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Summary sedimentologie - sedimentaire systemen (UU)
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SEDIMENTOLOGIE 2020/2021CHAPTER 4: PROCESSES OF TRANSPORT AND SEDIMENTARY STRUCTURES
The simplest mechanism of sediment transport is due to gravity, which causes rock falls and
accumulations called scree, which build up as talus cones. The angle of rest varies for the clast size
and varies from 30 to 36 degrees, with larger clast sizes being able to rest at a bigger angle. Transport
by water is by far the most significant and can transport over large distances. Wind blowing over land
can pick up dust and sand and carry it large distances. Over longer time periods ice also functions as a
transport mechanism. When there is a very high concentration of sediment in water the mixture forms
a debris flow and is gravity driven.
There are two types of fluid flow: laminar flows, all molecules within the fluid move parallel to each
other in the direction of transport; turbulent flows, molecules in the fluid move in all directions but
with a net movement in the transport direction. The Reynold number is obtained by relating the
velocity of flow (v), the ratio between the density of the fluid and the viscosity of the fluid (V) and the
depth of flow (l). Re= (v*l)/V, this flow will be laminar if Re<500 and turbulent when Re>2000. There’re
three mechanisms of particles moving: rolling, saltation and suspension. Particles being caries by
rolling or saltation are called bedload, and the material in suspension is called suspended load. Higher
energy is needed rolling → suspension. Rolling grains are moved by frictional drag between the flow
and the clasts. The fluid velocity at which a particle becomes entrained in the flow can be referred to
as critical velocity. The Hjülstrom diagram shows the relationship between water flow velocity and
grain size, it demonstrates important features of sediment movement in currents. The lower line
shows the velocity to keep a rock in motion while the lower line shows the velocity required to start
the motion from rest. The cohesive properties of clay particles mean that fine-grained sediments
require high velocities to re-erode them once they are deposited, especially once they are compacted.
A deaccelerating flow will form a normal graded bed (fining upwards), while a flow with an increasing
velocity through time may result in a reverse graded bed (coarsening upwards). The settling velocity
of particles in a fluid is given by stokes law where V=gD2(ρs-ρf)/18μ. Stokes law only accurately predicts
for small grains, because of the turbulence created by the drag of larger grains falling through the
fluid, same is true for plate-like clasts and micas, which will settle lasts because of the greater drag.
Higher viscosity fluids can transport bigger sediments and rocks.
A bedform is a morphological feature formed by the interaction between flow and cohesionless
sediments on a bed. We will focus on ripple marks and sand dunes, which leave distinctive layering in
the preserved strata. A fluid flowing over a surface can be divided into a free stream, which is the
portion of the flow unaffected by the boundary effect, a boundary layer, within which the velocity
starts to decrease due to friction with the bed and a viscous layer, a region of reduced turbulence that
is typically less than a millimetre thick. If all particles are contained within the viscous layer it is
hydraulically smooth, if parts stick out it is hydraulically rough. “steps” are formed by grains being
clustered by turbulent sweeps. Expansion of flow over the step result in an increase in pressure and
the sediment transport rate is reduced, resulting in deposition on the lee side of the step, which forms
a series of layers at the angle of the slope. These thin inclined layers of sand are called cross-laminae
and build the sedimentary structure called cross-lamination. When viewed from above the ripples
might be straight/sinuous ripples or linguoid ripples, which are unconnected arcuate forms. Straight
ripples tend to develop into linguoid ripples over time and at high velocities. A perfectly straight ripple
would generate planar cross-lamination. Linguoid ripples develop a pattern of trough cross-
lamination (see image pg. 55). Climbing ripples are indicators of rapid sedimentation as their
formation depends on the addition of sand to the flow at a rate equal to or greater than the rate of
,downstream migration of the ripples. Dunes are like big ripples. The formation of dunes ca n be related
to large-scale turbulence within the whole flow. The water depth controls the scale of the turbulent
eddies in the flow and this in turn controls the height and wavelength of the dunes. The sloping layers
formed by the avalanching are referred to as cross-beds. The roller vortex with reverse flow in front
of the lee slope is called the eddy, this vortex may create counter-flow ripples. Cross-beds bound by
horizontal surfaces are sometimes referred to as tabular cross-bedding.
Bars are bedforms occurring within channels that are of a larger scale than dunes, their dimensions
are of the same order of magnitude as the channel they were formed in. These bars may contain dune
bedforms which migrated over the bar surface to form units of cross-bedded sands. Horizontal
layering in sands deposited from a flow is referred to as plane bedding in sediments and produces
structures called planar lamination in sedimentary rocks. Primary current lineation is seen on the
surfaces of planar beds as parallel lines of main grains which form very slight ridges and may often be
rather indistinct. Flow may be subcritical, often with a smooth water surface or supercritical, with an
uneven surface. These flow states relate to the Froude number which is a relationship between the
flow velocity (v), flow depth (h), and gravity (g) Fr = v/√g * h. The Froude number can be considered
to be a ratio of the flow velocity to the velocity of a wave in the flow. IF the value is less then one the
flow is subcritical, and a wave can propagate upstream because it is travelling faster than the flow. If
the value is greater than one this indicates that the flow is too fast for a wave to propagate upstream
and the flow is supercritical, anti-dunes may form. The bedform stability diagram indicates the
bedform that will occur for a given grain size and velocity. Two general flow regimes are recognised:
a lower flow regime in which ripples dunes and lower plane beds are stable due to a subcritical flow,
and a upper flow regime where plane beds and anti-dunes form, the change to supercritical flow lies
in this section.
The height and energy of waves is determined by the strength of the wind and the fetch, the expanse
of water across which the wave-generating wind blows. The depth to which surface waves affect a
waterbody is referred to as the wave base. If the water motion is purely oscillatory, the ripples formed
are symmetrical, but a superimposed current can result in asymmetrical wave ripples. At low energies
rolling grain ripples form, which are characterised by broad troughs and sharp crests. Wave ripples
are formed only in relatively shallow water in the absence of strong current, whereas current ripples
may form because of water flow in any depth in any subaqueous environment.
Different types of mass flows might be formed in different circumstances:
- debris flow, a moving mass of loose mud, sand, soil, rock, water, and air that travels down a slope
under the influence of gravity.
- turbidity currents, gravity-driven turbid mixtures of sediment temporarily suspended in water,
often turbulent flows. The deposit is called a turbidite. The first material to be deposited from a
turbidity current will be the coarsest as this will fall out of suspension first. Therefore a turbidite
is characteristically normally graded.
, - Grain flows, avalanche of mass transport down a steep slope.
Mudcracks form as clay-rich
sediments lose water the volume is
reduced, and clusters of clay
minerals pull apart developing
cracks in the surface. Under
subaerial conditions a polygonal
pattern of cracks develops when
muddy sediments dry out
completely, desiccation cracks.
Syneresis cracks are shrinkage
cracks that form underwater, as
the clay layer settles and compacts it shrinks to form single cracks. Scouring mat form a channel which
confines the flow, the presence of an erosional scour surface marks the base of the channel. Small-
scale erosional features on a bed surface are referred to as sole marks. Which are divided into scour
marks, form because of turbulence in the water, and tool marks, impressions formed by objects
carried in water flow. A bed is a unit of sediment which is generally uniform in character and contains
no distinctive breaks. Alternations of thin layers of different lithologies are described as interbedded.
Cross-stratification is any layering in a sediment or sedimentary rock that is oriented at an angle to
the depositional horizontal. If the bedform is a ripple the resulting structure is referred to as cross-
lamination. Migration of dune bedforms produces cross-bedding, which may be tens of centimetres
to tens of metres in thickness. A single unit of cross-laminated, cross-bedded or cross-stratified
sediment is referred to as a bed-set. Where a bed contains more than one set of the same type of
structure, the stack of sets is called a co-set. Flaser bedding is characterised by isolated thin drapes of
mud amongst the cross-laminae of a sand. Lenticular bedding is composed of isolated ripples of sand
surrounded by mud, and intermediate forms made up of approximately equal proportions of sand and
mud are called wavy bedding.
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