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4.2 · Foliations 83

Box 4.3 Stylolites
Pressure solution is common in low-grade deformation and is usu-
ally active throughout a rock volume on the grain scale, leading to
development of foliations and grain-scale dissolution and deposi-
tion features. Pressure solution can locally be enhanced, for exam-
ple in strain caps aside a rigid object. Localised pressure solution is
often concentrated along surfaces that may originate as joints or
fractures, particularly in limestone (Stockdale 1922; Dunne and Han-
cock 1994; Petit and Matthauer 1995; Renard et al. 2004), but also in
other macroscopically homogenous, fine grained rocks (Dewers and
Ortoleva 1990; Railsback and Andrews 1995; Railsback 1998; Karcz
and Scholz 2003; Gratier et al. 2005). Such surfaces are normally
highly indented and consist in three dimensions of interlocking teeth
of wall rock. These surfaces are therefore known as stylolites (from
Latin stylus, a stake or pen). Stylolites can be subdivided into bed-
ding parallel and transverse stylolites. Teeth in the stylolite surface
commonly have secondary phases such as mica grains along the
crowns, while teeth walls are commonly parallel so that the stylolite
might be pulled apart without breaking the teeth. The indented
shape of stylolites forms by preferred pressure solution along one
side of the surface, usually due to a concentration of non-soluble
phases on the opposite side (Fig. B.4.3a). A difference between
stylolites and amoeboid grain boundaries (Box 3.9) formed by grain
boundary migration is the concentration of material on the crowns
of the teeth, and protrusions with inward sloping walls (Fig. B.4.3b).
Stylolites are generally enriched in insoluble material such as opaques
and mica with respect to the wall rock. Where the wall rock is locally
rich in insoluble material, the stylolite is also locally enriched in
such material (Borradaile et al. 1982). Stylolites are most common
in carbonate rocks with a certain clay content, but can also occur in Fig. B.4.3. a Development of a stylolite in rocks with insoluble inclu-
some sandstones. Bedding parallel stylolites are commonly con- sions; material behind inclusions may be protected from solution
sidered to be diagenetic (Andrews and Railsback 1997). Transverse and form interlocking teeth. b Stylolites formed by pressure solu-
stylolites occur in rocks with insoluble residue material of 2–20 wt%. tion differ from amoeboid grain boundaries formed by grain
In many texts, the word stylolite is used for pressure solution boundary migration in that they have teeth with parallel sides that
generated surfaces with teeth normal to the stylolite surface, while allow the two halves to be “pulled apart”. c Explanation of the terms
slickolites have teeth oblique to the surface (Fig. B.4.3c). A classi- stylolite, slickolite and slickenside. In a stylolite, teeth and inferred
fication of stylolites was presented by Guzetta (1984) and Andrews shortening direction are normal to the plane, in slickolites oblique
and Railsback (1997). and in slickensides parallel
Fig. B.4.4.
Stylolite in limestone. A late
calcite-filled vein transects
part of the stylolite. Width of
view 2 mm. PPL. (Courtesy
Daniel Köhn)