Page 22 - Petroleum Geology

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Fig. 1-1. Sediment transported in the central physiographic basin of North America ac-
cumulated mainly in the coastal area, not the deep part of the Gulf of Mexico.

physiography and climate of the basin. The nature of the sediments that
accumulate depends on these factors, the processes of transportation within
the bqsin, and the position of the sedimentary basin within the physiographic
basin. If the sedimentary basin is in the coastal region, paralic sediments will
accumulate; if.in the coastal plain, fluvial sediments will accumulate; if off-
shore, marine sediments will accumulate.
Sedimentation is one thing: sediment accumulation is another. The mouth
of a river may well be the site of heavy sedimentation; but if the energy of
the environment in which the sediment is deposited (the energy of the waves
and qurrents) is sufficient, the sediment will be transported elsewhere and
the accumulation of sediment near the river mouth may be nil or very little.
The redistribution of sediment depends on its physical properties - the den-
sity, size and shape of the particles - and the energy available to move it.
Sediment will be moved along the sea floor under the influence of waves and
currents until it arrives in a position where the available energy is insufficient
to move it further. There it will accumulate.
But if the accumulation of sediment raises the depositional surface to a
level of higher energy, where the waves and currents are stronger in the shal-
lower water, it will only accumulate to the level at which the energy is suffi-
cient to move the sediment elsewhere. This is the concept of baseleuel in