Engineering Geology

E n g i n e e r i n g G e o l o g y
correct way up can be discerned. However, if fossil evidence is lacking, the correct way up
of the succession may be determined from evidence provided by the presence of “way-up”
structures such as graded bedding, cross bedding and ripple marks (Shrock, 1948).
Unconformities
An unconformity represents a break in the stratigraphical record and occurs when changes
in the palaeogeographical conditions lead to a cessation of deposition for a period of time.
Such a break may correspond to a relatively short interval of geological time or a very long
one. An unconformity normally means that uplift and erosion have taken place, resulting in
some previously formed strata being removed. The beds above and below the surface of
unconformity are described as unconformable.
The structural relationship between unconformable units allows four types of unconformity
to be distinguished. In Figure 1.24a, stratified rocks rest upon igneous or metamorphic rocks.
This type of feature has been referred to as a nonconformity (it also has been called
a heterolithic unconformity). An angular unconformity is shown in Figure 1.24b, where an
angular discordance separates the two units of stratified rocks. In an angular unconformity,
Figure 1.24
Types of unconformities: (a) nonconformity or heterolithic unconformity, (b) angular unconformity, (c) disconformity and
(d) paraconformity.
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