencil)
between levels of possible time equivalence, such as formation contacts, etc.
Figure 2: Correlation by lithology
Example 2: Correlate the units in Figure 3 on the basis of lithologic similarity
and position in sequence. Note that this forces you to correlate two units of
dissimilar lithology. This indicates different environments of deposition at the
two locations at the time when the original sediments were being deposited
(shallow water at one location, deeper water at another).
Figure 3: Two stratigraphic columns showing lateral variation
Example 3: Figure 4 shows three stratigraphic columns A, B, and C.
a) Correlate the columns on the basis of lithologic similarity and position in
sequence.
b) Which lettered unit pinches out laterally?
c) Fossil evidence indicates that the line drawn between positions 7 and 8
is a time line (i.e., same age everywhere). We will assume that surfaces
parallel to a time line are also time lines. Are lines 1-2 and 3-4 time lines?
d) What does the above imply about unit d?
Figure 4: Three stratigraphic columns showing a “pinch out”
Example 4: Relationships to Unconformities and Structures
It is generally possible to correlate lithic units on the basis of unconformities.
Typically one correlates units that lie directly above an unconformity. Lithic units
which have structures (such as mud cracks and rain prints) not found in the rest
of the section may also be correlated.
Figure 5 shows two stratigraphic columns with an unconformity.
a) Correlate the two columns. Note that the cherty limestone is thicker in column
B.
b) Which column experienced the most erosion?
c) Which column might have had more uplift during erosion?
d) Which column might have experienced a longer period of erosion?
Figure 5: Two columns showing an unconformity
Example of correlation using geophysical data
Using Geopjhysical Data for Correlating sections.
Exercises:
1. Correlate the following two sections by lithologies
2. Correlate the three coumns shown and answer the following questions.
a) What kind of lateral stratigraphic relationshio exists between the
sequences of columns A-B-C at time 3?
b) If all the rocks were deposited in a marine environment, which column is
the one most likely to have been deposited farthest from land?
c) Which one was deposited closest to land and in the direction of the
sourceland.
d) Indicate on the columns where transgressions and regressions may
have occured.
3. More Correlation Problems
Problem A
Problem B:
Problem C:
Problem D:
Problem E:
Problem F:
Questions:
1) Which rock layer is the oldest? The youngest?
2) At location Y, the boundary between the red sandstone and the black shale
marks the
end of what geologic era?
3) An unconformity can be observed at location Z. Which rock layer was most
probably removed by erosion during the time represented by the unconformity?
Did the faulting occur before or after the unconformity?
4) The fossils in the rock formations at location X indicate that this area was often
covered by
a) tropical rain forests
b) glacial ice
c) desert sand
d) seawater
5) Which rock layer was formed by the compaction and cementation of particles
that were all less than 0.0004 centimeter in diameter?
6) Are there any transgressions/regressions recorded in the columns?
7) What is the evidence for mountain building?