STRUCTURAL GEOLOGY EXERCISES
with Glaciotectonic Examples

James S. Aber

1. INTRODUCTION

What is structural geology?

Structural geology is the architecture of the Earth's crust. More specifically, structural geology deals with those geometric elements of the crust that were produced as a result of deformation. Deformation can range in scale from microscopic fractures to enormous, complexly folded and faulted mountain ranges 1000s m high and 1000s km long.

Tectonic movement of lithospheric plates driven by mantle convection gives rise to many structures within the crust. These structures fall into orogenic (mountain building) and epeirogenic (crustal warping) categories depending on the magnitude of deformation. Tectonic structures comprise the majority of features usually dealt with in structural geology.

Many other crustal structures are created by forces which have nothing to do with tectonic movements. Deformations of the crust by meteorite impact, salt-dome uplift, ice pushing, soft sediment collapse, and other mechanisms are an important part of structural geology. These other structures are often considered only briefly in most structural geology lab manuals, but are of paramount significance in many situations.

This lab manual has a special emphasis on glaciotectonic structures produced by ice shoving, in addition to the more usual geologic structures (Aber 1988). This emphasis should be of particular interest to those living in formerly glaciated areas. Because of their moderate size, glaciotectonic structures are appropriate for beginning students to develop techniques that can later be applied to larger crustal features.

Nature of glaciotectonism

The glacial theory was born more than 150 years ago based on two categories of field evidence: (1) features formed by glacial erosion and (2) features created by glacial deposition. The possibility that glaciers and ice sheets could deform shallow crustal rocks and sediments was not widely recognized until this century. It now appears that glaciotectonic features are omnipresent in glaciated regions underlain by sedimentary bedrock or thick drift (Sauer 1978) and are even common in thin drift resting on crystalline bedrock.

A variety of distinctive structures and landforms are now attributed either wholly or partly to glaciotectonic genesis. Hence, glaciotectonic features must be included with erosional and depositional features as primary field evidence for former glaciation. The modern glacial theory is, therefore, supported by a triad of field evidence, including erosional, deformational, and depositional features--see Fig. 1-1.

Glaciotectonic features may be defined as those structures and landforms produced by deformation and dislocation of soft bedrock and drift masses as a direct result of glacier-ice movement (Aber et al. 1989). The identification of glaciotectonic structures and landforms is based on two fundamental criteria.

  1. Presence of recognizable masses of pre-existing (preglacial) bedrock or sediment.

  2. Presence of glacially induced deformations within those masses. Such deformations must be the result of glacially imposed stresses and are not merely pre-existing structures inherited from the parent material.

Structural and terrain analysis

Glaciotectonic structures produced in soft bedrock and drift by ice-pushing are really no different from folds, faults, and dislocations found in mountain zones. In fact, glaciotectonic structures can be regarded as natural scale-models of true mountains (Berthelsen 1979). Thus, the techniques used by structural geologists can be applied directly to analysis of glaciotectonic features. These techniques fall into two general categories.

  1. Structural analysis -- emphasizing three-dimensional and subsurface geometry.

  2. Terrain analysis - emphasizing geomorphic expression.

Structural analysis of deformed bedrock and drift has three objectives. First, the geometry of the structure must be determined--its size, shape, depth or thickness, and orientation. This is best done in the field using a variety of techniques ranging from simple surveying methods for surface exposures to geophysical measurements in the subsurface.

The resulting geometric data are then displayed graphically in the form of maps, cross sections, block diagrams, or stereographic projections for further analysis. Several lab exercises deal with graphical techniques commonly used in structural geology. From this geometry, the second and third objectives of analysis--determining the age and genesis of the structure--follow.

Terrain analysis for structural purposes is based on the assumption that landscape topography, drainage, vegetation, and soils often faithfully reflect the nature of subsurface structures. In fact, folds, faults, and other structures are usually mapped on the basis of topographic expression, even where the bedrock is itself not visible at the surface.

Organization of exercises

Most exercises in this lab manual involve actual geological examples from North America and Europe. This case-history approach seems preferable to made-up exercises that may bear little relationship to real geology. Because of the use of actual situations, some exercises do not have a single right solution. Multiple correct answers are possible in some cases, and the student will have to use some judgement in selecting the most reasonable possibility given the data at hand. Far from being a drawback, this is a more realistic representation of the daily work of structural geologists.

Several of the exercises are amenable to computer analysis with various commercial software programs. If the necessary software and hardware are available, students are encouraged to work through exercises using both traditional graphical techniques and computer analysis. Metric and English units are both used in this lab manual, and students should become familiar with conversion between the two.

The following materials or equipment will be necessary to complete all the lab exercises for this course. Each student should provide the following.

References

2. PLANAR STRATA

Geometric elements in structural geology

Geologists must deal with structures developed on all scales from microscopic to continental within all manner of materials from loose sediments to high-grade metamorphic complexes. Some of these structures are geometrically simple, but many are irregular and complex in form due to multiple phases of deformation. All structures, regardless of their complexity, may be reduced to combinations of two basic geometric elements--planes and lines, the orientations of which can be determined.

Planar features within rocks include: bedding planes and planar cross beds; crystal faces; joints, faults, dikes, fissures and other fractures; veins, foliation, schistosity, cleavage and partings; fold axial planes; seismic discontinuities; water table; formation and other stratigraphic boundaries; and unconformities.

Linear features within rocks include: striations, grooves, troughs and channels; yardangs; axes of pebbles, shells, augens, and of other elongated objects; crests of ripples, dunes, drumlins, and of other elongated sedimentary forms; crystallographic and optic axes of minerals; lineations; intersections of fractures or other planar features; fold axes; paleomagnetic axes; rotation axes of plates; strike and dip lines; and lineaments.

Geometric elements of structural geology.

Orientations of planes and lines

The orientation of a plane relative to the earth's surface is determined by two measurements: strike and dip. Strike is defined as the compass direction of a horizontal line in the plane. Compass directions are customarily measured in degrees from 0 to 360°, or can be recorded as N50E, N45W, S15E, etc. Dip is the direction and angle of maximum downward tilting, which is always perpendicular (90°) to strike.

The house roof shown in Figure 2-1 illustrates strike and dip in a tilted plane. The horizontal crest of the roof represents the strike with a compass direction of 50° (or N50E). The dip direction of the roof is SE, perpendicular to the strike: 50 + 90 = 140° (or S40E), and the dip angle measured from horizontal is 45°.

Measurements are always made relative to true north rather than magnetic north. The most common compasses carried by field geologists are the Brunton Pocket Transit or the Silva Ranger. Both include built-in mechanisms for making magnetic declination adjustments and levels for measuring dip angles. With one of these relatively simple instruments, the field geologist can collect a great deal of information concerning the planar and linear elements for all kinds of structures.

The "T" symbol shown on Figure 2-1 is used to indicate strike-and-dip measurements on maps and diagrams. The long cross-line represents strike, and the shorter stem represents the dip direction. The dip angle is sometimes given next to the dip line. Variations of the basic strike-and-dip symbol are used for different planar features--see Fig. 2-2.

The orientation of a line relative to the earth's surface is likewise determined by two measurements: trend and plunge. Trend is defined as the compass direction of the line, and plunge is the direction and angle of downward tilting of the line. In Figure 2-1, the strike line has a trend of 50° and a plunge of 0°. The dip line has a trend of 140° and a plunge of 45°SE. The basic map symbol for a linear measurement is simply a short line with or without an arrow at one end to indicate the direction of plunge (fig. 2-2). A word of caution: trend and plunge refer to linear features, whereas strike and dip refer to planar features--do not confuse them.

Finding the orientation of a plane

Geometrically, any three points, which are not in the same line, define a plane. Given a recognizible planar feature, such as a marker bed or formation boundary, the strike and dip of that planar feature can be determined if its elevation is known at three points. The elevation control points can be in either surface or subsurface locations.

One common situation involves three nearby wells drilled into the same tilted horizon. The orientation of that horizon can be easily calculated and projected into the surrounding area. The same procedure can also be applied to surface outcrops or to a combination of surface and subsurface control points. Note that deep subsurface elevations are often below sealevel, and so are negative.

Figure 2-3 shows a map view of three points for which the elevations of a distinctive bed are known. The three points are labelled as follows: A = low point, B = intermediate point, and C = high point. In this special case, B and C are equal in elevation. To find the strike and dip, first connect the three points with straight lines forming a triangle.

This planar triangle represents the dipping bed. Recall that strike is a horizontal line in the plane, that is a line of constant elevation; thus line CB is the strike. Its compass direction can be measured off the map. Next, draw a line which is perpendicular to the strike line CB and which passes through point A, the low point. This is the dip line (AE) and its compass direction equals strike direction plus 90°. The dip angle is given by a simple expression:

     elevation E - elevation A
     -------------------------  =  tan (dip angle).
         map distance AE
In this case, 300/1020 = tan 16°. So, strike = 64° and dip = 16°SE. This example was easy to work with because two points are at the same elevation and automatically define the strike line.

In the more general case, as illustrated in Figure 2-4, all three points have different elevations. Again, the points are connected to form a triangle and labelled with A lowest, B intermediate, and C highest. Point B will serve as one end of the strike line; the other end will be point D located somewhere along line AC. The exact location of point D can be found by a ratio of distances to elevations:

     elevation B - elevation A     distance AD
     -------------------------  =  -----------
     elevation C - elevation A     distance AC
Point D found in this manner is equal in elevation to point B; thus line BD is the strike. Next draw the line AE, which is perpendicular to line BD, and continue with the same steps as before to find the dip. In Figure 2-4, distance AD = 1000 m, strike = 15°, and dip = 25°NW.

Apparent dip versus true dip

The true or maximum dip angle can only be measured perpendicular to the strike line, as shown by the house roof (Fig. 2-1). A dip angle measured in any other direction will be less than true dip and is called apparent dip.

Consider, for example, a vertical, E-W cross section through the house of Figure 2-1. The section will cut diagonally across the roof, which will appear to dip in the section at an angle less than 45°. Another cross section running parallel to strike (N50E) would show no dip; the roof would appear horizontal. The relationship between true dip and apparent dip is defined by the following functions:

tan AD = tan TD x cos A, or
tan AD = tan TD x sin B,

where:

TD = true dip angle.
AD = apparent dip angle.
A = angle between true dip and apparent dip trends.
B = angle between apparent dip trend and strike.

For an E-W cross section cutting through the house, A = 50°, B = 40°, and AD = 33°E. The concept of apparent dip is often not understood by students beginning in structural geology. Take time to comprehend apparent dip now and avoid difficulty later. Work both equations to practice calculating apparent dip.

Problem

The large-scale map shows three control points on a distinctive mineralized vein, which outcrops in a roadcut at the eastern point and has been encountered below hills in drill holes to the west--see Fig. 2-5. The vein is assumed to be planar and is of considerable economic interest, as it is known to locally contain gold.

  1. Determine the strike and dip of the mineralized vein.

  2. What apparent dip does the vein display where it is exposed in the vertical roadcut, which strikes N15E, at the eastern point (145)?

  3. The ficticious "Amerigold Company" owns a mining lease at the point labelled AMG. If this site has a land elevation of 158 m, how deep would the miners have to dig in order to reach the mineralized vein at that point?

3. PRIMARY STRUCTURE

Introduction

The Lower Cretaceous, Dakota and Kiowa Formations of north-central Kansas are famous for their cross bedding. These formations were deposited in various coastal, deltaic, and shallow marine environments--see Fig. 3-1. Cross-bedded sandstones fill large channels within finer siltstone and shale. The cross bedding consists of small- and medium-scale, planar or lenticular sets. The planar style with high angle of dip (> 20°) is most common (Franks et al. 1959). Such cross bedding is particularly well displayed where it crops out in large concretions of the Kiowa Formation, as at Rock City, near Minneapolis and at Mushroom Rock, near Kanapolis, Kansas.

Rock City, near Minneapolis, Kansas. Numerous large sandstone concretions are exposed on the side of the Solomon River valley. Photo date 10/78, © J.S. Aber.
Closeup view of large sandstone concretion. Notice cross-bedded layers within the concretion. Most cross beds dip to the left in this example. Rock City, near Minneapolis, Kansas. Photo date 10/78, © J.S. Aber.
Mushroom Rock, near Kanapolis, Kansas. Sandstone concretion stands on a pillar of softer sandstone, and other concretions lie on the ground. Photo date 4/91 © J.S. Aber.

Detailed investigation of sandstone cross bedding was undertaken in Ottawa County by Franks et al. (1959), who constructed the accompanying map. The map shows average cross bed directions (vectors) for 79 sites, the number of measurements at each site, and the standard deviation of measurements at each site. A table of data for each site is also included (Table 3-1). For this lab, you will make a rose (compass) diagram of the data and interpret the depositional environment from the rose diagram and other information.

Table 3-1. Cross-bedding vector data in azimuth degrees. Adapted from Franks et al. (1959, Table 1).
1. 82 2. 54 3. 53 4. 54
5. 70 6. 95 7. 142 8. 113
9. 174 10. 92 11. 169 12. 175
13. 164 14. 117 15. 133 16. 131
17. 137 18. 167 19. 136 20. 149
21. 175 22. 174 23. 121 24. 233
25. 236 26. 216 27. 233 28. 222
29. 212 30. 219 31. 202 32. 265
33. 203 34. 204 35. 199 36. 270
37. 206 38. 221 39. 230 40. 239
41. 231 42. 236 43. 268 44. 186
45. 192 46. 205 47. 235 48. 260
49. 199 50. 252 51. 215 52. 267
53. 210 54. 290 55. 294 56. 297
57. 284 58. 315 59. 274 60. 273
61. 274 62. 280 63. 291 64. 326
65. 340 66. 306 67. 270 68. 275
69. 290 70. 305 71. 320 72. 325
73. 298 74. 321 75. 282 76. 305
77. 282 78. 279 79. 316

Lab procedures

  1. Use RockWorks to create the necessary files. See "linears" in the sample folder for general structure of files. After viewing "linears" open a new file for data entry. You can modify the column headings as needed. Enter your results--trend (compass bearing, 000 to 359°) and plunge (00°). For entering data you will create two files--data file (*.adt) and a template file (*.tem). Store these files on your floppy disk (a:\).

  2. Your data file should have 79 vectors. Use RockWorks to display the data (from your floppy disk). Select the "rose diagram" function from the button bar. Refer to the "help" section for directions to construct a rose diagram display of the sandstone cross-bedding vectors. When you have completed your rose diagram, print out the result to turn in.

  3. Interpret the regional pattern of paleocurrents as displayed on the rose diagram and the map. Briefly describe the likely depositional environment and paleogeography for the Kiowa and Dakota Formations. The paleocurrent patterns of some common sedimentary environments are given below (Table 3-2).

Table 3-2. Paleocurrent patterns of deposition.
Environment Local current vector Regional pattern
Alluvial, braided unimodal, low variability diverging
Alluvial, meandering unimodal, high variability converging
Delta unimodal, high variability radiating
Aeolian uni-, bi-, or polymodal large arc
Shore line/shelf bimodal (tides) or other 90° or parallel
to coast line
Continenal slope unimodal (turbidites) radiating

Reference

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Notice: Structural Geology is presented for the use and benefit of students enrolled at Emporia State University. Others are welcome to view the course webpages. Any other use of text, imagery or curriculum materials is prohibited without permission of the instructor. GO 568 © J.S. Aber (2003).