Page 55 - Petroleum Geology

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When related to the development and evolution of a sedimentary basin, such
faults may move - intermittently, perhaps - over significant spans of geo-
logical time. Fault movement can rarely (if ever) be regarded as an instan-
taneous event. A fault is one form of adjustment to the forces acting on a
rock mass. Instantaneous forces can only be visualized in catastrophic situa-
tions. Whether a fault moved during the accumulation of sediment in a sedi-
mentary basin, or subsequently, the movement must have occupied a span of
time, and brought different rock units together across the fault during that
time.
It matters to the petroleum geologist when a fault moved, because it may
be significant for the migration of fluids and the accumulation of petroleum,
as well as important for the understanding of the geology of an area. We shall
take this topic into Chapter 9, on fluid migration.
The rate of growth-fault movement is a teasing problem of some interest.
Taking an extreme example, assume that while 1000 m of sedimentary rocks
accumulated in the upthrowing block, 1500 m accumulated in the down-
throwing block at the rate of 500 yrs/m. The fault threw 500 m in 750,000
yrs, or about 7 X m/yr - rather less than 1 mm/yr. This is very slow
movement, and many growth faults would grow at a much slower rate.
Maintenance of a fracture in a thick plastic mudstone is unlikely.

GROWTH ANTICLINES

Growth anticlines are anticlines in which the thicknesses of rock units are
greater on the flanks than on the crest (Fig. 2-6). These thickness changes

Fig. 2-6. Diagrammatic cross-section through growth anticline.