GEOS1050 3D geology

8/7/2019 GEOS1050 3D geology

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Strike & Dip Strikeand dipare used to specify the orientation of a

geological surfaces, such as the top of a layer ofsedimentary rock.

The strikeof bedding is the orientation of any imaginaryhorizontal line running along a planar bed. It has no position,just orientation.

It is usually recorded in degrees measured clockwise from northand quoted as threefigures, say 045

The angle ofdipis the maximum inclination of the bed indegrees from the horizontal. To avoid confusion with strike it isquoted as twofigures (e.g. 08 or 30)

In addition to the angle of dip, there is the direction towardswhich the surface is inclined, called the dip direction.


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Strike & Dip Strikeand dipare used to specify the orientation of a

geological surfaces, such as the top of a layer ofsedimentary rock.

Dip direction will always be at right angles to the strike, and inconjunction with the strike and dip angles, is usually referred toas the sense, and is normally quoted as an octant (e.g. S, NW).

When a surface is measured then, these three pieces ofinformation are usually quoted together in what is referred to asthe strike/dip/sensenotation,

e.g. 045/30 SE is a plane with a strike of 045 which is dipping at 30towards the southeast.

Note that the sense part of the notation is necessary, since aplane with a strike of 045 could be dipping in the opposite

direction (i.e. NW). This then, gives the orientation of the planein three-dimensional space


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Plunge & Trend The strike and dip concept will not work for linear geological

features such as the intersections of two planes (e.g. bedding andcleavage) or the plunge of a fold axis etc. In this case plungeand

trendare normally used (Fig. 3). Plungeis the inclination of the line from the horizontal, measured in a vertica

plane.

Trendis the downward direction of the line (opposite to azimuth), measuredin the horizontal as a compass bearing.

The recorded orientation of a linear geological feature is then recorded asplunge and trend (e.g. 35155 is a line plunging at an inclination of 35

towards a direction of 155).

The two systems of recording orientations exist because it is not possible tofind the strike and dip of a line, nor the trend and plunge of a plane.


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Mapping Symbols

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

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Strike & dip of bedding (S0)

Strike of beddingwith vertical dip (S0)

Plunge & trend ofprimary lineation

Plunge & trendof fold axis

Synclinalfold axial trace

Anticlinalfold 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, creatinga variety of outcrop patterns.

Horizontal geological units have no dip and strike, so in an areawith little or no topographic relief and horizontal geology, it islikely only one rock unit will be exposed.

If the area in question has greater relief, an outcrop pattern wilemerge where geological boundaries are parallel to topographic

contours, since both are horizontal (Fig 4a).

In contrast, an area with vertically dipping units will display whais often referred to as tramline geology, where the map patter

is a series of straight lines that are completely unaffected bytopography (Fig. 4b).


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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, creatinga variety of outcrop patterns.

More complicated map patterns produced by the interaction oftopography and geological units that are dipping at an angleother than 0 or 90.

For example figure 4c, which depicts an easterly flowing creekand a series of geological units dipping west at 45, a map

pattern is produced in which the beds appear to be folded eventhough they are not.

In this case the rule of Vs can be used - in the case of valley

incision, inclined units will V in the direction in which they aredipping.


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Construction of Geological Cross-sections

Construction of geological cross-sections can vary fromvery simple exercises to exceedingly complicated ones.

Generally, the orientation and position of a section line is pickedso that it will portray the geological structure of an areaeffectively.

In figure 3 for example, the map pattern (top surface of theblock) reflects the 3-dimensional fold structure that is veryapparent in the block diagram, as does the vertical cross-section

at the front of the figure. The section at the side of the diagram however, merely shows a

series of dipping, but apparently unfolded units - and hence doe

not portray the true structure.


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Construction of Geological Cross sections


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Construction of Geological Cross-sections

Once the section line is chosen, it is a simple matter of:

aligning the edge of a piece of graph paper (folded so that the edge of theprinted graph forms the edge of the sheet) along your section line.

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 boundariesfaults, fold axes etc.) and relevant structural information such as dip anglesetc.

The vertical geology is then projected to a sensible depth using the geologicfeatures and dip angles.

One golden rule that should be applied when constructing geologic

cross-sections is that vertical exaggeration should never be used. 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 othe scale and legend information that the plan map displays.

True and apparent dip


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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 wibe portrayed as the true dip angle.

In contrast, a cross-section parallel to the direction of strike willshow each unit as horizontal, since strike is a horizontal line runninalong the bed.

Therefore, a section along a line somewhere in between the dip direction anthe strike direction will show the units inclined at an angle that is somewherein between the true dip angle and the horizontal.

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

In the actual dip direction the inclination reaches its maximum and istherefore the true dip.

Angles of inclination less than the true dip are referred to as apparent dip.


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True and apparent dip


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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 notparallel to the dip direction in question, the apparent dip shouldalways be depicted. Hence the true dip - apparent dip correctionmust be made.

Note that you will have to accurately measure the angle between your sectioline and the dip direction (which is referred to as the direction angleorobliquity angle).

Alternatively, since strike is represented by the outcrop pattern of units on thmap (provided the area is relatively flat), it is sometimes easier to measurethe angle between your section line and strike - in this case use the scale onthe right hand side of figure 5.

Note that since strike an dip directions are always 90 apart, this angle will be thcomplimentof the direction angle.


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GEOS1050 3D geology

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