Geology AS - Geological structures (Deformation of rocks (Beds and bedding…
Geology AS - Geological structures
Deformation of rocks
Beds and bedding planes
- A unit of sedimentation, which can vary considerably in thickness.
- Marks the break between beds. Represents a break in sedimentation, a change in the composition or grain size, or a change in colour of the sediments.
Beds will be different in a variety of ways:
- the maximum inclination of a bed measured from the horizontal using a clinometer.
- the direction at right angles to the dip. There is zero dip in this direction. It is a bearing measured from north using a compass.
- a dip that is measured to be less than the maximum inclination.
Stress and strain
- Change of length of line / Original length of line.
- The force applied to rocks.
- Strong and brittle and tend to join and fault.
- Weak and plastic and tend to fold and develop cleavage.
Factors affecting stress and strain:
Rocks behave in different ways to stress, depending on their physical properties:
Competent rocks stay the same thickness when deformed and react in a brittle way.
Incompetent rocks vary in thickness when deformed, as they behave in a plastic way.
The relationship between forces and geological structures
The forces which act along a plane in the rock and promote sliding along that plane.
Results in deformation of rocks in one plane - usually horizontally.
May result in faults or folds.
The force trying to push rocks together - the Earth's crust will be shortened.
results in either fracturing or folding of rocks.
Cold rocks tend to fracture and
Warm rocks tend to fold.
The force trying to pull rocks apart - the Earth's crust will be lengthened.
Results in fracturing of rocks and crustal extension.
If forces result in a displacement of rocks on either side of the fracture plane, structure described as a
If not displacement, structure described as a
Using fossils and ooliths to measure strain
To measure rock strain, bilaterally symmetrical fossils or ooliths can be used.
Undeformed fossils used to establish original shape and then the amount and direction of deformation can be analysed.
Ooliths originally spherical but after deformation, become ellipsoid.
Recognition of geological structures
Represent a break in time - a period when no sediment was deposited.
May result in either a change in the environment or earth movements and erosion.
Angular unconformities occur where rocks above have a different dip, and possibly strike, to rocks beneath.
How to recognise an unconformity:
Difference in strike and dip of beds below and above plane of unconformity.
Presence of a basal conglomerate.
Older beds and structures cut off by younger rocks above. Structures do not stop at the plane of unconformity, but continue beneath it.
Rocks above are young than those beneath.
Joints only form in competent rocks, which are brittle and break when put under tension.
Joints will not form in incompetent rocks.
Joints in sedimentary rocks normally found perpendicular to the beds.
- A fracture in competent rocks along which no observable movement has occurred. Produced by tensional forces. These forces may be the result of folding, cooling or unloading rocks.
- Form as a result of folding and cause
parallel to the axial plane and
on the limbs.
- Form as a result of contraction on cooling of igneous rocks.
- Often horizontal as well as vertical and form as a result of lower pressure near to the surface.
- A fracture in a rock along which there has been an observable amount of displacement.
- The plane of fracture, along which the rocks have been displaced.
- The vertical displacement of rocks along the fault plane.
- The maximum inclination of the fault plane as measured from the horizontal.
- The rocks that lie about the fault plane if the fault is not vertical.
- The rocks that lie beneath the fault plane if the fault is not vertical.
- Side of the fault where the movement is upwards in relation to the other side.
- Side of the fault where the movement is downward in relation to the other side.
Largely vertical movement.
Where movement along the fault plane is parallel to the dip of the fault plane.
A fault where the hanging wall has moved upward relative to the footwall.
Occur when two blocks of rock are forced together by compressional forces.
Type of reverse fault, where the dip is less than 45 degrees.
Formed by compressional forces.
A fault where the hanging wall has moved downward relative to the footwall.
Occur when two blocks of rock are pulled apart by tensional forces.
Graben and horst:
A Graben forms when two normal faults face each other, both dipping towards each other.
A Horst forms when two normal faults face away from each other, both dipping away from each other
Largely horizontal movement.
Where movement along the fault plane is parallel to the strike of the fault plane.
The result of different rates of movement within a plate and allow the rigid plates to adjust for these differences in the rate of movement.
The result of shearing forces applied to the rocks.
Dextral tear fault
- When the block of rock opposite the fault moves right.
Sinistral tear fault
- When the block of rock opposite the fault moves left.
Features associated with fault planes
- The polishing and striations found on a fault plane indicating the direction of relative movement.
- Composed of fragments produced by rocks fracturing during faulting.
Folds formed by high levels of compression
Formed by high compressive pressures from one side.
Axial plane of the fold and its limbs are close to horizontal and always less than 30 degrees.
Rocks usually incompetent or plastic, in order to absorb high stress levels.
Recognised by low angle limbs and axial plane + one inverted limb.
Huge recumbent folds that have broken along thrust planes.
Thrust plane movement is horizontal and part of the fold has been moved forward along the thrust.
Form some of the world's largest tectonic features.
Formed by compressional forces, stronger from one direction.
Characterised having both fold limbs dipping in the same direction, but by different amounts.
Axial plane is inclined.
Can be either anticlinal or synclinal.
Formed by large amounts of pressure equally from both sides.
Recognised by parallel limbs that are nearly vertical and in very tight folds.
Axial planes usually vertical.
Recognised on maps by outcrops of repeated, parallel beds.
Domes and basins: