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  S 0 – Oldest surface, S 1 – next oldest, etc.  Fold Sets – F 1, F 2, etc.  Linear Structures – L 1, L 1, etc.  Deformation Structures – D 1, D 2, etc.

Cleavage Surface at Multiple Scales

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 Stylolitic Seam Residues of insoluble materials Anastomosing slaty cleavageRough Cleavage Smooth 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 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

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 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

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