reconstructing the geologic history of painted canyon and...

Lab 3. Reconstructing Geologic History

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Reconstructing the Geologic History of Painted Canyon and Planetary Surfaces (Lab 3)

Synopsis

You will use the information you accumulated in the previous labs to reconstruct the

geologic history of Painted Canyon. You will learn how to tell the sequence of geologic

events that formed the landscape and then apply these principles to reconstruct the

geologic history of Painted Canyon. You will use these and other principles to

reconstruct the relative ages of different surfaces on planetary objects.

Northern Painted Canyon, looking west.

Introduction You have all the information you need to reconstruct the geologic history of Painted Canyon, except

for some principles of how to determine the relative sequence of events.

Goals for This Week

Learn how to reconstruct the sequence of events that formed the geology.

Reconstruct the geologic history of Painted Canyon.

Reconstruct the relative ages of different regions of a planetary object in our Solar System.


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Exercise 3A: Determining the Sequence of Geologic Events

Read Box 3-1, which outlines the principles you will use to determine the sequence of events in an

area. As you read the box, examine Figure 3-1 and Figure 3-3, which illustrate these same principles.

Each cross section in Figure 3-3 illustrates a different principle or rule for determining the sequence

of events the rule is what is important, not the specific example.

Discuss key points with your team member.

Note: Your instructor may choose to do the following bullet as a whole-class exercise.

Using the principles of relative dating, determine the sequence of events that occurred to form the

rock sequence shown in the cross section in Figure 3-4. Use a pencil to write your answers in

Worksheet 3A-1 at the end of this lab, and discuss your answers with your partner.

Your two-person team should go to the computer, click on the SES123 home page and click on the

Principles of Relative Dating link, or use the second link below:

SES123 Home Page

Principles of Relative Dating

Next, visit the Key Relative Age Relations page, also on the SES 123 home page (or the link below)

Key Relative Age Relations

Proceed through the module, and put your observations and answers in the appropriate spaces on

Worksheet 3A-2 and Worksheet 3A-3. Black and white versions of some photographs in this module

are in Figure 3-5.

Exercise 3B: Reconstructing the Geologic History of Painted Canyon

Take all the information you have developed in the past Painted Canyon exercises (Lab 1 and Lab 2)

and work with your team summarizing the geologic history of Painted Canyon in Worksheet 3B. This

worksheet will be handed in and graded. The SES 123 webpage has links to the sample location numbers

of igneous, sedimentary, and metamorphic rocks. From these locations you can determine the relative

ages of each rock type. You will need to know the descriptions and environments of all the samples you

examined in Labs 1 and 2. Your history should incorporate the following:

The sequence in which all the unknown rock samples were formed. The relative ages of your samples

is in part suggested by the relative elevations of the samples. You can also re-examine the images of

the sample sites to determine which sample is higher and which one is lower. Relative elevation may

not work for the very oldest rocks (look for cross-cutting relations too). In any event, a great place to

start is to list on a sheet a scratch paper the samples in order of elevation.

The name of each sample and the environment in which it formed. You may decide to combine

several samples into one event, like the different sedimentary rock types that occur in the red- and

gray-layered rock unit half way up the canyon.

A possible explanation for how the environment changed from one time (sampled rock unit) to

another (e.g., the seas went out).

There are other features that fit into the history of the canyon. These include (1) a fault, which is a

fracture along which rocks on either side are offset; (2) pegmatite dikes, which are coarsely

crystalline igneous rocks that are expressed in the bottom of the canyon as light-colored streaks that

cross cut the layering in metamorphic rocks; (3) a dike of dark basalt, which is expressed as a vertical

http://reynolds.asu.edu/SES123/index.html

http://reynolds.asu.edu/glg103/relative_age_principles.htm

http://reynolds.asu.edu/glg103/key_relative_ages.htm


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sheet cutting across most of the layers. Within the canyon there is also an unconformity, which

represents an ancient erosion surface. An unconformity separates older rocks below from younger

rocks above, such as metamorphic rocks below and sedimentary rocks above. You should also

consider the erosion of the modern canyon.

Important: You will need to look at the computer images or print outs that show relative age relations, or

else you will miss key features, such as the dike and fault.


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The two figures below illustrate how different types of sediments (e.g., limestone versus sandstone)

can be deposited at the same time. We use the term facies to describe these different environments, the

sediments that are deposited in those environments, and the rocks that are formed from those sediments.

For example, we can talk about a beach facies versus a deeper water facies. The top set of diagrams

shows how changes in the environment, in this case rising sea level, can deposit one facies on top of

another facies. The bottom photograph illustrates how such a sequence of stacked facies is expressed as

layers of different types of sedimentary rocks.

Figure 3-1. Sedimentary environments change from place to place and can migrate over the surface of

the Earth, depositing a sequence of different sedimentary layers. Such processes help explain why there

are different layers in Painted Canyon. When the seas move in across the land, it is a transgression, and

when the seas retreat off the land it is a regression. Figures 3-1 and 3-2 both show a transgression.

Figure 3-2. Sequence of layers in the Grand

Canyon formed by seas that came in across the

area (transgression), depositing layers of

sandstone, then mud rock, then limestone.

Compare this sequence with Figure 3-1. The

actual canyon was later eroded down into the

layers, exposing them.

Limestone depositedoffshore in clear water

Mud depositedaway from beach

Beach sanddeposited

along shoreline

Erosion on land

Time 1: Different sediments deposited in different environments at the same time.

Limestone depositedcontinuously offshore

Limestone depositedover mud

Mud depositedover beach sand

Beach sanddeposited overerosion surface

Time 2: Rise of sea level causes environments to move toward land. At any place, one type of sediment can be overlain by another type of sediment.


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A. The youngest layers are on the top and

oldest are on the bottom. The layers are

numbered from oldest to youngest. Feature 2 is

an old erosion surface (unconformity), which

developed after rock unit 1 was formed, but

before rock layer 3 was deposited on top.

B. Layers are deposited nearly horizontal, and if

they are no longer so, something has happened

(tilting, folding, etc.). Wiggly lines, like feature 2,

depict an unconformity in this and all other figures.

C. A feature, such as a fault (9), is younger than

rocks or other features it crosscuts. Such an age

constraint is called a crosscutting relationship.

D. An intrusion of magma (8) will crosscut older

units as a dike or inject parallel to them as a sill.

Figure 3-3. Geologic cross sections illustrating the principles of relative dating. Each cross section

illustrates a different principle of relative dating. In all figures, units are numbered from oldest (1) to

youngest. Figure continues on the next page.

1

5

2

4

3

67

1

5

2

43

6

78

1

52 4

3

67

54

3

6

9

8

7

5

2

1

4 8

6

7

3

Dike

Sill


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E. An angular unconformity (9) is an old erosion

surface, which represents erosion of underlying

tilted layers, then burial by overlying subhorizontal

layers. Feature 2 is an older unconformity.

F. River gravels (9) are younger than the erosion

that cut the canyon (8). This erosion and the

resulting canyon are younger than the rock layers

exposed in the canyon walls.

G. Folded layers can be eroded and then overlain by

younger layers. The old erosion surface (5) is an

angular unconformity. The folding occurred after

layers 1 through 4, but before development of the

unconformity (5) and the deposition of layers 6

through 9.

Figure 3-3 (continued). Geologic cross sections illustrating the principles of relative dating.

In all figures, units are numbered from oldest (1) to youngest.

5

2

14

67 89

10

3

1112

River gravels

5

21

4

3

67

9

Formationof canyonby erosion

8

5

2

1

4

3

6

7

8

9


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Helpful Hint: It is easier to determine the sequence of events by starting with the oldest

rock unit and working toward younger and younger events.

Figure 3-4. Use the principles of relative dating to reconstruct the sequence of events on this

geologic cross section. Put your answer in Worksheet 3A-1.


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Youngest layer is on top, oldest on the bottom

Canyonlands National Park, Utah.

Younger features (dike in this case) crosscut older

rocks Hance Rapids, Grand Canyon.

Younger rock units can contain pieces of older

rocks, such as these cobbles of gray granite and dark

basalt in a conglomerate Camp Verde, Arizona.

Rocks generally are deposited in nearly horizontal

layers. If the layers are no longer so, then they

must have been titled or folded Southern Utah.

An angular unconformity is where older rocks were tilted, eroded, and overlain by younger rock layers

Eastern Grand Canyon. Unconformity is in center of scene.

Figure 3-5. Photographs illustrating principles of relative dating.


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Exercise 3C: Determining the Relative Ages of Planetary Surfaces

Many of the same strategies we use to reconstruct geologic history on Earth, such as cross-cutting

relations, also apply to other planetary objects, such as the Moon, Mars, and solid moons of other planets.

There are other strategies that are specific to objects with surfaces that are more ancient than any

preserved on Earth. For example, we can determine the relative ages of different types of surfaces by

comparing the number of impact craters each type of surface preserves (older surfaces have a higher

density of craters). In this exercise, you will use relative-dating strategies to determine the relative ages of

different structures on Europa, an icy moon of Jupiter. Follow the steps below and answer questions on

the associated worksheets.

Figure 3-6. Close-up of part of the surface of Europa. All of the surface is some type of ice.

Observe the image above, which shows a close-up of part of the surface of Europa. Broadly survey

the image and figure out how many different types of features are expressed. Give each type of

feature a name that is descriptive and makes sense to you and list them in Worksheet 3C-1. On the

worksheet, include a short description of each type of feature.


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Use the principles of relative dating to determine the relative ages of the lettered features on the

image below. Explain your reasons for the relative ages between any two features. Enter your relative

order and observations/interpretations in Worksheet 3C-2.

Figure 3-7. Close-up of part of the surface of Europa. All of the surface is some type of ice.


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Box 3-1. Geologic Time, Relative Dating, and Unconformities

Sedimentary, igneous, and metamorphic rocks each form in a different way and over millions of

years. For these reasons, it is important to develop a perspective of geologic processes and geologic time.

Geologic processes can be seen in sedimentary rocks, which record events that occur on the Earth’s

surface, such as sea-level changes, climate changes, and periods of deposition and erosion. Igneous rocks

record volcanic eruptions or the solidification of magma (molten rock) at depth. Metamorphic rocks form

when heat, pressure, and deformation change one rock type into another, commonly where tectonic plates

are moving together (i.e., converging, the collision of India and Asia).

In reconstructing the geologic history of an area, we first determine the sequence in which events

occurred, using the principles of relative dating. We can also use absolute dating methods, such as

radioactive dating, to determine the actual time, before present, when each event occurred. Absolute dating

is quantitative and assigns a specific age to each unit. Relative dating is simply saying that one event

occurred before or after another event. It is analogous to stacking mail you have received on a desk each

day – the oldest mail is at the bottom of the stack, and the most recent mail is on the top. In this same

manner, rocks record relative ages, and geologists are able to establish a rock unit’s relative age using

several basic principles. In this lab, we are only concerned with relative dating.

Relative Dating

Geologic events are recorded in many different ways – the cutting of a canyon, deposition of

sediments, erosion, volcanism, and deformation of the Earth’s crust. Each of these processes leave a record

that can be recognized out in the field, and their relative age can be established by applying several

principles of relative dating. The following are some common-sense principles to help you determine the

relative age relationships between rock units (see Figure 3-3).

Sedimentary rocks and some volcanic rocks are deposited in layers that are horizontal or nearly so. If

the layers are no longer horizontal, then some process, like folding or tilting, has affected them.

Younger sedimentary and volcanic rocks are deposited on top of older rocks. This means that in a

series of rock layers, the oldest rock unit is usually on the bottom and the youngest rock layer is on

top. A possible exception is where a sheet of magma is squeezed between existing rock layers.

Sedimentary deposits (like river gravels) and volcanic rocks deposited in the bottom of a valley must

be younger than (1) rocks that form the canyon walls, and (2) erosion that carved the valley.

A younger rock can contain pieces of an older rock. This most commonly occurs where one rock unit

is deposited over existing rock and picks up pieces of the underlying rock unit. Magmas can also

incorporate pieces of older rocks from the sides and top of the magma chamber.

Igneous intrusions and structures, such as faults and fractures, are younger than the rocks they cut.

Older rocks can be baked (heated up and changed) if they are in contact with later magmas, such as at

the base of lava flows and near igneous intrusions.

Unconformities

At times in the Earth’s past, some areas were being eroded, rather than being covered with sediments

or volcanic layers. Such erosion formed irregular topography, such as mountain ranges and broad plains.

When such an erosion surface is buried and preserved by younger deposits, it is called an unconformity.

An unconformity represents a break or gap in the geologic record and it is difficult to tell what events,

except erosion, occurred during this time gap. Unconformities are the result of weathering and erosion,

and generally record drops in sea level or uplift of the rock units. They are usually recognized as an

irregular surface between two rock units, commonly with rounded or angular pieces of the underlying,

older rock incorporated into the overlying, younger rock. When an unconformity separates layered rocks

with different orientations, it is an angular unconformity.

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