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3- Sedimentation in the ocean :
Water depth varies with distance from the continent. Away from a
continent , two things happen ; the water gets deeper and the sea bottom
is farther from the source of terrigenous sediment, which is mainly sand
and mud eroded from the land and supplied by rivers. On the basis of
water depth , the ocean can be subdivided into two major areas of
sedimentation . The continental shelf is shallow and close to sources of
sediment from the land , and the deep sea is deep and far from river
supplies of terrigenous sediment. Sedimentation processes are quite
different in these two areas , and therefore each is examined separately.
a- Shelf sedimentation :
A continental shelf is a relatively broad , essentially flat platform 70
to 100 kilometers wide that represents the submerged edge of a
continent . water depths on the shelf are shallow , varying from zero
at the shoreline to 120 to 150 meters at shelf break , where the
gradient of the sea bottom steepens to about 4 degrees and marks the
beginning of the continental slope.
Energy for eroding and transporting sediment grains is provided by
the tides and wind-generated waves and currents.
The shoreline is affected by breaking
waves, and these high-energy
conditions suspend and remove all the fine sediment and allow
mainly medium and coarse sand and gravel to be deposited on the
beach and near-shore zone .
Seaward from the nearshore zone , bottom energy induced by waves
decreases because of increasing water depths. This decrease of
bottom energy with water depth results in a systematic decrease in
grain size with distance offshore. The beach , which is composed of
coarse to medium sand and gravel , grades into fine sand farther
offshore and , moving seaward , into muddy sand (sand with some
mud) ,sandy mud (mud with some sand ) , and finally mud far
offshore.