Lab10biostratigraphyi
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Geology
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Dec 21, 2024
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EARTH AND ENVIRONMENT THROUGH TIME LABORATORY - EES 1005 LABORATORY TEN FOSSILS, MAPS, AND STRATIGRAPHIC COLUMNS Introduction In this lab, we will see how the fossil record is used to formulate a sequence of events in Earth history. Sedimentary strata alone sometimes cannot provide a precise sequence of events because the rock cycle produces many of the same rock types over and over and radiometric dating is not always an option. Fossils can be used to determine the ages of rock beds and the history of an area. Biostratigraphyis the correlation of stratigraphic units based on fossil content. Each section of strata contains a certain fossil assemblageof coexisting species. Because the assemblage waswas deposited together, the fossils must have all exsisted at the same time. By determining the time when the age rangesof each particular fossils overlap, the precise age of the beds can be determined. Of course, index fossils are especially useful in biostratigraphy. For example, if we want to determine the geologic ages of beds 1, 2, and 3, we need to first identify each of the species and their age ranges within each of the formations. We won't get too specific for this example, so we'll call them species A, B, C, D, and E.
Fossil AssemblageFrom the data set on page 46 we can begin to chart out the age ranges of the species we find in each stratigraphic unit. We will be focusing on the geologic period that contains the overlap of the species age ranges. This overlap indicates when the species coexisted in earth history.NOTICE:The formations follow the Principle of Superposition. The oldest bed is on the bottom and they get progressively younger towards the top. Note that we have now found an unconformity between formations 2 and 3. The Pennsylvanian is not represented in this column. Also note that species A and C are found in more than one time period. As a whole, the species existed for an extended geologic time period. The individual organisms that were found and identified by the geologist, however, only lived for a limited period of time within onegeologic time period. Question for thought: If a formation was found between formations 2 and 3, and it contained no fossils, could you determine what geologic time period it belonged to? Why or why not?
BiostratigraphySome students find biostratigraphy abstract. Let's illustrate the logic through an example closer to home. You have purchased an old abandoned schoolhouse which you intend on renovating into a luxury condominium. The schoolhouse was built in 1932 and finally abandoned in 1972. While inspecting the building you find an old school locker that is still locked. You cut the lock and sift through the contents because you are curious to find out how long the locker has been abandoned. Among the contents you find: A wristwatch An 8-track tape of Elvis (issued in 1968) A pair of Levis blue jeans A white t-shirt When was the locker last used? Some items will be useful and others will not. For example, white t-shirts and blue jeans have been around for decades. In fact, they were commonly worn before the school was built and are still worn today. As a result, the blue jeans and white t-shirt do not provide any useful information. The wristwatch might be useful if only we knew more about wristwatches, but we don't, so this is not useful either. On the other hand, the 8-track tape of Elvis tells us that the locker was sealed no earlier than 1968. Since the school itself closed in 1972, we can safely conclude that the locker last saw light during President Nixon's first term in office- 1968- 1972. Here is the time line:Therefore, the blue jeans and the t-shirt are not very useful indicators for determining the time the locker was in use due to their extensive time range. In addition, since we cannot determine the age of the watch, it also is not much help. However, the Elvis 8-track is an important indicator since we know when the tape was released. This information coupled with the fact that the school closed in 1972 allows us to determine the age range for when the locker was last visited.
Stratigraphic CorrelationNow that we know how to construct stratigraphic columns, we will learn some of the uses for them. The principle of lateral continuity states that rock beds can be traced across a wide area. We can look at stratigraphic sections from different areas and correlatethem. This means that we can connect similar rock beds and reconstruct the history of the area. Rock beds can be correlated one of two ways: lithologic correlationuses lithology (rock type) to match like rock units, while biostratigraphic correlation uses age (via index fossils and fossil assemblages) to match like rock units. The formations may be of different thickness from one column to the next due to erosion or other processes. Sometimes rock beds pinch out, or kind of taper off in thickness as it approaches the edges of its lateral depositional area, and will not be found in other sections.
FaciesFaciesare different types of rock deposited at the same time but representing different environments. Along a beach, sand is deposited, which may eventually form a sandstone; clays are deposited in deeper waters which form shale; calcareous clays are deposited around reefs which form limestone; and small rocks and pebbles are deposited at mountain bases which forms conglomerates. These rock types grade into each other from one facies to the nest, but may represent the facies change across an area. FACIES AND CHANGES IN SEA LEVELA static sea level results in a thick sequence of the same rock type. However, in areas where one facies grade into one another, the rock types may be mixed (ex: a coarse-grained sandstone, a sandy limestone, ect.), or the rock types may be interbedded. Changes in sea level causes the facies to either shift landward or seaward, and produces a predictable pattern that we often see in a stratigraphic column. When a sea level rises, it is called a transgression, and the facies shift landward. The stratigraphic column exhibits a "fining up" sequence as the grain sizes decreases as you move up the column. Falling sea levels result in a regression. Facies shift seaward, and the stratigraphic column exhibits a "coarsening up" sequence. A static sea level results in a thick sequence of the same rock type.
Stratigraphic Columns
Laboratory ExerciseDIRECTIONS: Using the fossils provided by your instructor, identify each fossil and indicate the age range. Once you have given all the age ranges, determine the age of each bed by using overlap, the index fossils and the principle of superposition. When asked the age range, use the information on pages 32-33, however if the species is "most common" during a certain time period, use that period ONLY. Each formation represents onegeologic period, and there may be unconformities present. PART 1: IDENTIFICATION PART 2: STRATIGRAPHIC COLUMN Now construct a stratigraphic column on the back of this sheet using this information. Use the scale of 1"=100' (one inch = one hundred feet). Use the map name, lithologies, formation names and ages, and include any unconformities. PART 3: Sleepy Hills, Vermont Quadrangle MapUsing the ages you obtained above, what structure is this? When did the folding occur?