Page 51 - Petroleum Geology

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side a growth fault. It would not be a prolongation of the fault plane at 60"
to the horizontal because this is unstable in unconsolidated sediments. Per-
haps a sequence of foreset beds inclined at about 30" to the horizontal, bi-
secting the bedding and the fault plane, would accumulate on the upthrow-
ing side.
The history of movement of a growth fault is recorded in the thicknesses of
each rock unit across the fault. This typically shows a gradual acceleration in
the rate of movement to reach a peak, and then a gradual deceleration (Fig.
2-4). When there is no movement on the fault, there is no difference of thick-
ness of the rock units that accumulated across the fault during that time; and
when all movement ceases, rock units are continuous across the top of the
fault.

GROWTH INDEX
'Y'''';

Fig. 2-4. Plot of growth, or expansion, index. The ratio of downthrown to upthrown thick-
ness of each rock unit across a growth fault is plotted at the centre of each unit, regarded
as a time series.

In major regressive sequences it is found that the growth fault on the
downthrown side of another is further away from the orogeny that caused
the regression, and is younger in the sense that it started to move later, reached
its peak later, and died later (Fig. 2-3; see also Fig. 15-13 of Seria field, Brunei,
and note how the tops of the faults move progressively up the section towards
the north-west). Thorsen (1963), in a nice study of growth faults in south-
east Louisiana, showed that the maximum rate of movement occurred pro-
gressively later in faults towards the south, towards the Gulf of Mexico. He
also noted that the maximum rate of growth anticline movement occurred at