G136 SP18 Lab3 - G136 SP18 Lab3 PDF

Preview

Summary

G136 SP18 Lab3...

Description

Lab 3: Telling Time - Geochronology Objectives • • • • • •

Understand the difference between relative and absolute (radiometric) age dating. List and apply the geologic principles used to determine the relative ages of geologic events including: original horizontality, superposition, and cross-cutting relationships. Recognize the three types of unconformities (nonconformity, angular unconformity, disconformity) and describe what they can tell us about the geologic history of a unit of rocks. Explain and apply the use of fossils in dating rocks and correlating rock units, including the ideas of fossil succession and index fossils. Apply methods of relative and absolute age dating to analyze rock outcrops and infer geologic history. Analyze rock units of Indiana and apply age dating principles to determine a timescale of geologic history in Indiana.

Introduction to this Lab Exercise In this lab, you will continue to “think like a geologist." We introduced this idea in our previous lab. Here, we want to think about what rocks and earth materials can tell us about the geologic history of an area. We will explore ideas of geologic time, relative and absolute dating principles, and interpreting grouping of rocks.

If you have difficulty with ANY part of this lab, use the Canvas Discussion forum to ask questions!

A. Relative Dating Principles In your readings you examined several principles of relative dating – the process of putting rock units and events in chronological order without knowing their exact age. Here, we want to review some of the terminology related to relative dating. Use your readings, or the internet, to determine the meaning of the following terms: 1) Write out a brief definition of the following terms related to relative dating: A. Geologic Time _______________________________________________________________________________ B. Law of Superposition _________________________________________________________________________ C. Principle of Original Horizontality _______________________________________________________________ D. Principle of Inclusions _________________________________________________________________________ E. Principle of Cross Cutting Relationships ___________________________________________________________ F. Disconformity _______________________________________________________________________________ G. Nonconformity ______________________________________________________________________________ H. Angular Unconformity ________________________________________________________________________ I.

Paleontology ________________________________________________________________________________

J.

Fossil ______________________________________________________________________________________

K. Index Fossil _________________________________________________________________________________ L. Fossil Assemblage ____________________________________________________________________________ M. Principle of Fossil Succession ___________________________________________________________________ N. Stratigraphy ________________________________________________________________________________ O. Stratigraphic Column _________________________________________________________________________

The figure below is a cross section of a hypothetical area. Cross sections are a way for geologists to visualize the geology under the surface of a given area. We will learn to create these in our next lab.

The letters represent different geologic units of rock or sediment. (H) is a fault – a break in the rocks that moves them relative to one another. Letter (A) is an igneous intrusion – a mass of igneous (volcanic ) material that pushed up through the layers and solidified into rock. (Review your rock cycle from the previous lab if you need a refresher on rock types.) If you have difficulty with ANY Use the cross section to answer the questions below. Use relative age dating principles to help you answer the questions.

part of this lab, use the Canvas Discussion forum to ask questions!

2) Place the lettered features in proper sequence, from youngest to oldest, in the spaces to the right of the figure. 3) Which relative age dating principle did you apply to determine that layer K is older than layer B? 4) Which relative age dating principle did you apply to determine that Fault H is older than rock layer M? 5) Which relative age dating principle did you apply to determine that intrusion A is older than layer N? 6) What type of unconformity is identified by line R? 7) What type of unconformity is identified by line S? 8) What type of unconformity separates intrusion A from layers M, D, and J?

2

This figure is a portion of a cross section of the rocks of the Grand Canyon. This is a generalized cross section, the actual Grand Canyon has many more minor layers and features. The gray boxes to the right of the cross section give the general geologic time period of the layers.

PERMIAN

P ENNSYLVANIAN COLORADO RIVER MISSISSIPPIAN DEVONIAN

C AMBRIAN

PRECAMBRIAN

Using this image, please answer the questions below. 9) What is the oldest labeled unit in this cross section? ___________________________________________________ 10) What is the youngest labeled unit in this cross section? ________________________________________________ 11) What type of unconformity separates the Dox Formation (sedimentary) from the Tapeats Sandstone?___________ 12) What type of unconformity separates the Muav Limestone from the Temple Butte Limestone? _________________ 13) The Vishnu Complex has been dated to be 1.7 billion years old and is composed of metamorphic and igneous rocks. The Bass Limestone has been dated to be 1.2 billion years old. Therefore, 500 million years of time is missing between the Vishnu Complex and the Bass Limestone. What type of unconformity does this contact represent?

14) The Kaibab Limestone is about 270 million years old. How much geologic time separates the Vishnu Complex and the Kaibab limestone? Report your answer in millions (or billions) of years.

15) The Colorado River flows through the area marked on the cross section above. Although not depicted in the diagram above, the rocks you see on the right are also found on the other side (left) of the canyon. Based on this information, and on the evidence in the figure above and your answers to questions 9, 10, and 14, during what general timeframe might the Colorado River have begun carving the Canyon? Explain your answer.

3

The image to the right is from the Billings Gazette and shows coal mining operations near Kemmerer, Wyoming. To give a scale, the dump truck to the left of the image stands about 20 feet tall. In this figure, you should see layered rocks behind the equipment, labeled A, B, and C. Layer B is the coal layer that the mining operations are targeting, and layers A and C are sandstone and siltstone units. More than three distinct layers likely exist here but for simplicity we will only discuss three of them. Surrounding the equipment you see piles of coal. Please answer these questions regarding this image. 16) What are the relative ages of layers A, B, and C above? Explain.

17) Were layers A, B, and C deposited at an angle as we see them now? Why or why not? What relative dating principles helped you decide?

18) Carefully examine the image above. Then, make a list of events that have occurred, starting with the deposition of Layer A and ending with the loading of coal into the dump truck. You should recognize at least five events, if not more.

B. Fossils and Fossil Succession Geologists use fossil assemblages (groups of fossils) to determine the relative ages of rocks. Many of the rocks on Earth’s surface are sedimentary rocks and many contain fossils (evidence of past life). Geologists have learned that certain fossils existed during certain time periods – these fossils are called index fossils. Geologist have also learned that fossils originated, co-existed, and disappeared in a recognizable sequence in rocks worldwide (fossil succession). So, when a geologist finds fossils in a rock unit, he or she can do a bit of research to determine when those fossils lived and roughly determine the age of the rock. An example of this is shown above. Here, a geologist determined the age ranges of six fossils found in a rock unit (listed below the chart). Then, the geologist plotted the age range of each fossil as arrows on a simple geologic timescale. The geologist looked at the lines and determined that all of the fossils were present, at the same time, in the Permian (shaded section of the chart). Therefore, these fossils indicate the rock that the geologist was examining is Permian in age.

4

Here, you will practice using fossil assemblages to determine rock ages. 19) Suppose that a geologist has collected the following fossils from a rock outcrop and determined their age range through research. Plot the age ranges given for each fossil as a line on the geologic time scale below. The first one is done for you as an example. Fossil A: Ordovician – Permian Fossil B: Silurian – Mississippian Fossil C: Cambrian – Triassic Fossil D: Devonian – Tertiary Fossil E: Ordovician – Jurassic Fossil F: Mississippian – Quaternary

Quaternary Tertiary Jurrasic Triassic Permian Pennsylvanian Mississippian Devonian Silurian Ordovician Cambrian

Fossil A

Once you’ve plotted the fossils, you should see some overlap between the fossil groups. On the timescale, shade in the time period(s) when ALL the fossils existed. This is the age of the rock. You will need this figure to upload!

20) Given the ranges the geologist determined for each fossil, what is the age of the rocks in the outcrop? __________ 21) A good index fossil has a narrow range of time that it existed, making it easier to narrow down a specific time period for the rock that contains it. Which fossil is the best index fossil? ____________________________________________ 22) A poor index fossil has a broad range of time that it existed, making it difficult to pinpoint a specific age for the rock unit that contains it. Which of the fossils is a poor index fossil? _______________________________________________ 23) If you were only given two fossils to determine the age of the rock unit, which two would you pick? That is, which two would give you the age of the rock without needing any of the others? ____________________________________

C. Correlating Rock Layers No single location on Earth displays all of geologic time in its rocks. Instead, geologists have to depend on correlation, or matching, of rock layers from place to place. Often this must occur over very long distances; therefore, geologists depend on principles of relative dating, absolute ages, AND characteristics of rock units to help them understand geologic history. The idea is that if rocks units in different locations have the same characteristics and ages, they must have been deposited or formed by the same processes. You looked at these ideas in your reading from Tulane University, in the trash pit example (above).

5

To correlate rocks from one location to another, geologists will examine the characteristics of rocks such as color, mineral composition, and grain size, shape, and sorting. If the rocks in two different locations have similar characteristics and the order (sequence) of the layers is similar, geologists might determine that the rock layers were continuous at one time and represent similar periods of geologic time. We call this process stratigraphic correlation. Below is a diagram of three hypothetical rock units. These rock units are composed of limestone, sandstone , and shale, in the same general sequence from top to bottom. Some of the units have unconformities (missing rocks). The legend describes what the colors and symbols of the unit mean. Using the idea of correlation, draw lines between the rock units that are the same, similar to the Tulane/UNO trash pit example. The first set of lines (dashed) are drawn for you as an example. After you have correlated the layers, answer the questions below the figure. You will need to upload the figure.

24) What rocks are missing from Outcrop B? ____________________________________________________________ 25) What rocks are missing from Outcrop C? _____________________________________________________________ 26) Geologically, why might some rock layers be missing between the units? __________________________________ ______________________________________________________________________________________________ Geologists can use correlation information to determine the succession of rocks over time, using the principles of relative dating we discussed in Part A. We call this process stratigraphy. 27) Using your correlation lines, put the rock units in order from oldest to youngest, using the names given: Oldest ____________ ____________ ____________ ____________ ____________ ____________ Youngest

6

D. Using Fossils AND Rock Characteristics to Correlate Rock Units Geologists use the principles we explored in sections B and C above to determine the sequence of deposition of rocks and their ages in a given area. Here, we want to explore fossil assemblages AND rock characteristics to determine the evolution of an area through geologic time. This exercise is a small-scale model of how geologists created the geologic time scale using correlations over great distances.

If you have difficulty with ANY part of this lab, use the Canvas Discussion forum for Lab 3 to ask questions!

The figure on page 10 of this worksheet shows four outcrops from widely separated areas on Earth. The rock layers were assumed to be continuous over the area at one time in the geologic past. Geological processes like uplift and erosion have broken this continuity. Your job is to determine the correct sequence of the rock layers (stratigraphy) using the evidence in the figure and then create a stratigraphic column in the diagram to the left of the figure. You will upload this figure on your quiz! To complete the figure, 1. Draw lines connecting the boundaries of similar rock units from one outcrop to the next, similar to what you did in part C of this exercise. The correlation of unit S has been done for you as an example. 2. Determine what rock layers are missing in each outcrop, and list the missing layers above each outcrop. 3. In the blank stratigraphic column to the left of the outcrop drawings, construct a stratigraphic column showing the correlation between the four rock outcrops. Every rock layer should be included on this column. Draw the layers, or write the letter for each layer, in the proper order. Oldest rocks should be at the bottom of the column and youngest at the top. Layer P has been added to the column for you. 4. The diagram along the bottom of Page 9 shows the age ranges for fossils in our hypothetical area. Many of the fossils overlap each other in time. Given that all these rock layers are from the Paleozoic, assign a time period(s) to the rock layers in your stratigraphic column. P does not have fossils, so you will not be able to determine its time period, but the evidence should indicate that P is the oldest rock unit. Once you have completed the steps above, please answer the following questions using your diagram and stratigraphic column: 27) List the missing rock layers for each region. Region 1: _____________ Region 2: _____________ Region 3: _____________ Region 4: _____________ 28) There are several rock layers that have more than one designated time period based on the fossil evidence. Which layers have more than one time period?_____________________________________________________________ 29) Thinking geologically, how can fossils extending over more than one time period be found in one rock layer? _________________________________________________________________________________________________ _________________________________________________________________________________________________ 30) The fish fossil in layers Z and X has a time range extending from the Devonian to the Permian. What other evidence from the rock units could we use to assign a specific (ONE) time period to layer X? _________________________________________________________________________________________________ _________________________________________________________________________________________________ 31) The rocks here are sandstones, mudstones, and limestones. Using this information and the fossils you see, what type of environment might these rock have formed in? How did you decide on this environment? What was the evidence? ________________________________________________________________________________________________ _________________________________________________________________________________________________

7

E. Indiana Rocks We now have a good understanding of geologic time and the methods used to determine the ages of rocks, sediments, and events. Here, we will apply these concepts to Indiana rocks and sediments to explore If you have difficulty with our state’s place in geologic time. You will explore images and online resources to help ANY part of this lab, use you answer these questions. In Lab 2, we learned about some of the common rock units the Canvas Discussion of Indiana. Here, I would like you to determine their ages and decide their stratigraphic forum for Lab 3 to ask correlation. I have added some new rock unit names to the list we examined in Lab 2 below. questions! 32) At any one place in Indiana, you may find several units of rock from the same time period but with different characteristics. For example, at McCormicks Creek State Park, we actually see several rock units: The Salem Limestone, the Ste. Genevieve Limestone, and the St. Louis Limestone. From the IGS website: "THE ST. LOUIS CONFORMABLY OVERLIES THE SALEM LIMESTONE AND UNDERLIES..[THE] STE. GENEVIEVE LIMESTONE". Based on this sentence, which of these three units is oldest? Youngest? How do you know? ________________________________________________________________________________________________ ________________________________________________________________________________________________ ________________________________________________________________________________________________ 33) Using the information you can gather from this page (http://igs.indiana.edu/IGNIS/search.cfm) determine the age of each of the following rock units. (HINT: Enter the names of the rocks in the search field) Mansfield Formation (sandstone)__________________ Brassfield Limestone__________________ Ste. Genevive Limestone__________________ Coal __________________ Jeffersonville Limestone__________________

Salem Limestone__________________ New Albany Shale__________________ Trenton Limestone __________________ Big Clifty Formation (sandstone) __________________ Antrim Shale________________

34) Using the time periods you determined above, what is the range of ages of the common rocks of Indiana? In other words, Indiana rocks come from what geologic time periods? ___________________________________________ 35) Here, list the rock units above in order from oldest to youngest. Note that several rock units may be from the same time period, so list those together. _________________________________________________________________________________________________ _________________________________________________________________________________________________ _________________________________________________________________________________________________ 36) Sitting atop the bedrock in central Indiana is sediment, mainly glacial till and outwash. We broadly call this the Tipton Till Plain. Knowing that the till sits on top of the bedrock, what is the age relationship between the two? In other words, which is younger, till or bedrock? What stratigraphic principle helped you ...