Student Name: Grade: _____
Physical Geology 101 Laboratory
Structural Geology and Analysis
Introduction & Purpose: Structural geology is the study of how geologic rock units are initially arranged and later deformed. Changing spatial relations between geologic units and the stress and strain that occur during deformation events are key aspects in understanding geologic structures. The purpose of this lab is to both learn and apply the concepts of structural geology to reading and interpreting geologic structures, including tilted beds, folds, and faults. The terms and concepts of geologic structures, the application of structural geology to mountain building events, and the techniques used to interpret geologic structures will be presented and discussed. The three types of graphic representations of geologic structures: 1) geologic maps, 2) geologic cross sections, and 3) block diagrams will also be highlighted and discussed.
The objective of this laboratory is to become successful at applying the principles of structural geology for both, interpreting surface and subsurface structural and geologic relations, stress and strain regimes, and solving structural problems, concerning geographic regions that expose a rock record of igneous, metamorphic, and sedimentary events, folding and faulting, and surface erosion.
Part I. Pre-Lab - Knowing and Understanding Structural Terms and Symbols:
A. Measuring the Attitude of Rock Units
Attitude is the spatial orientation of planar rock structures. Two aspects of attitude are needed to constrain a rock unit or surface orientation in three-dimensional space: 1) Strike and 2) Dip. Strike is the compass bearing of a line formed by the intersection of a horizontal plane and the (inclined) plane of the layered rock feature. Strike can be expressed as either a quadrant, or an azimuth bearing.
Dip is the angle between the horizontal plane and the planar rock unit or feature. Dip direction is always down the inclined plane and is perpendicular to the strike. Strike and dip are drawn on geologic maps as a “T-like” symbol – the long segment is the strike; the short segment the dip. A number nest to the short segment represents the dip angle. In the field, geologists measure attitude with a compass (strike) and an inclinometer (dip).
B. Geologic Map Symbols
Geologic symbols are used on geology maps to indicate one or more characteristics of the rock formation at the point on the map that they (the symbols) are placed. Some commonly used map symbols are found in Figure 10.3 on page 197 (you will refer to these symbols for interpreting and making geologic maps, cross sections, and block diagrams). Map symbols indicate 1) attitude (e.g. strike and dip of either, bedding or foliation), 2) formation contacts, 3) fault lines (rock type, location, and planar orientation), 4) fold axes (type, location, and their limb orientations), and 5) rock formation information (type, name, and age). You will need to be able to recognize and interpret these symbols while working on geologic maps and diagrams.
C. Major Types of Geologic Structures
Mappable rock units are called formations. Locations where rock formations are exposed at the earth’s surface are called outcrops. Undisturbed rock formations such as sedimentary beds and lava flows are typically horizontal and planar in spatial orientation. However, shifting tectonic plates produce a variety of stresses in the crust that will, over time, cause crustal deformation such as uplift, tilting, erosion, faulting, and folding of formations. Faults and folds exposed at the earth’s surface in outcrops have unique structural characteristics that can be recorded, mapped, identified, categorized, and analyzed. Carefully study the major structural features listed and described on pages 198 through 202. You will get to analyze these structures in Part II. Three-dimensional visualization of folds and faults are found on the web link at http://geology.asu.edu/~reynolds/blocks/menulist.htm. At this web site, you to be able to interactively manipulate the fold and fault blocks.
Know these basic terms used in structural geology below (from lab lecture and lab text):
1) Outcrop - _________________________________________________________________
2) Formation - _________________________________________________________________
3) Strike - ______________________________________________________________________
4) Dip -________________________________________________________________________
5) Contact - ___________________________________________________________________
6) Unconformity - _______________________________________________________________
7) Footwall - ___________________________________________________________________
8) Hanging wall - ________________________________________________________________
9) Normal Fault - _______________________________________________________________
10) Reverse Fault - ______________________________________________________________
11) Thrust Fault - _______________________________________________________________
12) Strike-Slip Fault - _____________________________________________________________
13) Slickensides - ________________________________________________________________
14) Anticlines - ___________________________________________________________________
15) Synclines - ___________________________________________________________________
16) (Fold) Plunge - ________________________________________________________________
17) Monoclines - _________________________________________________________________________
18) Dome - _____________________________________________________________________
19) Basin - _______________________________________________________________
E. Rules for Interpreting Geologic Structures
There is a set of 12 simple rules for observing and interpreting geologic structures found in Figure 10.11 in your lab manual. Carefully study and make use of these rules for completing Part II. The most important rules are listed below.
1) Layers of rock dip downward in the same direction as the which the youngest rock layers are
exposed at the surface.
2) The older rocks are exposed in the center of eroded anticlines and domes.
3) The younger rocks are exposed in the center of eroded synclines and basins.
4) Plunging anticlines form "U" shaped outcrop belts that point in the same direction that the fold plunges.
5) Plunging synclines form "U" shaped outcrop belts that point in the opposite direction that the fold plunges.
6) The steeper the dip of the layer, the more narrow the width of its outcrop belt.
7) In compressional faults, the hanging wall tends to move up relative to the foot wall (pushed together).
8) In tensional faults, the hanging wall tends to move down relative to the footwall (pulled apart).
Part II - In-Lab: Drawing and Interpreting Geologic Structures in Block Diagrams
Introduction: Three-dimensional geologic block diagrams are scaled-down, abstract, simple representations, or models of Earth's crustal rock structures, which include 1) formations, 2) unconformities, 3) faults, 4) folds, and 5) topography. Block diagrams are a 3-dimensional composite of both, a geologic map (horizontal map-view) and geologic cross-sections (vertical side-views). The key to successfully completing the block diagrams lies in visualizing the 2-D representations as 3-D structure.
A. STRIKE AND DIP BED AND BLOCK EXERCISES
1. Measure the strike and dip of the two planar objects the instructor has set up in the class:
a) Planar Object #X - Strike = _______ Dip = _____
b) Planar Object #Z - Strike = _______ Dip = _____
2. a) Number the layers on all sides of the block (below). b) Place a strike and dip symbol on top of layer 2. c) Complete the geological map showing the outcrop of layer 2 and its strike and dip symbol.
Estimate and record the strike and dip here: _________________
3. a) Complete the block (below). b) Number all layers on all surfaces.
4. a) Complete the block (below) using the strike and dip shown on block’s top. b) Number all the
beds. c) Estimate the dip amount and write it next to the strike and dip symbol. d) State the
strike and dip in words here: ____________________
5. a) Complete the block below using a 60o dip amount. Note that the dip (tilt) direction must be determined from the age of the layers. b) Place a strike and dip symbol on the top of the block. Estimate the strike and dip of the beds. ____________________
B. FAULT BLOCK EXERCISES
1. a) In map view (top of block), put strike and dip symbols on the beds and a fault dip symbol on fault. b) Also put appropriate displacement arrows on the fault in the frontal cross-section.
What type of fault is shown below? ________________________.
2. a) Put appropriate fault displacement arrows, fault dip symbol, and strike and dip of beds symbols in both the map view and cross-section. What type of fault is shown below if the slickensides are parallel to the dip? _____________________
3. a) Put appropriate fault displacement arrows, fault dip symbol, and strike and dip of beds symbols in both the map view and cross-section. What kind of fault is shown below if slickensides are parallel to the dip? ________________
4. a) Draw the appropriate symbols for the fault in both the map view and cross-section. b) Draw
strike and dip symbols for the beds. What type of fault is shown below if the slickensides are
oriented horizontally? ______________
Estimate the strike and dip of the fault: ________________ Estimate dip of beds. _____
5. What type of fault is shown below if the slickensides are oriented horizontally? __________
Estimate the strike and dip of the fault: _____________ Estimate the dip of beds. _____
C. FOLD BLOCK EXERCISES
- a) Complete the block (below). b) Number the beds from oldest to youngest (1 being oldest). c) Indicate the axial plane and fold axis with appropriate symbols in both the map view and cross section. What is the name of this structure? ________________
- a) Complete the block diagram (below). b) Place strike and dip symbols on the map view (top of block) to indicate the structure. c) Label the axial planes and fold axes with the appropriate symbols. What is the name of this structure? ____________________________
- a) Complete the block (below). Dip amount is arbitrary, but dip direction is not. b) Number the beds from oldest to youngest (1 being oldest). c) Indicate the axial plane and fold axis with appropriate symbols in the map view and cross-section of the block. d) Place strike and dip symbols on top. What is the name of this structure? ____________________________
4. a) Complete the block (below). b) Number the beds from oldest to youngest (1 being oldest).
c) Indicate axial plane and fold axis with appropriate symbols in the map view and cross-section.
What is the dip angle? _____. What is the name of this structure? ___________
5. a) Complete the block (below). b) Number the beds from oldest to youngest (1 being oldest).
c) Indicate the axial plane and fold axis with appropriate symbols in the map view and cross-
section face of the block. d) Draw strike and symbols on the map view.
What is the fold plunge angle? _____. What is the name of this structure? ___________
6. a) Complete the block (below). b) Number the beds from oldest to youngest (1 being oldest).
c) Indicate the axial plane and fold axis with appropriate symbols in the map view and cross-
section face of the block. d) Draw strike and symbols on the map view. Note bed width for #2.
What is the bed dip angle? _____. What is the name of this structure? ______________
7. a) Complete the block (below). b) Number the beds from oldest to youngest (1 being oldest).
c) Indicate the axial plane and fold axis with appropriate symbols in the map view and cross-
section face of the block. d) Draw strike and symbols on the map view.
What is the bed dip angle? _____. What is the name of this structure? _____________
Part III – Structural Geology Laboratory Reflection
Directions: Write a reflection (minimum 100 words in length) about your experience in doing the structure exercises today. Include the following: 1) What was the purpose of the lab?; 2) What did you learned from this laboratory?; 3) What did you find interesting? 4) What were the problems and challenges you encountered; and 5) Your opinion on how this lab was designed – good and bad?
Part IV Extra Credit: Completing & Interpreting Geologic Structures in 3D Block Diagrams
Introduction: Three-dimensional geologic block diagrams are scaled-down, abstract, simple representations, or models of Earth's crustal rock structures, which include 1) formations, 2) unconformities, 3) faults, 4) folds, and 5) topography. Block diagrams are a 3-dimensional composite of both, a geologic map (horizontal map-view) and geologic cross-sections (vertical side-views).
Directions: In this exercise, you will construct and analyze geologic block models using the six cardboard models provided in the very back of your lab manual. These models are very easy to construct, requiring simple cutting and folding. Be sure to cut and fold your cardboard models before
proceeding to the block model activities below. Instructions for constructing your block diagrams are found on in Figure 10.14 on page 207. Answer the Model Activity Questions in your lab manual on a separate piece of paper.