Cleavage and Foliations

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Chapter 17

• Cleavage and Foliations

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Cleavage

• Cleavage A prominent planar structure that may
  differ in orientation from the bedding and
  indicate subsequent deformation and metamorphism

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Cleavage

• Continuous Cleavage Pervades the rock mass
• Spaced Cleavage Domains of uncleaved rocks
  separated by cleavage planes
• Microliths Uncleaved zones between cleavage
  surfaces

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Continuous Cleavage

• Gradation of continuous cleavage intensity
• Slaty Cleavage In fine grained rocks and
  associated with low grade metamorphism
• Phyllitic Structure - Phyllitic texture with
  satiny luster
• Schistosity Forms in medium grained rocks under
  medium to high grades of metamorphism

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S-Surfaces

• S-Surface All planar and some curved structures
  in deformed rocks
• i.e. Cleavage, foliations, and bedding
• S0 Oldest surface, S1 next oldest, etc.
• Fold Sets F1, F2, etc.
• Linear Structures L1, L1, etc.
• Deformation Structures D1, D2, etc.

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Cleavage Surface at Multiple Scales
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Spaced Cleavage

• Disjunctive Crosscutting and not related to
  original layering
• Stylotic Columnar pattern
• Anastomosing Interconnected paths
• Rough Irregular cleavage
• Smooth Regular cleavage pattern
• Crenulation Cleavage Small-scale crinkling of
  existing layers.
• Discrete Broken distinct cleavage areas
• Zonal More ductile continuous cleavage areas

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Types of Cleavage
Stylolitic Seam

• Residues of insoluble materials

Anastomosing slaty cleavage
Rough Cleavage
Smooth Cleavage
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Discrete Versus Zonal Crenulations
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Discrete 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.
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Progressive Cleavage Development in Fine-Grained
Sediment

1. Undeformed condition
2. Earliest deformation stage Volume loss from
   reorientation and expulsion of water
3. Pencil structure Formed from intersection of
   bedding and cleavage
4. Embryonic cleavage stage Parallel to fold axis
   resulting from pressure solution and
   recrystalization
5. Cleavage stage Strong planar fabric from
   reorientation and recrystallization of clays
6. Strong cleavage with mineral lineation More of
   above with mineral-elongation parallel to the X
   direction of strain

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Zonal 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.
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Stages of Cleavage Development

• Pencil structure Embryonic Cleavage Stage
  Cleavage stage
  Well-developed Cleavage
  stage with mineral lineations

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Pencil Structures in Siltstone in TN
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Cleavage Formation

• Cleavage is related to folding and it frequently
  parallels the axial planes of folds.
• The maximum principal shortening direction is
  perpendicular to cleavage planes.
• The XY plane in the strain ellipse parallels
  cleavage planes.
• As much as 75 shortening has been observed

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Cleavage in Slate
Mudstone 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
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Cleavage Formation and the Strain Ellipse
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