Lesson 7: Basin and Range and Mojave Deserts
In this week's lesson, as in last week's, we'll be studying the processes that have shaped the deserts of eastern California. Although this week's lesson emphasizes rocks that were formed long ago (like the strata exposed in the Titus Canyon Anticline, right), crustal extension and the Cenozoic sedimentation and volcanism associated with it continue to be important processes across the Basin and Range and Mojave provinces. This week we'll also begin working on the first of our four writing assignments. During the coming weeks these assignments will give you a chance to read two scientific papers that present recent ideas about California geology that are not in our text, and learn how geologists organize and interpret their observations to come up with new models for how the Earth works.
In chapter 7 Harden describes the bedrock geology of the Basin and Range and Mojave deserts. She begins by describing how young extensional faults have broken the landscapes of these provinces into parallel basins and ranges during the past 20 million years and, in so doing, produced local sedimentary basins and young volcanoes. Next, she takes us back in time to learn how rocks exposed across this region were deposited on a continental shelf formed by the break-up of an ancient supercontinent several hundred million years ago. Finally, she describes how the beginning of subduction along the western margin of North America about 225 m.y. ago crumpled the ancient sediments into folds and produced granitic magmas that were emplaced into the crust during Mesozoic time.
As you read about the complex geologic history of California's deserts it will be useful to take detailed notes. The region described in this chapter has experienced several major events—Proterozoic rifting; Paleozoic passive margin sedimentation; Mesozoic folding, faulting and intrusion due to subduction; and Cenozoic extension and volcanism—and writing out key facts in your own words or making neatly labeled drawings will help you better understand the significance of what you've read and keep everything straight. Having good notes will also make it easier for you to review for this week's quiz and and access what you've learned when you want to refer back to it for future assignments. Be sure that you are prepared to meet the learning objectives outlined below before you move on to the 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:
- Distinguish between a normal fault and a detachment fault on either a cross-section or outcrop photograph based on the dip of the structure and the types of rock found along it.
- Based on their compositions, indicate whether volcanic rocks from the Mojave and Basin and Range provinces are more similar to those from the High Cascades or the Modoc Plateau (e.g., Medicine Lake volcano) and explain this similarity in terms of tectonic settings of these areas.
- Explain why finding a thick section of Paleozoic limestone in eastern California implies to geologists that this region was once: (1) a broad passive margin; and (2) was covered by a tropical sea.
- Suggest two things a geologist might look for to determine whether a sequence of Paleozoic sedimentary rocks in eastern California had been changed mostly by Mesozoic or Cenozoic geologic events. (Hint: How do styles of deformation (folding and faulting) and associated igneous activity differ between the Mesozoic and Cenozoic in this region?)
- Order a list of the major events in the geologic history of eastern California from Late Proterozoic to Cenozoic time in correct chronological sequence.
Reading and Browsing Assignment
- Read Chapter 7 and the first part of chapter 18 (p. 483-490) on the late Proterozoic rifting of Rodinia that created a passive continental margin with a broad continental shelf in eastern California. As you read and take notes, be sure to focus on the topics outlined in the learning objectives above.
- Browse through the U.S. Geological Survey and National Park Service's brief introduction to the geology of the Basin and Range Province.
- To learn more details about the pre-Late Cenozoic geologic history of the Great Basin-Mojave desert region check out some of the "field trip stops" in the U.S. Geological Survey and National Park Service's "Geology in the Parks" sites for Death Valley National Park and Mojave National Preserve. This week, visit the 'Death Valley Dawn' through 'Dante's View' sections at Death Valley, as well as the 'Evidence of Early Life' through 'Mitchell Caverns State Park' stops at Mojave. Remember, your goal is to learn to what the pre-Late Cenozoic rock units, as well as the young faults and volcanoes that cross-cut or overlie them, tell us about the changing geographic and geologic setting of this region during the past several hundred million years.
- Finally, for an overview (literally) of how the ancient landscape of California has changed since Proterozoic time, check out the Paleogeographic Maps of Western North America created by Robert Blakey at Northern Arizona University. To get a bird's-eye view of North America rifting away from Australia and Antarctica, the carbonate platform that dominated eastern California during the early Paleozoic, or the subduction zone that developed along the margin of the continent during Mesozoic time just click on the period you're interested in. These maps can really help you see how the various geologic features Harden describes "fit together", and you may want to come back to them later in the semester as we discuss the geologic histories of other provinces.
Exercise 7: Outline of First Article (Due by 9:00 AM on 28-Feb-2011)
This week's exercise is to complete the first of our four writing assignments. To learn about the assignment read through the pointers below and then click on the "Resources" link on the left side of this page, scroll down, and click on "Outline 1" under "Writing Assignment" near the bottom of the page. This week we'll be completing steps one and two and you'll be turning in the outline of the article you choose from the first set.
- Your first task is to choose one of the two articles on plate boundary processes in Set 1 and to read that article carefully. You'll find PDFs of the articles we'll be using under the "Resources" tab on our class Etudes site, not on this website. Just choose the article that interests you most, download it, and read it carefully.
- After you've read through the article once to get the jist of it, go back, read through it a second time, and write down notes on your key observations and conclusions. Do not hestitate to post a note to the discussion board or contact me via private message if you have questions about an unfamiliar term or idea. If you don't understand it there's a good chance other people will have questions too.
- Next, draft an outline of the article's major conclusions and supporting evidence that follows the format of the "sample outline" linked to the Writing Assignment page. In order to organize your outline imagine that you are telling a friend who hasn't read the article what its three or four key conclusions are and what data or observations are presented to support each of them. You will want to do your best to focus on the main points of the article and not get sidetracked by peripheral details.
- Your outline does not need to follow the style of the sample outline exactly (e.g., you don't need to use the same font and bullets), but be sure that it includes the complete article citation, your "PIN" (last 5 digits of your Etudes user id), and the course name and date at the top. Also, be sure that your text is written in complete sentences (not single words or phrases) and that it is well organized and coherent so that a reader can easily follow the main themes of the article.
- Finally, before you submit your outline you should check out the outline grading rubric that is linked to the Writing Assignment page. It will show you what criteria I will use to evaluate your outline. There is no set length for your outline, but if it's longer than about a page and a half it won't be as helpful when you go to write your abstract. (Abstracts are very short, so the more you can winnow the article's main themes from its peripheral details as you prepare your outline the less work you'll have to do later). Also, if you print a copy of the article be sure to save that copy so that you'll have it to refer to again in a couple of weeks when you'll be writing the corresponding abstract.
- When you have completed your outline go to the Etudes site for our class, go to Exercise 7 in the Etudes "Assignments, Tasks, and Tests" tool and submit your outline as an attached file. Outlines are due by 9:00 AM next Monday (28-Feb-2011.) I ask that you to send it as an attachment rather than pasting it into the text box because this will preserve your formatting. You should send it as an MS Word document or, if you write it in another program, as a Word-compatible file. If you have any problems just let me know. I will return your outline with comments and a completed rubric by 7-Mar-2011 in time for you to get started on the second part of this assignment.
Quiz 7: Basin and Range and Mojave Deserts (Due by 9:00 AM on 28-Feb-2011.)
After you feel you have met the learning outcomes outlined above, please complete Quiz 7 in the Etudes "Assignments, Tests, and Surveys" tool. There are ten questions about California's deserts and each is worth one point. If you can answer all of them correctly it means that you know your way around ancient landscape of California's Basin and Range and Mojave deserts pretty well and are ready to start learning about one of California's most iconic regions—the Sierra Nevada—next week. Like all of our weekly quizzes, this one is timed (you'll have 30 minutes) and must be completed in one "sitting". (That is, you will only be granted access once.) So, be sure you're ready to complete your quiz when you start it—and be sure you're using Firefox. Good luck.