with Glaciotectonic Examples
James S. Aber
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.
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.
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.
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.
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.
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.
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 AEIn 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 ACPoint 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.
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:
|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.
|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|
|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|
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