Cleavage and FoliationsCleavage and Foliations

CleavageCleavage

►CleavageCleavage – A prominent planar structure – A prominent planar structure that may differ in orientation from the that may differ in orientation from the bedding and indicate subsequent bedding and indicate subsequent deformation and metamorphismdeformation and metamorphism

CleavageCleavage

►Continuous CleavageContinuous Cleavage – Pervades the rock – Pervades the rock massmass

►Spaced CleavageSpaced Cleavage – Domains of uncleaved – Domains of uncleaved rocks separated by cleavage planesrocks separated by cleavage planes

►MicrolithsMicroliths – Uncleaved zones between – Uncleaved zones between cleavage surfacescleavage surfaces

Continuous CleavageContinuous Cleavage

►Gradation of continuous cleavage intensityGradation of continuous cleavage intensity Slaty CleavageSlaty Cleavage – In fine grained rocks and – In fine grained rocks and

associated with low grade metamorphismassociated with low grade metamorphism Phyllitic StructurePhyllitic Structure - Phyllitic texture with satiny - Phyllitic texture with satiny

lusterluster SchistositySchistosity – Forms in medium grained rocks – Forms in medium grained rocks

under medium to high grades of metamorphismunder medium to high grades of metamorphism

S-SurfacesS-Surfaces

►S-SurfaceS-Surface – All planar and some curved – All planar and some curved structures in deformed rocksstructures in deformed rocks i.e.i.e. Cleavage, foliations, and bedding Cleavage, foliations, and bedding SS00 – Oldest surface, S – Oldest surface, S11 – next oldest, etc. – next oldest, etc.

Fold SetsFold Sets – F – F11, F, F22, etc., etc.

Linear StructuresLinear Structures – L – L11, L, L11, etc., etc.

Deformation StructuresDeformation Structures – D – D11, D, D22, etc., etc.

Cleavage Surface at Multiple Cleavage Surface at Multiple ScalesScales

Spaced CleavageSpaced Cleavage

► DisjunctiveDisjunctive – Crosscutting and not related to – Crosscutting and not related to original layeringoriginal layering Stylotic Stylotic – Columnar pattern– Columnar pattern Anastomosing Anastomosing – Interconnected paths– Interconnected paths RoughRough – Irregular cleavage – Irregular cleavage Smooth Smooth – Regular cleavage pattern– Regular cleavage pattern

► Crenulation CleavageCrenulation Cleavage – Small-scale crinkling of – Small-scale crinkling of existing layers.existing layers. Discrete Discrete –– Broken distinct cleavage areasBroken distinct cleavage areas Zonal Zonal – More ductile continuous cleavage areas– More ductile continuous cleavage areas

Types of CleavageTypes of Cleavage

Stylolitic SeamResidues of insoluble materialsResidues of insoluble materials

Anastomosing slaty cleavage Rough Cleavage

Smooth Cleavage

Discrete Versus Zonal CrenulationsDiscrete Versus Zonal Crenulations

Discrete CrenulationsDiscrete Crenulations

Scale 1mm; Discrete crenulated truncating an earlier crenulated fabric. Ordovician rock New South Wales; Seyfert 1987

Scale 125 micrometer; Discrete crenulations in slate fabric. Thin films of muscovite. Seyfert 1987.

Progressive Cleavage Development in Fine-Progressive Cleavage Development in Fine-Grained SedimentGrained Sediment

1.1. Undeformed conditionUndeformed condition2.2. Earliest deformation stageEarliest deformation stage – Volume loss from – Volume loss from

reorientation and expulsion of waterreorientation and expulsion of water3.3. Pencil structurePencil structure – Formed from intersection of bedding – Formed from intersection of bedding

and cleavageand cleavage4.4. Embryonic cleavage stageEmbryonic cleavage stage – Parallel to fold axis resulting – Parallel to fold axis resulting

from pressure solution and recrystalizationfrom pressure solution and recrystalization5.5. Cleavage stageCleavage stage – Strong planar fabric from reorientation – Strong planar fabric from reorientation

and recrystallization of claysand recrystallization of clays6.6. Strong cleavage with mineral lineationStrong cleavage with mineral lineation – More of above – More of above

with mineral-elongation parallel to the X direction of strainwith mineral-elongation parallel to the X direction of strain

Zonal CrenulationsZonal Crenulations

Scale 500 micrometers; Zonal crenulations in schist fabric from South Australia. Dissolution of quartz and albite in microlithons and increase in biotite. Seyfert 1987.

Scale 2mm; Zonal crenulations of carbonaceous and micaceous domains with dissolution of quartz in the initial fabric from New South Wales; Seyfert 1987.

Stages of Cleavage DevelopmentStages of Cleavage Development

Pencil structure Pencil structure Embryonic Cleavage Stage Cleavage stage Well- Embryonic Cleavage Stage Cleavage stage Well-developed Cleavage developed Cleavage stage with mineral stage with mineral lineationslineations

Cleavage FormationCleavage Formation

►Cleavage is related to folding and it Cleavage is related to folding and it frequently parallels the axial planes of folds.frequently parallels the axial planes of folds.

►The maximum principal shortening direction The maximum principal shortening direction is perpendicular to cleavage planes.is perpendicular to cleavage planes.

►The XY plane in the strain ellipse parallels The XY plane in the strain ellipse parallels cleavage planes.cleavage planes.

►As much as 75% shortening has been As much as 75% shortening has been observedobserved

Cleavage in SlateCleavage in SlateMudstone devoid of cleavage from Wales; Clay flakes mainly chlorite with random arrangement

Ill-cleaved mudstone from Wales; Planar structure developing

Highly cleaved slate from Wales; Closely spaced layers with thin lenticels of randomly arranged flakes

Borradaile et al. 1982; 20 micrometer scale SEM

• Foliation:Foliation: and planar fabric element and planar fabric element• Lineation:Lineation: any linear fabric elements any linear fabric elements

They have no genetic connotationsThey have no genetic connotations Some high-strain rocks may be foliated, but Some high-strain rocks may be foliated, but

they are treated separatelythey are treated separately

FoliatedFoliated Metamorphic Rocks Metamorphic RocksSchistositySchistosity

• A preferred orientation of inequaint mineral A preferred orientation of inequaint mineral grains or grain aggregates produced by grains or grain aggregates produced by metamorphic processesmetamorphic processes

• Aligned minerals are coarse grained enough to Aligned minerals are coarse grained enough to see with the unaided eyesee with the unaided eye

• The orientation is generally planar, but linear The orientation is generally planar, but linear orientations are not excluded orientations are not excluded

FoliatedFoliated Metamorphic Rocks Metamorphic RocksGneissose structureGneissose structure

• Either a poorly-developed schistosity or Either a poorly-developed schistosity or segregated into layers by metamorphic segregated into layers by metamorphic processesprocesses

• Gneissose rocks are generally coarse grained Gneissose rocks are generally coarse grained

Figure 22.1. Examples of foliated metamorphic rocks. a. Slate. b. Phyllite. Note the difference in reflectance on the foliation surfaces between a and b: phyllite is characterized by a satiny sheen. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

a: a: SlateSlate

b: b: PhyllitePhyllite

Slate: compact, very fine-grained, metamorphic rock with a well-developed cleavage. Freshly cleaved surfaces are dull

Phyllite: a rock with a schistosity in which very fine phyllosilicates (sericite/phengite and/or chlorite), although rarely coarse enough to see unaided, impart a silky sheen to the foliation surface. Phyllites with both a foliation and lineation are very common.

FoliatedFoliated Metamorphic Rocks Metamorphic Rocks

Figure 22.1c. Garnet muscovite schist. Muscovite crystals are visible and silvery, garnets occur as large dark porphyroblasts. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Schist: a metamorphic rock exhibiting a schistosity. By this definition schist is a broad term, and slates and phyllites are also types of schists. In common usage, schists are restricted to those metamorphic rocks in which the foliated minerals are coarse enough to see easily in hand specimen.

FoliatedFoliated Metamorphic Metamorphic RocksRocks

The EndThe End