bedforms lecture 1

8/8/2019 Bedforms Lecture 1

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Grain Motion Rolling/sliding

Continuous bed contact Saltation

Ballistic jumps: steepascent of a few grain

diameters, shallow descent In air, hundreds of grain

diameters can be achieved:lower resistance

Rebound efficiency ishigher on coarse surface

Suspension

Grains permanently

suspended in fluid

Grainscolliding

with each

other as

well asbeds


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Sediment Transport &

Load

Bedload: all grains in partial contact with bed

surface

Suspended load: weight is balanced by

turbulence in water (fine particles)

Washload: suspension of clay grade particles in

water

Dustload: v. fine suspension of particles in air

Gravity Flows: grain aggregates transported

without overlying media

Water: particles transported in aqueous flow

Air: wind movement

Waves


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Gravity Flows

For gravity flows to occur: frictionmust be overcome.

Grain Flow.

Debris Flow.

Liquefied Flow.

Composite Tubidite.


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Grain Flow

Grains avalanchingdownslope

Grain-grain collisions

Slope of 30-35+needed

Well sorted sand

layers produced May be massive parts

Reverse grading


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Debris

Flow

Slurry of particles inwater

Silt & boulders

Only gentle sloperequired

E.g. arid region with

surface sediment:rainfall


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Very concentrateddispersion of grains in

moving pore water

Moves like a liquid Requires shock to

initiate

Common inearthquake zones

LiquifiedFlow


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Bouma Sequence

River entering sea:

high particle content

can be denser.

Flows under sea

Deposits coarsefine

4 river deposits, 1

sea.

Need not be

complete

Distal turb. sea

Proximal turb. nearsource

Fast moving: erosion

of sea floor at base

CompositeTurbidite


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Calculating Flow I Reynolds Number

Change from laminar to turbulentflow occurs at some given

Reynolds Number


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Froude Number

Inertial force / gravity F > 1 = rapid flow

F < 1 = tranquil flow

Calculating Flow II


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A relationship exists between

grain size and the velocity of flowneeded to move it : Critical

Erosion Velocity

Critical Erosion Velocity


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Deduced experimentally from flows of 1m depth

Hjulstroms Diagram


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Stratification &Bedforms- StructuresFormed bySedimentation


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tabular to lenticular layers ofsedimentary rock that have

lithologic, textural or structural

unity that clearly distinguishesthem from layers above and below

Bedding = Change

Sediment compositionGrain size

Sedimentation pattern

Bedding and Lamination


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Thinly laminated planar bedding

Massive beddingPoorly bedded

Types of bedding


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--------1 mm----------------10 mm--------

Thin laminaThin bed

--------3 mm----------------100 cm--------

Medium laminaMedium bed

--------10 mm----------------300 mm--------

Thick laminaThick bed

--------30 mm--------------1000 mm-------

Very thick laminaVery thick bed

LaminaeBeds

Rule of thumb:

Beds > 1cm

Laminae < 1 cm

Scales


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After deposition, beds may bemodified by:

Erosion

Compaction Chemical dissolution (especially

due to pressure)

Modification of bedding


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Types of Bedforms

A relationship exists between

sediment grain size, flow velocity

and bedforms.

Based on flume experiments using

flow depths of approx. 20 cm.


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Bedforms, Flow andGrain Size


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Possible by:

Settling of fines from suspension

(FB)

E.g. salt/ calcite precipitatingas thin parallel laminae.

High flow rate removing

irregularities (UFB) Most sand grade parallel

bedding: turbulent flow

Primary Current Lineation Turbulent flow causes Taylor

vortices : ridges normal to

flow.

Flat

parallelbedding


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Increasing flow velocity:

topography on bed surfaceincreases.

Ripples Dunes Sand

Waves. Aqueous forms: move

downstream under the

influence of unidirectional

aqueous flows.

Tides = bi-directional flow.

Ripples and Dunes


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Usually a few tens of cm high Ripple Index (R.I.) = length/height = 8 to

10

Ripple Wavelength () = distancebetween two troughs/ crests

Lee side: facing flow direction

Stoss side: opposing flow direction

(sheltered)

Ripples


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Ripple structure andterminology


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Wavelength may be 1m+

Max height : few tens of cm

R.I. = 8 - 20

Ripples and Dunes: similar crosssection thus similar Ripple Indices

Megaripples


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Wavelength may be 100 m+

height : low

R.I. = much higher

Sand Waves


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Determined by flow velocity

Ripples

Straight

Sinuous

Catenary

DunesStraight Sinuous Catenary

Lunate/ Linguoid

INCREASING FLOW RATE

N.B. shapes

are those seen

looking down

on to the

bedding

surface

Ripple/ Dune Shapes


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Straight-crestedripples


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Sinuous Ripples


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Linguoid Ripples


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Wave Ripples


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Cross-bedding andCross-lamination

Downstream migration of ripples/dunes under conditions of netsedimentation results in cross-stratification

Cross-strata (foresets) = formerposition of ripple/dune lee face

2 types: Tabular cross-strat (2D source)

Trough cross-strat (3D source)

Cross-bedding =

migration of

dunes

Cross-

lamnation =

migration of

ripples


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Whilst we discuss cross-bedding

mainly in sandstones, it is alsopossible to find it in conglomerates.

Braided river systems (high

energy) often have cross beddedgravel bars.


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Produced by straight bedforms

Foresets (sloping beds) dip up to approx 30

Few 10s of cm to m+ foreset thickness

Tabular Cross-bedding


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Tabular Cross-beds

T h C b ddi


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Produced by 3D bedforms especially sinuous

and lunate

Scoop-shaped beds generally have tangential

bases ad dip 20 30

Changes in flow velocity or depth can cause

erosion surface. After recommencement of

deposition this forms a Reactivation Surface

N.B.

Linguoid

ripples

producetrough

cross-

lamination

Trough Cross-bedding


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Trough Cross Beds


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Climbing Ripple CrossLamination Material moved from stoss slope

and avalanched down lee slope. In addition to forward movement,

sand builds up, forming climbing

ripple cross-lamination.


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Climbing Ripple CrossLamination


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Mixed Deposition

Mixed sandstone/ mudstone deposits

2 extremes

Mainly sand deposition, small lenses ofmud at base of sand troughs : FlaserBedding

Mainly mud deposition with small lensesof sand: Lenticular Bedding

Occurrence:

Not very common: needs contrastingdepositional regimes

High energy/ strong currents: sand

Low energy/ quiescent: muds

Energy fluctuations common in TidalAreas


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Flaser Bedding


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Lenticular Bedding


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Tidal Areas


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Tidal Areas

Avalanching sand.

Tidal movements create small

foresets dipping in oppositedirections.

Herring-bone Cross

Stratification


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Herringbonecross-stratification


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Tidal Areas

Meandering tidal channels (also rivers) Erosion and deposition

Erosion: outer-side of meander Deposition: inner side of meander Point Bar

As point bar moves laterally: large scaleCross-bedding deposited: Lateral AccretionSurface

Lateral AccretionSurface


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Lateral Accretion


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Tidal Areas

Wave-formed ripples

Wave formed Ripples


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Tidal AreasWave-formed Ripples

Common in shallow seas, deltas andlakes

In section: crest are more pointed thanunidirectional current crests, symmetrical

In plan: continuous, straight andcommonly bifurcate with symmetry,diagnostic for wave ripples

R.I = 6 10 (current = 8-20)

R.I = f (water depth, grain size) : onlyform in shallow waters as waves only

affect sed down to half the wave length(wave base)

Ripple formation depth = / 2

= Wave Base

Waveripples

complex internal structures


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Tidal Areascomplex internal structures

Waveripple

structures

bedforms lecture 1

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