Page 49 - Petroleum Geology

In the Federal Republic of Germany there are some faults that appear to
have begun as normal growth faults, to have their sense of movement reversed.
There is evidence of thickening of rock units on the overthrust block.
It is evident that growth faults have various expressions and appear in vari-
ous geological contexts, but there are two main classes:
(a) Those in sedimentary basins formed by rifting, which affect the trans-
gressive and the regressive sequence that usually follows closely in time.These
are basement faults that continued to move during the early stages of the
development of the basin.
(b) Those in the regressive sequence of a major sedimentary cycle. These
are not upward extensions of basement faults, but faults confined to the re-
gressive sequence that die out downwards and many also upwards.
We shall examine here the nature of growth structures as such, leaving dis-
cussion of their geological contexts to later chapters.

The nature of growth faults

The quality of correlation across a growth fault varies from poor to excel-
lent. This must be seen as a measure of the degree to which the movement of
the fault affected the sediments and their accumulation. Where rock-unit cor-
relation across a growth fault is good, the process was clearly one of transport
of sediment over a subsiding sedimentary column, with greater potential for
accumulation on the more rapidly subsiding downthrowing block. The con-
tinuity of lithologies implied in good correlation suggests that the supply of
sediment generally exceeded the capacity of the area to accumulate it. This
is a characteristic of regressions. It is instructive to examine the nature of a
growth fault in some detail.
Clearly, the deposition of sediment from suspension in water cannot lead
to the observed results by itself because there is no reason why the rate of
deposition from suspension should be different across a subsurface feature,
or even a surface feature of modest relief.
These difficulties are removed if the sediment accumulates from the trac-
tion load on the sea floor. The clue to this lies with Barrell’s concept of base-
level, discussed at some length in Chapter 1. Sedimentary particles are moved
by the water until they reach a position in which the energy is insufficient to
move them further. There they accumulate. Some particles reach that posi-
tion near a growth fault, but of these, more reach it on the downthrowing
side than on the upthrowing side because of the differing rates of subsidence
relative to baselevel. Note that this applies to muds as well as sands because
mudstones also show thickness contrasts across growth faults.
The accumulation of sediment on the upthrowing side of a growth fault
is also evidence that it too was subsiding relative to baselevel, so we are led
to the conclusion that both blocks of a growth fault were subsiding during
fault movement, but one side was subsiding faster than the other, and the