GEOS1050 3D geology

Strike & Dip • Strike and dip are used to specify the orientation of a geological surfaces, such as the top of a layer of sedimentary rock. ƒ The strike of bedding is the orientation of any imaginary horizontal line running along a planar bed. It has no position, just orientation. ƒ It is usually recorded in degrees measured clockwise from north and quoted as three figures, say 045° ƒ The angle of dip is the maximum inclination of the bed in degrees from the horizontal. To avoid confusion with strike it is quoted as two figures (e.g. 08° or 30°) ƒ In addition to the angle of dip, there is the direction towards which the surface is inclined, called the dip direction.

Strike & Dip • Strike and dip are used to specify the orientation of a geological surfaces, such as the top of a layer of sedimentary rock. ƒ Dip direction will always be at right angles to the strike, and in conjunction with the strike and dip angles, is usually referred to as the sense, and is normally quoted as an octant (e.g. S, NW). ƒ When a surface is measured then, these three pieces of information are usually quoted together in what is referred to as the strike/dip/sense notation, • e.g. 045/30 SE is a plane with a strike of 045° which is dipping at 30° towards the southeast.

ƒ Note that the sense part of the notation is necessary, since a plane with a strike of 045° could be dipping in the opposite direction (i.e. NW). This then, gives the orientation of the plane in three-dimensional space

Plunge & Trend • The strike and dip concept will not work for linear geological features such as the intersections of two planes (e.g. bedding and cleavage) or the plunge of a fold axis etc. In this case plunge and trend are normally used (Fig. 3). ƒ Plunge is the inclination of the line from the horizontal, measured in a vertical plane. ƒ Trend is the downward direction of the line (opposite to azimuth), measured in the horizontal as a compass bearing. ƒ The recorded orientation of a linear geological feature is then recorded as plunge and trend (e.g. 35→155 is a line plunging at an inclination of 35° towards a direction of 155°). ƒ The two systems of recording orientations exist because it is not possible to find the strike and dip of a line, nor the trend and plunge of a plane.

Mapping Symbols • In addition to being recorded as data, the orientation of geological planes can be added to maps by means of symbols. A variety of different symbols have evolved, both for bedding surfaces and for the various other structures such as axial planes of folds, cleavage etc.

52 Strike & dip of bedding (S ) 0 Strike of bedding with vertical dip (S0)

32 Plunge & trend of fold axis

Synclinal fold axial trace

Plunge & trend of primary lineation

Anticlinal fold axial trace

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Interaction of Geological surfaces with the land surface (topography). • Geological surfaces with different attitudes (strike and dip) will interact differently with topographic surfaces, creating a variety of outcrop patterns. ƒ Horizontal geological units have no dip and strike, so in an area with little or no topographic relief and horizontal geology, it is likely only one rock unit will be exposed. ƒ If the area in question has greater relief, an outcrop pattern will emerge where geological boundaries are parallel to topographic contours, since both are horizontal (Fig 4a). ƒ In contrast, an area with vertically dipping units will display what is often referred to as “tramline geology”, where the map pattern is a series of straight lines that are completely unaffected by topography (Fig. 4b).

Interaction of Geological surfaces with the land surface (topography). • Geological surfaces with different attitudes (strike and dip) will interact differently with topographic surfaces, creating a variety of outcrop patterns. ƒ More complicated map patterns produced by the interaction of topography and geological units that are dipping at an angle other than 0° or 90°. ƒ For example figure 4c, which depicts an easterly flowing creek and a series of geological units dipping west at 45°, a map pattern is produced in which the beds appear to be folded even though they are not. ƒ In this case the “rule of V’s” can be used - in the case of valley incision, inclined units will “V” in the direction in which they are dipping.

Construction of Geological Cross-sections • Construction of geological cross-sections can vary from very simple exercises to exceedingly complicated ones. ƒ Generally, the orientation and position of a section line is picked so that it will portray the geological structure of an area effectively. ƒ In figure 3 for example, the map pattern (top surface of the block) reflects the 3-dimensional fold structure that is very apparent in the block diagram, as does the vertical cross-section at the front of the figure. ƒ The section at the side of the diagram however, merely shows a series of dipping, but apparently unfolded units - and hence does not portray the true structure.

Construction of Geological Cross-sections • Once the section line is chosen, it is a simple matter of: ƒ aligning the edge of a piece of graph paper (folded so that the edge of the printed graph forms the edge of the sheet) along your section line. ƒ marking the ends of the section (often denoted as points on the map like “A” and “B”) together with all geological features (such as lithological boundaries, faults, fold axes etc.) and relevant structural information such as dip angles etc. ƒ The vertical geology is then projected to a sensible depth using the geological features and dip angles.

• One golden rule that should be applied when constructing geological cross-sections is that vertical exaggeration should never be used. ƒ The primary reason for this rule is that vertical exaggeration changes the dip angle of inclined units, just as it changes surface slopes.

• Also remember that your cross-section should also include a copy of the scale and legend information that the plan map displays.

True and apparent dip • In geological cross-sections that are constructed where the section line is parallel to the dip direction, the dip of the geological units will be portrayed as the true dip angle. • In contrast, a cross-section parallel to the direction of strike will show each unit as horizontal, since strike is a horizontal line running along the bed. ƒ Therefore, a section along a line somewhere in between the dip direction and the strike direction will show the units inclined at an angle that is somewhere in between the true dip angle and the horizontal. ƒ The closer the section line is to the strike direction, the more gently dipping the beds will look. The closer the dip direction, the steeper they will appear. ƒ In the actual dip direction the inclination reaches its maximum and is therefore the true dip. ƒ Angles of inclination less than the true dip are referred to as apparent dip.

True and apparent dip • The conversion between true and apparent dips can be made in numerous ways, involving various permutations of trigonometry, construction, and conversion charts (e.g. Fig. 5). • If you are constructing a cross-section, and your section line is not parallel to the dip direction in question, the apparent dip should always be depicted. Hence the true dip - apparent dip correction must be made. ƒ Note that you will have to accurately measure the angle between your section line and the dip direction (which is referred to as the direction angle or obliquity angle). ƒ Alternatively, since strike is represented by the outcrop pattern of units on the map (provided the area is relatively flat), it is sometimes easier to measure the angle between your section line and strike - in this case use the scale on the right hand side of figure 5. • Note that since strike an dip directions are always 90° apart, this angle will be the compliment of the direction angle.