Figure 5. Procedure for counting pole density using a counting circle (left) and a peripheral counter (right).
Place the counting circle directly over a grid intersection on the equal area plot. Count the number of poles that fall within the circle. Write this number on the grid intersection. Repeat with all the intersections in the plot.
For intersections that fall near the edge of the plot, part of the counting circle will fall outside the primitive. The points that ‘should’ be in this part of the counting circle will appear on the opposite side of the net. We therefore use a second device, called a peripheral counter, which has two counting circles, their centres spaced apart by the same distance as the diameter of the net. A slot in the centre of the peripheral counter fits over the thumb tack in the middle of the plot, so as to keep the counter correctly aligned. When one hole is centred over a grid intersection near the primitive, the circle on the opposite side contains the extra points that should be counted.
Finally, make a smooth contour map of the results. As in all contouring, make sure that higher numbers all appear on one side of a contour, and lower numbers on the other. Contours should never branch. In contouring, remember that contour lines that disappear over the primitive should reappear on the opposite side of the net. Number the contour lines in percent, meaning “percent of the data occurring in each 1 % of the plot area”.
Assignment
1. A mineral exploration company drilled three diamond drill holes to intersect a vein set in the subsurface. The angles between the axes of the cores and the veins are shown in the following table.
Drill hole # Trend & plunge of hole Core-vein angle Core-pole angle
1 340-70 NW 40 50
2 080-76 NE 65 25
3 210-68 SW 54 36
Lab 7. Fractures | 129
Calculate the strike and dip of the vein set.
2. You are provided with an equal area projection of 100 poles to conjugate shear fractures in the Cadomin Formation from the Alberta foothills. To find the maximum densities of points, it is helpful to construct a contoured plot. A grid has been constructed across the projection to assist with contouring.
a) First, cut out the centre counter and peripheral counting circles precisely. The counting circles have a diameter exactly 10% of the plot, which means that their area covers 1% of the plot. Use the counting circles to mark densities at the each grid intersection. Contour the resulting densities on the grid. Remember the principles of contouring that you used in the first lab: each contour should separate higher from lower values; make the contours as smooth as possible while honouring the data; contours should never branch. In addition, remember you are really contouring on the surface of a sphere, and the pattern is symmetrical on the lower and upper hemispheres. Therefore, if a contour disappears over the primitive, it should reappear on the opposite side of the projection. When you are done, make a clean copy with selected contours on a separate tracing sheet.
b) Mark estimated centres, or modes, of the two density clusters. These are poles to the typical conjugate shear planes. From these poles, determine the strike and dip of the two planes, and draw the planes as great circles on your plot.
The maximum principal stress is predicted to bisect the acute angle between the two planes. The minimum principal stress bisects the obtuse angle, and the intermediate stress is parallel to their intersection.
c) Mark the principal stresses on your contoured projection and determine the plunge and trend of each.
d) An oil company is interested in flow through extension joints in the Brazeau formation. Predict the orientation of these joints based on your estimates of the directions of the principal stresses.
Lab 7. Question 2. Plot of poles to 100 shear fractures in Cardium Sandstone, Alberta Foothills.
Lab 7. Question 2. Counting Tool
3. *Look at the geological map of Canmore (east half) which shows numerous faults. Notice the band of dark blue Palliser Formation in the region north of Exshaw.
a) Without using the cross-section, what is the evidence that the Palliser Formation dips to the SW in this area?
b) Now look at the Palliser Formation on cross-section 1, which extends into the sheet Canmore (west half). Place two sheets of tracing paper so as to cover the cross-section, and trace only the base of the Palliser Formation (DPa) and the faults that offset it. For each of the named thrust faults, answer the following questions:
c) Does the fault place older over younger or younger over older strata?
d) Estimate the the dip separation of the base of the Palliser formation. Note – if the fault is curved, you can approximate the separation with a series of straight-line measurements with a ruler.
e) Estimate the throw and the heave. (These are straight line measurements even if the fault is curved.) 4. *Strictly speaking, you have no evidence for the direction of movement on the faults: – it could be exactly parallel
to the cross-section or there could be a component of strike-slip movement, in and out of the page. However, it is clear that there has been substantial overall shortening of the rocks in the hanging wall of the lowest, McConnell Thrust. Estimate this shortening in the line of the cross-section, as follows:
130 | Lab 7. Fractures
a) Measure the total original length l0 of the segments of the base of the Palliser Formation in the cross-section.
b) Measure the present-day distance l between the easternmost outcrop of the base of the Palliser, and the western edge of the section.
c) Calculate the shortening in kilometres: l0 – l.
d) Calculate the longitudinal strain as a value of stretch: s = l / l0.
e) Calculate the longitudinal strain as a fractional change in length or extension: e = (l – l0) / l
Map. Geology of Canmore (East Half)
Cross-sections. Geology of Canmore (East Half)
Lab 7. Fractures | 131
K. Faults
Introduction
Fractures are known as faults if there has been significant displacement of one side relative to the other, parallel to the fracture plane. Faults have had enormous economic impact in the exploration for natural resources.
Faults affect the flow of fluids in the Earth’s crust, and thereby control the distribution of water, oil and natural gas. Fractured material along a fault plane may form a porous breccia (pronounced “bretchya”). Fluid passing through breccia may deposit valuable minerals. Faults are also important to humans because they generate earthquakes.
An extensive terminology has developed around faults, their geometry, and movement (kinematics). It is important to distinguish between descriptive (geometric) terms, which tell us about the orientation of a fault and the offset of layers on either side, and kinematic terms, which describe the distance and direction of fault movement.