Structure Contours

STRUCTURAL GEOLOGY

Map Skills (Summarised)2012

KAMAL’ OMOSANYA, University of [email protected]

Structure contours?• Topographic contours connect points of equal height above sea level

• Structure contours (also known as strike lines because they can run parallel to strike) connect points of equal elevation (height or depth) on a geological surface (eg. bedding)

• A plane with a consistent strike and dip has evenly spaced straight structure contours

• Steep dipping planes have closely spaced contours

• Shallow dipping planes have widely spaced contours

• Structure contours are used to:

• Map the subsurface and anything related to it (cross section, depth, horizon of interest)

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Topographic Contours• Topographic contours join all points of equal elevation

• Hills have closed contours

• Widely spaced contours - shallow slopes

• Closely spaced contours – steep slopes

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• Hills have closed contours

• Widely spaced contours - shallow slopes

• Closely spaced contours – steep slopes

• Streams and rivers follows vees in contours

Topographic maps

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200 m

N

400 m

300 m

200 m

100 m

A

B

550 m 300 m

A B

= Tan-1 (300/550)

= 28.6o

= 29o S

Horizontal distance from map scale

Vertical distance from struct cont spacing

Calculating dip

550 m

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200 m

N

400 m

300 m

200 m

100 m

Calculating strike

Structure contours are parallel to strike (they are also known as strike lines). To measure strike find north and measure round clockwise. In this case, the plane strikes east-west, so a strike of 090o

Strike and dip of the plane is 090/29S

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400 m

300 m

200 m

100 m

Calculating apparent dip from structure contour spacing

Apparent dip angle

’ YX

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200 m

Y

X

N

400 m

300 m

200 m

100 m

X Y

300 m

= Tan-1 (300/800)

= 20.6o in direction 135o

’

Calculating apparent dip

A

B

XY > AB

800 m

800 m

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T A

B

To calculate the true thickness T between boundaries A and B

Tv

1. Measure vertical thickness Tv from structure contours

2. Calculate dip from struct. cont. spacing

3. This angle is also

4. cos = T/Tv T = Tvcos

Method 1- Cross section

Method 2-Map

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100

200

300400 500

600

700

400

500

600

700

500m

How to draw a cross section: Part 1 - Topography

A B

10

100

300 400 500

600

700

400

500

600

700

500m

200

How to draw a cross section: Part 2 - Geology

A B

27o

27o

27o

27o

27o

27o

27o 27o

27o

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True scale vs vertical exaggeration500m

500m

True Scale:

• Horizontal and vertical scales are the same

• Directly measure angles and thicknesses

500m

500m

Vertical Exaggeration:

• Horizontal and vertical thicknesses are different – usually vertical scale is greater than the horizontal

• Steepens angles and increases thicknesses

• Can be used in areas of subdued topography or to make subtle structures clearer

Always use true scale cross sections (unless instructed otherwise) as these give the best

indication of the structure12

‘V’ in the direction of dip

Dipping beds

Veeing in valleys

Dip?

Outcrop Pattern(finding the dip direction)

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Geological history.

•All geological histories start and finish the same way:

•First: The oldest layers in the sequence were deposited

•Last: Uplift and erosion to the present day

•Include, where possible, information on sedimentary environments, metamorphic conditions, stresses (direction of compression/extension) etc

My way (EROPTE)

•E- Environment (Basement/Sedimentary/ contact)

•R- Rock types

•O-Order (order of deposition of rock/ emplacement/ unconformities)

•P- Processes involved (Traction, Saltation, Bed load, condition of sea level etc)

•T- Tectonism (Faulting, Folding, others)

•E- Erosion (Till now, erosion carved the area into the present topography) 14

3D diagram

Seeing cross section as a volume

My Way:

Initial 2D, Cross Section

Now! a 3D!!!!!

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Thanks to

•Structural Geology group (University of Leeds)

•Google earth

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