Lab: Biostratigraphy , geology homework help

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Lab 12 Biostratigraphy (modified from Bryce and Levin)
Purpose: Biostratigraphy is the subdiscipline of geology that is concerned with
determining the relative ages of sedimentary rocks on the basis of their fossil content.
The practical application of biostratigraphy is biostratigraphic correlation: i.e.,
establishing the temporal equivalence of widely separate rock units on the basis of
fossils.
Fossils are useful in relative age determination because the processes of evolution
have produced a unique sequence of life forms through time. Every species of fossil
plant, animal and protist has a definite stratigraphic range, the range in geologic time
from its evolutionary origin to its extinction. Similarly, every interval of geologic time has
been characterized by its own distinctive faunas and floras.
The age of a fossil-bearing sedimentary rock can be determined if the stratigraphic
ranges of its contained fossils are known. For example, suppose that a particular
trilobite species is known to have lived in late Cambrian time. It follows that any rock
containing fossils of that particular trilobite must be late Cambrian in age. In practice,
determining the precise stratigraphic ranges of fossil species can be quite involved.
Nevertheless, the stratigraphic ranges of thousands of species are well known, and they
can be used to correlate rocks with a precision that generally exceeds that of radiometric
dating.
Part 1 Fossil Assemblages
Background: Before fossils can be used to help determine the relative age of a
sedimentary rock, their stratigraphic ranges must be known. The following exercise is a
simplified example of how one might go about documenting the stratigraphic ranges of
some fossil species in rocks of known ages, and then using that information to infer the
ages of rocks in previously unexplored areas.
Figure 1 – Occurrences of two fossil species at five separate localities. Ages of rocks in regions I,
II, and III are known on the basis of independent evidence. Ages of rocks in regions IV and V must
be determined on the basis of their contained fossils. Abbreviations: C = Cambrian; O =
Ordovician; S = Silurian; D = Devonian; M = Mississippian (from Brice et al. 2001).
Part 1 Exercise:
a) Use letter abbreviations to complete the geologic column at the left in Figure 1, with
Cambrian at the bottom and Mississippian at the top.
b) Now, with your knowledge of the geologic time scale and the Principle of Superposition,
use heavy vertical lines to show the stratigraphic ranges of species F-1 and F-2 in the two
columns under the heading “Fossil Ranges.” You can determine the stratigraphic ranges of
these species by observing their occurrences in rocks of known ages in regions I, II and III.
c. Using the stratigraphic ranges of species F-1 and F-2, what inference can you make
about the age of the fossil-bearing layers in region IV?
d. What inference can you make about the age of fossil-bearing strata in region V?
Part 2 Biozones
Background: Biostratigraphic correlation is usually accomplished by means of
biozones, defined as bodies of rock strata that are characterized by their distinctive
association of fossils species. The assumption is that a given biozone in one region is
approximately the same age as the same biozone in a separate region, even if the
regions are quite distant from one another.
Many kinds of biozones are recognized. The most widely used are the taxon range
biozone, concurrent range biozone, and interval biozone.
• Taxon range biozone: body of strata corresponding to the total stratigraphic
range of a specified fossil taxon (e.g., species or genus)
• Concurrent range biozone: body of strata corresponding to the overlapping
stratigraphic ranges of two or more specified fossil taxa
• Interval biozone: body of strata corresponding to the interval between any two
specified evolutionary events (e.g., interval between two extinction events;
interval between two origination events; interval between an origination event
and an extinction event).
Examples of these kinds of biozones are illustrated in Figure 2.
Figure 2 – Different kinds of biozones defined on the basis of stratigraphic ranges of hypothetical
fossil species A–G, depicted by heavy vertical lines. The interval biozone corresponds to the body
of strata from the evolutionary origin of species D to the origin of species B.
Part 2 Exercise:
a) Illustrations of Paleozoic brachiopods, along with their known stratigraphic ranges,
are given on the following two pages. Use the information on these pages to help you
complete Table 1. Using pencil, shade in the stratigraphic range of each brachiopod
genus listed.
b) Once you have recorded the stratigraphic ranges of each genus, identify examples
of: (1) a taxon range biozone; (2) a concurrent range biozone; and (3) an interval
biozone. Draw the boundaries of each biozone and label it appropriately.
c. What is the geologic age of a rock sample that contains the brachiopods
Cyrtospirifer, Atrypa, Composita and Leptaena?
Set I
Set II
Part 3: Graphic Correlation
Background: Graphic correlation is a special biostratigraphic technique for correlating
pairs of stratigraphic sections. The technique does not rely on biozones for correlation,
but rather it utilizes the ranges of all species that occur in both of the stratigraphic
sections being correlated.
In order to perform graphic correlation, one must ascertain the lowest observed
occurrence and the highest observed occurrence of all fossil species that occur in both
sections. These data, referred to as fossil “bases” and “tops,” respectively, are then
plotted on an X-Y graph and a line of correlation is fitted through the points. The line of
correlation can be interpreted to relate a given level within one stratigraphic section to
the exact temporal equivalent in the other section.
Examine Figures 3 – 6 for a clearer description of graphic correlation.
Figure 3 – Two sections, A and B, can be correlated using traditional biozones, but precise
correlation within biozones is not possible using traditional techniques.
Figure 4 – Graphic correlation plot of the same data as in Figure 3. Note that fossil “bases” (or
evolutionary appearances) are plotted as circles and fossil “tops” (or extinctions) are plotted as
pluses.
Figure 5 – Line of correlation fitted through the data points from sections A and B. Any level in
section A can be correlated with its exact temporal equivalent in section B by projecting to the
line of correlation and then over to the other section.
Figure 6—The line of correlation is not always a straight line. A change in the slope of the line of
correlation signifies that sedimentation rates changed in one section relative to the other. For
example, a perfect 45° line of correlation means that sedimentation rates in the two sections are
exactly the same. A slope less than 45° means that sedimentation at Locality A (horizontal axis)
occurred faster than at Locality B (vertical axis). Conversely, a slope greater than 45° means that
sedimentation at Locality B was faster than at Locality A. A horizontal segment in the line of
correlation signifies an unconformity or gap in sedimentation at Locality B. In other words, a finite
thickness of strata at Locality A accumulated during an episode of non-deposition or erosion at
Locality B.
Part 3 Exercise:
a) Table 2 contains information on “bases” and “tops” of 11 fossil species that occur in
two sections, X and Y. Plot these data on Graph A and then draw a line of correlation
that best fits the distribution of data points.
Table 2 – Bases and tops of 11 species at sections X and Y. Values in meters above base of
section.
b) At which section, X or Y, was sedimentation occurring at the faster rate?
c) What level in section Y is exactly the same age as 50m in Section X?
d) Table 3 contains information on “bases” and “tops” for another group of 11species
from two more hypothetical sections, X and Y. Plot the data on Graph B, as before, and
draw a line of correlation.
Table 3 – Bases and tops of 11 species at sections X and Y. Values in meters above base of
section
e) Is the line of correlation a straight line, or is there a change in slope?
f) What does the line of correlation tell you about the rate of sedimentation in the lower
part of section Y relative to the rate of sedimentation in the lower part of section X?
g) Examine the simple graphic correlation plot below. Assume that the line of
correlation has been drawn correctly. Explain why some of the data points do
not fall exactly on the line of correlation?

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