Cross sections of folded and dipping beds

Июль 31, 2022

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Cross sections of folded and dipping beds

Содержание

2. This presentation is to be completed in conjunction with exercise worksheet 4. Objectives: By the end
3. Folded and dipping cross sections: Problems As we have previously covered the techniques necessary to complete
4. Folded and dipping cross sections: Problems Answers for problem 1: Following the law of “V’s” that:
5. Answers for problem 1: Remember: to draw structure contours, the geological boundary is known where it
6. Then using simple trigonometry: tan(θ) = (opp/adj) tan(θ) = (100m/250m) tan-1(100m/250m) = θ = 22° True
7. Folded and dipping cross sections: Problems Answers for problem 1: First mark on the topographic points.
8. Folded and dipping cross sections: Problems Answers for problem 1: Then mark on the geological interface
9. Answers for problem 1: Then using a protractor, measure a dip of 22o and draw on
10. Folded and dipping cross sections: Problems Answers for problem 1: Now fill in the beds lithological
11. Folded and dipping cross sections: Problems Answers for problem 1: Finally extend the structure contours, to
12. Using exercise worksheet 4, complete problem 2 before continuing onto the next slide. Questions for problem
13. Folded and dipping cross sections: Problems Answers for problem 2: Remember: to draw structure contours, the
14. Folded and dipping cross sections: Problems Answers for problem 2: Remember to calculate true thickness: True
15. Folded and dipping cross sections: Problems Answers for problem 2: First mark on the topographic points.
16. Folded and dipping cross sections: Problems Answers for problem 2: Then mark on the lithological interfaces.
17. Answers for problem 2: Then assuming constant dip, structure contour points from other areas of the
18. Folded and dipping cross sections: Problems Answers for problem 2: Now the beds can be drawn
19. Folded and dipping cross sections: Problems Answers for problem 2: Now you have drawn the cross
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This presentation is to be completed in conjunction with exercise

worksheet 4.
Objectives:
By the end of this exercise you should:
Be able to construct cross sections of dipping beds.
Be able to construct cross sections of folded beds.

Folded and dipping cross sections

This exercise will build on many of the concepts you have learnt so far, utilising:
Folded structures.
Law of “V’s”.
Drawing cross sections.
Calculating true thickness.

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Folded and dipping cross sections: Problems
As we have previously covered

the techniques necessary to complete these exercises, we will move straight onto problems instead of going through an example.
Using exercise worksheet 4, attempt problem 1 before continuing onto the next slide.
Questions for problem 1:
Look at the outcrop pattern and deduce the direction of dip. Which is the oldest bed and which is the youngest?
Draw structure contours for each geological interface and calculate the strike and amount of dip from a representative area of the map.
Draw a cross section from A to B assuming constant dip.

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Folded and dipping cross sections: Problems
Answers for problem 1:
Following the law

of “V’s” that: Beds dipping downstream V-downstream ; the beds must dip to the South.

Then following the law of superposition stating that:
“In any un-overturned sequence of rocks deposited in
layers, the youngest layer is on top and the oldest on
the bottom; each layer being younger than the one
beneath it and older than the one above”
Then the series of deposition must be:

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Answers for problem 1:
Remember: to draw structure contours, the geological

boundary is known where it crosses a topographic contour line. For example a line can be drawn through the four geological boundaries where they cross the 600m contour.
Then use a protractor to measure strike;
Strike = 85o

Folded and dipping cross sections: Problems

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Then using simple trigonometry:
tan(θ) = (opp/adj) tan(θ) = (100m/250m) tan-1(100m/250m) = θ =

22°
True dip = 22°

Folded and dipping cross sections: Problems

Answers for problem 1:
Then to calculate dip, choose two representative structure contours, eg. 400m and 500m. Using your ruler measure the distance between these on the map (250m), this gives you the adjacent length of the triangle (see below). Then calculate the difference in height from the structure contours (100m) , this gives you the opposite side of the triangle.

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Folded and dipping cross sections: Problems
Answers for problem 1:
First mark

on the topographic points.

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Folded and dipping cross sections: Problems
Answers for problem 1:
Then mark

on the geological interface points.

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Answers for problem 1:
Then using a protractor, measure a dip

of 22o and draw on the dipping beds.

Folded and dipping cross sections: Problems

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Folded and dipping cross sections: Problems
Answers for problem 1:
Now fill

in the beds lithological patterns.

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Folded and dipping cross sections: Problems
Answers for problem 1:
Finally extend

the structure contours, to show where the beds would extend to if they hadn’t been eroded.

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Using exercise worksheet 4, complete problem 2 before continuing onto

the next slide.
Questions for problem 2:
On the map draw structure contours for each boundary (i.e. Siltstone-Shale boundary; Shale-Grit boundary).
Calculate the true thickness of the shale bed.
Using the topographic and structure contours, construct a cross section through A to B.
Indicate on the map the position of an anticlinal axis with the symbol:
and a synclinal axis with the symbol:

Folded and dipping cross sections: Problems

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Folded and dipping cross sections: Problems
Answers for problem 2:
Remember: to

draw structure contours, the geological boundary is known where it crosses a topographic contour line.

Blue = Siltstone-shale boundaries; Green = Grit-shale boundaries

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Folded and dipping cross sections: Problems
Answers for problem 2:
Remember to

calculate true thickness:
True thickness (t) = width of outcrop (w) x sin(θ) (angle of dip)
So first we must calculate the angle of dip using structure contours (e.g. the most Westerly shale structure contours: 700m and 600m.)
The distance between these is: ~9mm = 180m.
The difference in height is: 700m-600m = 100m
Therefore:
tan(θ) = (opp/adj) tan(θ) = (100m/180m) tan-1(100m/180m) = θ = 29°
True dip = 29°

So: True thickness (t) = 370m x sin(29°)

True thickness (t) = 179 m

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Folded and dipping cross sections: Problems
Answers for problem 2:
First mark on

the topographic points.

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Folded and dipping cross sections: Problems
Answers for problem 2:
Then mark on

the lithological interfaces.

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Answers for problem 2:
Then assuming constant dip, structure contour points from

other areas of the map can be added to this.

Folded and dipping cross sections: Problems

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Folded and dipping cross sections: Problems
Answers for problem 2:
Now the beds

can be drawn on. Remember to use solid lines where the actual boundaries are and dotted lines for where the boundaries are unknown as well as where they would of extended to above ground level prior to erosion.
How does the measured thickness of the cross section compare with your calculated actual thickness?

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