Page 46 - Petroleum Geology

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deformation of unconsolidated sediments by slumping and sliding than to
folding and faulting on the scale of petroleum accumulations.
The terminology of growth faults is confused. There is little doubt that
concurrent fault (Tiddeman, 1890) has priority, when applied to a fault that
has different facies of correlative units across it and, by analogy, to a fault that
has a thicker sedimentary sequence on the downthrown side than the upthrown
side. Currie (1956) used both concurrent and contemporaneous; Liechti
et al. (1960) used depositional, following common U.S. Gulf Coast usage;
Ocamb (1961) used growth, but Hardin and Hardin (1961) stated that con-
temporaneous had “some claim to priority”. In addition to these, synsedi-
mentary has been and is still widely used. Less desirable synonyms include
progressive and Gulf Coast type (!). Dennis (1967) accepted growth fault for
the International Tectonic Dictionary, and recommended that all synonyms
be dropped.
Because concurrent cannot be revived with any hope of acceptance, and
the multiplicity of synonyms can serve no useful purpose, we bow to the
International Tectonic Dictionary here, and accept growth fault as the term
to be applied to a fault that separates correlative sequences of different thick-
nesses, with the thicker sequence on the downthrown side.
The terminology for growth structures other than faults has not received
much attention. Growth anticlines are sometimes called growth structures,
but “structure” is a wide term that is not synonymous with “anticline”. It is
desirable to use the same adjective for analogous geological features, so we
use the term growth structure to embrace all structures that affected the ac-
cumulation of sediment in them; and for specific structures, we use the spe-
cific terms growth fault, growth anticline and growth syncline.

GROWTH FAULTS

A typical, but idealized, growth fault is shown in Fig. 2-1. It is interpreted
as a fault that was moving during the accumulation of those rock units that
show a thickness contrast across the fault. This contrast is not to be expected
over the whole fault, and will not exist in those units that had accumulated
before the fault was created. The throw of a growth fault generally increases
with depth over the interval or intervals of thickness contrast, because of the
thickness contrast; but any antithetic faults reduce the throw with depth.
The diagnostic feature is therefore the contrast in bed thicknesses: the increase
of throw with depth is a consequence of this and is not in itself diagnostic.
Growth faults occur in many - perhaps most, possibly all - sedimentary
basins of the world, and in rocks of all ages. Tiddeman’s brief description of
the Craven fault in England is apparently the first description of a growth
fault (Tiddeman, 1890). This fault affected Lower Carboniferous (Mississip-