Color Coding of Grand Canyon Rock

Create section 1 in your virtual field journal. Title it “Section 1: Preliminary Investigation of Cambrian Strata in the Western U.S.” This section must include the following:

1. An annotated version of Figure 1: Geologic Columns from Two Grand Canyon Locations (Lochman-Balkk; 1971)

a. On each column, color the formations as indicated in Table 1.

Table 1: Color Coding of Grand Canyon Rock Formations

Formation Color

Muav Limestone Blue

Bright Angel Shale Green

Tapeats Sandstone Yellow

Contacts (Boundaries between formations) Black Line

b. Include a legend showing each formation’s color

c. Label each formation with its name

2. A one–paragraph written description of each formation including

a. The rock types present

b. The depositional environment for each rock type

3. An analysis of the facies present in each column.

a. What sort of change over time is represented by each of the rock sequences? This is a specific question with a specific answer. Refer back to the Depositional Environments Lab if you are not sure.

4. A one-paragraph answer of the following question

a. Based on your observations, what was occurring on this piece of crust during the Cambrian?

Section 2 Overview

You are excited by the observations that you made while studying Grand Canyon stratigraphy and you want to find out whether a similar phenomenon was occurring at that time throughout Western North America. You plan to investigate this by studying Cambrian system sedimentary rock formations throughout the west.

The phenomenon that you observed at the Grand Canyon has an important time component. You know that you will need a method for determining the ages of the rocks that you plan to study. Fortunately, these rocks are from the Cambrian, so Trilobite fossils abound!

Trilobites were macroscopic, hard shelled organisms that flourished on the shallow continental shelfs during the Cambrian. Their abundance, hard shells (which they shed multiple times throughout life), and the fact that they lived in environments with a lot of sedimentation make their fossils relatively easy to find and very helpful for determining rock ages. Your goal is to create a record of Trilobite evolution throughout the Cambrian. This will be a table showing the order in which Trilobite genera appeared (evolved) and disappeared (went extinct) during the Cambrian. You set out to achieve this by visiting four locations where there are excellent exposures of Trilobite fossil-containing Cambrian-system rocks. You fastidiously diagram the stratigraphic column at each location, including the Trilobite fossils present in each layer

Section 3 Assignment

Create section 3 in your virtual field journal. Title it “Section 3: Extensive Analysis of Western U.S. Cambrian Strata and Trilobite Fossils.” This section must include the following:

1. Answers to the following questions

a. What geologic change over time is recorded in the rocks at nearly all of the seventeen sites? (Hint, it is the same phenomenon at all sites and the same phenomenon that you observed at the original Grand Canyon sites.)

b. Consider the Trilobite fossils you found at your seventeen sites. Consider the Trilobite Faunal Succession chart. Did the geologic phenomenon from the previous question occur simultaneously at your sixteen sites? Use Trilobite Fossil Evidence and the Principle of Faunal Succession to explain your answer.

c. As you examine your seventeen sites, you notice that the oldest trilobite species at each site tends to be found either high in the sandstone layer or near the contact of the sandstone and siltstone/shale/mudstone layer. What does this precise location represent? Why would we expect to find our oldest Trilobite fossils there?

2. Create a table in which you cross reference each of your 17 sites with the oldest trilobite fossil found at that site and the position designation of that fossil in the Cambrian Sequence (see Table 3 for an example).

a. At site 15 the only fossil present is from the Upper Cambrian Brachiopod Bilingsella.

Section 5 Overview

Now that you have mapped your data you are finally able to draw conclusions about this enormous riddle you set out to solve. If your conclusions stand up to scrutiny you might be able to advance our understanding of Earth History.

Section 5 Assignment

Create section 5 in your virtual field journal. Title it “Section 5: Data Review and Conclusions.” This section must include the following:

1. The answers to these questions. These are complex questions and each will require at least one paragraph to answer sufficiently.

a. Were the sandstone layers at the various sites deposited at the same time or at different times?

b. When you constructed your map, did you have any reservations about sites 15, 14, 4, and 7? Why or why not?

c. According to your data and your map, what continent-scale geologic phenomenon unfolded over about 50 million years during the Cambrian?

d. What global processes could have caused this phenomenon to occur?

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