Chapter 8 - Structural Geology Flashcards
What is a bed?
A unit of sedimentation which can vary considerably in thickness
What is a bedding plane?
Marks a break between beds. They represent a break in sedimentation, a change in the composition or grain size, or a change in colour of sediments
What is dip?
The maximum inclination of a bed measured from the horizontal using a clinometer
What is true dip?
The actual angle dip measured at right angles to the strike
What is apparent dip?
A dip that is measured to be less than the maximum inclination
What is strike?
The horizontal line on a bedding plane, measured as a bearing from north using a compass. It is at right angles to the dip.
What is a stratigraphic method?
A study of strata or layers of rock
What are the 6 laws of stratigraphy?
- Law of superposition
- Law of original horizontality
- Law of cross-cutting relationships
- Law of lateral continuity
- Law of inclusion
- Law of faunal sucession
What does the law of superposition mean?
Younger layers of rock sit atop older layers
What does the law of original horizontality mean?
- Most sedimentary rocks are laid down horizontally
—-> presumed if tilted moved from original horizontal position ( older )
What does the law of cross-cutting relationships mean?
Features which cut through rocks must be younger than the rocks they cut through
What does the law of lateral continuity mean?
Layers of rock are continous until they encounter other solid bodies that block their deposition or until they are acted upon by agents that appeared after deposition took place
——-> valleys
What does the law of inclusion mean?
Oldest rocks are inside younger rock
e.g Xenoliths/fossils
What does the law of faunal succession mean?
If fossils go extinct they cant be found in younger rocks
How can beds be different to eachother?
- Colour
- Grain size
- Grain shape
- Sorting
- Composition
- Thickness
How can colour vary in beds?
- Major changes —-> red/green
- Subtle changes —-> grey/brown
How can grain size differ in beds?
- Major changes —> fine clay/coarse pebbles
- Minor changes —> fine/medium sand
How can grain shape differ in beds?
-Angular/rounded
How can sorting differ in beds?
- Well/poorly sorted
How can composition differ in beds?
Each bed will contIn different amounts of minerals and the cement/matrix will vary
How can bed thickness differ in beds?
Thin = lamination
Thick = massive
What is a fault?
Brittle fracture where there is displacement
Characteristics of faults:
-Horizontal, vertical or combination
-Very common structures
-Areas of weakness where EQ takes place when stored stress is released
-cm—> km in length
What are faults formed by?
Tension, compression or shear forces
What direction are dip-slip faults?
Vertical
What are the 4 dip-slip faults?
- Normal faults
- Gaben and horst
- Reverse faults
- Thrust fault
Normal faults:
- Dip-slip fault
- Earth’s crust is being stretched
- Crust lengthening
- Clear gap in beds
- Principle stress is vertical due to weight of overlying rocks
Graben and horst:
- Two normal faults face each other —> graben or rift valley is formed
- Centre of the MOR and form the East African Rift Valley
- Two normal faults fave away from each other —> horst is formed
Reverse faults:
- Compressional faults
- Shortening of the Earth’s crust
- Overlap in beds
- Principal stress direction is horizontal
- Near surface sue to less overlying rock
Thrust fault:
- Fault dip <45 degrees
- Compressional forces
- Associated with major fold mountain systems
- Displacement measured in km
What direction are strike slip faults?
Horizontal
Strike-slip faults:
- Vertical fault plane
- Horizontal movement along fault (parallel to strike of fault plane)
- Large scale structures —> large displacement
3 types of strike-slip faults:
- Tear
- Dextral and sinistral
- Transform
Tear fault:
- Result of shearing forces
Example: Great Glen Fault, Highlands of Scotland
—> 100km displacement from Caledonian Orogeny
What direction does sinistral mean?
left
What direction does dextral mean?
Right
