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horizontal. It was perhaps a normal fault originally, with the movement sub-
sequently reversed by mudstone diapirism acting on the original downthrown
block.
Miri is perhaps at a more advanced stage of deformation than Seria. It is
onshore, and the land surface has topographic relief.
It appears therefore that Gesa, Seria, and Miri represent stages in the devel-
opment of growth anticlines in regressive sequences, and that this growth is
vertical, due to mechanical instability in the stratigraphic sequence. The co-
existence of growth faults in growth anticlines in Seria and Miri are evidence
of a protracted period of extensional stress during subsidence. We shall pursue
this line of reasoning in the last chapter.
It is clear that by its very nature, a regressive sequence tends to be “open-
ended”, and that the load on the abnormally pressured mudstones will tend
to squeeze them out laterally (Fig. 15-15). Dailly (1976) has argued cogently
for the reality of this on a large scale in the Niger delta; and, on a small scale,
the evident lateral movements of folds and overthrusts under the mudlumps
of the Mississippi delta (Morgan et al., 1968) is further support. This bulk
flow may therefore be the additional process tending to flatten growth faults
with depth in a regressive sequence and, indeed, may be the principal cause
of growth faults. Hence Bruce’s (1973, p. 884) observation that the abnor-
mality of pore pressure is greater where the dip of the growth fault is flatter
is probably ascribable to mass flow (rather than the direct mechanical effect
that Bruce inferred).

Fig. 15-15, A regressive sequence will tend to extrude the relatively plastic mudstone DY
virtue of the load (diagrammatic).

REFERENCES

Atwater, G.I. and Forman, M.J., 1959. Nature of growth of southern Louisiana salt domes
and its effect on petroleum accumulation. Bull. Am. Ass. Petrol. Geol., 43: 2592-
2622.
Barton, D.C., 1931. Effect of salt domes on accumulation of petroleum. Bull. Am. ASS.
Petrol. Geol., 15: 61-66.
Barton, D.C., 1933. Mechanics of formation of salt domes with special reference to Gulf