1 Unconformities, Joints, Faults and Folds

Unconformities

1. The diagram above shows an angular unconformity below the Gault Clay on the coast of southern Britain. Describe the sequence of events, starting with the deposition of the oldest rock group.

2. The picture below shows part of a cliff face exposing sedimentary strata. Trace over the angular unconformity in a bright colour.

3. In geology the term joint refers to a fracture in rock where there has been no lateral movement in the plane of the fracture (up, down or sideways) of one side relative to the other. How does this differ from a fault?

Gault clay

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Joints

Joints form in solid, hard (competent) rock that is stretched such that its brittle strength is exceeded (the point at which it breaks). When this happens, the rock fractures in a plane parallel to the maximum principle stress and perpendicular to the minimum principle stress (the direction in which the rock is being stretched). This leads to the development of a single sub‐parallel joint set. Continued deformation may lead to development of one or more further joint sets.

Joint sets are commonly observed to have relatively constant spacing and the spacing is roughly proportional to the thickness of the bed.

Find out the origins of the following types of joints and the types of rock where they are likely to be found. In each case, describe at least one location where you might find the type of jointing.

a. Tectonic joints

b. Tension and cross joints,

c. Cooling joints

d. Unloading joints.

3 Faults

On each of the following diagrams add labels to show (where applicable): fault plane, throw, fault dip, hanging wall, footwall, upthrow and downthrow.

Since faults do not usually consist of a single, clean fracture, so the term fault zone is used when referring to the zone of complex deformation that is associated with the fault plane.

Define what is a fault?

The two sides of a non‐vertical fault are called the hanging wall and footwall. By definition, the hanging wall occurs above the fault and the footwall occurs below the fault. This terminology comes from mining. When working rock face, the miner stood with the footwall under his feet and with the hanging wall hanging above him.

4 Fault Types

The creation and behaviour of faults is controlled by the relative motion of rocks on either side of the fault surface. Because of friction and the rigidity of the rock, the rocks cannot simply glide or flow past each other. Rather, stress builds up in rocks and when it reaches a level that exceeds the strain threshold, the accumulated potential energy is released as strain, which is focused into a plane along which relative motion is accommodated — the fault.

For each of the fault types illustrated below, explain its formation in terms of the type (compression, tension or shearing) and direction of the forces involved. Label the diagrams and add arrows.

Dip‐slip faults (Normal and Reverse)

Graben (rift), and Horst Thrust Fault

Strike‐slip fault Transform Fault 

TRANSFORM FAULT

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Slickensides are naturally polished rock surfaces that are produced by motion on faults. They may be scattered surfaces as small as your hand or, in rare cases, thousands of square meters in extent. The corrugations show the direction of motion along the fault. Unusual minerals may occur given the combinations of fluids and pressures along slickensides. But even familiar rocks, as we will see, take on unusual features too.

Fault breccia is a breccia (a rock type consisting of angular clasts) that was formed by tectonic forces. Fault breccias usually have no cohesion; they are normally unconsolidated rocks, unless cementation has taken place at a later stage.

You should be able to recognise a fault in the field. To help you, study the slideshow on the VLE on Mapping Faults and Unconformities . There is also a handout that you can print out for your notes.

6 Folds

Label the following features on this diagram: fold limbs, hinge, crest, trough, axial plane, axial plane trace (The intersection of the axial plane of a fold with the surface of the earth or any other specified surface; sometimes such a line is loosely and incorrectly called the axis), plunge (the direction in which the fold axis dips), antiform and synform.

The diagram below shows part of a fold mountain range. Label the diagram to show the direction of the compressional forces that will have resulted in the folds.

State THREE factors, other than variations in the compressional forces, will determine the degree to which rocks can be folded.

1

2

3

7 In the spaces below, sketch block diagrams or cross sections of the following features

A symmetrical anticline An asymmetrical anticline and syncline

A syncline An overfold

A recumbent folds, A nappe

An isoclinal fold A monocline fold

A dome A basin

8 Study the following geological map carefully

1. Mark on the map the axial plane trace of an antiform using the correct symbol.

2. Mark on the map the axial plane trace of a synform using the correct symbol.

3. There are two faults shown on the map, F1 and F2. Which fault is the younger of the two?

4. How do you know?

5. All the rocks, except Q and P are conformable sedimentary rocks. Which is the oldest?

6. Q is a sedimentary rock that is horizontally bedded.

7. Draw a sketch of what a cross section from X to Y might look like (Assume that the land surface is flat).

8. P is an igneous intrusion. How old is it in relation to the other rocks?

9. Write a summary of the sequence of events revealed by the map and your sketch section.

P

Q

Q