Page 62 - Sedimentology and Stratigraphy

Nichols/Sedimentology and Stratigraphy 9781405193795_4_004
26.2.2009 8:16pm Compositor Name: ARaju
Final Proof page 49
The Behaviour of Fluids and Particles in Fluids 49
velocity and particles that are already in motion. The settling velocity of particles in a fluid is deter-
This shows that a pebble will come to rest at around mined by the size of the particle, the difference in the
1
1
20 to 30 cm s , a medium sand grain at 2 to 3 cm s , density between the particle and the fluid, and the
and a clay particle when the flow velocity is effect- fluid viscosity. The relationship, known as Stokes
ively zero. The grain size of the particles in a flow Law, can be expressed in an equation:
therefore can be used as an indicator of the velocity
at the time of deposition of the sediment if depos-
2
ited as isolated particles. The upper, curved line V ¼ g D (r r )=18m
s
f
shows the flow velocity required to move a particle
from rest. On the right half of the graph this line
parallels the first but at any given grain size the where V is the terminal settling velocity, D is the grain
velocity required to initiate motion is higher than diameter, (r r ) is the difference between the den-
f
s
that to keep a particle moving. On the left side of the sity of the particle (r ) and the density of the fluid (r )
f
s
diagram, there is a sharp divergence of the lines: and m is the fluid viscosity; g is the acceleration due to
counter-intuitively, the smaller particles require a gravity. One of the implications of this for sedimentary
higher velocity to move them below coarse silt size. processes is that larger diameter clasts reach higher
This is due to the properties of clay minerals that velocities and therefore grading of particles results
will dominate the fine fraction in a sediment. Clay from sediment falling out of suspension in standing
minerals are cohesive (2.4.5) and once they are water. Stokes Law only accurately predicts the set-
deposited they tend to stick together making it diffi- tling velocity of small grains (fine sand or less)
cult to entrain them in a flow. Note that there are because turbulence created by the drag of larger
two lines for cohesive material. ‘Unconsolidated’ mud grains falling through the fluid reduces the velocity.
has settled but remains a sticky, plastic material. The shape of the particle is also a factor because the
‘Consolidated’ mud has had much more water drag effect is greater for plate-like clasts and they
expelled from it and is rigid. therefore fall more slowly. It is for this reason that
The behaviour of fine particles in a flow as indicated mica grains are commonly found concentrated at the
by the Hju ¨lstrom diagram has important conse- tops of bed because they settle more slowly than
quences for deposition in natural depositional envi- quartz and other grains of equivalent mass.
ronments. Were it not for this behaviour, clay would A flow decreasing in velocity from 20 cm s 1 to
be eroded in all conditions except standing water, but 1cm s 1 will initially deposit coarse sand but will
mud can accumulate in any setting where the flow progressively deposit medium and fine sand as the
stops for long enough for the clay particles to be velocity drops. The sand bed formed from this decel-
deposited: resumption of flow does not re-entrain the erating flow will be normally graded, showing a
deposited clay unless the velocity is relatively high. reduction in grain size from coarse at the bottom to
Alternations of mud and sand deposition are seen in fine at the top. Conversely, an increase in flow veloc-
environments where flow is intermittent, such as tidal ity through time may result in an increase in grain
settings (11.2). size up through a bed, reverse grading, but flows that
gradually increase in strength through time to pro-
duce reverse grading are less frequent. Grading can
4.2.5 Clast-size variations: graded bedding occur in a wide variety of depositional settings: nor-
mal grading is an important characteristic of many
The grain size in a bed is usually variable (2.5) and turbidity current deposits (4.5.2), but may also result
may show a pattern of an overall decrease in grain from storms on continental shelves (14.2.1), over-
size from base to top, known as normal grading,ora bank flooding in fluvial environments (9.3) and in
pattern of increase in average size from base to top, delta-top settings (12.3.1).
called reverse grading (Fig. 4.6). Normal grading is It is useful to draw a distinction between grading
the more commonly observed pattern and can result that is a trend in grain size within a single bed and
from the settling of particles out of suspension or as a trends in grain size that occur through a number of
consequence of a decrease in flow strength through beds. A pattern of several beds that start with a coarse
time. clast size in the lowest bed and finer material in the