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Chapter 7: A Surface Veneer: Sediments, Soils, and Sedimentary Rocks

Geotours

Download Geotours.

Getting Started :

If you haven’t done it already, download Google Earth™ and install it on your computer.
If you haven’t done it already, download the Geotours.kmz file and save a copy to your desktop. (The Geotours.kmz file contains the Geotours for all chapters, so you only need to download this once!)
By downloading Geotours.kmz you acknowledge that it was created solely to accompany Steve Marshak's Earth: Portrait of a Planet and Essentials of Geology and is limited to use with only Steve Marshak's Earth: Portrait of a Planet and Essentials of Geology and may not be reproduced, stored in a retrieval system, or transmitted in any form by any means for any other purpose without the written permission of the publisher.
Double-click the Geotours.kmz file and Google Earth™ will open automatically.
In the left-hand sidebar you will see a Places menu, and in the Temporary Places folder you will see an EARTH_3e.kmz file. Double-click the file and you will see a list of Geotours for each chapter.
Then open the Geotour folder you want to explore! If you’d like to read more about the features of Geotours see Using Geotours, or go to our Helpful Resources section.

STOPS ON GEOTOUR 7: Sedimentary Rocks and Environments

Grand Canyon, Arizona

See also Worksheet Problems 1-3
"What a Geologist Sees" Geofeatures: North Rim, South Rim, Bright Angel Canyon, Kaibab Limestone, Toroweap Formation, Coconino Sandstone, Hermit Shale, Supai Group, Redwall Limestone, Muav Limestone, Bright Angel Shale, Tapeats Sandstone, Zoroaster Granite (dike), and Vishnu Schist; Trace: The Great Unconformity
Colorado River Tour

Lewis Range, Montana

See also Worksheet Problems 4-5

Death Valley, California

See also Worksheet Problem 6

Great Exhuma, Bahamas
Sand Dunes, Namibia

See also Worksheet Problem 7

Niger Delta, Nigeria

See also Worksheet Problem 8
"What a Geologist Sees" Geofeatures: Cutbank, Sandbar, Meander Scars, Yazoo Tributary, Oxbow Lake; and Polygon Outline: Niger Delta

GEOTOUR 7 WORKSHEET

You can see dramatic exposures of sedimentary rocks at many localities across the planet. Stratification in these exposures tends to be better where climate is drier and vegetation sparse. With a little searching, you can also find many examples of places where sediment is now accumulating.

Grand Canyon, Arizona
1. Fly to Image G7.1–2, which provides a view of a point in the Grand Canyon about 10.8 km NW of Plateau Point.

(a) About how wide (in km) is the Grand Canyon as measured between the Problem 1a placemarks?

(b) Double-click on the placemark for Problem 1b. You can see the narrow “Inner Gorge” adjacent to the Colorado River. The inner gorge formed where the river cut down into Precambrian basement metamorphic rock. How wide is the inner gorge at this site (in km)?

(c) Open the WAGS folder within the Grand Canyon, Arizona, folder. Double-click on the “Great Unconformity” to see the contact between basement and cover at Plateau Point. Check the box to make the line showing the unconformity appear. What is the spatial relationship between the Great Unconformity and the Inner Gorge?

(d) From the tilted viewpoint in Problem 1c you can see the color banding of the sedimentary beds from the Great Unconformity up to the top of the canyon. Are the beds tilted at an angle, horizontal, or vertical? (Hint: Use the flat surface at the top of the canyon as a reference plane—are the beds parallel to the surface, at an angle to this surface, or perpendicular to this surface?)

(e) Double-click on the placemark for Problem 1e. You can see that the river narrows at the mouth of the canyon and there are some rapids. Why?

What kind of sedimentary material makes up the apron at the end of the side canyon?

Why was the debris apron deposited at this location?

(f) The placemarks labeled Problem 1fa and 1fb highlight two different sedimentary rock formations in the Plateau Point region. These formations weather differently—one forms a prominent cliff throughout the Grand Canyon (Problem 1fa), whereas the other forms a relatively flat bench (Problem 1fb). Based on your knowledge of rock types and how they weather, match the placemarks with the correct rock unit:
• Bright Angel Shale
• Redwall Limestone

2. Bright Angel Canyon (delineated by placemarks labeled Problem 2) is remarkably straight because the canyon follows a straight fault. During faulting, slip of one block of rock past another breaks and crushes rock along the fault plane into smaller pieces. Why does Bright Angel Canyon follow a fault?

3. In the folder labeled Problem 3, two placemarks highlight the Kaibab Plateau on the north side of the canyon and the Coconino Plateau on the south side.

(a) Which of the two plateaus is higher?

(b) Click on the North Rim and South Rim labels in the WAGS for Grand Canyon, Arizona. Is the rock unit underlying the North Rim the same or different from the one underlying the South Rim? (Hint: Zoom in on both canyon walls and determine if the color and thickness of beds at the top of the canyon on one wall look like those on the other wall.)

Lewis Range, Montana
4. When you fly to the Lewis Range in Montana (double-click on Image G7.3), you see a landscape that is very different from that of the Grand Canyon. In the Lewis Range, beds of sedimentary rock were tilted in response to mountain-building forces (you will learn about this process in Chapter 11). Tilted sedimentary rocks typically erode to form an asymmetric ridge called a cuesta, or flat iron—one slope of the ridge lies parallel to bedding (and is called a dip slope), whereas the other cuts across bedding (and is called a scarp). With this concept in mind, you can recognize the tilt (dip) of bedding by looking at the landscape.

(a) Fly to the placemark labeled Problem 4. Is the dip slope tilted to the east or to the west (put another way: if you poured water on the dip slope, in which direction would the water flow)?

5. Click on the two placemarks labeled Problem 5. Do the beds at the two placemarks dip in the same direction or opposite directions? (Hint: You can get a good sense of the orientation of the layers by looking at stream cuts.)

Considering your answer, have the layers been bent into an arch-like shape or a trough (canoe)-like shape?

Considering your answer, have the layers been bent into an arch-like shape or a trough (canoe)-like shape?

Death Valley, California
6. Death Valley lies within a continental rift called the Basin and Range, where regional stretching caused faulting, and slip on faults produced down-dropped valleys (basins) and tilted blocks of strata that stand out as ridges (ranges).

(a) Is the placemark for Problem 6 positioned on a basin or on a range?

(b) Does this feature serve as a source of sediment, or a repository (collection site) for sediment?

(c) Double-click on Image G7.4. Approximately how many small canyons feed sediment into the alluvial fans that occupy the region east of the white playa?

Sand Dunes, Namibia
7. Double-click the placemark labeled Problem 7, and you’ll end up over the red sand dunes of Namibia. Here, the crests of dunes are perpendicular to the wind direction.

(a) What is the predominant wind direction producing dunes in this desert environment (choose one)?
• from west to east
• from east to west

(b) If the sand dunes were to be buried, the resulting sandstone would contain cross beds. Would the cross beds tilt to the east or to the west?

Niger Delta, Nigeria
8. Double-click on Image G7.8–9 to fly to the Niger Delta. The Niger Delta consists of sediment carried to the sea by the Niger River. Click on the Niger Delta overlay in the WAGS folder to see the approximate present-day outline of the delta. Deltas and river systems contain many distinct depositional environments. The WAGS folder provides the names of the different landscape features of the area (discussed further in Chapter 17).

Each of the following placemarks (Problems 8a, b, and c) mark a distinct depositional environment. Match the placemark to the type of sedimentary rock that could form from the sediment deposited at the site.
• sandstone
• coal
• shale

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StudySpace: Chemistry 2e.