rocks being eroded or weathered (provenance), sediment transport, and depositional environments. The
grains-size distribution is very much a function of
the mineralogy in the source rock, and shales and
volcanic rocks will produce clay-rich sediments.
Rapid erosion of granite and gneisses produces
feldspar-rich sand (arkoses) while weathering results
in sand corresponding to the quartz crystals in the
granite and clay composed of kaolinite from
weathering of feldspar. Volcanic rocks produce smectite-rich clays which are very fine grained and have
cohesive properties very different from kaolinitic
clays. The grain-size distribution and the sand/shale
ratio as well as the composition of clay play an important role in the depositional processes.
Descriptions of outcrops and cores should not be
limited to studies of lithology and sedimentary
structures but should also include sampling and
analyses of the textural and mineral composition by
microscope (optical microscope and SEM) and by Xray diffraction (XRD). This will provide a basis for the
interpretation of diagenetic reactions occurring during
progressive burial in a sedimentary basin and also the
distribution of pore space between the minerals.
2.1.2 Textures
The textures of clastic sediments include external
characteristics of sediment grains, such as size, shape
and orientation. These properties can be described
relatively objectively and say a great deal about the
origin and conditions of sediment transport and
deposition.
By grain size we normally mean grain diameter, but
the two are only strictly synonymous in the case of
completely spherical particles. Most grains are not
spherical, however, and it is difficult to identify a
representative diameter, particularly in the case of
elongated or flat grains. For this reason we have
adopted the concept “nominal” diameter (d n ), defined
as the diameter of a spherical body which has the same
volume as the grain. In practice we are seldom in a
position to measure the volume of individual grains,
and we therefore use indirect methods to measure the
distribution of grain size within a sample.
Sand and gravel can most simply be analysed by
means of mechanical sieving. A bank of sieves
consists of sieves with mesh sizes which decrease
downwards. A sample is put in the uppermost sieve
and the bank of sieves is shaken (Fig. 2.1a). Grains
which are larger than the mesh size will remain, while
smaller grains will fall through and perhaps remain
lying on the next sieve. By weighing the fraction of the
sample which remains on each sieve, we can construct
a grain-size distribution curve. The lower practical
limit for sieve analyses is 0.04–0.03 mm; finer
particles exhibit much more cohesion, which makes
it difficult for them to become separated and pass
through the finer sieves.
Fine silt and clay fractions can be analysed in a
number of ways. Most classic methods are based on
measurements of settling velocity in liquids, and are
based on Stokes’ Law:
v ¼ cgR
2
Δρ=μ
Here c is a constant (2/9) and μ is the viscosity of the
water. R is the radius (cm) of the grain and Δρ is the
density difference between the grain and the fluid (water).
When the settling velocity of grains (falling
through water, for example) is constant, the resistance
to the movement (friction), which acts upwards, must
be equal to the force of gravity, which acts downwards
(Fig. 2.2).
6πRvμ friction
ð
Þ¼4=3πgR
3
Δρ gravity
ð
Þ
v ¼ c gR
2
Δρ=μ
Log v ¼ 2 log R þ c a constant
ð
Þ
The settling velocity is sensitive to temperature
variations, which affect the viscosity of the water (μ).
We can measure the settling velocities of sediment
grains indirectly by measuring the density of the water
with suspended sediment sample with a hydrometer,
which registers the fluid density. We disperse the
sample in a cylinder with a mixer so that at a start
time T o we have an even distribution of all grain sizes,
and therefore of density, throughout the cylinder. The
individual sediment grains then sink to the bottom at a
rate which is a function of their size. The change in the
fluid density as progressively fewer grains remain in
suspension is therefore a function of the grain-size
distribution. This applies for small particles, where
the flow of the liquid around the grain is laminar and
the concentration of grains is low.
34
K. Bjørlykke
grains-size distribution is very much a function of
the mineralogy in the source rock, and shales and
volcanic rocks will produce clay-rich sediments.
Rapid erosion of granite and gneisses produces
feldspar-rich sand (arkoses) while weathering results
in sand corresponding to the quartz crystals in the
granite and clay composed of kaolinite from
weathering of feldspar. Volcanic rocks produce smectite-rich clays which are very fine grained and have
cohesive properties very different from kaolinitic
clays. The grain-size distribution and the sand/shale
ratio as well as the composition of clay play an important role in the depositional processes.
Descriptions of outcrops and cores should not be
limited to studies of lithology and sedimentary
structures but should also include sampling and
analyses of the textural and mineral composition by
microscope (optical microscope and SEM) and by Xray diffraction (XRD). This will provide a basis for the
interpretation of diagenetic reactions occurring during
progressive burial in a sedimentary basin and also the
distribution of pore space between the minerals.
2.1.2 Textures
The textures of clastic sediments include external
characteristics of sediment grains, such as size, shape
and orientation. These properties can be described
relatively objectively and say a great deal about the
origin and conditions of sediment transport and
deposition.
By grain size we normally mean grain diameter, but
the two are only strictly synonymous in the case of
completely spherical particles. Most grains are not
spherical, however, and it is difficult to identify a
representative diameter, particularly in the case of
elongated or flat grains. For this reason we have
adopted the concept “nominal” diameter (d n ), defined
as the diameter of a spherical body which has the same
volume as the grain. In practice we are seldom in a
position to measure the volume of individual grains,
and we therefore use indirect methods to measure the
distribution of grain size within a sample.
Sand and gravel can most simply be analysed by
means of mechanical sieving. A bank of sieves
consists of sieves with mesh sizes which decrease
downwards. A sample is put in the uppermost sieve
and the bank of sieves is shaken (Fig. 2.1a). Grains
which are larger than the mesh size will remain, while
smaller grains will fall through and perhaps remain
lying on the next sieve. By weighing the fraction of the
sample which remains on each sieve, we can construct
a grain-size distribution curve. The lower practical
limit for sieve analyses is 0.04–0.03 mm; finer
particles exhibit much more cohesion, which makes
it difficult for them to become separated and pass
through the finer sieves.
Fine silt and clay fractions can be analysed in a
number of ways. Most classic methods are based on
measurements of settling velocity in liquids, and are
based on Stokes’ Law:
v ¼ cgR
2
Δρ=μ
Here c is a constant (2/9) and μ is the viscosity of the
water. R is the radius (cm) of the grain and Δρ is the
density difference between the grain and the fluid (water).
When the settling velocity of grains (falling
through water, for example) is constant, the resistance
to the movement (friction), which acts upwards, must
be equal to the force of gravity, which acts downwards
(Fig. 2.2).
6πRvμ friction
ð
Þ¼4=3πgR
3
Δρ gravity
ð
Þ
v ¼ c gR
2
Δρ=μ
Log v ¼ 2 log R þ c a constant
ð
Þ
The settling velocity is sensitive to temperature
variations, which affect the viscosity of the water (μ).
We can measure the settling velocities of sediment
grains indirectly by measuring the density of the water
with suspended sediment sample with a hydrometer,
which registers the fluid density. We disperse the
sample in a cylinder with a mixer so that at a start
time T o we have an even distribution of all grain sizes,
and therefore of density, throughout the cylinder. The
individual sediment grains then sink to the bottom at a
rate which is a function of their size. The change in the
fluid density as progressively fewer grains remain in
suspension is therefore a function of the grain-size
distribution. This applies for small particles, where
the flow of the liquid around the grain is laminar and
the concentration of grains is low.
34
K. Bjørlykke