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Lesson 3: Geologic Dating and Crustal Deformation

Before we dive into the details of the geologic processes that have shaped the North American continent and America's national parklands we're going to spend a little time learning about some key geologic principles. This week's lesson and the two that follow are drawn from Chapter 2 and introduce geologic concepts that we'll come back to repeatedly throughout the semester.

As you read about relative and absolute dating and about the folds and faults produced by crustal deformation this week it will be useful to take careful notes so that you'll be able to keep track of major ideas and apply them to the specific examples we'll encounter during the coming weeks. Be sure that you are prepared to meet the learning objectives outlined below before you move on to the exercise and quiz at the bottom of the page.

Weekly Learning Objectives

Upon successful completion of this week's lesson, a student is expected to be able to:

Reading and Browsing Assignment

Exercise 3 (Due by 9:00 AM on Monday, 1-Feb-2010)

This exercise will give you practice applying the relative dating principles you've learned to work out the geologic history of a famous national park region: the Grand Canyon. Your task is to study the accompanying cross-section of the canyon and, based on what you observe, answer the questions below. Be sure to write your answers down and then go to Exercise 3 in the Etudes "Assignments, Tasks, and Tests" tool to record them.

  1. Which type of unconformity separates the Tapeats Sandstone from the underlying granite and metamorphic rocks of the Vishnu Group?
  2. Which type of unconformity separates the Tapeats Sandstone from the underlying sediments of the Grand Canyon Supergroup (the tilted beds of the Unkar and Chuar Groups)?
  3. Which is older, the Tapeats Sandstone or the Bright Angel Shale?
  4. Which relative dating principle enables you to infer the relative ages of the Tapeats Sandstone and Bright Angel Shale?
  5. In the Vishnu Group which is older, the schist and gneiss or the Zoroaster Granite (labeled simply "granite" in the cross-section)? Note that the granite cuts across the metamorphic fabric (denoted by short dashed lines) in the schist and gneiss.
  6. Which relative dating principle enables you to determine the relative ages of the Vishnu Schist and the Zoroaster Granite?
  7. Order the following events in sequence, from oldest (1) to youngest (5):
    • Cutting of the Grand Canyon by the Colorado River.
    • Formation of the metamorphic and granitic basement rocks.
    • Deposition of the sequence of Paleozoic strata (Tapeats Ss [sandstone] to Kaibab Ls [limestone]).
    • Deposition of the Unkar and Chuar Group strata in a rift basin in the ancient basement rocks.
    • Period of uplift and erosion that produced the unconformity that now lies beneath the Tapeats Sandstone.
Grand Canyon x-section
Click on the picture above to view a larger version. You may want to print this larger version (scaled to ~50% so that it will fit on a page) in order to have it to refer to as you answer the accompanying questions.

Quiz 3 (Due by 9:00 AM on Monday, 1-Feb-2010)

After you feel you have met the learning outcomes outlined above, please complete Quiz 3a in the Etudes "Assignments, Tasks, and Tests" tool. Remember, this quiz covers all of the learning outcomes above, not just relative dating. There are ten questions, each worth one point. If you can answer all of them correctly it means that you know your way around geologic dating and crustal deformation pretty well and are ready to move on to a more detailed look at earth materials (rocks and minerals) next week.