Page 84 - Microtectonics

72 4 · Foliations, Lineations and Lattice Preferred Orientation

Fig. 4.6. Bedding-parallel diagenetic foliation defined by elongate detrital micas. Collio Formation. Southern Alps, Italy. Width of view 1.8 mm. PPL

frayed edges (Fig. 4.6). These micas are usually subparallel lation cleavage, differentiated layering, fracture cleavage,
to bedding. Diagenetic foliation is thought to be the result schistosity etc. (see definitions in the glossary). Unfortu-
of diagenetic compaction of sediment that contains detri- nately, the use of these names is not uniform and some have
tal micas (Williams 1972a; Borradaile et al. 1982; Sintubin been used with genetic implications. For example, the name
1994a). The micas have rotated passively into an orienta- fracture cleavage has been used for a discontinuous folia-
tion parallel to bedding during compaction. Diagenetic fo- tion with finely spaced compositional layering that possi-
liation is an example of a foliation defined by the preferred bly originated by preferential dissolution along fractures
orientation of micas that is not associated with folds. It is that are no longer visible; other interpretations of such
thought to precede and play an important role in develop- structures that do not involve fractures are possible and
ment of secondary foliations in pelitic rocks (see below). the use of such genetic names should therefore be avoided.
Maxwell (1962) and Roy (1978) have postulated that For this reason, we aim to use purely descriptive terms.
diagenetic or dewatering foliations may also be oblique The concepts of cleavage and schistosity are so widely
to bedding and associated with synsedimentary folding, used that we maintain them as general terms for foliations
and may even be the initial stage of slaty cleavage. How- thought to be of secondary origin. Cleavage is generally used
ever, this idea is now largely abandoned since such foli- for fine-grained rocks up to the scale where individual cleav-
ations can usually be shown to have formed after the rock age forming minerals (e.g. micas) can be distinguished
lithified. Oblique synsedimentary foliations do occur but with the naked eye; schistosity for more coarse-grained
seem to be extremely rare. secondary foliations. Cleavage and schistosity therefore
cover the complete range of secondary foliations, espe-
4.2.3 cially in field descriptions. Notice, however, that this dis-
Secondary Foliations tinction by grain size is not expanded consistently to all
parts of foliation terminology; terms like crenulation cleav-
Below, we present a morphological classification of sec- age, shear band cleavage, cleavage domain and cleavage
ondary foliations and discuss the main processes involved lamellae (Sect. 4.2.6; Fig. 4.7) are generally used regard-
in their development. Secondary foliations may show a less of grain size. Another term that is occasionally used
large variation of morphological features. On the basis of is gneissosity, for a coarse-grained secondary foliation in
these characteristics, a number of more or less descrip- gneiss; use of this term is not recommended because of
tive names have been used such as slaty cleavage, crenu- possible confusion with compositional layering in gneiss.