2. INTRODUCTION
Foliation ( Latin word folium, meaning leaf) is any
fabric-forming planar or curvi-planar structure in a
metamorphic rock, but may also include primary
sedimentary bedding or magmatic layering.
Some geologists reserve the term for planar
structures formed by tectonic strain.
It is now common to include depositional bedding
and other primary planar structures in the
definition of this term.
3. Primary foliations: form during the deposition of
sediments and formation of magmatic rocks
eg: bedding in sedimentary rocks, flow banding in
lavas and magmatic layering in intrusive rocks.
Secondary foliations: products of stress and strain
and most are tectonic foliations. they form in
response to tectonic stress.
Eg: axial plane cleavages in metamorphic rocks
4. Foliations have a structure characterised by
alternating domains.
Domains show marked difference in preferred
orientation of mineral grains, in structure, or in
composition.
5. SPACED FOLIATION
defined by domains having a spacing of 10
micrometer or more.
Categorised into;
1. Compositional foliation
2. Disjunctive foliation
3. Crenulation foliation
6. COMPOSITIONAL FOLIATION
Marked by layers or laminae
Planar alignment of platy or needle-shaped crystals
may be also present.
Rock has weak tendency to cleave parallel to the
foliation.
subdivisions;
1. Diffuse foliation
2. Banded foliation
8. DIFFUSE FOLIATION
Defined by widely spaced weak concentrations of
mineral in a rock of predominantly one lithology.
Common in ultramafic rocks.
Eg: dunites in which sparse layer concentration of
pyroxene crystals.
Common in deformed granites in which concentrtions
of mafic minerals define the foliation.
9. BANDED FOLIATION
Defined by relatively closely spaced compositional
layers that are mineralogically distinct and are of
comparable adundance.
Common in high grade metamorphic rocks.
Fig: banded
foliation in
gneiss.s
10. DISJUNCTIVE FOLIATION
Contain thin cleavage domains or seams marked by
concentrations of oxides and strongly aligned platy
minerals.
Cleavage domains are seperated by tabular or
lenticular domains called microlithons in which platy
minerals are less abundant or more randomly
oriented.
Commonly form in previously unfoliated rocks such as
limestone or mudstone
Also develop in some foliated rocks cross cutting an
earlier foliation.
Subdivided intofour;
11. 1.STYLOLITIC FOLIATION
Consist of long continuous,but very irregular cleavage
domains.
A distinct tooth like geometry in cross section.
Individual domains are called stylolites.
Common in limestones.
Spacing of the cleavage domains is commmonly
from1-5cm or more.
12. 2.ANASTOMOSING FOLIATION
Distinguished by long , continuous , wavy cleavage
domains.
Domains form an irregular network outlining lenticular
microlithons.
Common in phyllites and schists.
The spacing of the clevage domain tend to be smaller than
for stylolitic foliation,averaging perhaps 0.5-1cm.
The clevage domain contain concentrations of platy
minerals with strong preferred orientation parallel to
domain boundaries.
13. 3.ROUGH FOLIATIONS
Typically develops in rocks containing abundant sand-
sized material.
The cleavage domains are short, discontinuous
concentrations of highly oriented platy minerals that
envelop the coarse grains.
The spacing of the cleavage domains are generally
less than a millimetre.
Within the microlithons, preferred orientation of
mineral grains may vary widely from random to
strongly oriented.
14. 4.SMOOTH FOLIATIONS
Represents the end member of the spectrum from
irregular to planar cleavage domains.
Characteristic of some slates.
Cleavage domains are long, continuous and smooth
with concentrations of highly oriented platy minerals.
Cleavage domain spacing is generally less than a
millimeter.
Fabric develop in microlithons commonly range from
random to completely oriented.
15.
16. CRENULATION FOLIATION
Formed by harmonic wrinkles or chevron folds that
develop in pre-existingfoliation.
The new foliation cut across the old foliation.
Defined by;
Both limbs of symmetric crenulations
Long limbs of asymmetric crenulations.
Microlithon width is comparable with the half-
wavelength or the wavelength of crenulations.
Subdivided into;
18. 1.ZONAL CRENULATION
FOLIATION
Platy minerals in the new cleavage domains are
oriented at a small angle to the domain.
Form a continuous variation of orientations from platy
minerals in microlithons.
The microlithon boundaries are gradational.
Compositional difference occur between the clevage
domains and microlithons.
Proportion of platy minerals are relatively high in the
clevage domains and low in microlithons.
20. 2.DISCRETE CRENULATION
FOLIATION
Orientation of platy minerals in the new clevage
domains is parallel to the domains.
Sharply discordant with the orientation of platy
minerals in the microlithons.
Crenulations are preserved in the microlithons.
Cleavage domains are generally narrow and may do
not neccessarily , correspond to the limbs of
crenulation and microlithons.
Difference in mineralogy between the two domains
are similar to those of zonal crenulation foliations.
21. Fig:Discrete cleavage in phyllite, axial planar to mesoscopic folds. Joma
area, Central Norwegian Caledonides
22. CONTINUOUS FOLIATION
Defined either by domains with spacing less than
10micrometer.
Also by nondomainal struture.
Divisible by grain size into;
1.Fine continuous
2.Coarse continuous.
23. 1.FINE CONTINUOUS
FOLIATIONS
Microdomainal or micro continuous
Micro dominal:
fine foliations, may be micro crenulation or micro
disjunctive, microdomainal spacing is less than 10
micrometer.
Micro continuous: fine foliation characterised by
parallel allignment of all platy or inequant grains, lack
any domainal struture.
These two are impractical to use as field
classificationterms.
In fine grained rock, only electron microscope can
reveal these sstrutures.
24. 2.COARSE CONTINUOUS
FOLIATIONS
Characterised by complete orientation of
homogeneously distributed platy minerals
Or by alignment of flattened mineral grains.
No domainal structure.
Easily revealed by coarse grain size.
Defined by preferred orientation of deformed objects
distributed within the rock.
25. RELATIONSHIP WITH FOLDS
AXIALSURFACEFOLIATIONS: parallel or subparallel
to axial surface off olds.
Fans across the fold.
Foliation fans are convergent or divergent.
Orientation of foliation changes significantly at the
lithologic contact –refracted foliation.
Help to determine the geometry of folding.
28. RELATIONSHIP WITH DUCTILE
SHEARZONE
Ductile shearzone contains;
1.S-foliation
Continuous ,coarse foliation.
Defined by preferred orientation of mica grains and
elongate quartz grains
Orientation is oblique to dutile shearzone.
29. 2.C-foliation
Defined by very thin seam of very fine-grained mica
connected to the ends of the micafish.
31. SPECIAL TYPES
1.Slaty cleavage:
Fine ,continuous foliations.
Characteristic of slate.
Continuous or microspaced.
Microdomain spacing is not recognisable in the field.
Foliation provides very strong clevage.
32. 2.PHYLLITIC CLEVAGE
Resembles slaty cleavage.
Grain size is slightly coarser.
Characteristic of phyllite.
In handspecimen foliation surface has a sheen to it.
Intermediate between fine and coarse continuous
foliation.
Some may be smmoth disjunctive.
36. 4.FLOW CLEAVAGE
Term applied to continuous axial surface foliation.
Resul to a large amount of ductile deformation in
rocks.
Represented the orientation of ductile
deformation.
37. 5.FRACTURE CLEAVAGE
A variety of disjunctive foliations or discrete
crenulation foliation.
Term has applied to disjunctive foliations in which
microlithon has no fabric.
38.
39. 6.Shear/solution/strainslip cleavage
Used to describe a variety of spaced foliations.
Refers to disjunctive foliations.
None of these term is well defined and none
should be used descriptively.
40. SUMMARY
Foliations are common in metamorphic rocks and
are often used in the definition of the different
types of metamorphic rocks.
Without foliations it would be difficult to do proper
structural analysis
Most foliations form perpendicular to or at a high
angle to the shortening direction.
give us important strain information where regular
strain markers are absent.
Different types of foliations reflect variations in
lithology and temperature or depth of burial
during deformation.
