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INTRODUCTION Geology, one of the core science

disciplines with physics, chemistry, and biology, is the search for knowledge about the Earth, how it formed, evolved, and how it works. Geology is often presented in the broader context of Geosciences; a grouping of disciplines specifically looking for knowledge about the interaction between Earth

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Types of GEOLOGICAL STRUCTURES Foliation Folds Faults Joints Unconformities

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geologic structures up until now, we have focused our attention mostly on!

flat-lying rocks, i.e. sedimentary or volcanic layers!not all

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Geological structures. dynamically produced patterns or

arrangements of rock or sediment that result from forces acting within the Earth

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Strike Direction of a line formed by the

intersection of a foliation plane & a horizontal plane.

It is a horizontal line on a surface of rock beds.

If the beds are not horizontal, lines joining the points with same elevation (on one foliation plane) are called strike lines.

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Measuring the strike

Strike is usually reported as a measurement of compass direction. Due north is considered to be 000°, due east is 090°, due south is 180 °, due west 270°, northwest 045° etc.

The compass was held parallel to the strike line & the orientation read relative to magnetic north.

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By definition, a strike line trends in 2 directions exactly 180° apart. It is needed to specify only one direction. By convention, the smaller of the 2 numbers is considered.

Line 1 strikes at 45°, line 2 strikes at 55°, and line 3 strikes at 60°.

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N 300 E N 550 W

900

00

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Dip

The amount and direction of inclination is also used to describe orientation of bedding.

The dip is defined as the amount of inclination measured from the horizontal plane downwards.

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Measurement of dip The dip is reported as degrees

measured downward from a horizontal plane.

By convention, the dip of an inclined bed cannot exceed 90°.

Beds with dips of 90° are said to be vertical. Beds that have been overturned have technically been rotated more than 90°, but their dips are still reported as less than 90°.

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CIVIL ENGINEERING The long side of the symbol records the strike direction (N45°E) and the smaller tick shows the direction of dip (SE).

The number gives the amount of dip (30°)

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Determination of strike A strike line joins points of equal elevation on a

particular bedding plane. The strike line is always at right angles to the dip

direction. In determining the strike line, two points of equal

elevation of a particular boundary of a bed on a geological map are located.

The line formed by joining these two points will give a strike line. Its altitude will be the same as that of the corresponding contour.

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(A) Determination of strike & dip directionon a map(B) Determination of dip by graphical method

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Determination of dip direction

The dip will be at right angles to the strike direction.

In order to find the dip direction, a line xy is drawn at right angles to the strike lines.

Dip direction is from the strike of higher elevation to that of lower elevation. It is indicated by the arrow on the xy line.

Amount of dip

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What are Folds? Warps in rock layers and occur bended

upwards, downwards, or sideways. Compression forces are the main causes of

folds. Folds are important causes of mountain

formation.

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Fold Terminology

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crest

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Folds Can be defined as the curved or zig-zag

structure shown by rock beds. They consist of arches & troughs

alternatively & generally occur in groups. Two main types.

Anticline Syncline

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Anticline

An up-fold where the limbs dip away from the axis of fold on either side.

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Syncline

A down-fold where the limbs dip towards the axis of fold on either side.

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Geometry of a fold Fold axis – Line joining planes of sharpest

folding. Axial plane – Includes axis and divides the

fold symmetrically as possible Plunging fold – If the fold axis is not

horizontal, fold is called plunging. Plunge – Angle between horizontal plane &

axis Limbs – Sloping sides of a fold

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Plunging folds

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RPK

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Classification of folds

Symmetrical fold – Axial plane is vertical, limbs dip in opposite direction in the same angle

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Asymmetrical fold - Axial plane is inclined, Limbs dip in opposite direction at different angles

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Overturned fold – An asymmetrical fold whose one limb is turned past the vertical. Axial plane is inclined & both limbs dip in the same direction.

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Recumbent fold - The folding is so intense both limbs become almost horizontal. The Axial plane also becomes nearly horizontal.

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Recumbent folds

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Overturned folds

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Faultfault, in geology, a planar or gently curved fracture in the rocks of the Earth’s crust, where compressional or tensional forces cause relative displacement of the rocks on the opposite sides of the fractureFaults may be vertical, horizontal, or inclined at any angle

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• When rocks slip past each other in faulting, the upper or overlying block along the fault plane is called the hanging wall, or headwall;

• the block below is called the footwall. • The fault strike is the direction of the line

of intersection between the fault plane and the surface of the Earth.

• The dip of a fault plane is its angle of inclination measured from the horizontal.

• Faults are classified according to their angle of dip and their relative displacementz

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There are three kind of fault call as,

• Normal dip-slip faults• Reverse dip-slip faults• Strike-slip faults

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Normal faultNormal dip-slip faults are produced by vertical compression as the Earth’s crust lengthens. The hanging wall slides down relative to the footwall. Normal faults are common; they bound many of the mountain ranges of the world and many of the rift valleys found along spreading margins of tectonic plates

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Reverse dip-slip faults Reverse dip-slip faults result from horizontal compressional forces caused by a shortening, or contraction, of the Earth’s crust. The hanging wall moves up and over the footwall.

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Strike-slip faultStrike-slip (also called transcurrent, wrench, or lateral) faults are similarly caused by horizontal compression, but they release their energy by rock displacement in a horizontal direction almost parallel to the compressional force

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Joints

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TYPES OF JOINT Tension Joints – Due to tensional forces.

Relatively open & have rough & irregular surfaces.

Shear Joints – Due to compression forces involved in folding & faulting of rocks. Rather clean cut & tightly closed.

Strike Joints – Run parallel to the strike of the country rocks.

Dip Joints – Run parallel to the direction of dip of the country rocks.

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JOINTS… Advantages

Good source of ground water Disadvantages

In rock quarries blasting become difficult & rock quality is poor Outer & Inner breaks in tunnels More liable to landslides, rockslides & mudflows Foundation problems Excavation need more supports

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CIVIL ENGINEERINGUNCONFORMITIES

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Unconformities An unconfAormity is an old

erosion surface which separates younger series of rocks from the older series. Angular Unconformities Disconformities Nonconformities

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Development of an Unconformity

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Angular Unconformity

An unconformity in which the upper and lower layers are not parallel

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DisconformitiesAn unconformity in which the upper and lower

layers are horizontal

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Disconformities

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Nonconforty

When bedded sedimentary rocks overlie the non-bedded igneous mass, the structure is called “Nonconforty”.

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Angular unconformity on San Miguel Island, May, 2004

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Nonconformity in Canada Halcon, February, 2007

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Tilted nonconformity, Canada Halcon, February, 2007