Lab 11 – Interpreting the Geologic Map of the Grand Canyon

Pamela J. W. Gore
Department of Geology, Georgia Perimeter College
Clarkston, GA 30021
Copyright © 1982-1998-2004 Pamela J. W. Gore

For this lab we will be looking at the southeast corner of the Geologic Map of the Grand Canyon 30’ x60’ Quadrangle, Coconino and Mohave Counties, Northwestern Arizona. In order to get yourselves familiar with geologic maps, please, be sure to take a look at this website to help you in understanding and interpreting geologic maps.
www.nature.nps.gov/geology/usgsnps/gmap/gmap1.html

In this lab you will be using the geologic map of the Grand Canyon. You will examine the geologic map to look for evidence of unconformities, and to determine the relative sequence of geologic events.

Unconformities may be recognized on a geologic map in several ways.
1. An angular relationship between sequences of beds is called an angular unconformity. In cross-section an angular unconformity looks like this:

In map view, when the bottom of one formation is in contact with two or more underlying formations, the contact is an angular unconformity.

2. A big difference in age between two adjacent formations, such as a Cretaceous sandstone overlying a Cambrian limestone (with ages determined from fossils contained in the rocks, or from radiometric dates) indicates the presence of a time gap or unconformity in the sequence. The ages of the formations are given in the legend of the geologic map.
Relative ages of geologic events can be determined by using the principles of superposition (one of Steno’s Laws) and cross- cutting relationships. (See Lab 8 on Relative Dating).
Important first steps:
1. Examine the geologic map of the Grand Canyon (Geologic Map of the Grand Canyon 30’ x60’ Quadrangle, Coconino and Mohave Counties, Northwestern Arizona) with map legend and D-D’ structure cross sections across the canyon (GrandCanyonMap.doc or GrandCanyon.rtf). Be sure to take a look at the “List of Map Units” located attached with the map. You will need to know what contact lines look like. You will find an example of these lines located here. These lines separate rock formations from one another. We will be focusing in on the southeastern corner of this map.

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2. Now take a look at the map scale located at the bottom center of the map. You will find the contour interval for the map here.

Time scale:

1. Each colored unit on the geologic map represents a formation. See the “List of Map Units” on the side of the map for formation names. The ages of the rocks are given also. Each formation has a symbol or abbreviation that will help you to read the map. The first letter of the symbol is capitalized, and represents the age of the unit. The other letters are lowercase and represent the formation or group name, or provide lithologic information. Example:
Cba = Cambrian Bright Angel Formation
2. Take a look at the geologic time scale that is attached to this lab. Start to familiarize yourself with the geologic period names from the geologic time scale. Compare this time scale with the time scale listed on the map.
a. What are the ages of rocks are present on this map? Put an X in the box next to each period abbreviation if it is on the map or in the legend.

Q
T
K
J
Tr
P
IP
M
D
S
O
C
X, Y, and/or Z

b. What ages (periods) are missing from this sequence? Put an X in the boxes of those, which you cannot find on the map or in the legend.

Q
T
K
J
Tr
P
IP
M
D
S
O
C
X, Y, and/or Z

Focusing on southeast corner of map:
2. The fine gray lines on this map are topographic contour lines. Some of these contour lines are numbered with the elevation (in meters) above sea level. The contour interval is defined as the difference in elevation between two adjacent contour lines. Find the scale for the map. It is located at the bottom center of the map.
a. What is the contour interval on your map? _______ meters

b. What is the approximate depth of the canyon? Follow the procedure below to arrive at your answer.
(1) Zoom in on the map to the point where you can read the map symbols and the contour lines.
(2) Find Shoshone Point on the southern rim of the Grand Canyon.
(3) From Shoshone Point, find the elevation at the top of the rim of the canyon. (This is the place where the contour lines change from being closely spaced to being widely spaced.) This is the top of the canyon.
Elevation at top of canyon = __________________________ meters
(4) Find the elevation at the bottom of the canyon at the Colorado River.
Elevation of river at bottom of canyon = _________________________ meters
(5) Subtract these two elevations to find the depth of the canyon.
Depth of canyon = __________________ meters
c. What is the approximate thickness of the sequence of Paleozoic rocks in the Grand Canyon (i.e., from the rim, to the base of the Paleozoic section)? Do not include any additional thickness of Paleozoic rocks that may be located elsewhere on the map that are not actually in the canyon. You will note that the topographic contour lines parallel most of the contacts between the geologic formations (at least in the Paleozoic part of the section). Use your elevation for the top (or rim) of the canyon from the previous question, and the elevation at the base of the Cambrian Tapeats Sandstone (Ct), and subtract.

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Thickness of Paleozoic rocks in the Grand Canyon = _________________ meters
d. What is the general structural orientation of the Paleozoic rock units in the Grand Canyon? (Possibilities include nearly horizontal or flat lying), nearly vertical (standing on edge), dipping, or folded.) Hint: If the answer is not obvious from the map with a little thought, take a look for photographs of the Grand Canyon at the beginning of this lab.
___________________________________
e. In some places at the bottom of the canyon (near the river), the topographic contours do NOT parallel the formation contacts. These formations are not horizontal. List at least four formations, which are not horizontal, from an examination of the MAP or LEGEND. (Do not use the structure sections for this question.)

1.
2.
3.
4.

3. The contact between the Cambrian Tapeats Sandstone and these underlying units is called an unconformity.
a. What type of unconformity is it? _______________________________
b. This type of unconformity implies that the rocks below the unconformity surface were (1) deposited, (2) tilted and uplifted, and (3) eroded. Then, (4) the rocks above the unconformity were deposited.
Examine the structure cross section D-D’. It is located at the bottom the map. What is the approximate AGE of the unconformity? (Bracket it between two formations, and give their ages (period names) – older than X but younger than Y.)
Name of formation directly above unconformity ______________
Age of formation directly above unconformity _______________
Name of youngest formation below unconformity ______________
Age of youngest formation below unconformity _______________
Therefore, you now know that an episode of crustal deformation (tilting and uplifting), followed by erosion, occurred between the
___________________________ period and the
____________________________ period.