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Sedimentologi Kamal Roslan Mohamed. PROCESSES OF TRANSPORT & SEDIMENTARY STRUCTURES. INTRODUCTION.

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Sedimentologi

Kamal Roslan Mohamed

PROCESSES OF TRANSPORT & SEDIMENTARY STRUCTURES


INTRODUCTION

Most sedimentary deposits are the result of transport of material as particles. Movement of detritus may be purely due to gravity but more commonly it is the result of flow in water, air, ice or dense mixtures of sediment and water.

The interaction of the sedimentary material with the transporting media results in the formation of bedforms, which may be preserved as sedimentary structures in rocks and hence provide a record of the processes occurring at the time of deposition.

Understanding these processes and their products is therefore fundamental to sedimentology.


TRANSPORT MEDIA

Gravity

The simplest mechanism of sediment transport is the movement of particles under gravity down a slope.

Water

Transport of material in water is by far the most significant of all transport mechanisms.

Air

Wind blowing over the land can pick up dust and sand and carry it large distances.

Ice

Water and air are clearly fluid media but we can also consider ice as a fluid because over long time periods it moves across the land surface, albeit very slowly.

Dense sediment and water mixtures

When there is a very high concentration of sediment in water the mixture forms a debris flow, which can be thought of as a slurry with a consistency similar to that of wet concrete.

Movie

GLACIEL


THE BEHAVIOUR OF FLUIDS AND PARTICLES IN FLUIDS

Laminar flow / arus lamina

all molecules within the fluid move parallel to each other in the direction of transport: in a heterogeneous fluid almost no mixing occurs during laminar flow.

Turbulent flow / arus gelora

molecules in the fluid move in all directions but with a net movement in the transport direction: heterogeneous fluids are thoroughly mixed in turbulent flows.


the velocity of flow

the diameter of a pipe or depth of flow in an open channel

The fluid kinematic viscosity

REYNOLDS NUMBER (Re)

Fluid flow in pipes and channels is found to be laminar when the Reynolds value is low (<500) and turbulent at higher values (>2000).

Laminar flow occurs in debris flows, in moving ice and in lava flows.

Fluids with low kinematic viscosity, such as air, are turbulent at low velocities so all natural flows in air that can transport particles are turbulent.

Water flows are only laminar at very low velocities or very shallow water depths, so turbulent flows are much more common in aqueous sediment transport and deposition processes.


Transport of particles in a fluid

Particles move in a flow by rolling and saltating (bedload) and in suspension (suspended load).

Rolling/Golekan: the clasts move by rolling along at the bottom of the air or water flow without losing contact with the bed surface.

Saltation/loncatan: the particles move in a series of jumps,periodically leaving the bed surface, and carried short distances within the body of the fluid before returning to the bed again.

Suspension/ampaian: turbulence within the flow produces sufficient upward motion to keep particles in the moving fluid more-or-less continually.


Entraining/seretan particles in a flow

Rolling grains are moved as a result of frictional drag between the flow and the clasts. However, to make grains saltate and therefore temporarily move upwards from the base of the flow a further force is required.

The lift force resulting from the Bernoulli effect causes grains to be moved up from the base of the flow.


  • Melantum

  • Berguling


Grain size and flow velocity

The fluid velocity at which a particle becomes entrained in the flow can be referred to as the critical velocity / halaju genting.

The drag force required to move a particle along in a flow will increase with mass, as will the lift force required to bring it up into the flow.

A simple linear relationship between the flow velocity and the drag and lift forces can be applied to sand and gravel, but when fine grain sizes are involved things are more complicated.


Grain size and flow velocity

The Hju¨ lstrom diagram


Grain size and flow velocity

The fluid velocity at which a particle becomes entrained in the flow can be referred to as the critical velocity / halaju genting.


Grain size and flow velocity

The drag force required to move a particle along in a flow will increase with mass, as will the lift force required to bring it up into the flow.


Grain size and flow velocity

A simple linear relationship between the flow velocity and the drag and lift forces can be applied to sand and gravel, but when fine grain sizes are involved things are more complicated.


Clast-size variations: graded bedding/Lapisan bergred

The grain size in a bed is usually variable and may show a pattern of an overall decrease in grain size from base to top, known as normal grading, or a pattern of increase in average size from base to top, called reverse grading/bergred songsang.

The settling velocity of particles in a fluid is determined by the size of the particle, the difference in the density between the particle and the fluid, and the fluid viscosity. The relationship, known as Stokes Law.


FLOWS, SEDIMENT AND BEDFORMS

A bedform is a morphological feature formed by the interaction between a flow and cohesionless sediment on a bed.

Current ripples/riak arus

Ripples in sand in a flowing stream and sand dunes in deserts are both examples of bedforms, the former resulting from flow in water, the latter by airflow.

Flow over a bedform: imaginary streamlines within the flow illustrate the separation of the flow at the brink of the bedform and the attachment point where the streamline meets the bed surface, where there is increased turbulence and erosion. A separation eddy may form in the lee of the bedform and produce a minor counter-current (reverse) flow.

Movie

riak


Current ripples/riak arus

In plan view current ripples may have straight, sinuous or isolated crests.

Migrating straight crested ripples form planar cross-lamination. Sinuous or isolated (linguoid or lunate) ripples produce trough cross-lamination.

Climbing ripples: in the lower part of the figure, more of the stoss side of the ripple is preserved, resulting in a steeper ‘angle of climb’.


Dunes/gumuk

Beds of sand in rivers, estuaries, beaches and marine environments also have bedforms that are distinctly larger than ripples. These large bedforms are called dunes.

Dune bedforms in an estuary: the most recent flow was from left to right and the upstream side of the dunes is covered with current ripples.



Bar forms/bentuk beting

Bars are bedforms occurring within channels that are of a larger scale than dunes.

Bars can be made up of sandy sediment, gravelly material or mixtures of coarse grain sizes.

In a sandy channel the surfaces of bar forms are covered with subaqueous dune bedforms, which migrate over the bar surface and result in the formation of units ofcross-bedded sands.

Planar tabular cross-stratification with tangential bases to the cross-beds


Plane bedding and planar lamination/lapisan meyatah

Horizontal layering in sands deposited from a flow is referred to as plane bedding in sediments and produces a sedimentary structure called planar lamination in sedimentary rocks.

Horizontal lamination in sandstone beds.


Bedform stability diagram

This bedform stability diagram indicates the bedform that will occur for a given grain size and velocity and has been constructed from experimental data.

Two general flow regimes are recognised: a lower flow regime in which ripples, dunes and lower plane beds are stable and an upper flow regime where plane beds and antidunes form.

A bedform stability diagram which shows how the type of bedform that is stable varies with both the grain size of the sediment and the velocity of the flow.


WAVES/ombak

A wave is a disturbance travelling through a gas, liquid or solid which involves the transfer of energy between particles.

In their simplest form, waves do not involve transport of mass, and a wave form involves an oscillatory motion of the surface of the water without any net horizontal water movement.

The waveform moves across the water surface in the manner seen when a pebble is dropped into still water.

When a wave enters very shallow water the amplitude increases and then the wave breaks creating the horizontal movement of waves seen on the beaches of lakes and seas.


Formation of wave ripples

The oscillatory motion of the top surface of a water body produced by waves generates a circular pathway for water molecules in the top layer. This motion sets up a series of circular cells in the water below.

In shallow water, the base of the water body interacts with the waves. Friction causes the circular motion at the surface to become transformed into an elliptical pathway, which is flattened at the base into a horizontal oscillation.

This horizontal oscillation may generate wave ripples in sediment.

The formation of wave ripples in sediment is produced by oscillatory motion in the water column due to wave ripples on the surface of the water. Note that there is no overall lateral movement of the water, or of the sediment. In deep water the internal friction reduces the oscillation and wave ripples do not form in the sediment.


Characteristics of wave ripples

In plan view wave ripples have long, straight to gently sinuous crests which may bifurcate (split); these characteristics may be seen on the bedding planes of sedimentary rocks.

In cross-section wave ripples are generally symmetrical in profile, laminae within each ripple dip in both directions and are overlapping.

Forms of wave ripple: rolling grain ripples produced when the oscillatory motion is capable only of moving the grains on the bed surface and vortex ripples are formed by higher energy waves relative to the grain size of the sediment.

Movie

OMBAK


MASS FLOWS

Mixtures of detritus and fluid that move under gravity are known collectively as mass flows, gravity flows or density currents.

A number of different mechanisms are involved and all require a slope to provide the potential energy to drive the flow.

Movie

Aliran debris

Mechanisms of gravity-driven transport on slopes. Rock falls and slides do not necessarily include water, whereas slumps, debris flows and turbidity currents all include water to increasing degrees.


Debris flows

Debris flows are dense, viscous mixtures of sediment and water in which the volume and mass of sediment exceeds that of water.

A dense, viscous mixture of this sort will typically have a low Reynolds number so the flow is likely to be laminar.

In the absence of turbulence no dynamic sorting of material into different sizes occurs during flow and the resulting deposit is very poorly sorted.

Material of any size from clay to large boulders may be present.

A debris-flow deposit is haracteristically poorly sorted, matrix-supported conglomerate.


Turbidity currents

Turbidity currents are gravity-driven turbid mixtures of sediment temporarily suspended in water. They are less dense mixtures than debris flows and with a relatively high Reynolds number are usually turbulent flows.

They flow down slopes or over a horizontal surface provided that the thickness of the flow is greater upflow than it is downflow.

Turbidity currents, and hence turbidites, can occur in water anywhere that there is a supply of sediment and a slope.

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.

A turbidity current is a turbulent mixture of sediment and water that deposits a graded bed – a turbidite.

Movie

TURBIDIT


Bouma sequence

Ta This lowest part consists of poorly sorted, structureless sand: on the scoured base deposition occurs rapidly from suspension with reduced turbulence inhibiting the formation of bedforms.

Tb Laminated sand characterises this layer, the grain size is normally finer than in ‘a’ and the material is better sorted: the parallel laminae are generated by the separation of grains in upper flow regime transport.

Tc Cross-laminated medium to fine sand, sometimes with climbing ripple lamination, form the middle division of the Bouma sequence: these characteristics indicate moderate flow velocities within the ripple bedform stability field and high sedimentation rates. Convolute lamination can also occur in this division.

Td Fine sand and silt in this layer are the products of waning flow in the turbidity current: horizontal laminae may occur but the lamination is commonly less well defined than in the ‘b’ layer.

Te The top part of the turbidite consists of finegrained sediment of silt and clay grade: it is deposited from suspension after the turbidity current has come to rest and is therefore a hemipelagic deposit


THE BEHAVIOUR OF FLUIDS AND PARTICLES IN FLUIDS

The lower parts of the Bouma sequence are only present in the more proximal parts of the flow. With distance the lower divisions are progressively lost as the flow carries only finer sediment (Fig. 4.30) and only the ‘c’ to ‘e’ or perhaps just ‘d’ and ‘e’ parts of the Bouma sequence are deposited.

Proximal to distal changes in the deposits formed by turbidity currents. The lower, coarser parts of the Bouma sequence are only deposited in the more proximal regions where the flow also has a greater tendency to scour into the underlying beds.



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