Using the power of Google Earth, GeoTours take you on flyovers of key locations discussed in the text.
StudySpace student website and ebook icons in each GeoTour connect your to the text and online review materials.
Dialogue boxes accompanying each site include text, figures, photos.
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Geotour 6: Sedimentary Rocks and Environments
Sedimentary Rocks Exposed in the Grand Canyon
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| 1a. The Grand Canyon is a marvelous place to study geology. Here, weathering and erosion by the Colorado River have revealed an immense span of time as recorded in the rock record. Turn on the Grand Canyon Geologic Map (Note: This overlay is 4 MB and takes time to be downloaded into Google Earth’s memory cache. Slight mismatches exist because the map is draped over topography.). In this worksheet, we’re going to focus on the cover sequence of sedimentary rocks that were deposited on top of the metamorphic and igneous basement rocks.
In order to get a sense of horizontal scale, check and double-click the placemarks for Problem 1a. Use the Measuring Tool to determine the distance (in km) between the North and South Rims.
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| 1b. To get a sense of relief in this area, check and double-click the placemarks for Problem 1b. Hover the hand cursor over each placemark and find the relief by subtracting the elevations shown at the bottom of the screen in the information bar (in m).
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| 1c. To identify the boundary between the cover rocks and the ancient basement rocks, check and double-click the placemarks for Problem 1c. These placemarks delineate the steep and narrow Inner Gorge adjacent to the Colorado River. Which of the following statements is incorrect?
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| 1d. Locate the lavender-colored Redwall Limestone (Mr) using the geologic map (placemark Problem 1d). What kind of slope does it form?
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| 1e. Locate the white-colored Bright Angel Shale (Cba) using the geologic map (placemark Problem 1e). What kind of slope does it form?
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| 1f. Locate the tan-colored Coconino Sandstone (Pc) using the geologic map (placemark Problem 1f). What kind of slope does it form?
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| 1g. What general statement can you now make about the slopes of the Grand Canyon based on the different sedimentary rock types?
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| 1h. Check and double-click the placemark for Problem 1h (you also may want to look at the “Rock Units” folder in Geotour Sites “Grand Canyon, AZ” folder for this chapter in addition to the geologic map). Identify the following sequence of Cambrian rock units: Tapeats Sandstone (near shore), Bright Angel Shale (intermediate distance from shore), and Muav Limestone (distal from shore sediments, but still shallow water). The stacking of these units represents either a regression (fall) or transgression (rise) of sea level. Which is occurring in this area?
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| 1i. Check and double-click Problem 1i. Here, the Colorado River narrows at the mouth of a side canyon, and there are some rapids. What causes this?
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Tilted and Folded Sedimentary Rocks
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| 2a. As we discovered from Problem 1 in the Grand Canyon, one of the most easily observable sedimentary structures is bedding or layering. However, not all regions have experienced the spectacular erosion that the Grand Canyon has. In mountainous areas (such as the Lewis Range in Montana), plate tectonic convergence has exposed various sedimentary layers by tilting, folding, and/or faulting them. Resistant rocks still erode to form cliffs, and less resistant rocks still form low slopes or valleys. Folding and faulting can repeat the same sedimentary layers multiple times, creating a series of repeating ridges (resistant) and valleys (less resistant).
Check and double-click the placemarks for Problems 2a-i and -ii. Which of the following is true regarding the resistance to erosion of the layers specified by the placemarks?
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| 2b. Based on the geometry of these layers, which direction are the rocks tilted (geoscientists might ask which way do the rock layers “dip”)? Hint: Tilted sedimentary rocks typically erode to form an asymmetric ridge called a flat iron—the dip slope of the ridge lies parallel to bedding, forming a somewhat planar surface with a pointed or rounded edge pointing in the direction opposite of the tilt (geoscientists thought that this geometry resembled the flat bottom of a laundry iron). Put another way: if you poured water on the flat dip slope, in which direction would the water flow? Problem 2b is a polygon outlining one of the flat irons.
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| 2c. Now that you’ve seen some flat irons, check and double-click the placemarks for Problems 2c-i and -ii. Which choice best describes the dip/tilt of the layers for each placemark (we’ll learn more about this feature in Chapter 9)?
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Arid Depositional Environments
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| 3a. Check and double-click placemark Problem 3a to fly to Death Valley, CA.
What depositional environment does placemark Problem 3a represent?
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| 3b. What depositional environment does placemark Problem 3b represent?
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Fluvial Depositional Environments
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| 4a. Check and double-click the placemarks for Problems 4a-i, -ii, and -iii to fly to the Niger Delta, Nigeria. The Niger Delta consists of sediment carried to the sea by the Niger River. Check the “What a Geologist Sees” folder in the Problem 4 folder to activate a Niger Delta overlay that provides the approximate present-day outline of the delta with labeled landscape features.
What type of sedimentary rock will likely form at each of the placemarks?
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| 4b. If you drilled at placemark Problem 4b, which rock units would you likely see?
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