Microsoft PowerPoint - AGH Sed 4 sed transport & deposition EN ver2 HANDOUT.PPT

SED

IMENTOLOGY

Flow regime, sediment transport & deposition

Sediment transport and deposition

Transport modes in a turbulent fluid

•

Traction (rolling over the bed surface)

•

Saltation (jumping over the bed surface)

•

Suspension (permanent transport within the fluid)

•

Solution (chemical transport)

Hjulstrom

diagram

Sediment transport and deposition

comment

•

Fluid density and viscosity play a key role in determining which

particle sizes can be transported

•

The amount of sediment transport is not only related to flow

velocity

(or bed shear stress)

and grain size, but also to:

•

Grain density

•

Grain shape

BED FORMS AND

SEDIMENTARY

STRUCTURES

(

processes and results

)

The flow regime concept

... is the he result of experimental research

into fluid flows and their depositional
results

… relates flow energy with the deposited

bed forms and their internal structures

Flume tank experiments

Straight crest

Curved crest

POJĘCIE REŻIMU

PRZEPŁYWU:

Sed. structures:
Cross-lamination
(= small-scale)

Cross-bedding
(= large scale)

Parallel lamination
with
parting lineation

Cross-lamination
/cross-bedding
dipping upstream

Bedforms:
Current megaripples with megaripples-dunes
ripples

small ripples on top without small ripples

Bedforms:

antidunes

planar bed (upper)

'shooting flow'

Lower flow regime

Current

ripplemarks

and dunes

(

bed forms

)

& their sedimentary structures

cross

lamination and cross

bedding

Features:

- asymmetrical bed forms: stoss side is gentle, lee
side : much steeper (up to 30 degr.)

- height: 0.5 – 3 cm; wave length 5-40 cm

- grain size: <0.7 mm (<medium sand)

- structure: cross-lamination

Origin:

Origin of cross lamination

in current

ripplemark

Sedimentation
on the lee side:
grain avalanching.

Origin: key points:

- Stoss side (windward): grains rolled up the slope

- Temporary accumulation at the crest

- Lee-side: grain avalanching (initiated when the

slope reaches the angle of repose) & accumulation
of a lamina/bed inclined with regard to the
depositional surface (= horizontal bottom)

This way foresets originate

Slope of the foreset beds is a function of the
grain size of the sediments:

• coarse sediments (sand and gravel) result in

steep slopes

• fine sediments (fine sand and silt) result in

shallow slopes

Classification

SET

COSET

(= composite set)

The lower boundary of each set:

- is erosional

- its shape defines the type of

cross-laminated

or cross-bedded set:

Sets:

Tabular - have planar bounding
surfaces

Trough - lower surfaces curved
or scoop-shaped and truncate the
underlying beds

Wedge (a variety of tabular)

The shape of the crest defines the shape of the

lower boundary of the set – see classification 2-D & 3-D forms

Tabular - 2D
Crest straight

Trough – 3D
Crest curved and
bedfrom height
decreases laterally

Size classification: ripples vs. dunes

(& frequency of occurrence)

megaripples

Megaripplemarks

•

Grain size: >0.2 mm (= >fine sand)

•

Height usu up to a few dcm

Climbing current ripples
Origin:

•

Deposition out of traction associated with
suspension

•

The higher the intensity of deposition out of
suspension, the steeper the angle of climb

Backflow

with formation of

backflow ripples

STOPPED HERE

SAND WAVES : S. Francisco Bay (Golden

Gate)

Aeolian dunes

Barchan

Atacama Desert, Chile

Avalanches of sand grains
(sandy grain flows) down
the stoss side

Aeolian versus water

born cross

bedding/lamination

Aeolian:
Steeper dips of foresets (high inter-granular friction of

dry sediment)

Inversely graded (dispersive pressure in dry sand

avalanches: dry grain-flow – grain-to-grain collisions)

Water-born:
Lower dips of foresets (low inter-granular friction of

water-lubricated sediment)

Inverse grading absent (sorting in water-saturated

avalanches of sand grains)

Upper flow regime

Sediment transport and deposition

Plane beds and antidunes

•

In coarse sands (>0.7 mm) lower-stage plane beds develop instead of current ripples

•

At high (

but still subcritical

) flow velocities upper-stage plane beds

are formed in all sand grain sizes

•

At still higher flow velocities (

supercritical flow conditions, Fr

≈1 or higher

)

antidunes are formed, characterized by bedform accretion in
an upstream direction

Parting lineation: plane bed, upper flow regime

Upper flow regime (supercritical flow): standing

waves – here antidunes are formed

• eg. – a current ripplemark is a bed form

• Its internal organisation of sediment:

sedimentary structure = cross lamination

REMEMBER

DISTINGUISH BETWEEN THE BED FORMS AND

SEDIMENTARY STRUCTURES

Sediment transport and deposition

by waves

Waves

•

Waves are wind-generated oscillatory motions

of water

•

Wave height is dependent on wind strength

•

The depth to which the oscillatory motion due

to wave action extends is known as the

wave base

•

Shallow water leads to breaking waves

•

Wave ripples are distinct from current ripples

due to their symmetry and include low-

energy ‘rolling grain ripples’ and high-energy

‘vortex ripples’

Sediment transport and deposition

Waves

•

Waves are wind-generated oscillatory motions of

water

•

Wave height is dependent on wind strength

•

The depth to which the oscillatory motion due to

wave action extends is known as the wave base;

shallow water leads to breaking waves

•

Wave ripples are distinct from current ripples due to

their symmetry, and include low-energy ‘rolling grain

ripples’ and high-energy ‘vortex ripples’

oscillation waves

translation waves

swash &
backwash

breakers

Animation wave ripples

Internal structure of wave ripplemarks; Tumlin quarry
(Gradzinski)

Features:

- symmetrical bed forms
- height: 0.5 – 3 cm
- grain size: <0.7 mm (<medium sand)
- structure: composite lamination, bi-directional

------------------------------------------------------------
Important bathymetry indicator: are formed above

the wave base!!!!

Wave

ripplemarks

Interference ripples

Sediment transport and deposition

Tides

Tides result from the gravitational attraction of the

Moon and Sun on the Earth, combined with the

centrifugal force caused by movement of the Earth

around the center of mass of the Earth-Moon system

•

Semi-diurnal (= every half-a-day) or diurnal tidal cycles

•

Neap-spring tidal cycles

•

Annual tidal cycles

Tide changes proceed via the following stages:

•

Sea level rises over several hours, covering the intertidal zone:

flood tide.

•

The water rises to its highest level, reaching high tide & stays at

this level (slack water).

•

Sea level falls over several hours, revealing the intertidal

zone: ebb tide.

•

The water stops falling, reaching low tide (slack water).

Neap & spring cycles

Neap-spring tidal cycles are controlled by the position of

the Moon relative to the Sun and Earth

•

Spring tides (meaning: 'rise'): when the Earth, Moon, and Sun
are all in a line (Full and New Moon Phases) the high tides
are MUCH higher than at other times

•

In brief: high waters are higher than average, low waters are

lower than average

•

Neap tides (unknown origin/meaning): when the Moon and Sun
are at right angles to each other the high tides are lower
than at other times

•

In brief: Neaps result in less extreme tidal conditions

•

There is about a seven-day interval between springs and
neaps

Tidal currents (

cont from here on 8/05

)

Tidal ranges around UK

erringbone

cross stratification

•

Origin: deposition of current ripples or dunes by
the currents of alternating opposite flow directions

•

Are characteristic for tidal conditions (tide-ebb-
…-...)

NOTE:

a) Ripplemarks originate only in non-

cohesive sediment (sand, coarse silt)
transported by traction

b) Muds are deposited out of suspension

only (being cohesive, cannot be
transported by traction)

c) Therefore: interbeds of ripple cross

laminated sand and mud reflect
significant oscillations of the current
energy: traction – suspension (V~0 =
slack water) – traction – suspension -
(V~0 = slack water) ….

Association

Ripplemar

& mud

Bedding types:
A. Flaser

B. Wavy

C. Lenticular

D. Starved ripples

Sedimentary environment – most often tidal;

but also: fluvial – flood plain area

Ocean currents

•

The circulation of sea water in the world’s oceans is driven by

wind and contrasts in density due to variable temperature and

salinity (thermohaline circulation), combined with the Coriolis
effect

•

Ocean currents transport clay and silt in suspension, and sand

as bed load, and their effects are especially important in deep
waters, where storms and tides are less important

Global system of winds

– Solar energy: variations in temperature

– Coriolis effect:

deflection of fluid flow in

motion = winds (also water currents

– Major mountain ranges: deflection of

winds

Gravity flows in general

Two groups of flows generated by

gravity:

1. Fluid gravity flows – the motion of fluid powered

by gravity sets sediment grains in motion (e.g

rivers)

2. Sediment gravity flows: gravity sets sediment in

motion, which in turn sets the ambient fluid in

motion due to friction.

Four mechanisms supporting grains in sediment

gravity flows

•

Grain flow. Mechanism: (explain) Result: very well sorted sand

•

Debris flow. Mechanism: (explain) Result: Poorly sorted, matrix-rich,

massive or inversely-graded bed

•

Liquefied flow. Mechanism: pore-fluid escape. Result: dish structures

water escape pipes, sand volcanoes

•

Turbidity current. Mechanism: turbulence. Result: normally-graded bed;

Bouma sequence

Sediment transport and deposition

Gravity flows

•

Debris flows have a high (>50%) proportion of sediment to

water and can be both subaerial and subaqueous

•

Low Reynolds numbers

• Turbidity currents have a higher proportion of water, are

always subaqueous, and move due to density contrasts

•

Higher Reynolds numbers

Sediment transport and deposition

(

cont from

here

)

Gravity flows

• Debris flows have a high (>50%) proportion of sediment to

water and can be both subaerial and subaqueous

•

Low Reynolds numbers

•

Turbidity currents have a higher proportion of water, most

commonly are subaqueous, and move due to density contrasts

•

Higher Reynolds numbers

Autosuspension

Motion

Turbulence

Suspension

a part of
mud
remains
in
suspen-
sion
after the
turbidity
current

V = 0

with sole marks

Bouma sequeence
intervals (Ta – Te)

Newfoundland continental slope

Erosional

structures

• Channels

Erosional

structures

• Small channel

• Scour-and-fill

Sole marks

origins

Conditions of origin:
1. Bottom - covered with cohesive sediment (mud)
2. Erosion - caused by turbulence cells in, or objectes carried

by, turbidity current (shale clasts, wood fragments, fish
bones, itp.)

3. Sand deposition of the overlying bed – immediately after

erosion, preferably out of the same turbidity currrent that
caused erosion

Note – the erosional feature is a mold and the sole mark, which

we see at the base of the corresponding sandstone bed, is a
cast

Other sedimentary structures

Resulting from liquefaction of sandy sediment:

•

Sand volcanoes

•

Clastic dikes (sand dikes)

Deformational structures

Convolute lamination
Recumbent folds
Slump folds
Slump beds
Load casts
Flame structures
Ball-and-pillow

Dessication cracks (mud cracks)

Bioturbations

& trace fossils

(

ichnofossils

)

• Bioturbations (general term): Disturbances of

sediment by organisms

• Preserved traces of activity of organisms
Traces of:
Resting, crawling, walking, feeding, hiding,

burrowing, etc

• Traces of infauna: living within sediment
• Traces of epifauna: living on sediment surface