Page 59 - Petroleum Geology

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reservoirs. The growth component due to faulting may obscure the compo-
nent due to anticlinal growth, so that the recognition of the growth anticline
depends on the recognition of convergence of stratigraphic units towards the
crest within fault blocks.
The proper use of isopach maps and sections is essential for the elucida-
tion of even simple growth structures. For instance, the axis of minimum
sediment accumulation may change position during the development of a
growth anticline. Such shifts of position can rarely be seen in structural maps
or sections.
We cannot leave the subject of growth anticlines without mention of an
alternative hypothesis for the thinning of stratigraphic units: that is, the
mechanical attenuation of the beds due to the enlargement of their area, or
stretching, over a growing anticline. Confidence in the sediment-accumula-
tion hypothesis rests largely on the analogy with growth faults, the evidence
of which is not consistent with mechanical attenuation of the sedimentary
rocks because of the abrupt changes of thickness across the fault. Mechanical
attenuation is not mutually incompatible with sedimentary thinning, and
model studies have shown attenuation of layers over a growing diapir. The
key to the problem may lie in the thinning of individual beds of similar lithol-
ogy. If mudstones, for example, show consistent and greater thinning than
sandstones or limestones, then attenuation may be suspected. On the other
hand, if the patterns of thinning are variable and not associated with lithology,
sedimentary thinning may be the dominant cause. The problem is complex
and the evidence ambiguous. However, when growth faults cut an anticline
in which the rock units tend to thin towards the crest, the ambiguity for all
practical purposes is removed and at least some of the crestal thinning is due
to thinner accumulation of sediment there.
Growth anticlines have hardly ever been reported from surface surveys. It
is possible that they hardly ever occur at the surface. In active sedimentary
basins they will, of course, have no surface expression. In older sedimentary
basins where they have been exposed by uplift and erosion, they will be
characterized by steepening of dips towards the crest. Such anticlines when
mapped may be attributed to diapirism (and we shall see that diapirism is a
common cause of growth anticlines), but the feature of crestal steepening of
dips is a feature of growth anticlines.

REFERENCES

Billings, M.P., 1954. StructuraZ geology (2nd ed.). Prentice Hall, Englewood Cliffs, N.J.,
514 pp.
Bottcher. H., 1925. Die Tektonik der Bochumer Mulde zwischen Dortmund und Bochum
und das Problem der westfalischen Karbonfaltung. Gliickauf, 61: 1145-1153 and
61: 1189-1194.
Bottcher, H., 1927. Faltungsformen und primare Diskordanzen im niederrheinisch-west-