Stereographic Projection Techniques Second Edition

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CONTENT PG 20

Geological structures of planar type
The rocks ai the vast majority of exposures possess some kind of planar structure. In most sedimentary rocks a planar structure known as bedding is visible (Fig, la). This is a primary feature formed at the time of deposition and is a layering characterized by compositional, textural or grain-size variations. Some igneous rocks possess an equivalent structure called primary igneous layering produced by the accumulation of crystals settling out front a magma. The orientation of these primary planar structures reflects the mechanics of the deposition process, and measurements of their orientation can yield information about the paiaeohorizontal, the direction of flow of currents, etc.

Foliation is a general term for all pervasively developed planar structures found in rocks. Sedimentary bedding falls under this heading as do planar structures resulting from deformational and metamorphic processes. The latter are secondary foliations and include rock cleavage and schistosity. Some foliations are defined by compositional variations; others, such as slaty cleavage, by a parallel alignment of grains or mineral aggregates.

The directions of cleavage planes are frequently measured for the purpose of estimating the directions of geological strains in rocks. Figure 1b illustrates rocks containing two foliations together: a primary foliation (bedding) and a secondary foliation (cleavage).

Gneissic banding (Fig. If) is a common feature of coarse-grained metamorphic rocks such as gneisses, litis secondary foliation is a compositional layering defined by the concentration of particular minerals.

Other structures in rocks have planar geometry but are discrete features and are therefore not examples of foliation. Joints (Fig. 1d) are fractures in rocks along which little or no movement has taken place. Although they are produced by only minor tectonic strain, they represent discontinuities in the rock mass and as such are important to its mechanical behaviour. Assessment of the orientations of joints present might form an
essential part of any stability analysts of surface slopes or underground excavations (see p. 86).

Faults (Fig. le) are planes along which the rock on one side is displaced relative to the other. By collecting, in a region, directional data on a number of fault planes it is sometimes possible to estimate the directions of principal stress axes at the time of faulting (p, 58).

The surfaces of contact between geological units can often be considered planar, at least on a local scale. These contacts are often parallel to the bedding in the rocks on either side of the contact, but could form the margins of intrusive igneous rocks or could be erosional, e.g. surfaces of unconformity.

Other examples of planes are defined geometrically with respect to other features. For example, the axial surface (or axial plane) of a fold (Fig. If) can be determined as the plane which bisects the angle between two limbs of a fold. 1Ъе profile plane of a fold is one that is perpendicular to the fold axis. Planes of this type may not appear as measurable planar surfaces at the outcrop.