Page 82 - Microtectonics

70 4 · Foliations, Lineations and Lattice Preferred Orientation
foliations may also develop in some cataclasite zones from secondary foliations are listed in Box 4.1. In fact, only
(Sect. 5.2). Secondary foliations that are not homogene- the first and last of these criteria are conclusive: the pres-
ous may be difficult to distinguish from primary layering ence of sedimentary structures is good evidence for bed-
(Box 4.1). The recognition of primary foliation is impor- ding, and the relation of a compositional layering with
tant, however, because in metasediments it allows recon- the axial surface of folds clearly demonstrates the second-
struction of the structural evolution from sedimentation ary nature of a layering (contemporaneous with the fold-
onwards (e.g. S (bedding), S , S , etc.). If bedding is not ing). The presence of two crosscutting layering structures
1
2
0
recognised, only the last part of the evolution can be re- in a metamorphic rock is also good evidence that one of
constructed (e.g. S , S , S , etc.). them must be secondary (Fig. 4.5).
n n+1 n+2
Unfortunately, it is often impossible to recognise bed-
4.2.1 ding, especially in rocks of medium to high metamorphic
Primary Foliation grade. Transposition processes (Box 4.9) may have oblit-
erated angular relationships, or sedimentary structures
In very low-grade metamorphic rocks, which are not in- may have disappeared by intense deformation and recrys-
tensely deformed, the recognition of bedding may be a tallisation. In such cases, the oldest compositional layer-
straightforward matter, since the main characteristics of ing has to be labelled S , keeping in mind that it may, at a
n
a sedimentary sequence, including sedimentary struc- certain scale, reflect remnants of bedding.
tures, may be well preserved. In the case of more intense Structures in primary sedimentary layering can be
deformation and higher metamorphic grade, it is usually used to establish younging direction in thin section. In
more difficult to distinguish between primary and sec- many cases this can be done by recognition of asymmet-
ondary layering. In many metamorphic rocks such as ric refraction of a secondary foliation through bedding.
gneisses a compositional layering may have a sedimen- Care should be taken, however, since in some cases the
tary, igneous or metamorphic/deformation origin, or may growth of metamorphic minerals may invert graded bed-
have a complex nature combining several of these origins ding if large micas or other minerals grow in originally
(e.g. Passchier et al. 1990b). fine-grained pelitic layers.
Primary layering in sediments results generally from
discontinuous processes, causing considerable variation 4.2.2
in thickness and composition of individual beds or lay- Diagenetic Foliation
ers, with low symmetry about planes parallel to layers
(Fig. 4.4). Secondary layering, however, forms by some Diagenetic foliation, also referred to as bedding-parallel
kind of differentiation process in a stress field, usually foliation, is commonly observed in very low-grade and
producing a more monotonous bimodal structure with a low-grade pelitic sediments, which have undergone little
symmetry plane parallel to the layering (Fig. 4.5). Some or no deformation (Borradaile et al. 1982). It is defined
empirical criteria that may help to distinguish primary by parallel orientation of thin elongate mica grains with

Box 4.1 Criteria to distinguish between primary and secondary foliations
The arguments given below are useful to distinguish a primary
foliation such as bedding from a spaced secondary foliation de-
veloped as a compositional layering.