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6. Sum mary and Discussion

Textural and compositional data generated during this study for the NWMR and NNMR

permits

a

quantitative

comparison

of

sedimentology

for

Australias

Northwest

and

Northern

Margin.Thedatabuildsonpreviouspredominantlyqualitativestudiesthatcurrentlyexistfor

theregion.Previousworkfortheareahasfocussedprincipallyontheshelf,andconsequently

thefollowingdiscussionislargelyrestrictedtoacomparisonofourfindingswiththeresultsof

previous sedimentmodels for the shelf.The implicationsof seabed sedimentdistribution for

marinehabitatmappingarealsodiscussed.

6.1.SEDIMENTTRENDSOFTHENWMRNew consistent quantitative data for the NWMR have revealed regional scale patterns in

sedimentdistribution

not

apparent

in

previous

studies

and

forms

aframework

within

which

localscalepatternscanbeunderstoodinaregionalcontext.Newdatarevealsomeoftheseabed

complexity. At a regional scale our data show that seabed sediments become finer with

increasing water depth in a margin parallel pattern. Variation in sediment texture and

compositiongenerallydecreaseswith increasingwaterdepth,withsedimentson the riseand

abyssalplain/deepocean floorbeing relativelyhomogeneouscompared to thoseon theslope

and shelf. The fining and homogenisation of sediment texture with distance offshore is

predominantlytheresultofdecreasingenergylevelsandsedimenttransportmechanisms(fora

detailedexplanationseeSection3).

Theshelf

is

predominantly

composed

of

sand,

and

the

abyssal

plain/deep

ocean

floor

is

composedofmud.Areasofgraveloccurmostlyontheinner,middleandoutershelf/slopeand

aregenerallyabsentfrom theabyssalplain/deepoceanfloor.Thisbroadzoningandfiningof

sediment type with water depth has alsobeen observed in similar reportsby Geoscience

Australia,notablyontheSouthWestAustralianmargin(Potteretal,2006).Calciumcarbonate

concentrationsarehigheston the inner,middleandoutershelf,and representeither relictor

recentcarbonatedeposits.Thehighconcentrationofcarbonatewithinthisregionhasalsobeen

observedbyCarrigyandFairbridge(1954),ColwellandvonStakelberg(1981),Dix(1989),Dix

etal.(2005),Jamesetal.(2004),Jones(1970,1973),andvanAndelandVeevers(1967).

Atalocal

scale

our

results

agree

with

previous

sedimentological

work

on

the

inner,

middle

and

outer shelvesand slope.Ourdata indicatedistinctchanges in sediment characteristics to the

northoftheLevequeRisewherehighmudconcentrationsandlowsandconcentrationsoccur,

similar to thatnoted inprevious studies (vanderKaars,2003;CarrigyandFairbridge, 1954;

Jones, 1971;Jones, 1973).Newdatahave allowedus tomore accuratelymap the extent and

recognisetheregionalsignificanceofthischange.

High resolution data generated by Geoscience Australia for the seabed in the EEZ have

indicatedthatgeomorphicfeaturesarecharacterisedbyacombinationofseveralenvironments

withzonesoftransitionbetweenthese.Forsomegeomorphicfeatures,thenewdataallowusto

moreaccurately

predict

and

distinguish

between

the

range

of

environments

present

and,

where

data are adequate, estimate the relative proportions of these. Distinct sedimentary

environmentsoccurinsomegeomorphicfeaturesandtheseincluding:abyssalplain/deepocean


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floor, ridges, terraces, slope,deeps/holes/valleys,andbanks/shoals.Establishing relationships

between substrate type and geomorphic feature will assist in creating a more accurate

predictionofthesedimentaryenvironmentinregionswithsparsesamplecoverage.

6.1.1.InnerShelfSeabedsedimentsoftheinnershelfaresanddominated,withlocalisedaccumulationsofgravel

andalargecarbonatecomponent(40100%).Bulkcarbonatecontentincreaseswithsandcontent

andsandcontentdecreaseswithwaterdepth.Ourdataandindicatethatlocaliseddepositsof

mudandgraveloccurinthevicinityoftheDampierArchipelago,PortHedland,Broomeandto

the north of theLevequeRise. To the north of the LevequeRise, inner shelf sediments are

dominatedbyhigherproportionsofmud(2040%)andgravel(4080%)andlowerproportions

ofsand(2040%)andcarbonate(2040%).Thispatterncorrespondstothesedimentologyofthe

regionasobservedbyCarrigyandFairbridge(1954).Wealsodetectedadditionalcomparable

areasof

gravel

(~40

80%)

present

locally

on

the

inner

shelf

of

the

Bonaparte

Gulf.

Dix

(1989)

and

Dixetal.(2005)observedthatseabedsedimentsontheinnerDirkHartogShelfaredominated

by carbonate.Our results agreewith this and show that this trend also extends across the

RolweyShelftotheLevequeRise.

Associationsbetweenoursedimentdataandpreviousfaciesmodelsforsomeareasoftheinner

shelfaredifficulttoresolveduetolocalareasofsparsedata.Jamesetal.(1999)observed low

amountsof terrigenous sedimenton the inner shelf,withan increase inmaterialproximal to

riverdischarge sites (e.g. sediments of theDirkHartog Shelf that reflectdischarge from the

GascoyneRiver).Althoughelevatedterrigenouscontentofsedimentsmaybesignificantatthe

scaleof

these

studies

and

should

be

noted

for

planning

purposes,

our

data

indicate

they

are

confined to relatively small areas located near the coast and do not significantly affect the

sedimentologyoftheshelfwhenassessedataregionalscale.

6.1.2.MiddleShelfAtaregionalscale,sedimentsofthemiddleshelfmostlycomprisecarbonatesand.Carbonate

content increases with sand content. Gravel content is generally low, however large

aggregationsoccurwithintheBonaparteGulf.Jones(1970,1974)notedthatseabedsedimentsof

themiddleshelfarecoarseandsanddominated,with~90%carbonatecomposition. Ourresults

agreewith

his

study

and

show

that

the

trend

extends

further

south

to

the

extent

of

the

Dirk

HartogShelf.

At a regional scale the greatest variety of sediments occur in areas containing several

geomorphic features (i.e.,basinswithinbanks, pinnacleswithinbasins). This is particularly

evidentwherefeatureswithadistinctsedimentologyareinterspersedwithotherfeatureswith

adistinctsedimentology(i.e.,graveldominatedpinnacleslocatedwithinthehomogenous,sand

dominatedunassignedshelf).

Our data confirm the aggregation of carbonate on themiddleRowley Shelf. This carbonate

depositcontains

80

100%

bulk

calcium

carbonate

and

is

inferred

from

previous

literature

as

a

relictcarbonatedepositfromtheLateQuaternary(James,2004).


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Newdata for theNorthWestShelfTransition (NWST) indicates that the areaof themiddle

shelf included in this bioregion has a unique sedimentology compared to the rest of the

Northwestmargin. For themajority of theNWMR, themiddle shelf zone isdominatedby

carbonatesand.TheNWSThowever ischaracterisedbyahighproportionofmudand lower

proportionof

carbonate.

Our

data

provide

further

evidence

in

support

of

the

facies

model

of

CarrigyandFairbridge(1954),whodescribedthisregionasshowingadistinctchangeinseabed

sedimentsnorthoftheLevequeRise.Apossibleexplanationforthedistinctchange inseabed

sediments in the regionmaybe the result of high levels of river discharge and subsequent

terrigenoussedimentintotheBonaparteGulf.

6.1.3.OuterShelfandSlopeOurdata indicate thatat a regional scale, seabed sedimentsof theouter shelf and slope are

dominatedby

carbonate

sands.

Further,

mud

content

increases

with

water

depth

on

the

mid

to

lower slope.Offshore the shelfbreak in theNWMR, preexisting sediment data and facies

modelsarerelativelyscarce,andourdatahavecharacterisedmuchofthisenvironmentforthe

firsttime.Additionofdata ingeomorphicfeaturesoccurringontheoutershelfandslopehas

facilitated the first quantitative analysis of the sedimentology of features occurring at these

water depths in the NWMR, including trenches, reefs and large plateaus and terraces.

Sediment data show that some features in this zone are characterised by a distinct

sedimentology thatdifferentiateseach featureandalso formsoccurrencesof thesame feature

elsewhereintheNWMR.Thesefeaturesincludeterraces,ridgesandslope.

Ourdata

provides

further

evidence

for

extensive

carbonate

sand

deposits

at

the

shelf

edge

(CarrigyandFairbridge,1954).Theslopecontainsahigherproportionofmud (4080%) than

foundontheshelf.TheExmouthPlateauisasignificantgeomorphicfeatureoftheoutershelf

and slope and contains 2060% sand and 4080% carbonate, also noted inColwell and von

Stackelbergs(1981)faciesmodel.

6.1.4.AbyssalPlain/DeepOceanFloorSediment samples procured for this task from the abyssal plain/deep ocean floor have

significantlyincreasedthesamplecoverageandunderstandingofthesedimentproperties.The

abyssalplain/deep

ocean

floor

is

arelatively

homogenous

sedimentary

environment

dominated

bysiliclasticmudwithsmallinclusionsofsandandnogravel.Particlesizeandbulkcarbonate

contentdecreaseswith increasingwaterdepth, as observedbyColwell andvon Stackelberg

(1981)andVeeversetal.(1974).OurdatasupportthefindingsofColwellandvonStackelberg

(1981)whodescribedthesedimentologyoftheabyssalplain/deepoceanfloorassiliceousclay.

However, the carbonate content of sediments in deepwater areas of theNWMR are now

knowntobemorespatiallyvariablethanpreviouslyreportedwithcarbonatecontentashighas

70%locally.


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6.2SEDIMENTTRENDSOFTHENOMINATEDAREAOFTHENNMRNew

consistent

quantitative

data

for

the

NNMR

have

revealed

regional

scale

patterns

in

sedimentdistributionnotapparent inpreviousstudiesandhave formeda frameworkwithin

which local scalepatterns canbeunderstood in a regional context. Seabed sediments of the

NNMRreflectpresentdayoceanographicconditionsandvaryinproximitytosourcesoffluvial

and terrigenousmaterial.Sedimentsare composedof fine to coarsegrained sand andmud,

withcarbonatecontenthighestinareaswithahighsandcontentsuchastheJosephBonaparte

Gulf.ThesedimentologyoftheArafuraandSahulShelvesaredistinguishedfromoneanother

basedonsedimenttextureandcomposition,asdiscussedbelow.

OurdataindicatesthatseabedsedimentsoftheSahulShelfaresanddominatedwithlocalised

depositsof

gravel

on

the

inner

shelf.

Carbonate

content

is

high

due

to

the

relict

carbonate

materialandextensiveoutershelfcarbonatebanksandfaciesthatcharacterisetheregion(van

Andel,1965).Sedimentdistributionreflectsoceanographicconditionsandsedimentsfinewith

distance offshore. Our data supports the findings of Lees (1992) who described the

sedimentologyof theBonaparteGulfasgrading fromgravely sandand sandygravelon the

innershelftosandonthemiddleshelfandclayeysandontheoutershelf,apatternthatreflects

aseawarddecreaseintidalvelocity(Lees,1992).

ThesedimentologyoftheArafuraShelf,asdescribedfromoursedimentdata,isdominatedby

mudwith localised accumulations of sand and gravel. Carbonate content is relatively low

(mostly


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relationships between physical datasets that are useful in developing more effective

interpolationtechniques.

Benthicbiota havebeen shown to havemeasurable relationshipswith the gravel andmud

content of seabed sediments (Post et al., 2006; Bax, 2001). Our data show that where thesedimentology

is

relatively

diverse,

such

as

on

the

inner

shelf

and

in

submarine

canyons,

the

sedimentproperties includinggravelandmudcontentvaryoverrelativelysmalldistances.A

much higher sample density is required in these environments tomore accuratelymap the

spatialdistributionofthesedimentproperties(andbyassociationbenthicbiota).Ourdatahave

improved samplecoverage in theseareas;however,additionalcoveragewill further increase

thereliabilityinwhichthiscanbemapped.Inareaswhereseabedenvironmentsarerelatively

uniform,suchasovermostoftheabyssalplain/deepoceanfloor,sedimentpropertiesaremore

constantoverlargerdistancesandcanbeaccuratelymappedfromfewersamples.

Oursynthesisofsedimentologyandgeomorphologyhas;1)providedamorecomprehensive

understandingof

the

range

of

seabed

sedimentary

environments

present

in

the

NWMR,

2)

allowed comparison between sedimentary environments occurring in different areas

culminating in the identificationof rareorunique areasof seabed thatmaybe ofparticular

interest for conservation,and 3)described relationshipsbetweenphysicaldatasetsproviding

full coverage of the NWMR, such as bathymetry and geomorphology, and sediment

distribution.These canbeused topredict the sedimentary environments that occur in areas

where sedimentdatapoints are relatively scarce.Newdataon the abyssalplain/deepocean

floorhaveallowedcharacterisationatahigherconfidence.

6.4.LIMITATIONSAlthoughwe have added significant detail to the regional sedimentology of the northwest

margin, includingbetterdefined localand regional trends, thedataare still relatively sparse

which limits thedegree towhichwe can fullydescribe the sedimentology. It is important to

recognisesomeofthelimitationsofthedata.

DataintheNWMRisclusteredaroundthemiddleshelfregion,withapaucityofdatalocated

on the inner shelf and abyssalplain/deepocean floor.Thismeans that sedimentspresent in

areaswithmost data are likely tobe overstated in descriptive statistics at a regional scale.

Uneven

distribution

of

data

also

makes

it

difficult

to

statistically

quantify

relationships

that

are

observed visually in data and means that existing relationshipsmay notbe detected and

utilisedwhen interpolatingdata to rasters for input intoseascapesprocesses.While thismay

causesomeinaccuracyorbiasataregionalscale,thestructureofouranalysiswithobservations

and statistics generated for individual bioregions, provinces and features means that

sedimentologyat thesescales isnotsignificantlyaffected.Becausedatadensity isgreateston

themiddle to outer shelfwe are confident that sediment patterns in this location are real.

However, complexity elsewheremay not havebeen detected due to relatively low sample

density.

In

this

study

we

have

used

the

inverse

distance

weighted

method

with

a

fixed

interpolation

parameter,which hasbeenusedbyGeoscienceAustralia to interpolate all of itspointdata

acrossAustraliasmarinejurisdiction.Thisprovides foracomparableandconsistentdataset.


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Themaximumdistancethatanydatawereextrapolatedwas45km.Thismethodisadequate,

wherelargerangesindatadensityoccur,toproducemapsthatallowidentificationoftrendsin

sediment distribution occurring at a regional scale,but it is likely to inaccurately represent

sedimentdistributionatfinerscales.

Thekey

question

in

modelling

studies

is

Howmuchsimplificationisacceptable?Alinearinverse

distanceweightedmethod(withafixedinterpolationparameter)doesnotnecessarilyrepresent

all trends insedimentdistribution.However,no interpolationmethod isabletopickupsuch

trends if sample density is inadequate. Trends in sediment distribution in theNWMR are

knowntooccuronscalesfromcentimeterstohundredsofkilometers.Withoutknowingatwhat

scalevariationsinsedimentcharacteristicsaresignificantinmappingdistributionofspecies,it

isdifficulttocommentonhowmuchuncertaintyininterpolateddataaffectsresultsgenerated

forseabedhabitatmapping.

SampledensityfortheNWMRis1:700km2ontheshelf,1:1,200km2ontheslope,1:19,100km2

onthe

rise

and

1:10,100

km2

on

the

abyssal

plain/deep

ocean

floor.

This

provides

the

minimum

distances over which variations in the sediment properties can be detected. Interpolation

imagesmustbeusedwithcautionwhendrawingcomparisonbetweenseabedcompositionin

differentareasoftheNWMRastheydonotnecessarily;1)representtherelativeproportionsof

environments present in an area; or 2) theway sedimentary environments are interspersed

spatially,asresolutionoftheinterpolation ismoreareflectionofsampledensitythandiverse

sedimentology.

6.5.RECOMMENDATIONSToimproveinterpolateddataitisimportanttoimprovesampledensitiesinareasoftheseabed

thatcontainsignificantvariationsinsedimentcharacteristicsoverrelativelysmalldistances.As

collectingsedimentsamplesfromtheseabedishighlytimeconsumingandcostly,information

about seabed complexity and the relationship togeomorphology canbeused to target areas

wheredatacoverage is likely tobe inadequate.Newdatagenerated for theNWMRand the

SWMR (Potter, 2006) allow recognition of relationships between relatively diverse seabed

sedimentology and geomorphic features including canyons and pinnacles. In the NWMR,

sampledensitiesinthesefeaturesremainrelativelylow.NewdatafortheNWMRalsoindicate

that although sediments aremorehomogenous indeepwater areas (e.g., abyssalplain/deep

ocean

floor),

greater

variation

may

be

captured

in

these

areas

than

is

captured

in

the

current

data.Datageneratedforthisstudyhavesignificantlyimprovedsampledensitiesfortheseareas

and thisworkshouldbecontinued,particularly for theabyssalplain/deepocean floor, lower

slopeandrise.

Data collection, advances in interpolation methods, and improved understanding of

relationshipsbetween geomorphic features and sediment typewill improve the accuracy of

futuresedimentologyworkconductedataregionalmarineplanningareascale.An improved

understandingofgeomorphicfeaturessuchastheabyssalplain/deepoceanfloorisrequiredto

mostaccuratelymapsedimentdistribution.Wheresamplecoverageissparse,theinclusionof

secondary

datasets

in

the

interpolation

process

will

allow

the

prediction

of

sediment

type.

Secondary datasets such as energy level, tidal regime, sediment transport pathways, and

previoussedimentmodelswillimprovetheaccuracyoffutureseabedsedimentmapping.Our


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studyhasshownthatfuturesamplingintheNWMRshouldfocusonareaswithpoorsample

coveragesuchastheabyssalplain/deepoceanfloor,pinnacles,trenches,innershelf,lowerslope

andrise.

Geoscience Australia has a research program to assess the accuracy and precision of

interpolationtechniques

and

is

investigating

the

usefulness

of

secondary

datasets

during

interpolation.

6.6.SUMMARYTheNWMRandNNMRare characterisedby avariablegeomorphologyand sedimentology.

Sedimenttextureandcompositiondisplaysazoningwithdepth,andsandandgraveldominate

the shelf area whilstmud dominates the lower slope and abyssal plain/deep ocean floor.

Calciumcarbonateconcentrationsthroughouttheregionaregenerallyhighestalongtheshelfto

the

shelf

edge

and

are

associated

with

reefs.

Significant

geomorphic

features

of

the

NWMR

and

NNMRinclude;pinnacles,shallowwateranddeepwaterterraces,slope,rise,trenches/troughs,

shallowwateranddeepwaterplateaus,deeps/holes/valleys, slope, shelfandbanks/shoalsof

theJosephBonaparteGulf.

GeosciencedataplaysavitalroleinthemanagementofAustraliasoceanresourcesbecausewe

may never have a full inventory of allbiota found on the seabed particularly for deep sea

regions.Geomorphology and sedimentologydata canbemapped economically and thedata

canbeusedtoinferrelationshipsbetweenthedistributionandabundanceofbenthicbiota.The

relationship(s)betweengeomorphologyandsediment/substratetypeandbiotaisakeypriority

for

research

associated

with

the

implementation

of

Bioregional

Marine

Planning

Areas.

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