EASC2124:EASC2124: Geological Maps and Geological Maps and Air PhotosAir Photos

Lecture/Practical 4:

Introduction to Structure Contours in Geological Maps

Exercise 6 and 7

TodayToday’’s Learning Outcome:s Learning Outcome:You should be able to understand and apply:You should be able to understand and apply:

• Outcrop patterns of Uniformly Inclined Strata – Law of V

• Structure Contour Lines to

- Draw a Geological Cross Section

- Determine/Calculate strike, dip direction and dip angle

- Determine the true (stratigraphic) thickness of a bed

Concept of outcrop pattern of a Concept of outcrop pattern of a geological contactgeological contact

• Flat, planar topography – geological contacts of tilted strata are straight lines on the map.

• Non-planar topography – geological contacts of tilted strata have irregular shapes on the map although the contact between the two beds is a planar surface.

Their shapes on the geological map is

produced by the intersection of two surfaces:

• Geological planar surface of the contact

between two different beds (or formations)

• The irregular topographic surface

Concept of outcrop Concept of outcrop pattern of a pattern of a

geological contactgeological contact

The extent to which the topography influences the shape of the contact line depends on the dip angle of the beds.

Review: Review: Strike = direction of a horizontal line drawn on Strike = direction of a horizontal line drawn on

the surface of the tilted bedthe surface of the tilted bed relative to north relative to north

Dip Direction: Dip Direction: Dip direction is always perpendicular to the Dip direction is always perpendicular to the

strike direction.strike direction. Dip AngleDip Angle:: Angle of the dipping bed with the horizontalAngle of the dipping bed with the horizontal

Structure ContourStructure Contour = = horizontal line on a planehorizontal line on a plane at a known elevation at a known elevation= = Strike lineStrike line of known elevation of known elevation

Dip Direction = perpendicular to the strike in the direction where the

structure contour reduce in elevation

400m

500m

Law of V:When the outcrop of dippingstrata crosses a valleythe outcrop pattern shows a V-shape with its apex pointingtowards the dip direction.

Outrcrop Patterns of dipping / tilted strata

Spacing/distance between strike lines is a measure forthe dip angle.

Structure contours and Exercise 6Structure contours and Exercise 6

Structure contour of the basalt-shale contact at -100 m, 0 m, +100 m, …, +400 m

Structure contour of the sandstone-shale contact at -100 m, 0 m, +100 m, 200 m

100m

200m

0m

0m100m 200m

-100m

300m

General Rules with theGeneral Rules with theoverlay of structure and topographic contoursoverlay of structure and topographic contours

•(1) In areas where topographic contours are at higher elevations than structure contours, then the bedding plane is "buried at depth" below the earths surface.

•(2) In areas where structure contours are at higher elevations than topographic contours, then the bedding plane is above the topography of the earths surface (i.e. it has been eroded away).

•(3) In areas where structure contours and topographic contours intersect at the same elevation, the bedding plane will outcrop at that point on the earth's surface.

A line connecting the intersection points of structure contour and topographic contour line of equal elevation on the map, will define the “outcrop line" of that bedding plane or geological contact on the earth's surface.

Structure contours and Exercise 6Structure contours and Exercise 6

•Draw a topographic profile

•Draw a geological cross- section with the help of given structure contours

•Notate full attitude of beds (strike and dip direction, dip angle)

•Add a legend

Completion of aGeological cross section

with the help of Structure Contours

Upper contact (Red)

Completion of a

Geological cross section

with the help of

Structure Contours

Lower contact (Blue)

LS/MS600

LS/MS500

LS/MS400

LS/MS300

MS/CO500

MS/CO400

Determination of Strike, Dip and Dip Directionwith the help of Construction of Structure Contours

Draw a Cross section

Determination of the Dip angle from Contour Lines

Tan (Dip angle)=

b-c / a-b

b-c = vertical thickness

a-b = distance between the two related strikelines

= outcrop width if the topography is horizontal

True Thickness = Ts =

Vertical Thickness Tv x cos (dip angle)

Vertical Thickness = Tvread from the Structure Contours

True Thickness = TsStratigraphic Thickness