Geology Worksheet
Biostratigraphy
Biostratigraphy is the subdiscipline of geology that is concerned with determining the relative ages of sedimentary rocks on the basis of their contained fossils. 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
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.
1) Use letter abbreviations to complete the geologic column at the left in Figure 1, with Cambrian at the bottom and Mississippian at the top. (So C, O, S, D, M in those boxes to the right of “Geologic time”, bottom up) (5 points)
2) 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. (5 points)
3) 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? (5 points)
4) What inference can you make about the age of fossil-bearing strata in region V? (5 points)
Part 2
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.
5) 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 then answer the questions on the next page. (10 points) A few notes.
· Just do the brachiopods listed on the chart. There are more brachiopods in the pictures than there are on the chart.
· Some of the brachiopods have ranges that extend off the top of the chart (say into the Jurassic) just take them to the end of the Permian.
· If a range goes from the Silurian to the Permian, for example, that includes all of the Silurian through the end of the Permian. As an example I’ve done Chonetes for you.
6) Identiy a taxon range biozone, concurrent range biozone and an interval biozone. Circle them and label them on the chart. (5 points)
7) Which would be more useful for determining the age of a rock, the taxon range zone for Stringocephalus or Chonetes? (5 points) Why? (3 points)
8) How old is a rock that contains Derbyia? (3 points)
9) How old is a rock that contains Leptaena? (3 points)
10) How old is a rock that contains Derbyia AND Leptaena? (3 points)
11) Given your answers to 7, 8 and 9 why is it usually better to do biostratigraphy with fossil assemblages (groups of fossils) than individual fossils? (5 points)
12) On the next page there is a very simple map, the lines represent boundaries between different rocks. Fossils have been collected from each of the rocks. Fill in the chart with the ages of the rocks, then label the map with the age of the rocks. (9 points)
| Site | Fossils | Age |
| A | Rafinesquina, Strophomena | |
| B | Rensselaeria | |
| C | Penicularis, Juresania |
13) As you move to the northwest on the map does the rock generally get younger or older? (5 points)
14) If you wanted to look for dinosaurs would you look generally to the northwest or southeast of the map area? (5 points)
C
B
A
N