FOLIATION AND LINEATION.pptx

Fabric
 The fabric of a rock is the geometric arrangement of component features in
the rock.
 If there is no preferred orientation (i.e., alignment) of the fabric elements,
then we say that the rock has a random fabric. Undeformed sandstone,
granite, or basalt are rocks with random fabrics.
 Deformed rocks typically have a non-random fabric or a preferred fabric, in
which the fabric elements are aligned in some manner and/or are repeated
at an approximately regular spacing.

 There are two main classes of preferred fabrics in rock.
 A planar fabric, or foliation, is one in which the fabric element is a planar
or tabular feature.
 A linear fabric, or lineation, is one in which the fabric element is
effectively a linear feature.
 Foliations and lineations can be subdivided into primary (pre-
deformational) ones, such as bedding, and secondary (deformational)
ones.
 If the fabric elements are closer than 1 mm (that is, below the resolution of
the eye), the fabric is continuous. When there is an obvious spacing between
fabric elements, we say that the fabric is spaced.

 Fabrics that form as a consequence of tectonic deformation of rock are
called tectonic fabrics, and fabrics that form during the formation of the
rock are called primary fabrics.
 Rocks with a penetrative tectonic fabric are also called tectonites.
 When linear fabric elements dominate, the rock is called an L-tectonite,
 A rock with dominantly planar fabrics is called an S-tectonite.
 The rocks with both types of fabric elements are called LS-tectonites.

Foliation
 A foliation is any type of planar fabric in a rock, property of rocks whereby
break along approximately parallel surfaces.
 Sedimentary rocks shows a property called bedding fissility in which rocks
split parallel to stratification or along bedding planes.
 Foliation plane is usually reffered as S plane.
 The term has no genetic implications and may include sedimentary bedding
or igneous layering, etc.

 Metamorphic foliations are divided into cleavages, schistosity and
gneissose structure.
1. Cleavage or rock cleavage – it is the property of rocks whereby they break
along approximately parallel surfaces of secondary origin. Cleavages are
foliations of secondary origin and usually parallel to bedding or inclined
to bedding. It is continuous cleavage or spaced cleavage. Slaty cleavage –
it is generally shown by slate due to parallel arrangement of platy minerals
lik mica, chlorite, etc.
.
Slaty cleavage

2. Schistosity – secondary foliation developed by recrystallisation, in which
rocks tend to split along parallel planes and is the general property of
schist. Common in high grade metamorphic rocks.
3. Gneissosity / gneissic banding – in case of gneissosity, alternate silicic and
mafic layers are common. Mafic is shown by ferromagnesian minerals and
felsic by light coloured minerals.

 Other types of foliation includes;-
 Fracture cleavage – these are closely spaced joints. The distance between
fractures exceed a few cms. In fracture cleavage, an individual block are
bounded by fractures called microlithon.
 Shear cleavage – it is synonym with slip cleavage. Fracture cleavage with
displacement. In the other hand, shear cleavages are closely spaced
fractures along with those has been some displacement.
 Bedding cleavage – in some bedding plane, cleavage is parallel to the
bedding, it is called bedding cleavage/bedding schistosity.
 Axial plane cleavage– those foliations that are parallel to the axial plane of
folds.

Lineations
 It is the result of the parallelism of some directional property in the rock,
any linear feature that appears on the surface of the rock.

Primary lineation
 It is found in both sedimentary and igneous rock and are formed along with
rock.
1. Linear flow structure – these are formed in igneous rocks as a result of
flowage of magma, longest axis of mineral align parallel to one another
and will be parallel to the flow direction.
2. Clots / aggregation – individual minerals when formed together clots may
develop, clots may allign parallel to themselves during the flowage of
magma.
3. Mineral lineation- linear parallelism of equigranular minerals develop
mineral lineation.

Secondary lineation
 These are superimposed on rock after they are originally deposited,
erupted or intruded.
 May be imposed more than once, so several lineation may be present in a
rock.
 May occur with or without foliation, but more commonly, associated with
foliation.
 A rock without cleavage or schistosity possess lineation.

1. Elongated / stretched pebbles - They usually result from the elongation of
minerals or mineral aggregates (stretching lineations). Stretched pebbles
in deformed conglomerates is a common example.
2. Intersection lineation - Intersection of bedding and cleavage produces a
lineation. This is because the intersection of two planes is a line and if the
rocks break parallel to the cleavage, the trace of bedding appears as
parallel; streaks on cleavage.

3. Slickenslides – polished striated surfaces that result from friction along the
fault plane. Scratches clearly indicate direction of movement.

4. Boudinage or sausage structure – In cross section, a competent bed thickens
and thins to form a string of sausages. It is parallel to bedding, each unit
look like sausages lying side by side. Line of junction of individual units
may be called boudin line and this is a lineation. Boudin line may be
occupied by quartz, feldspar or other mineral. Boudinage a result of
stretching at right angles to boudin line.

5. Pencil lineation – it is a very distinctive linear structure associated with
folded and cleaved mudstone and silt stones. It have a prefered orientation
and forms a lineation in unmetamorphosed and very low grade
metamorphic rocks.
6. Mullions - are linear deformation structure that are restricted to the
interface between a competent and incompetent rock.

FOLIATION AND LINEATION.pptx

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