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Fig. 16-1. Growth faults in the Niger delta reflect the growth of the delta. (After Weber,
1971, p. 560, fig. 2a.)

formed during the accumulation of sediment, and therefore also during sub-
sidence and the movement of associated growth faults. We infer, therefore,
that they too formed in a stress field with a component of horizontal tension.
It is part of the geological association in major young regressive sequences
that the thick mudstone unit underlying the sandier part of the sequence is
abnormally pressured. These abnormal pressures are found to be stratigraphi-
cally related, and closely related stratigraphically to the growth faults and
growth anticlines. The abnormally pressured mudstones are typically under-
compacted and less competent than the sandier, more compacted overburden.
Regionally, the anticlinal trends in such regressive sequences are found to
be narrow, sinuous, and separated by broad, gentle synclines. Some anticlinal
trends are steep, with complex cores.
We therefore infer that the deformation of regressive sequences of this
sort is a gravity-induced phenomenon, with mechanical instability in the se-
quence leading to incipient diapirism and diapirism, and a tendency for the
whole sequence to slide on the incompetent mudstones away from the land
when unequal subsidence imposes a gentle slope on the mechanical interface
at the top of abnormal pressures. The tendency to slide and form growth
faults is near the distal limit of sand accumulation, but in the early stages of
development, the slope on the mechanical surface is regionally down towards
the land from which the regression is coming (it is roughly parallel to the dia-
chronous surface separating the sandier sequence from the mudstones).
This deformation is seen as a contemporary deformation that results from
the accumulation of the regressive sequence. It is only indirectly a consequence
of orogeny. The role of orogeny is to create the mountains from which the
sediment will be generated in sufficient quantities to accumulate in neighbour-