Engineering Geology

Chapter 1
The extent and regularity of beds of sedimentary rocks vary within wide limits. This is because
lateral persistence and regularity of stratification reflect the persistence and regularity of the
agent responsible for deposition. For instance, sands may have been deposited in one area
whereas muds were being deposited in a neighbouring area. What is more, a formation with a
particular lithology, which is mappable as a stratigraphic unit, may not have been laid down at
the same time wherever it occurs. The base of such a formation is described as diachronous
(Fig. 1.23). Diachronism is brought about when a basin of deposition is advancing or retreating
as, for example, in marine transgression or regression. In an expanding basin, the lowest
sediments to accumulate are not as extensive as those succeeding. The latter are said to overlap
the lowermost deposits. Conversely, if the basin of deposition is shrinking, the opposite
situation arises in that succeeding beds are less extensive. This phenomenon is termed offlap.
Agents confined to channels or responsible for deposition over limited areas produce irregular
strata that are not persistent. By contrast, strata that are very persistent are produced
by agents operating over wide areas. In addition, folding and faulting of strata, along with
subsequent erosion, give rise to discontinuous outcrops.
Since sediments are deposited, it follows that the topmost layer in any succession of strata
is the youngest. Also, any particular stratum in a sequence can be dated by its position in the
sequence relative to other strata. This is the Law of Superposition. This principle applies
to all sedimentary rocks except, of course, those that have been overturned by folding or
where older strata have been thrust over younger rocks. Where strata are overfolded, the
stratigraphical succession is inverted. When fossils are present in the beds concerned, their
Figure 1.23
Diachronism of a lithological boundary and the migration time of a fossil assemblage. The fossiliferous horizon may be regarded
as a time plane if the localities (a), (b) and (c) are not far distant. As a rule, time planes cannot be identified.
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