F5nw-geomorphology-5a

7/29/2019 F5nw-geomorphology-5a

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f)

g)

Figure 4.37. Composition of bulk carbonate content in geomorphic features of the NNMR: a) bank/shoal; b)deep/hole/valley; c) basin; d) canyon; e) ridge; f) apron/fan; and g) terrace.

5. Geomorphology and Sedimentology ofBioregions

5.1INTRODUCTIONSampleswithquantitativetexturalandcompositionaldatarepresentalleightbioregionsinthe

NWMR (Fig.5.1;Table5.1).Newdatasignificantly improve sampledensities insevenof the

eightbioregionsandprovidethefirstsedimentdatafortheCentralWesternShelfTransitionin

theNWMR.


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A totalof 693 samples (72%)occur in the four shelfbioregionswhich comprise 1:2,550km infourShelfandtwooffshorebioregions(Table

5.1). Elsewhere in theNWMR average sample densities are lower. TheNorthwest Province

(NWP)covers178,700kmor17%of theNWMRandcontains58 samples; this results inan

averagesampledensityof80%mudcontentcomparedtoonly3%(14samples)ofsamplesinshelfbioregions.Changesin

the sedimentologyofshelfandoffshorebioregionsareattributed todecreasingenergy levels

andsediment

transport

mechanisms

with

distance

offshore

as

described

in

section

3.

Differencesbetween the texture of sediments in shelfbioregions aredifficult to quantify as

CWST andCWSP contain only nine and 13 samples, respectively.However, data from the

NWST indicate that the sediments in this bioregion differ significantly in texture and

composition from those in other shelfbioregions,withmud and gravel fractions regularly

attaining>20%andbulkcarbonatecontents80%).


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a)


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b) Figure 5.1. a) Sample coverage of bioregions within the NWMR; and b) sample density of each bioregion before and

after the task.


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a)

b)


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c)

d)


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e)

Figure 5.2. Textural composition (mud:sand:gravel ratio) of sediments in the shelf bioregions of the NWMR; a)Northwest Shelf Transition, b) Northwest Shelf Province, c) Central Western Shelf Transition, d) CentralWestern Shelf Province, and e) Carbonate content of all the above shelf bioregion sediments in the NWMR.

a)


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b)

c)


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d)

e) Figure 5.3. a) Textural composition (mud:sand:gravel ratio) of sediments in the offshore bioregions of the NWMR; a)

Central Western Transition, b) Timor Province, c) Northwest Transition, d) Northwest Province, and e)Carbonate content of all the above offshore bioregion sediments in the NWMR.


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Table 5.1. Description of change in sample coverage of the NWMR area of bioregions with task

BioregionNo. sample poin ts

(no. added for task)*% NWMR Area

Average sampledensity (km)

Northwest Shelf Transition 208 (62) 13 1:650

Northwest Shelf Province 463 (51) 20 1:450

Central Western Shelf Transition 9 (6)


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towardsthenorthofthebioregion.Samplesprovidecoverageofthemidrangeofwaterdepths

presentontheshelf,occurringatdepthsbetween50and250m.Assaysfromthesesamplesmay

notprovideagoodrepresentationoftherangeofsedimentsoccurringontheshelfduetosmall

sample numbers.Despite targeted sample acquisition, only six sampleswere collected from

reefsintheCWST;sedimentologyofreefsisdiscussedinChapters3and6.

a)

b)

Figure 5.4. a) Geomorphology of the Central Western Shelf Transition (CWST) with location of samples; and b)

Percentage area of each geomorphic feature within the CWST with number of corresponding sedimentsamples.


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a)

b)

Figure 5.5. a) Bathymetry of the Central Western Shelf Transition (CWST) with location of samples; and b)Percentage area of bathymetry class within the CWST with number of corresponding sediment samples.


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Table 5.2. Details of the geomorphology of the Central Western Shelf Transition.

Feature% of bioregionarea covered

% of NWMR areathis unit lies within

this bioregion

% of EEZ area thisunit lies within

this bioregionGeomorphic Feature

Unassigned shelf and slope 84.44 - -

Canyon 0.50 0.34 0.03

Terrace 15.05 0.49 0.19

Table 5.3. Distribution of water depths covered by the geomorphology in the Central Western Shelf Transition.

FeatureDepth Range

(m)Mean Depth

(m)

Geomorphic Province

Unassigned shelf and slope 35 - 210 105

Geomorphic Feature

Canyon 180 - 210 195

Terrace 45 - 240 125

5.2.3.SedimentologyoftheCentralWesternShelfTransitionA totalofnine sampleswere collected from theCWST, two from the shelf (unassigned), six

fromthe

slope

(unassigned)

and

one

from

the

terrace

(Figs.

5.4a

&

5.4b).

Texture

and

compositionof thesedimentsontheshelfarehomogenouswith thesandfractioncomprising

9899%,gravel


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Figure 5.6. Textural composition (mud:sand:gravel ratio) of slope sediments within the CWST.

Figure 5.7. Carbonate content of slope sediments within the CWST.

5.3.CENTRALWESTERNSHELFPROVINCE(CWSP)5.3.1

Geomorphology

and

bathymetry

TheCentralWesternShelfProvince (CWSP)coversa totalareaof33,000km2,allofwhich is

situated intheNWMR(Fig.5.1).Thisbioregionrepresents3%ofthetotalareaoftheNWMR


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(Table5.1).TheCWSPislocatedontheshelfoffshorebetweenKalbarriandCoalBay(Fig.5.8a).

TheBioregionliesonthesouthernboundaryoftheNWMRandisboundedtothenorthbythe

CWSTandtosouthbytheSouthwestShelfTransition(notincludedintheNWMR).

TheCWSPislocatedontheshelfandupperslope(Table5.4;Fig.5.8a).Theshelfvariesinwidth

from100m.(Fig.5.9b).Approximately10,000km

(>30%)occursinwaterdepthsof100

m.

Data

coverage

is

sparse

to

the

north

of

Shark

Bay

with

only

one

samplecollectedfromthisarea.

Average sample density across thebioregion is approximately 1:2,550 km. Samples attain

sufficient coverage todescribe the sedimentdistributionon the shelf (unassigned)and slope

(unassigned)only;otherfeaturescontain


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a)

b)

Figure 5.8. a) Geomorphology of the Central Western Shelf Province (CWSP) with location of sediment samples; andb) Percentage area of each geomorphic feature within the CWSP with number of corresponding sediment

samples.


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a)

b)

Figure 5.9. a) Bathymetry of the Central Western Shelf Province (CWSP) with location of sediment samples; and b)Percentage area of each bathymetry class within the CWSP with number of corresponding sediment samples.


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Table 5.4. Details of the geomorphology of the Central Western Shelf Province.



this bioregion


this bioregion

Geomorphic Feature


Bank/Shoals 0.99 1.12 0.65

Deep/Hole/Valley 2.37 0.84 0.47

Terrace 2.14 0.31 0.12

Table 5.5. Distribution of water depths covered by the geomorphology in the Central Western Shelf Province.

FeatureDepth Range

(m)Mean Depth

(m)

Geomorphic Province

Unassigned shelf and slope 35 235 105

Geomorphic Feature

Bank/Shoals 45 110 85

Deep/Hole/Valley 75 105 100

Terrace 50 175 120

5.3.3.SedimentologyoftheCentralWesternShelfProvinceAtotalof13grainsizeand15carbonateassaysareavailablefortheCWSPinwaterdepthsof

>10m.AllbutoneoftheseoccursontheshelfsouthofSharkBay. Sedimenttextureinthisarea

is relatively homogeneous. Sand is the dominant size fraction in all sampleswith contents

ranging from 79 to 100% (Fig 5.10). Sand content exceeds 96% in seven out of 13 samples.

Gravelcomprises


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a)

b)

Figure 5.10. Textural composition (mud:sand:gravel ratio) of a) shelf and b) slope sediments within the CWSP.


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a)

b)

Figure 5.11. Carbonate content of sediments within the CWSP on a) shelf and b) slope.

5.4.NORTHWESTSHELFPROVINCE(NWSP)5.4.1.GeomorphologyandbathymetryTheNorthwestShelfProvince(NWSP)coversatotalareaof209,740km2,allofwhichissituated

in theNWMR (Fig5.1).Thisbioregion represents20%of the totalareaof theNWMR (Table

5.1).TheNWSP is locatedon theshelfbetween theNorthWestCapeandCapeBougainville.

BetweenthesouthernmostpointandCapeLeveque,thebioregioncoverstheentirewidthofthe

shelf.NorthofCapeLeveque,thebioregionextendsfromtheshelfbreakto80kmofthecoast.

In thisarea it is separated from the coast andbounded to thenorthby theNorthwestShelf

Transition.Thewesternboundaryof theNorthwestShelfProvince coincideswith theTimor

ProvinceinthenorthandNorthwestTransitionandNorthwestProvinceinthesouth.


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TheNWSPislocatedpredominantlyontheshelfandincludesasectionoftheupperslope(Fig.

5.12a).Theshelf isnarrow in thesouth,approximately50kmadjacent toExmouthGulf,and

widensto>250kmnorthofCapeLeveque.Waterdepthsvaryfrom0toapproximately200m

along the shelfbreakandwithin thedeeps/holes/valleys to the southof thebioregion.More

than45%

of

the

bioregion

lies

in

depths

between

50

and

100

m

(Fig.

5.13b).

Unassigned shelf and slope in theNWSP cover122,990km2 (59%of theareaof theNWSP).

Terracescover45,570km, (22%of theareaof theNWSP)anddeep/holevalleys26,040km,

(12%).Plateaus cover 9,740 km, (5%),banks/shoals 2,310 km, (1%), andpinnacles 200 km,

(0.1%)ofthebioregions(Table5.6).

SixsignificantgeomorphicfeaturesareidentifiedfortheNWSP.TheRowleyShelfcovers47%

of thebioregion and forms >70%of the area of the shelf in theNWMR.Banks/shoals in theNWSPdisplay significantdifferences inmorphology anddimensions from those in adjacent

bioregions;they

are

therefore

likely

to

be

characterised

by

different

sedimentology.

Deeps/holes/valleys cover a significant area of thebioregion (26,040 km) and occur in two

separate locations, each displaying morphology that distinguishes them from

deeps/holes/valleyselsewhereintheNWMR.TheLevequeRise(alargeplateau)isoneofonlytwoplateausintheNWMRlocatedontheshelfandextendingintowaterdepthsasshallowas

100m. Terraces located inwater depths 40%)

samples

located>60kmfromthecoastand211(46%)inwaterdepthsof>50m.Averagesampledensity

across the bioregion is 1:450 km. Samples achieve sufficient coverage to describe the

sedimentologyinfiveofthesixsignificantgeomorphicfeaturesidentifiedforthisbioregion.A

totalof 178 sampleswere collected from theareaof theRowleyShelf; thisgivesanaverage

densityof1:560kminthisfeature.

Terraces in shallowwater (


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TheLevequeRisecontains10samplesrelativelyevenlydistributedacrossthefeature,givingan

averagedensityofapproximately1:970km. Deeps/holes/valleyscontain46samples,relatively

evenlydistributed,with an average sampledensity of 1:570 km. Banks/shoals contain four

samples,givinganaveragedensityof1:800km.

Onlytwo

samples

were

collected

from

pinnacles,

neither

of

these

occurs

within

the

area

of

pinnacles identifiedassignificant.Nosampleswereavailable for tidalsandwave/sandbanks.

SedimentologyforthesefeaturesisdiscussedinChapters3,4and6.

A totalof51 sampleswereadded to theNWSP for this study;and,despite targeted sample

addition,23of thesewerecollected from shelf (unassigned).Samples from the current study

increased coverage in deeps/holes/valley (4 samples added), plateau (1), ridge (1), slope

(unassigned)(8)andterraces(13).

Anunevendistributionofsampleswithhigherdensitiesontheoutershelf,particularlyonthe

RowleyShelf,

means

that

assays

are

likely

to

be

representative

of

the

range

of

sediments

occurringon theoutershelfbutareunlikely toaccuratelyrepresent thesedimentologyof the

innershelf. The few samples collected frombanks/shoalsmeans that the assaysmay not

representtherangeofsedimentspresentinthesefeatures.


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a)


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b)

Figure 5.12. a) Geomorphology of the Northwest Shelf Province with location of sediment samples; and b)Percentage area of each geomorphic feature within the NWSP with number of corresponding sedimentsamples.


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a)

b)

Figure 5.13. a) Bathymetry of the Northwest Shelf Province with location of sediment samples; and b) Percentagearea of each bathymetry within the NWSP with number of corresponding sediment samples.


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Table 5.6. Details of the geomorphology of the Northwest Shelf Province.



this bioregion


this bioregion

Geomorphic Feature


Bank/Shoals 1.10 7.93 4.57

Deep/Hole/Valley 12.41 27.91 15.77

Canyon 0.10 2.03 0.21

Ridge 1.23 44.50 2.31

Pinnacle 0.09 29.20 3.87

Plateau 4.64 10.34 0.65

Saddle 0.05 2.85 0.61Terrace 21.72 20.08 7.86

Table 5.7. Distribution of water depths covered by the geomorphology in the Northwest Shelf Province.

FeatureDepth Range

(m)Mean Depth

(m)

Geomorphic Province

Unassigned shelf and slope 0 250 95

Geomorphic Feature

Bank/Shoals 7 120 60

Deep/Hole/Valley 25 205 100

Canyon 130 190 165

Ridge 115 185 145

Pinnacle 5 160 60

Plateau 25 120 75

Saddle 150 165 160

Terrace 10 190 90

5.4.3.SedimentologyoftheNorthwestShelfProvinceAtotalof463grainsizeassaysand341carbonateassaysareavailablefortheNWSP.Sediments

in the south of the NWSP differ significantly from those in the north. South of Broome,

sedimenttextureisrelativelyhomogeneousanddominatedbysand. Sandcontentexceeds60%

inmostsamplesandexceeds70%in220(66%)samples.Sandcomprises


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coastandwithin100kmoftheshelfbreakandisgenerallyabsentfromsedimentsinthearea

between.

NorthofBroome,sedimenttextureismorevariable.Sandandgraveldominate,withcontents

forboth fractions rangingbetween2and 100%.Thedistributionsof sandandgraveldonot

showany

obvious

spatial

pattern.

Mud

content

ranges

from

0to

78%

in

this

area

and

exceeds

80% in three samplesoccurringon the slopeadjacent to adeeps/holes/valleys feature,but it

doesnotattain1%in73(50%)samples.

Bulk carbonate content exceeds 80% in 312 (91%) samples in the NWSP. South of Broome

carbonate contentsof 50%

sand (Fig. 5.14f).Gravel is thenextmost abundant fractionwith contents generally ranging

from50%.Mudcontentsgenerallyrangefrom50%.Areas adjacent toplateaushave consistentlyhighergravel content

thanotherareasoftheshelfandlowermudcontent.Themudcontentislow(50%

sand.

Gravel

is

the

next

most

abundant

fraction

with

contents

attaining

20

to

26%

in

three

of

the

foursamples.Mudcontent is50%sand.Gravel isthenextmostabundantfractionwithcontentsattaining


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50%.Gravelcontentofsediments isvariable, ranging

from3to80%. Mudcontentrangesbetween65%carbonate(Fig.5.15b).Carbonatesandcontentrangesbetween85and96%in

11samples,andcarbonatemudrangesfrom80to88%infoursamples.Carbonategravelranges

between55

and

100%

in

11

samples.

5.4.4.4.LevequeRise(plateau)

A total of 10 sampleswere obtained from theLevequeRise. Sediments characterising these

featureshave largevariations in the sand:gravel ratioandcontain relatively little (90%(Fig.5.15c).

5.4.4.5.ShallowwaterTerracesA total of 112 samples were obtained from terraces. Samples located adjacent to

deeps/holes/valleys showhighlyvariable sand:gravel ratios (Fig.5.14e). Samplesadjacent to

areasofunassignedshelfshowedlessvariablecharacteristicswithsanddominating(30 55%)

overgravel andmud (050%).Bulk carbonate content ranges from63 to100%,althoughone

samplecontains42%carbonate.Atotalof76sampleshavebulkcarbonatecontent>80%(Fig.

5.15e).5.4.4.6. Ridges

Atotal

of

18

samples

were

obtained

from

ridges.

Mud

is

the

most

dominant

sediment

fraction

withcontentsrangingbetween9and77%and11samplescontaining>50%(Fig.5.14d).Sandis

thenextmostdominantfractionwithcontentsrangingbetween23and99%andfivesamples

containing>50%.Gravelattains20%onaverageandattains36%inonesample.Bulkcarbonate

content ranges from 78 to 95% and attains >90% in six samples (Fig. 5.15d).Carbonate sand

rangesbetween93and98%, carbonategravel ranges from60 to100% in seven samplesand

carbonatemudrangesbetween88and89%insixsamples.


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a)


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b)

c)


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d)

e)


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Figure 5.14. Textural composition (mud:sand:gravel ratio) of a) bank/shoal; b) deep/hole/valley; c) plateau; d) ridge;e) shallow water terrace; and f) shelf/slope sediments within the NWSP.

a)

f)


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c)

d)

b)


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Figure 5.15. Carbonate content of a) bank/shoal; b) deep/hole/valley; c) plateau; d) ridge; e) shallow water terrace;and f) shelf/slope sediments within the NWSP.

5.5.NORTHWESTSHELFTRANSITION(NWST)5.5.1.GeomorphologyandbathymetryTheNorthwestShelfTransition(NWST)coversatotalareaof308,450kmofwhich136,660km2

(56%)issituatedintheNWMRandtheremainderisfoundinintheNorthMarineRegion(Fig

5.1).Thisbioregionrepresents13%ofthetotalareaoftheNWMR(Table5.1).

TheNWST extends from Cape Leveque to the eastern end ofMelville Island. TheNWMR

boundarydividesthebioregionthroughthecentreoftheJosephBonaparteGulf(Fig.5.1).The

entireNWSTislocatedontheshelfexceptforasingledeeps/holes/valleysfeaturethatextends

onto the slope (this isanextensionof thesame feature thatoccurs in theNWSP) (Fig.5.16a;

Table5.8).TheoffshoreboundariesofNWSTarecomplex.IntheNWMR, itabutstheNWSP,

Timor Province, and the unallocated area of the EEZ. The NWST extends > 300 km across the

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F5nw-geomorphology-5a