Rodo Diapositivas 40al 46

8/17/2019 Rodo Diapositivas 40al 46

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Sedimentary

Structures

I.G.Kenyon


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Give information about thedepositional environment

Allow the ‘way-up’ of

beds to be ascertained


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A Bed

A layer of roc separated from the layer

above and below by a beddin! plane

A bed represents a sin!le unbroen

episode of sediment accumulation

Beds vary in thicness from

"cm to many tens of metres

Beds # to $cm thic arecalled fla!s or fla!stones

Beds may occur in uniform thicnesses

over lar!e areas or pinch out laterally


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Beds % Beddin! &lanes' Blue (ills' )ear St.A!nes

*ne bedBeddin! &lanes

+" coin

for scale


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Beddin! &lane

,efines the top or bottom of a bed

epresents a chan!e in the

nature of sedimentation

a chan!e in the rate or

type of sedimentation

a pause where no sediment is deposited

a period of erosion where

some sediment is removed


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amination

A layer of sediment /"cm thic

0ommon in ar!illaceous rocs

such as siltstone and shaleIndividual laminations may be

1ust "mm thic or even lessIf the sedimentary unit

is 2"cm thic it is a bed


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aminations in ,evonian 3ylor Beds' &orthleven

"cm

aminations here are /"mm thic

,ifference in colour e4plained by variation in amount of

or!anic5carbonaceous matter incorporated into the sediment

3ineral content mainly clay minerals

such as aolinite' illite and serecite


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0ompetent Beds

A bed of roc' which durin! foldin!'fle4es and bends without appreciable

flow or internal shear to maintain its

ori!inal thicness

3echanically stron! rocs such

as limestones and sandstonescommonly show this behaviour


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Incompetent Beds

A bed of roc that deforms internally

durin! foldin!' resultin! in rapid

chan!es in lateral thicness

3echanically wea rocs suchas clays' mudstones and shales

commonly show this behaviour


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0ompetent and Incompetent Beds at 3illoo' )ear Bude

Sandstone is competent' retainin! ori!inal

thicness in the limbs and nose of the fold

6ension cracs occur in sandstonearound the nose of the fold

Shale deforms and thins

on the fold limbs

Shale is much thicer in

the nose5core of the fold

"m


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6he aw of Superposition

7irst proposed by )icolausSteno in the "8th 0entury

If one bed of sediment lies on top of another'

then the one above must be the youn!er

6his assumes the beds have not been

overturned due to earth movements

Sedimentary structures collectively nown

as ‘way-up criteria’ can be used to decide

if the beds have been overturned or not


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Graded Beddin!A bed which displays a finin!

upwards se9uence from the base.

6he finin! upward se9uence may

be produced in several ways

:cm

:cm


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6he 7ormation of Graded Beddin! "

&ro!ressive settlin! of !rade si;es from coarse to

fine in comparatively calm bodies of water


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7ormation of Graded Beddin! by 6urbidity 0urrents


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Graded Bed with an


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6he 7ormation of Graded Beddin! #

=ariations in the seasonal supply of

sediment' for e4ample deposition from

!lacial meltwater in a pro-!lacial lae

In Sprin!5Summer much meltwater is

available and coarse sand and !ravel may

be transported and deposited in the lae

In Autumn5>inter' the meltwater willbe !reatly reduced' the lae may even

free;e over allowin! only the finer silt

and clay to settle out from suspension


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3illstone Grit showin! Graded Beddin!

7inin! upwards

se9uence

&articles mainly #-?mm at the base

&articles @.$ to ".@mm at the top

"cm

,eltaic deposit with seasonal

fluctuations in ener!y conditions


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6he 7ormation of Graded Beddin! :

Seasonal variation in river dischar!e-in>inter coarse sand and !ravel may be

deposited durin! hi!h dischar!es' in summer

finer sand and silt may be deposited when low

flow conditions occur.


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0ross Beddin!

Also nown as 0urrent Beddin! and 7alse Beddin!

If very lar!e scale it is termed ,une Beddin!

If very small scale it is termed 0ross aminationIn each case the sediment is bein!

moved and accumulated at an an!le

to the principal beddin! direction

&roduced by a uni-directional current of

wind or water movin! sediment as a series

of asymmetrical ripples or dunes


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7oreset beds

6opset beds are truncated


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#m

,une Beddin! C ar!e Scale 0ross Beddin!

S l 0 B ddi , B ddi


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ar!e Scale 0ross Beddin! C ,une Beddin!

&eople for scale

&alaeo-wind direction

indicated by yellow arrows

6opset beds are truncated

7oreset and bottom

set beds preserved


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(errin! Bone 0ross Beddin!

&ennife for scale

Dpper Dnit

3iddle Dnit

ower Dnit

epresents a current reversal throu!h "E@F. Blue arrows indicate the

direction of sediment movement in each of the : units above

0 i ti = ll l b ddi


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0ross amination =ery small scale cross beddin!

6runcation5erosion surface of topset beds

&en top

for scale

7ine sandstone unit' 0ompass &oint near Bude

Appro4imate base of

cross laminated unit

0urrent

direction

Individual laminations # to ?mm thic

0 l B ddi 5Sl B ddi "


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0onvolute Beddin!5Slump Beddin! "

0ommon on deltas

where sediment is

saturated with water

and easily mobilised

*ccurs fre9uently in

interbedded sandstoneand shale se9uences

Shales deform internally

and flow showin!

incompetenceSandstone layers brea

into ri!id blocs which

become displaced and

show competenceIncompetent shale 0ompetent sandstone

0 l t B ddi 5Sl B ddi #


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0onvolute Beddin!5Slump Beddin! #

*ften initiated bya minor tectonic

disturbance or

slope failure

0an also be formedby the rapid e4pulsion

of pore water

6he e4ample here is

from the 0arboniferousbeds at 0ompass

&oint near Bude

i!id' competent sandstone blocs

Incompetent shale

which has flowed

or deformedinternally

I l d d5, i d 7 t


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Included5,erived 7ra!ments

unconformity

ower older series

oun!er upper series

*lder beds may beeroded before the

deposition of the ne4t

bed in the se9uence

6he eroded fra!mentsare then included as

clasts in the bed above

ower older series

oun!er upper series

,erived

fra!ments

,erived fra!ments from

older lower series

"m

I b i t St t


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Imbricate Structure

0ommon in

rudaceous rocs,eposited under

the influence of a

powerful current

on! a4es of clasts

lie sub-parallel with

one another ‘leadin!’

in direction ofcurrent flow

Arrows indicate

direction of flow

3 d 0


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3ud 0racs

7ormed when sediment is

e4posed to the atmosphere

0ommon in tidal flats'

mudflats and playa laes

3ud cracs form as

desiccation poly!ons

6he sediment dries out

and shrins as water

is evaporated from it

0ontraction centres develop

and a poly!onal patternof cracs develop

Analo!ous to columnar

1ointin! in coolin! lavas:@cm

)ote how the ed!es curl

up to accentuate the =

shaped !ap between them

3 d


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3udcracs

6he mud cracs are widest at

the surface taperin! to a point

at a depth of @.$ to #.@ cm

*ften later infilled with finer'

wind blown sediment of a

different colour or calcareousmaterial if in a playa lae

3 d 0 d i &it


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3ud 0racs and ain &its

ain pits formed by impact of raindrops on an e4posed

sediment surface. 6hey appear as small rounded depressions

up to "cm in diameter' sometimes with a small raised rim.ain pits mar the top of the sediment

>ash *ut in 7ine Grained Sediment


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>ash-*ut in 7ine Grained Sediment

Scremerston' )orthumberland

7ormed as a result of Scour and 7ill

*lder laminations

truncated

Small scale channel C base

is conve4 downwards

0oin for

scale


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oad 0asts and 7lame Structures

0ommon in sandstone and shale se9uences

ocally' the denser sandstone sins down into the

less dense shale below as bulbous protrusions

6he shale is incompetent and deforms5flows upwards intothe spaces between the bulbous sandstone protrusions

6he rounded protrusions mar the base of the sandstone

bed' whilst the flames mar the top of the shale bed

Sometimes a !lobule of sandstone becomes completely

detached from the bed above and sins into the shale below

distortin! the laminations to form a teardrop structure

d 0 t d 7l St t


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0ompetent

sandstone

Incompetent shale

d 0 t d 7l St t


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Bulbous protrusions from base ofoverlyin! sandstone bed

Shale s9uee;ed up between sandstone

protrusions as a flame structure

0ompetent sandstone

Incompetent shale

6he ife &osition of 7ossils


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*r!anisms preserved inlife position such as trees

can indicate if the beds are

the ‘ri!ht way up’


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?cm

*r!anisms preserved in life position such as stromatolites

al!al mounds can indicate if the beds are the ‘correct way up’

3odern day stromatolites

Al!al mounds are conve4 upwards

Bottom

6op

0onve4

upwards

!rowin!

towards theli!ht

Stromatolites #.$ billion years old

from 0ordoba &rovence' Ar!entina



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6he Great Barrier eef' Australia

ivin! corals form the

upper part of the reef

:cm

*r!anisms preserved in life

position such as corals can

indicate if the beds are the

‘correct way up’

0orals indicate clear water less

than $@m deep' well o4y!enated

envi. with normal salinity :.$H'

temperatures ##-#EF0 and located

within :@F latitude of the e9uator

0orals preserved

in limestone

Sole Structures


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Sole Structures

7ormed in interbedded

sandstone and shale se9uences6hese are preserved on the base

of the overlyin! sandstone bed

3ain processes are scourin! and

erosion of the soft shale accumulation

surface by currents and tools

0lassified accordin! to shape

6hey include 7lute' Groove'

Bounce and &rod56ool casts

7lute 0asts en la parte de un estrato de grauvaca


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7lute 0asts en la parte de un estrato de grauvaca

,ireccin paleocorriente"@cm

0owpeel Brid!e' &eebleshire' Scotland

moldes cnicos en arenisca


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moldes cnicos en arenisca

=ista en planta la seccin transversal 9ue

muestra el des!rasado de

huecos estriados en el barro

blando por vrtices actuales

Reparto surco en la superficie inferior de un estrato de


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Reparto surco en la superficie inferior de un estrato de

grauvaca, muelle, norte devoniano

la parte superior del

boli!rafo para la

escala&osibles direcciones

paleocurrentes

7ormado por un !ui1arro rodando a travJs de una

superficie del sedimento blando y cortar una ranura en

ella. &reservado como un yeso en la superficie inferior

del estrato superpuesto

&rod 5 (erramienta echa en la parte inferior de un


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&rod 5 (erramienta echa en la parte inferior de un

estrato !rauvaca' muelle' norte devoniano

$cm

&osibles direcciones

paleocurrentes

De rebote Surco y


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De rebote, Surco y

impresin en muro de capa"cm

ebote coladas formadas por una

vJrtebra peces de a!ua salada

surcos

rectilnios

&rod 0ast

Geopetal Structures


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Geopetal Structures

conchas parcialmente

llenos de or!anismos

marinos

6ambiJn conocido

como Lfsiles niveles

de burbu1aL

Indican la cantidad de

inclinacin ha tenido

lu!ar desde la

deposicin


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Cuando el barro originalmente entró en las

cavidades que se habría conformado

horizontal debido a la influencia de lagravedad

resultados de inclinación posterior en el nivel

de lodo se trasladaron a una nueva ydiferente de inclinación horizontal de hoy

Si las cavidades están vacías o

completamente lleno de sedimentos,entonces no pueden ser utilizados como

estructuras geopetalreferente al techo y

muro en momento de formarse!


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ipple 3ars-Symmetrical

0arboniferous sandstones' 0ompass &oint near Bude

3inibus ey for scale

3ar the top of the bed and

imply the sediment was underthe influence of wave action


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6race 7ossils - Burrows


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6race 7ossils - Burrows

*r!anisms such asbivalves and marine

worms burrow from the

surface downwards

into unconsolidatedsoft sediment

6he burrows are open

at the ancient sedimentsurface and taper

downwards to a point ‘i!ht way up’

*pen at the surface

6apers to a point

‘i!ht way up’

6race 7ossils C (ori;ontal Burrows


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imestone showin! tube-lie traces of

burrowin! animals' &ort Issol' 7rance

6race 7ossils C (ori;ontal Burrows

6race 7ossils-6rails and 7ootprints


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6race 7ossils 6rails and 7ootprints

6rilobite trail

0ru;iana>indin! trails-epichnia

and &asichnia

Sauropod footprints

epresents the upper

sediment surface overwhich or!anisms

waled or crawled

elationships between types of 6race


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elationships between types of 6race

7ossils and Sedimentary


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(alite &seudomorphs

"cm

&reserved on the base of the overlyin! bed as a cast

(alite pseudomorph' the ori!inal

crystal has been dissolved away and

the mould has been infilled by mud


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Rodo Diapositivas 40al 46