ESS 241
Lab 7: Folds
Due date: Nov. 18th, 2024, 11:00 pm for Monday Lab session
Nov. 21st, 2024 11:00 pm for Thursday Lab session
Submissions to be made on Quercus
Total: 50 points
For this lab you will need
Schmidt Net
Tracing Paper and Graph paper
Thumb Tack
Mechanical Pencil or regular pencil with a sharpened point for neatness
Ruler
Eraser
Exercises:
1: Simple folds (10 points total)
The maps below show folded sedimentary beds in a region of nearly flat topography. For both, complete the following tasks:
1. Mark the trace of the fold axial plane and add appropriate symbols (see appendix) to indicate fold type.
2. Mark the trend and plunge of the fold axis if different from the axial trace.
3. Construct vertical cross-sections along lines A-A’ and B-B’ on a separate sheet of paper. Draw the axial plane on your cross section.
Exercise 2: Plotting a fold on a stereonet (16 points total)
A few of your colleagues mapped an interesting fold last year at field school. The fold is on a cliff face of Lake Huron, with excellent exposure and access. The rocks are metamorphosed and folded sandstone and shale. The students took the following bedding readings:
A. 291/59
B. 138/40
C. 170/18
D. 127/66
E. 282/41
F. 256/22
Steps in analyzing the fold using a stereographic projection:
1. Plot the beds and the poles to the bedding measurements (6 points)
2. Find the great circle that all the poles to the bedding readings lie on (or are very close to). Mark the pole to this plane. The pole to this great circle is the fold axis. Write the trend and plunge of the fold axis below (4 points).
FOLD AXIS: _____________________________
3. Next, find the spread of the poles by counting the degrees between the two furthest bedding poles. Determine the mid-point of the bedding plane poles (on the great circle). Mark this mid-point on the great circle. (2 points)
4. Find the great circle that both the mid-point and the fold axis both lie on. This plane is the axial plane. Write the strike and dip of the axial plane using the right hand rule. (4 points)
AXIAL PLANE: _____________________________
Exercise 3: Cross-section of a fold (24 points total)
Construct a cross-section with no vertical exaggeration from points X to Y for the Geological map “Lab 7- Folds” (available on Quercus). The cross section is perfectly perpendicular to the strike of the fold axial plane, which makes this task easier.
Some advice…
Start by colouring the different units in the map. When drawing strike lines you will quickly notice that your map will become crowded immediately so make sure to be neat, label your strike lines well especially for contacts on either limbs of the folds. You will also notice that the contacts on each limb of the fold have different dip angles. For example- The CB contact west of unit A is shallower than the CB contact east of unit A.
Another approach……
Start by outlining different contacts on the map (between A and B, B and C, etc.) in different colors. For each contact, draw points in your cross-section every time your contact crosses a topographic contour (on the map) in the corresponding color. Connect the dots on your cross-section in a sensible way, maintaining bed thickness on the fold limbs, but allowing some thickening in the hinges. Once you have done this for a number of beds you should be able to discern the fold shape.
Draw the fold axial planes on the cross section and transfer their trace onto the topographic map using a heavy black line. Use appropriate symbols to show if the features are anticlines, synclines or if they’re overturned (see Appendix). Determine the strike and dip of the axial planes and record them here:
Syncline strike and dip of FAP: ____________Anticline strike and dip of FAP: ______________
(4 points)
Note that A is the oldest unit and H is the youngest unit on this map. Therefore use the correct FAP symbol for your map.
Your map and cross-section should show all the following information
Map (6 points)
- Strike lines for all the units
- Calculation for the dip of the units on either side of the fold axis
- Trace of Fold axial plane with appropriate symbol
Cross-section (14 points)
- No vertical exxageration
- Axes labeled, north arrow, E-W labeled
- Folded beds at depth (solid lines) and above the surface (dashed lines- eroded part of folds)
- Trace of Fold axial plane