6
e- Cosmogenous sediment :
Very tiny grains that originate from outer space and tend to be mixed
into terrigenous and biogenic sediment.
2-Factors that control sedimentation :
Two of the most important factors that determine the nature of
a
sediment deposit are particle-size distribution and energy conditions at
the site of deposition .because these two factors interact to produce the
properties of a sediment deposit , a geologist is able to deduce what
they were at the time of deposition by examining those properties of
even an ancient sediment.
If the rock eroded slowly, then sediment was supplied at a low rate and
likely was reworked by currents before i t was burried by ever younger
deposits.
A slow rate of sedimentation generally gives more opportunity for water
to sort the grains according to size, shape , and density. This can result
in deposits of mud and sand that are well sorted ( a small range of grain
sizes ) and have a uni-form appearance , such as a coarse sand ,or very
fine sand or silt.
In most cases , a clear relationship exists between the average grain size
and the strength (energy) of bottom currents at the time that sediment is
deposited.
Stated simply, the average particle size of a deposit is proportional to the
energy level present at the time of deposition. Under high –energy
conditions , water is swift and turbulent , keeping fine grains in
suspension
and resuspending those fine particles that momentarily
settled to the ocean floor.this constant agitation of the sea-bottom
separates small grains and transports them into quieter water , which
typically is deeper than turbulent water. Thus a coarse sand (rather than
medium or fine ) is deposited under high-energy conditions.