Geoscience Reference
In-Depth Information
Table 7.1
Possible landscape clues to the location of an unexposed
contact.
Aspect
Possible features
Topography
Break in slope; trough (or ridge); general change in
landscape
N
Weathering
Contact may weather back and erode more than average
7
Vegetation
Change in type (related to soil chemistry) or lushness
(related to drainage) across contact
(a)
c. 1km
It is often particularly informative to measure strike and dip of
discordant contacts, especially if there are other examples in
the area. Measurement principles are the same as those laid out
in Section 2.3.1. When it comes to making a map (Chapter 10),
an obvious example is the strike and dip pattern of dykes,
which will tell you whether you are dealing with a generally
parallel dyke swarm, or radial dykes coming from a common
centre (Figure 7.4), or cone sheets (circular or elliptical in
outcrop pattern but dipping inwards) or ring dykes (similar but
dipping outwards; Figure 7.5). If the edge of your igneous rock
body lies parallel to the stratifi cation of the country rock (a
concordant contact), then it may be extrusive (lava) or a layer
of pyroclastic rock. However, it could still be intrusive.
N
(b)
c. 1km
Figure 7.4
Schematic plan view
of the possible outcrop pattern of
radial dykes around a volcanic
centre: (a) in the absence of
regional stress and (b) with
east-west extension.
Intrusions into foliated metamorphic rock commonly exploit
the foliation, though in places they can sometimes be seen to
cut across it discordantly. In a migmatite, the leucosomes
(layers or lenses of sweated-out igneous material) form coevally
with the overall fabric, and so are generally concordant.
If you suspect that a concordant contact may be the top or
bottom of a sill (Figure 7.6), you should try to fi nd the opposite
contact. Sills are tabular bodies, typically intruded between
beds of horizontal (or gently dipping) strata. Although they
may extend horizontally for tens of hundreds of kilometres,
most sills are only a few tens of metres thick, so the evidence
needed to confi rm the concordant nature of both contacts may
not be far away.
When an intrusion is emplaced into wet, poorly consolidated,
sedimentary rocks, then water-rock reactions can lead to
margins that are locally brecciated, unlike the almost planar
contact in Figure 7.6. This texture is called peperite, and may
look like an inverse version of a stoped contact (such as Figure
7.2), having angular lumps of igneous rock enclosed within the
country rock instead of angular fragments of country rock
enclosed within the igneous rock.
BEWARE! A concordant
contact must be parallel to
the stratifi cation of the
country rock in three
dimensions, so be sure that
you are not misled by
looking only at one surface
in a specifi c orientation.
It can be particularly tricky to distinguish a sill from a lava
fl ow, but you will be in less doubt if you can see (or deduce
