Week 13

Cleavage I. Read pages 424-448 in Chapter 8:Cleavage, Foliation and Lineation.

• readings
• terms
• questions
• classnotes

You are expected to read all the sections listed below. Information from the sections in italics will be discussed in class. You are expected to read the other sections and you may be called on in class to answer questions based on that material.

Nature of Cleavage p.424-428

• General Outcrop Appearance
• Geometric Relationship of Cleavage to Folding
• Geometric Relationship of Cleavage to Shearing
• Domainal Character of Cleaved Rocks

Types of Cleavage p.429-433

• Continuous cleavage
• Disjunctive cleavage

Microscopic Properties of Cleavage p.433-436

• Slaty cleavage
• Schistosity and Phyllitic structure
• Crenulation cleavage
• Spaced cleavage

Strain significance of Cleavage p.436-448

• The issues
• Strain significance of slaty cleavage
• Strain significance of crenulation cleavage
• Strain significance of spaced cleavage

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You should become familiar with the following terms during this weeks lectures and readings:

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You should be able to answer the questions below following this week:

1. How can fold geometry be modified by cleavage development?
2. Why does cleavage "fan" around some folds whereas it remains parallel to the axial plane in others?
3. How can bedding/cleavage relationships be used to determine the character of folding?
4. What is the difference between disjunctive and continuous cleavage?
5. How is slaty cleavage formed?
6. Define the following terms: zonal crenulation cleavage, microlithon, schistosity, refracted cleavage, stylolite, anastomosing spaced cleavage.
7. What type of cleavage is illustrated in the image on the right?

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Cleavage, Foliation and Lineation

Cleavage - closely spaced, systematically oriented, planar surfaces that are discordant to bedding and are typically associated with folds

Cleavage is typically penetrative - pervasive at the scale of observation - at outcrop to microscopic scales

Cleavage and folding

• Cleavage rarely is present without folding
• Cleavage in the hinge area of the fold is parallel to the axial plane (axial plane cleavage)
• Cleavage on the fold limbs may "fan" around the axial plane

• Convergent fans - cleavage converges from the convex toward the concave side of the folded layer, e.g. in competent rocks, sandstone
• Divergent fans - cleavage diverges from the convex toward the concave side of the folded layer, e.g. in less competent rocks, shale, schist

• Cleavage should be more steeply inclined than fold limbs unless the fold has been overturned

• If S_o and S₁ dip in opposite directions, S_o is upright
• If S_o and S₁ dip in the same direction, S_o is upright if S₁ > S_o
• If S_o and S₁ dip in the same direction, S_o is overturned if S_o > S₁

Note: above rules only work for a single episode of folding

• Cleavage may be refracted at boundaries between layers of different lithology

Cleavage represents variations in mineralogy and fabric - grain shape, size and texture. Rocks with cleavage exhibit domainal structure:

• Cleavage domains - thin, subplanar, laminae where the fabric of the host rock has been altered so that minerals show a preferred physical or crystallographic orientation
• Microlithons - undeformed areas of rock between cleavage domains that contain original rock fabric

Types of Cleavage

Cleavage is divided into two types based on scale:

• Continuous cleavage - pervasive at all scales
• Disjunctive cleavage - domainal structure can be identified with the naked eye

Continuous cleavage (slaty cleavage, phyllitic structure, schistosity)

• Increasing grain size - slaty cleavage to phyllite to schist
• Slaty cleavage - fine grained rocks (<0.5 mm), products of low grade metamorphism
• Schistosity - larger grain size (1-10 mm), product of medium to high grade metamorphism
• Phyllitic structure - intermediate between slaty cleavage and schistosity

Disjunctive cleavage

Subdivided into two types: crenulation cleavage and spaced cleavage

Crenulation cleavage

• Small scale folding of earlier cleavage, spacing of domains is 0.1 mm to 1 cm

• Discrete crenulation cleavage - pre-existing cleavage truncates against crenulations (slate)
• Zonal crenulation cleavage - pre-existing cleavage can be followed from mircolithons through the crenulations (schist, phyllite)

Spaced cleavage

• Occurs in unmetamorphosed sedimentary rocks, especially limestones, marls, siltstones
• Cleavage seams are typically filled with clay-rich matter
• Spacing between cleavage planes is 1-10 cm, has been subdivided based on:

• Shape of cleavage domains (anastamosing, stylolitic, smooth)
• Spacing between cleavage surfaces
• Mircolithon fabric

Strain Significance of Cleavage (p.436-448)

Slaty cleavage

• distortion of the rock due to shortening
• slaty cleavage forms perpendicular to maximum shortening direction
• Origin of slaty cleavage - physical rotation of platey micas and clay minerals (overhead)
• But why are micas more densely distributed in cleavage seams? Directional crystallization - new mineral growth in the cleavage plane, e.g. pressure shadows
• Pressure solution and slaty cleavage - not constant volume, volume loss of 50% or more
• Concentration of clay-rich material in cleavage seams is natural result of dissolution of non-clay component of the rock

Crenulation cleavage

• Shortening by microfolding of preexisting cleavage and dissolution (overhead)

Spaced cleavage

• Stylolitic surfaces - perpendicular to maximum principal stress, teeth oriented parallel to maximum stress direction

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