1. Chapter 17 Cleavage and Foliations

2. Cleavage ► Cleavage – A prominent planar structure that may differ in orientation from the bedding and indicate subsequent deformation and metamorphism

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

4. 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

5. 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.

6. Cleavage Surface at Multiple Scales

7. 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

8. Types of Cleavage Stylolitic Seam Residues of insoluble materials Anastomosing slaty cleavageRough Cleavage Smooth Cleavage

9. Discrete Versus Zonal Crenulations

10. 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.

11. 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

12. 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.

13. Stages of Cleavage Development Pencil structure Embryonic Cleavage Stage Cleavage stage Well-developed Cleavage stage with mineral lineations

14. Pencil Structures in Siltstone in TN

15. 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

16. 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

17. Cleavage Formation and the Strain Ellipse