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trichtian. The minimum hiatus near highs seems to be late Albian-Aptian to
early Coniacian (Williams et al., 1975, p. 372, fig. 9). That faults were still
moving at this time is shown in Piper, where Coniacian and Santonian mark ac-
cumulated on the downthrown side of a southern fault, but not everywhere
on the upthrown side (as discussed in Chapter 13, pp. 279-302). This is evi-
dence that here, for about 5 m.y., the upthrown block was not subsiding
relative to base-level, but that the downthrown block subsided about 100 m
(i.e., about 50,000 yrs/m!). So fine was the balance here that it is hard to
escape the conclusion that accumulation was continuous in places (with all
the reservations discussed in Chapter 1 and in Chapter 13, pp. 295-196).
By the end of the Cretaceous, beginning of the Tertiary, the North Sea
basin accumulated sediment by virtue of subsidence without faulting. Exten-
sive and thick mudstones with some sands accumulated, the thicker sequences
in areas of greatest subsidence. At least some of the sands were to become
petroleum reservoirs.
The present relief on the Jurassic-Cretaceous disconformities and uncon-
formities, generally reported to be about 2000 m, has largely been induced
by later events. Although some relief certainly existed on the erosion surfaces
at the time, as shown by truncated strata on the present highs, it is unlikely to
have been great. The general tendency towards greater thicknesses and more
complete sequences in the lows is the result of a tendency to reduce relief by
sediment accumulation.
The consistency of structure and stratigraphy in such widely separated
areas as the North Sea and Australia, Arctic North America and South Africa,
indicates, as Kent (1977) so clearly expounded, a world-wide sequence of
events that is not to be explained simply by eustatic changes of sea level.
Doubtless the broad crustal events changed the volume of the ocean basins
so that there were concomitant eustatic sea-level changes, and there is evidence
of this in the stratigraphy; but these were a consequence of more fundamental
geological events that took place from late Palaeozoic times to the present in
large areas around the world. The significant events seem to be the inception
and cessation of normal faulting, because this is a more local phenomenon.
Some of the periods of non-accumulation and erosion may have been due to
eustatic sea-level changes caused by other events in the series in other parts
of the world.
In general, continental margins of the “passive” or “aseismic” type were
completed during the final post-unconformity stage of epeirogenic subsidence
without faulting; but it is not a universal consequence of the whole process
that continental margins were formed. The Sirte basin in Libya, for example,
is a rift basin that was completed with transgressive Cretaceous and Tertiary,
as we have seen. It is not a continental margin (and its geology must be taken
into account in any hypotheses concerning northward relative movement of
Africa against Europe). Nor was this great Mesozoic event necessarily the first