Laboratory 16

Interpreting Geologic History from Maps:
The Grand Canyon Geologic Map
Laboratory

Pamela J. W. Gore

Department of Geology, Georgia Perimeter College

Clarkston, GA 30021

Copyright 1982-1998-2004 Pamela J. W. Gore

 

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).

 

 

Laboratory 16

Interpreting Geologic History from Maps:
The Grand Canyon Geologic Map Lab
Exercises

Examine the geologic map of the Grand Canyon (Bright Angel Quadrangle), with structure sections across the canyon, and the specimens provided to answer the following questions.

1. Each colored unit on the geologic map represents a formation. See the "explanation" (or "legend" or "key") 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

You should be familiar with the geologic period names from the geologic time scale. There is a copy of the time scale in the front of your lab manual.

a. What are the ages of rocks are present on this map? Put an X in the box below each period abbreviation if it is on the map or in the legend.

 

pC

C

O

S

D

M

P

P

Tr

J

K

T

Q

                         

 

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.

 

pC

C

O

S

D

M

P

P

Tr

J

K

T

Q

                         

 

2. The fine gray lines on this map are topographic contour lines. Some of these contour lines are numbered with the elevation (in feet) above sea level. The contour interval is defined as the difference in elevation between two adjacent contour lines.

a. What is the contour interval on your map? _______ feet

 

b. What is the approximate depth of the canyon? Follow the procedure below to arrive at your answer.

(1) Find the elevation at the top (or rim) of the canyon. (This is the place where the contour lines change from being closely spaced to being widely spaced. A good point to use near the El Tovar Hotel on the south rim.)

Elevation at top of canyon = __________________________ feet

(2) Find the elevation at the bottom of the canyon (at the river).

Elevation of river at bottom of canyon = _________________________ feet

(3) Subtract these two elevations to find the depth of the canyon.

Depth of canyon = __________________ feet

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, and subtract.

Thickness of Paleozoic rocks in the Grand Canyon = _________________ feet

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, examine the structure sections or look for photographs of the Grand Canyon in your textbook.

___________________________________

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. (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. Sketch the MAP EVIDENCE for an unconformity. (See Figure 2 in this lab).

 

 

 

 

 

 

c. Sketch the unconformity as shown in the structure section A-A' near Widforss Point.

 

 

 

 

 

d. An angular 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 section A-A'. 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.

 

4. Locate the Vishnu Schist in the legend of the map.

a. What is its age? ____________________________

b. Is it located near the top or near the bottom of the canyon? (Circle one.)

 

5. The Tapeats Sandstone is separated from the Vishnu Schist by an unconformity.

a. What type of unconformity is it? ______________________________

b. Which formation (Tapeats or Vishnu) is younger? (Circle one.)

c. What other formations does the Tapeats overlie? (Give at least four other formations.)

  1.  

  2.  

  3.  

  4.  

6. What formation overlies the Tapeats Sandstone? ____________________________

7. The diagram below shows two stratigraphic sections with lithologic and fossil data, through the Tapeats Sandstone, Bright Angel Formation, and Muav Formation. The letters A, B, C, and D refer to the stratigraphic ranges of particular fossil species.

a. Is this a transgressive or a regressive sequence? (Circle one.)

 

 

 

 

 

b. Using the fossil specimens (labelled A, B, C, and D) provided, which are of types similar to those which may be found in strata of this age in the Grand Canyon, identify the fossils.

A ________________________________

B ________________________________

C ________________________________

D ________________________________

 

c. On the diagram above, draw lines which correspond to the top and bottom of the stratigraphic ranges of each of the fossils, A, B, C, and D.

d. Assume that the tops and bottoms of the stratigraphic ranges of each of these fossils (A, B, C, and D) represent time lines. Do the contacts between the three formations parallel time lines, or are they diachronous?

__________________________________

e. Did sedimentation begin earlier in the eastern section or in the western section?

__________________________________

f. From your answer to the question above, which direction (east or west) was more landward?

__________________________________

8. a. What is the age of the Muav Formation? (See legend on map.)

__________________________________

b. What is the age of the overlying Redwall Limestone?

__________________________________

c. The different ages of these two adjacent formations indicate the presence of an unconformity or time break. Is there any evidence ON THE MAP for and unconformity between these two formations?

__________________________________

d. How could the presence of this unconformity be detected? (Hint: See the explanatory information at the beginning of this lab.)

 

____________________________________________

Fossils of many marine invertebrates which lived during the time after the Muav Formation was deposited, but before the Redwall Limestone was deposited, are known from other parts of the world. But, all are missing here. The fossils indicate the presence of an unconformity.

9. a. Box E contains fossils like those from the Redwall Limestone. Identify three fossil groups.

_____________________, ___________________, _____________________

b. What is a probable depositional environment for these fossils?

_____________________________________________

c. Box F contains rocks and fossils (or trace fossils) like those from the Supai Formation.

(1) Identify the rocks _____________________________________________________

(2) Identify the fossils or trace fossils. _______________________________________

d. What is a probable depositional environment?

____________________________________

e. The Supai Formation overlies the Redwall Limestone. What happened to sea level? (Transgression or regression?)

_____________________________________

10. The Hermit Shale is red, with a few sandstone beds near its base (fining upwards). Ripples, mudcracks, plant fossils, and footprints are also present. Interpret the depositional environment.

__________________________________________

11. The Coconino Sandstone is a white to buff, cross-bedded, well sorted quartz sandstone with rounded grains, a few reptile tracks, and no marine fossils. Interpret the depositional environment.

___________________________________________

 

12. The Toroweap Formation and Kaibab Formation are marine limestones. The Kaibab also contains chert. Fossil brachiopods, corals, cephalopods, and crinoids are present.

The presence of marine deposits over continental deposits indicates that a (transgression? or regression?) occurred.

____________________________________________

Summary

Outline the events in the geologic history of the Grand Canyon area as recorded by the rocks, and as interpreted step-by-step, in order from oldest to youngest, in this lab exercise. Use the geologic map, structure sections, and your answers to the above questions to guide you. (You are basically just summarizing your findings, in order.) Use your own paper. Should be typed.

Start with the Precambrian Vishnu Schist (pCV), and indicate the units in order of deposition (with depositional environments mentioned for the Paleozoic units), and mentioning periods of tilting, uplift, erosion, transgression, and regression, in the proper order.


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This page created by Pamela J. W. Gore, pgore@gpc.edu
Georgia Perimeter College

December 22, 1998
Modified December 13, 2003


Instructor's Guide For Laboratory 16 on the Grand Canyon

Students need to be able to read elevations from contour lines.

You will need the following:

Copies of the geologic map of the Grand Canyon or Bright Angel Quadrangle for each pair or table of students. Many of these maps are sold in sets which come with structure sections or geologic cross sections along several lines. Several questions refer to the structure sections. If they are not available, the students can omit those questions.

To reinforce identification of invertebrate macrofossils, a set of Paleozoic marine fossils which might be found in the Cambrian Tapeats Sandstone, Bright Angel Formation (shale), or Muav Formation (limestone) can be assembled. If they are actually from these formations, so much the better. If Grand Canyon-type fossils are not available to you, it is possible to use appropriate substitutes (with the proper geologic ranges) for the sake of the exercise. Trilobites and brachiopods are characteristic of the Muav Formation.

(1) There should be four boxes of fossils labelled A, B, C, and D. (See question 7).

(2) There should be a box labelled E with fossils from the Mississippian Redwall Limestone (or substitutes). Various marine invertebrates can be used, including brachiopods, corals, and crinoids.

(3) There should be a box labelled F with Pennsylvanian- Permian Supai-type rocks (non-marine red cross-bedded sandstone, and red shale), and fossils or trace fossils such as tracks, or bones of amphibians or primitive reptiles).