Page 360 - Petroleum Geology

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15-4) - but it is not easy to determine the causal relationship. It seems likely
that the diapir caused the fault in many cases, but the two are closely related.

Fig. 15-4. Mud volcanism and mudstone diapirism associated with a fault (diagrammatic).

Generalizations

Diapirs, of whatever sedimentary material, are characteristically overlain
by a sequence of sedimentary rocks that is, of course, younger than the ma-
terial of the diapir. The accumulation of these sedimentary rocks must be
taken as clear evidence that they were subsiding during periods of sediment
accumulation. If the growth of a diapir is reasonably accurately depicted by
Fig. 15-5, which is consistent with the observations reported in an extensive
literature, most of the strata now penetrated by the diapir were once conti-
nuous across it, like those that have not yet been penetrated. If the growth
of a diapir is accelerating, there may come a time when there is absolute up-
ward movement at a sufficient rate to inhibit sediment accumulation, and
stratigraphic continuity will be broken. This is essentially the concept of
“downbuilding” that Barton (1933) proposed.

DIAPIRISM
Diapirism is a dynamic process that takes place under the force of gravity
during the accumulation of sediment in a developing sedimentary basin. Its
significance for the petroleum geologist is that it is a process that deforms
the sedimentary strata while they are compacting, during fluid expulsion
from the more compactible lithologies. Mudstone diapirism is probably more
significant than salt diapirism because the mudstone itself may be a petro-
leum source rock, and mudstone diapirism is essentially contemporaneous
with fluid expulsion from the mudstone.
Diapirism involves the flow of diapiric material. When referring to the
flow of rocks, we use the term “equivalent viscosity” because the term “vis-
cosity” may suggest that Newtonian viscosity is involved, with the velocity
of flow at a point proportional to the distance of the point from a static boun-
dary. Carey (1954) argued that the conventional classification of matter into