folds, faults &unconformities

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  • GEOL 121.3 Structure 2- Folds, faults and Unconformities This exercise is concerned with describing the three-dimensional orientation of rock units. It is a difficult exercise because it requires visualization of 3-dimensional representations. Following completion of this exercise, you should be able to: 1. Identify and properly label all types of fold structures. 2. Determine relative moments along faults 3. Identify angular unconformities Folding Originally planar layers of rock may be folded into curved surfaces by the forces acting on the earths crust. The folding of the earths crust may produce a wave-like ground surface, as in Fig. 1-a, but it is usually accompanied by erosion that removes the tops of the folds, Fig. 1-b. The parts of a fold are indicated in Fig. 1. Axial plane- the plane that divides the fold as symmetrically as possible Axis- the line of intersection between the axial plane and a folded surface Axial trace- the trace of the axial plane on the ground surface Limb- the parts of the fold on either side of the axial plane

    An up folded or arched layer produces a fold known as an anticline while a down folded layer is called a syncline, see Fig 1. There are two ways to distinguish anticlines from synclines in map view. 1. If the dip direction of beds is known, anticlines are easily distinguished. For an anticline the beds dip away from the axial trace. For a syncline the beds dip towards the axial trace. See Fig. 2.

  • 2. The Principle of Superposition states that in undisturbed sedimentary rocks, the beds at the top of a sequence are younger than the beds on the bottom. In an anticline, deeper and older beds have been moved up in the centre of the fold. IN a syncline, higher and younger beds have been moved down in the centre of the fold (Fig 1). As a result, when you look at a map the beds are older as you go towards the axial trace of an anticline and the beds are younger as you go towards the axial trace of a syncline (Fig 2).

    The folds in Fig 2 appear as if they would extend forever in the direction of the strike of the layers. This could not happen. All folded layers disappear from the surface and plunge into the ground. Figs 3 & 4 show the patters produced by plunging anticlines and synclines.

  • Faulting Rock layers may break rather than fold during deformation. When such a break occurs and the rocks on one side of the break move relative to those on the other side, the rocks are said to have faulted. The surface along which the movement has occurred is the fault plane. If the fault plane is inclined at an angle of less than 90, then the rock above the fault plane is called the hanging wall and the rock below the fault plane is called the foot wall. The trace of the fault along the surface of the ground is known as the fault trace (Fig. 5).

    Faults can be classified on the basis of the direction of the movement on the fault plane. If the movement was parallel to the dip of the fault plane, then the fault is a dip-slip fault. Similarly, if the movement was parallel to the strike direction, the fault is a strike-slip fault (Fig. 6).

    Dip-Slip Faults: There are two principal types of faults which are based on the relative movement of the hanging wall and the foot wall. 1. Normal Faults. All faults in which the hanging wall has moved downward relative to the foot wall.

  • 2. Reverse Faults: All faults in which the hanging wall has moved upward relative to the foot wall.

    If layers that are not horizontal are faulted, then the map pattern (as seen in map view) may be greatly modified. Figs 9-11 are block diagrams and maps of an area with inclined layers before and after faulting.

  • Note: In Figs 10-13, at any point along the fault the youngest beds occur on the downthrown side of the fault.

    The fault plane of a dip-slip fault may be vertical. In that case it is impossible to recognize a hanging wall or a foot wall, and the fault blocks are referred to as the upthrown and downthrown blocks (Fig 13).

    Strike-Slip Faults: Faults along which the movement is parallel to the strike of the fault plane are known as strike-slip faults. The map pattern produced by this kind of faulting is shown in Fig. 14. Strike-slip faults are named according to the sense of movement of one block when viewed from the other. When the displacement is to the right, the fault is right-handed. When the displacement is to the left, the fault is left-handed (Fig. 14).

  • Unconformities: An unconformity represents a period of erosion or non-deposition separating younger rocks from older rocks. An unconformity then represents a period of time during which no rocks were deposited in an area. Very frequently this period of non-deposition is also a period of erosion as the older rocks are exposed at the earths surface. Three types of unconformities are recognized: disconformities, angular unconformities and nonconformities. If no external forces have affected the older rocks during the period of non-deposition, then the attitude of the layers of rock above and below the unconformity will be the same, this is called a disconformity (Fig 15a). If, during or before the period of non-deposition, the older rocks have been folded or faulted or in any way suffered a change in attitude, the surface of unconformity then separates rocks with differing strike and/or dip, this is called an angular unconformity (Fig. 15b). When the surface of the unconformity lies on an intrusive igneous rock or a metamorphic rock it is called a nonconformity (Fig. 15c).

    Unconformities can be recognized on a geologic map in a number of ways. Some examples are shown in Figure 16.

  • Assignment: Diagram A & B. Complete the diagrams by filling in the front face. What name is given to this structure? Number the formations in order of decreasing relative age (1=oldest). Add the trace of the axial plane and label the top face with proper symbols. Diagram C & D. Complete the diagrams by filling in the blank face. What name is given to this structure? Add several strike and dip symbols to the top face. Add the trace of the axial plane and label the top face with proper symbols. Number the formations in order of decreasing relative age. Diagram E & F. Complete the block diagrams and indicate by letter on the map face the following: hanging wall (H), foot wall (F), upthrown side (U), downthrown

  • side (D). Show with an arrow the relative movement along the fault plane. Label the fault type.

    MAP 7 1. Outline the trace of the main unconformity in red. 2. Insert the five (5) fold axial traces and indicate with proper symbols whether anticline or syncline. All of these folds are plunging in the same direction. 3. Make a list of formations from oldest to youngest. 4. Indicate on the map which is the upthrown side of the fault. 5. The fault is a normal fault. In which direction does the fault plane dip? ______ 6. Draw structure sections of XY and XZ. 7. Give, in point form, the geological history of the map area.

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