Cleavage (DOC)

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 ―Fundamental internal grain‖
 Presence of cleavage permits    a rock to be split into thin
 plates and slabs

                 Relationship to Folding
 Cleavage   planes tend to be parallel to the axial plane of
 the fold
   Sometimes cleavage planes fan symmetrically about the axial
    plane of a fold
   In either case, we refer to it as axial plane cleavage

                   Axial Plane Cleavage
                     Using Cleavage
 Bedding-cleavagerelationships tell us where we are on a
 fold and which way the rock units face
                   Using Cleavage
                   Using Cleavage
                   Using Cleavage
                   Using Cleavage
            Bedding-Cleavage Relationships
   any one point, the bedding cleavage relationship does
 At
 not accurately reflect the axial plane of the fold
   Cleavage tends to fan, trying to remain nearly perpendicular to
   Cleavage refracts, staying more nearly perpendicular to bedding
    in sandy layers and more nearly parallel to the axial plane in
    shaly layers
                   Cleavage Refraction
                   Axial Plane Cleavage
                              Cleavage Domains
 Microscopic         variations in mineralogy and structure
       Cleavage domains
          Anastomosing to subparallel, mica-rich laminae
          Fabric of the host rock has been rearranged
          Strong preferred dimensional and crystallographic orientation

       Microlithon domains or microlithons
          Narrow zones in which the original mineralogy and texture of the rock
           are preserved
          Little preferred orientation of grains

               Microscopic Properties of Cleavage
 Slaty      cleavage
     Overgrowths
     Beards
     QF-domains
     M-domains

                         Domain Structure
                    Photomicrograph of Domains
                         Types of Cleavage
   Continuous—Present throughout the rock
       Slaty cleavage
       Phyllitic structure
       Schistosity
   Disjunctive
       Crenulation cleavage
       Spaced cleavage
   Slaty cleavage
       Typically associated with slates (fine-grained, pelitic rocks, subjected to
        low-grade metamorphism
       This is what makes roofing slate and pool table tops
                               Slaty Cleavage
                              Types of Cleavage
       Schistosity
          Medium grained rocks containing visible flakes of mica
          Schists seldom split cleanly
          Developed in metasedimentary rocks and certain volcanic rocks
           metamorphosed to medium-high grade
     Phyllitic Structure
        Intermediate between slaty cleavage and schistosity
        Rocks have a satiny luster

                       Domainal Fabric in Schist
                         Types of Cleavage
 Disjunctive
     Crenulation cleavage
        A pre-existing continuous cleavage is crenulated into microfolds
        Discrete
            – Very narrow cleavage domains sharply truncate the continuous cleavage of the
            – Typically forms in slates
          Zonal
            – Wider cleavage domains that correspond to the tight, appressed limbs of
              microfolds in the preexisting continuous cleavage preserved within the microlithons
            – Typically forms in phyllites and schists

                   Discrete Crenulation Cleavage
                   Discrete Crenulation Cleavage
                    Zonal Crenulation Cleavage
                   Zonal Crenulation Photomicrograph
                              Types of Cleavage
     Spaced cleavage
        Parallel to anastomosing, stylolitic to smooth, fracture-like partings that
         are often occupied by clayey and carbonaceous matter
        Billings used to say, ―closely spaced joints.‖
        Offsets of bedding due to pressure-solution

                    Overgrowth Beards
              Spaced Cleavage Photomicrograph
               SEM Photo of Spaced Cleavage