Page 44 - Petroleum Geology

CHAPTER 2

EARLY DEFORMATION OF SEDIMENTARY BASINS:
GROWTH STRUCTURES

SUMMARY
(1) Growth structures are folds and faults in which variations of rock-unit
thickness are closely related to the structure itself. They result from defor-
mation that took place during sediment accumulation, burial and compaction.
There is an element of interpretation in a short definition of growth struc-
tures. The structures that result solely from differential compaction are not
strictly growth structures, but some degree of differential compaction exists
in growth structures.
(2) Growth faults (there are many synonyms, most common of which are
contemporaneous and depositional) are faults in which the thickness of rock
units in the downthrown block is greater than that of the correlative unit in
the upthrown block.
(3) Both blocks of a growth fault, in general, had a capacity to accumulate
sediment, both were subsiding relative to baselevel; but the downthrowing
block subsided faster than the “upthrowing” block, and so had a capacity to
accumulate a greater thickness of sediment. The throw of a growth fault
tends to increase with depth on account of the thickness contrast across it,
but antithetic faults reduce the throw.
(4) The fault plane in transgressive sequences is commonly steep to vertical.
In regressive sequences, it is usually curved in plan and in section, concave to
the direction of regression and concave up.
(5) Growth faults occur in two major associations: (a) in basins formed by
rifting, where basement faulting continued and caused growth faults in the
overlying (initially transgressive) sequence. These faults tend to die out up-
wards, commonly against an unconformity or disconformity; and (b) in re-
gressive sequences, where growth faults occur in the upper, sandy part of the
sequence. These die out downwards, and commonly also upwards.
(6) Growth anticlines are anticlines in which rock units thicken from crest
to flanks. They grew while the sediment was accumulating and compacting
during burial. The whole area of the anticline was subsiding, but the flanks
subsided faster than the crest, and so accumulated a greater thickness of sedi-
ment. Growth synclines can be formed.
(7) These processes can and do lead to local stratigraphic hiatus, the recog-
nition of which can be extremely difficult - but essential, of course, for the
correct interpretation of such areas.
(8) Sedimentary basins are growth structures on a large scale.