Lab--Geologic Structures

Introduction
Advance Preparation and Materials Needed
Geology Learning Outcomes
Methods
Lab Procedures
Elements of a geologic map
Identifying geologic structures

Horizontal and tilted strata
Folds
Faults
Fault-bounded structures

Cross-sections (seeing beneath the map)
Table of Geologic Map Symbols (page will open in new window)

Introduction

Geologic structures, which include folds and faults, have been called the architecture of the earth and the anatomy of the lithosphere. Geologic structures result from tectonic forces and other sources of stress in the crust that deform the rocks. In some cases, rocks bend - undergo slow, solid-state flow - and are folded. In other cases, rocks break abruptly and shift along major fractures known as faults. (Abrupt breaks and shifts that occur along faults in the rocks are the source of earthquakes, a topic for another day.)

Understanding geologic structures will enable you to see how changes that occur inside the earth are expressed in the shape of the earth's surface. Conversely, understanding the surface expression of geologic structures will allow you to visualize them in three dimensions, including the parts hidden below ground level.

As the architecture of the earth, geologic structures form the mountains and basins of a landscape, determine geologic hazards such earthquake and landslide zones, and control the location of geologic resources in the crust. A geologist exploring for economically valuable resources in the earth - resources that are vital to society such as aquifers (useable reservoirs of groundwater), petroleum, or metal deposits - will begin by analyzing the geologic structures of the area.

To learn about geologic structures, you will work with block diagrams, cross-sections, and geologic maps. To develop your understanding of the architecture of the earth - its geologic structures - you will need to apply what you have learned previously, including the principles of relative geologic age and the rock cycle.

Advance Preparation and Materials Needed

Read this page completely. It is specifically designed to prepare you for completing the lab worksheet. The worksheet, which is separate, contains a different set of detailed questions that you will answer and turn in.
Read the Geologic Structures Basics page. It will help you become familiar with the specific structures that you will be studying with the maps and diagrams in this lab.
Read the maps Basics page, specifically the section on geologic maps. The maps Basics page includes an introduction to the Basics Table of geologic map symbols.
Reread the section on the Geologic Time Basics page that covers the principles of relative geologic age determination.
To perform the lab you will need:

a computer with access to the Internet
colored pencils and a ruler
the Geologic Structures Lab Worksheet

Geology Learning Outcomes

By performing and completing this lab, you will progress toward the following learning objectives for this course:

Think, behave, and communicate scientifically by writing sentences that describe geologic structures and sequences of geologic ages using correct terms in context, and by drawing correct symbols on geologic maps to depict geologic structures.
Gather, show, and explain evidence of the earth as a dynamic, ever-changing planet by naming and describing the development of geologic structures such as faults and folds and the stresses that cause them.Interpret spatial and quantitative information with maps and diagrams by interpreting geologic structures and relative age sequences from maps and cross-sections.
Put geologic events in order and determine the age of geologic materials by applying principles of relative geologic age to geologic maps and cross-sections.
Analyze, locate, and prepare for earth hazards by identifying different types of faults and the types of earthquakes that occur on them.

Methods

Read, draw, and interpret maps of the earth.
Examine, construct, and interpret diagrams, flow charts, and tables of information.
Apply principles of relative geologic age.

Lab Procedures

There are three parts of this lab: working with a simplified geologic map to better understand mappable rock units and mappable sediment units that cover up the rocks; interpreting geologic structures on cross-sections, block diagrams, and simplified geologic maps; and working with simplified cross-sections based on real geologic maps. In all parts of the lab, you will apply principles of relative geologic age to determine age sequences of rocks. In addition you will be drawing and reading standard geologic map symbols for faults, folds, and strikes and dips.

Refer to the Lab Assignments Grading Rubric for a reminder of what constitutes a well-performed lab.

Part 1. A Geologic Map Exercise

Below is a simplified map of the geology around the fictional town of Rioville. Compared to an actual geologic map, this geologic map is missing, among other things:

A map legend, also called a map key, identifies the four geological formations - the four numbered geologic units on the map - in terms of their rock or sediment type and the geologic age of each
A different color or pattern for each of the geologic units mapped
The colors or patterns for each geologic unit in the legend are depicted in a box and the boxes are stacked in age sequence with the oldest geologic unit at the bottom
Structure symbols (faults, folds, strikes and dips), which are explained in the map legend
A map scale - how distances on the map compare actual geographic distances, such as how many miles are represented by one inch on the map
A north arrow - however, lacking a north arrow, the tradition is to assume that north is to the top

Click on the map to open a larger version in a new window

Given the following instructions and information about Rioville, how could you complete this geologic map?

The rock or sediment types and ages of the rock units are as follows:

basalt of the Miocene epoch
dolostone of Jurassic period
shale of the Cretaceous period
unconsolidated alluvium (loose sediment deposited by streams and rivers) of the Holocene epoch

What would the map legend look like given this information? Which unit would be listed at the bottom of the sequence? Which unit would be on the top?

When coloring a geologic map, you can assign your own colors for most units, but Holocene alluvium, unit 4 on the Rioville map, would be colored yellow, following geologic map tradition.

The symbol for what type of geologic structure is used on the Rioville map? Should that symbol and an explanation of what it stands for be included in the map legend?

By creating the map legend and coloring the map to match the legend, you will have completed the Rioville geologic map as much as is possible with the information given. What other information would be included on an actual geologic map?

Part 2. Identifying Geologic Structures

Stratified rocks tell the story of the earth's architecture more clearly than plutonic or metamorphic rocks do. Because stratified rocks begin as horizontal layers with lateral continuity we know that when we find stratified rocks that are not flat or continuous they have been subjected to stresses that caused tilting, folding, or faulting. We can measure the horizontal run and tilt (strike and dip) of sedimentary strata, record that information on a map, and draw the contacts (lines) between the mappable units. From what we record on the maps we can recognize geologic structures. The map data allow us to project what the structures look like beneath earth's surface, which we depict on cross-section diagrams that are based on the map. See the Basics Table of geologic map symbols for more information on strike and dip.

The diagrams in Part 2 are block diagrams. The top of a block diagram is an oblique view of the earth's surface, in other words it is a geologic map that you are viewing at an angle from above. The vertical sides of a block diagram are cross-sections, cut-away views that show how the rocks and structures extend into the earth. Cross-sections typically don't contain geologic map symbols. However, to make sure that we understand relative motion on faults, we will use half-arrows on each side of any fault in the cross-sections. All other map symbols appear only on the map (top) of the block diagrams.

Consider the questions posed below each diagram. They will help prepare you for completing the worksheet.

Horizontal and tilted strata

Block diagram 1
block diagram of flat-lying strata

What strike and dip symbol belongs on the map in block diagram 1?
Which stratum (bed) is the oldest in this diagram? Which is the youngest?
Have the strata in this diagram been deformed since they originated?

Block diagram 2
block diagram showing tilted strata

How would you describe the strata in block diagram 2? Are they tilted, folded, or faulted?
What do the strike and dip symbols on the map indicate?
Based on the front cross-section, what is the angle of dip?

Block diagram 3
block diagram showing vertically tilted strata with strike and dip symbols on two of the three beds

How would you describe the strata in block diagram 3? Are they tilted, folded, or faulted?
What do the strike and dip symbols on the map indicate?
Can you apply the principle of superposition to determine the age sequence of the beds in the diagram?
What could you look for in sedimentary rocks to determine which way was originally up?

Folds

Refer to the Geologic Structure Basics page section on folds.

Block diagram 4
block diagram showing an fold with the fold axis marked in red and the beds in each limb (each side of the fold) dipping away from
the center; strike and dip symbols mark two beds in one limb of the fold

Which bed in block diagram 4 is the oldest? Which is the youngest?
What do the strike and dip symbols on the map indicate?
What do the cross-sections tell you about the fold?
What is the name of the type of fold depicted in the diagram?
What does the red line on the map indicate? What needs to be added to make it a complete map symbol?

Block diagram 5
block diagram showing a plunging fold with the fold axis marked in red and beds in each limb dipping away from the center; strike and dip symbols
mark a bed on each side of the fold axis

Which bed in block diagram 5 is the oldest? Which is the youngest?
What do the strike and dip symbols on the map indicate?
What do the cross-sections tell you about the fold?
What is the name of the type of fold depicted in the diagram?
What does the red line on the map indicate? What needs to be added to make it a complete map symbol?

Block diagram 6
block diagram showing a fold with the fold axis marked in red and beds in each limb dipping toward the center; strike and dip symbols mark two
beds of one limb of the fold

Which bed in block diagram 6 is the oldest? Which is the youngest?
What do the strike and dip symbols on the map indicate?
What do the cross-sections tell you about the fold?
What is the name of the type of fold depicted in the diagram?
What does the red line on the map indicate? What needs to be added to make it a complete map symbol?

Block diagram 7
block diagram showing a plunging fold with the fold axis marked in red and beds in each limb dipping toward the center; strike and dip symbols
mark two different beds on one side of the fold axis

Which bed in block diagram 7 is the oldest? Which is the youngest?
What do the strike and dip symbols on the map indicate?
What do the cross-sections tell you about the fold?
What is the name of the type of fold depicted in the diagram?
What does the red line on the map indicate? What needs to be added to make it a complete map symbol?

Block diagram 8
block diagram showing a circular geologic structure with beds dipping away from the center in all directions; two strike and dip symbols
are in the bed in contact with the center bed

Which bed in block diagram 8 is the oldest? Which is the youngest?
What do the strike and dip symbols on the map indicate?
What do the cross-sections tell you about the structure?
What is the name of the type of fold depicted in the diagram?
What map symbols belong in the circle that marks the bed in the center of the map?

Block diagram 9
block diagram showing a circular geologic structure with beds dipping toward the center in all directions; there are two strike and dip symbols, one
in the outer-most bed and one in the center bed

Which bed in block diagram 9 is the oldest? Which is the youngest?
What do the strike and dip symbols on the map indicate?
What do the cross-sections tell you about the structure?
What is the name of the type of fold depicted in the diagram?
What map symbols belong in the circle that marks the bed in the center of the map?

Faults

Refer to the Geologic Structure Basics page section on faults.

Block diagram 10
block diagram showing a four horizontal beds in cross section, offset by a high-angle fault along which the hanging wall has moved up relative to the footwall

Which bed in block diagram 10 is the oldest? Which is the youngest?
What is the dip of the fault? (How many degrees down from the horizontal is the fault dipping? The angle can be seen on the cross-section that forms the front of the block diagram.)
What type of fault offsets the strata in the diagram?
Which directions would half-arrows point on each side of the fault in the cross-section?
Which fault symbol belongs on the map?

Block diagram 11
block diagram showing a three horizontal beds in cross section, offset by a low-angle fault along which the hanging wall has moved up relative to the footwall

Which bed in block diagram 11 is the oldest? Which is the youngest?
What is the dip of the fault? (How many degrees down from the horizontal is the fault dipping? The angle can be seen on the cross-section that forms the front of the block diagram.)
What type of fault offsets the strata in the diagram?
Which directions would half-arrows point on each side of the fault in the cross-section?
Which fault symbol belongs on the map?

Fault-bounded structures

Refer to the Geologic Structure Basics page section on other types of structures.

Block diagram 12
block diagram showing a geologic structure in which a body of rock has dropped down along two normal faults that dip towards each other

There are two faults shown in the block diagram 12. What type of faults are they?
Which directions would half-arrows point on each side of the fault in the cross-section?
There is one complete fault-bounded geologic structure depicted in the diagram. What is the name of the structure?

Block diagram 13
block diagram showing a fault-bounded geologic structure that is cored by a body of plutonic and metamorphic rock that forms the footwalls of two
detachment faults that dip away from each other; the hanging walls of the detachment faults contain stratified rocks of the shallow crust

There are two faults shown in the block diagram 13. What type of faults are they?
Which directions would half-arrows point on each side of the fault in the cross-section?
What is the name of the fault-bounded geologic structure in the diagram?

Part 3. Cross-sections (seeing beneath the map)

To take a real-world look at geologic structures and how they appear on geologic maps, we will use the geologic map of Washington State published by the Washington State Department of Natural Resources. It is available for downloading at http://www.dnr.wa.gov/Publications/ger_geol_map_washington_pagesize.pdf or else at http://commons.wvc.edu/rdawes/G101OCL/Labs/ger_geol_map_washington_pagesize.pdf.

Notice the geologic map legend, or map key, on the right side of the geologic map of Washington state.

The geologic map legend has abbreviations, such as lTs for lower Tertiary sedimentary rock, along with colors, to key you in to each rock type (or group of sediments, not yet turned into rock) that is depicted on the geologic map.

Let's consider three areas on the map that show the types of structures we examined in Part 2:

thrust faults and thrust sheets of the Olympic Mountains
the Chiwaukum Graben, which contains the city of Wenatchee
the Republic Graben, Okanogan Metamorphic Core Complex, and Kettle Metamorphic Core Complex

Each of the three following map snippets centers on one of the structures in the list above. A blue cross-section line, with diamonds on the ends, is drawn on each of the maps. Below each map is a blank cross-section that corresponds to the blue line on the map. Each of the cross-sections has been enlarged from the map scale and simplified to make it easier to work with. Consider the questions below each cross-section blank. They will help prepare you for completing the worksheet.

The Olympic Peninsula - remember that thrust faults are marked on geologic maps by thick black lines with triangular "teeth" on the hanging wall side of the fault.

Click on the map to open a larger version in a new window

unlabeled cross-section with five faults that dip steeply eastward

What directions should the half arrows point on each of the thrust faults in the cross-section?
What three-letter abbreviations that label the rock units on the map belong in each of the footwalls and hanging walls on the cross-section?
What do the three-letter abbreviations stand for? This information is available on the map legend. See the link above to open the complete map.

The Chiwaukum Graben - The Chiwaukum graben has faults on three sides and is mainly filled with a rock unit colored light green and labeled lTs, representing lower Tertiary sedimentary rock.

portion of Washington State Geologic Map showing the Chiwaukum graben; the line of cross-section a fault bounded structure containing
Lower Tertiary sedimentary rocks; outside the faults are deep crustal metamorphic and plutonic rocks

Click on the map to open a larger version in a new window

unlabeled cross-section showing a pair of normal faults that dip toward each other

The cross-section shows two faults. What type are they?
What type of geologic structure is bounded by the two faults?
What directions should the half arrows point on each of the two faults in the cross-section?
What three-letter abbreviations that label the rock units on the map belong in each of the footwalls and hanging walls on the cross-section? Note: It may difficult to recognize which label applies to which rock unit on the geologic map. The labels for several of the smaller mappable units are placed outside those units with lines from the labels pointing to the rock units. The map legend comes in handy to match the color of a rock unit with its label. The map legend is on the complete geologic map of Washington state that you can open by clicking on the link at the beginning of this section.

Okanogan Metamorphic Core Complex, Republic Graben, and Kettle Metamorphic Core Complex - These adjacent structures in northeastern Washington define one large metamorphic core complex divided by a graben. The western half of the metamorphic core complex is the Okanogan side, the eastern half is the Kettle side. Within each half of the core complex, between the detachment fault that defines the outer edge of the core complex and the normal fault where the graben cuts out the middle, the rock consists of deep crustal plutonic and metamorphic rock. Within the Republic Graben the rock consists of volcanic and sedimentary strata of the shallow crust.

portion of the Washington State Geologic Map showing the Okanogan Metamorphic Core Complex, Republic Graben, and Kettle Metamorphic Core
Complex; the line of cross-section crosses the structures and extends beyond the bounding detachment faults to stata of Paleozoic sedimentary
rocks on each side

Click on the map to open a larger version in a new window

unlabeled cross-section showing the Okanogan metamorphic core complex on the west (left) and the Kettle metamorphic core complex
on the east (right) with the Republic graben between them. The cross-section line extends beyond the core complexes to the west
and east into the hanging walls of the detachment faults.

Two of the faults in the cross-section dip at low angles. What type of fault are they?
The footwall of each of the low-angle faults is a geologic structure. What is the name of this type of fault-bounded geologic structure?
Given the type of geologic structure you have just identified, which type of rock would you expect in the hanging walls of the two low-angle faults? (a) deep crustal metamorphic and plutonic rock OR (b) stratified rocks from the shallow crust
According to the two- or three-letter abbreviations that label the rock units on the map, what rock units are in the hanging walls of the low-angle faults? Note: Remember as mentioned earlier, it may difficult to recognize which label applies to which rock unit on the geologic map. Be sure to match the color of the rock unit with its label on the map legend. In addition, ignore the yellow unit labeled Qg, which consists of loose sediments that cover the bedrock.
Are the rocks in your answer to the previous question consistent with what would be expected in the hanging wall of a detachment fault overlying a metamorphic core complex?
According to the two- or three-letter abbreviations that label the rock units on the map, what are the rock units in the footwalls of the low-angle faults?
Are the rocks in the footwalls of the detachment faults consistent with the type of rock - metamorphic or plutonic - that would be expected in a metamorphic core complex?

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Geology 101 - Introduction to Physical Geology
Lab--Geologic Structures
Created by Ralph L. Dawes, Ph.D. and Cheryl D. Dawes, including figures unless otherwise noted
updated: 9/11/13

Unless otherwise specified, this work by Washington State Colleges is licensed under a Creative Commons Attribution 3.0 United States License.