Transcript Cleavage and Foliation

Chapter 17
Cleavage and Foliations
Cleavage
– A prominent planar structure
that may differ in orientation from the
bedding and indicate subsequent
deformation and metamorphism
► Cleavage
Cleavage
► Continuous
Cleavage – Pervades the rock
mass
► Spaced Cleavage – Domains of uncleaved
rocks separated by cleavage planes
► Microliths – Uncleaved zones between
cleavage surfaces
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
S-Surfaces
– All planar and some curved
structures in deformed rocks
► S-Surface
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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.
Cleavage Surface at Multiple
Scales
Spaced Cleavage
– Crosscutting and not related to
original layering
► Disjunctive
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Stylotic – Columnar pattern
Anastomosing – Interconnected paths
Rough – Irregular cleavage
Smooth – Regular cleavage pattern
Cleavage – Small-scale crinkling of
existing layers.
► Crenulation
 Discrete – Broken distinct cleavage areas
 Zonal – More ductile continuous cleavage areas
Types of Cleavage
Residues of insoluble materials
Anastomosing slaty cleavage
Smooth Cleavage
Stylolitic Seam
Rough Cleavage
Discrete Versus Zonal Crenulations
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.
Progressive Cleavage Development in FineGrained Sediment
1.
2.
3.
4.
5.
6.
Undeformed condition
Earliest deformation stage – Volume loss from
reorientation and expulsion of water
Pencil structure – Formed from intersection of bedding
and cleavage
Embryonic cleavage stage – Parallel to fold axis resulting
from pressure solution and recrystalization
Cleavage stage – Strong planar fabric from reorientation
and recrystallization of clays
Strong cleavage with mineral lineation – More of above
with mineral-elongation parallel to the X direction of
strain
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.
Stages of Cleavage Development
Pencil structure
Cleavage
Embryonic Cleavage Stage
Cleavage stage
Well-developed
stage with mineral lineations
Pencil Structures in Siltstone in
TN
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
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
Cleavage Formation and the Strain Ellipse