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Chapter 17. Cleavage and Foliations. Cleavage. Cleavage – A prominent planar structure that may differ in orientation from the bedding and indicate subsequent deformation and metamorphism. Cleavage. Continuous Cleavage – Pervades the rock mass
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Chapter 17 Cleavage and Foliations
Cleavage • Cleavage – A prominent planar structure that may differ in orientation from the bedding and indicate subsequent deformation and metamorphism
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 • 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.
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
Types of Cleavage Residues of insoluble materials Stylolitic Seam Anastomosing slaty cleavage Rough Cleavage Smooth Cleavage
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 Fine-Grained Sediment • 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 Embryonic Cleavage Stage Cleavage stage Well-developed Cleavage stage with mineral lineations
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