acoustic characterisation of seafloor sediments and its relationship to active sedimentary processes...

8/19/2019 Acoustic Characterisation of Seafloor Sediments and Its Relationship to Active Sedimentary Processes in Cook Stra…

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New Zealand Journal of G eology and

Geophysics,

1992, Vol. 35: 289-300

0028 -8306/92 /3503-02 89 2.50/0 © The Royal Society of New Zealand 1992

289

Acoustical

characterisation of seafloor sediments and its relationship to active

sedimentary

processes in Cook Strait, New Zealand

LIONEL CARTER

New Zealand Oceanographic Institute

DSIR M arine and Freshwater

Private Bag 14 90 1, Kilbirnie

Wellington, New Zealand

Abstract 3.5 kHz seismic profiles are used to characterise

the seabed in Cook Strait. The various acoustical responses

have been classified into nine groups or echo-types which,

together with sediment samples, photographs, and side-scan

sonography, provide an insight into modern erosional and

depositional.processes operating in the strait.

Much of northern Cook Strait is underlain by semi-

consolidated, late Pleistocene sediments that are eroded by

strong, tide-dominated currents even at depths >200 m.

Locally, erosion of these deposits is impeded by a lag gravel

pavement that occupies much of the 150 -350 m deep central

strait. The sam e strong currents effectively transport bedload

along the Wellington continental

shelf,

which is a rocky

platform with a patchy veneer of mobile sand and gravel.

Outside the main tidal stream, within semiprotected embay-

ments, deposition is manifest by prominent sediment bodies of

mud and sand prograding across the inner-middle

shelf.

Seaward of the

shelf,

in southern Cook Strait, the seafloor is

dissected by a complex of submarine canyons that appear to

syphon off tidally transported sand to the nearby Hikurangi

Trough. However, in at least one place, transport is impeded

by a slide blocking the canyon axis. Outside submarine

canyons, products of gravitational mass movement are not

conspicuous, even though Cook Strait lies across a zone of

high seism icity. This scarcity of eviden ce is, in part, attributed

to current modification of any such deposits.

Ke yw ords acoustical meth ods; echo-character; bottom

sediments; sedimentary processes; Cook Strait; New Zealand

INTRODUCTION

Cook Strait, separating the North and South Islands of New

Zealand, is a highly active sedimentary environment (Fig. 1).

Strong tidal flows and meteoro logically forced currents ensure

regular transport of sand and gravel in de pths of at least 225 m

(e.g., Pantin 1961; Black 1986). Furthermore, the strait is

subject to active faulting and frequent earthquakes because it

is adjacent to a major plate boundary (e.g., Robinson 1986;

Carter et al. 1988). Acco rdingly, gravitational mass mov emen t

of sediment is to be expected on seafloor slopes. Against this

G91029

Received 28 August 1991; accepted 27 March 1992

background of modern sedimentation has been a markedly

different regim e associated with the last major lowering o f sea

level. During the glacial maximum, Cook Strait was closed by

a land bridge that radically altered the tidal and sedimentary

regimes (Proctor & Carter 1989). The sum of these processes

is manifest in the surficial sedimen t cover w hich is a complex

of relict, palimpsest, and modern features.

Charts outlining the distribution of surficial sediments

indicate the complexity of Cook Strait sedimentation and its

relationship to modern water motions (e.g., Lewis & Eade

1974; Lewis & M itchell 1980; Black 1986). However, a more

realistic appreciation of Cook Strait sedimentation may be

obtained by combining available sediment data with 3.5 kHz,

high-resolution, seismic profiles. Such profiles significantly

enhance ou r interpretation of more traditional sediment charts

in tha t: (1) seismic reflection is in part a function of sediment

lithology and enables a more accurate mapping of substrates

(e.g., Smith & Li 1966; Embley & Langseth 1977; Damuth

1980); (2) profiles record morphological data that assist with

the identification of sedimentary processes (e.g., current-

induced bedforms; sediment body geometry); and (3) seismic

sections add a stratigraphic perspective to interpretation.

DATA AND METHOD

This study relies mainly upon 2000 line kilometres of high-

resolution, 3.5 kHz seismic records collected during New

Zealand Oceanographic Institute cruises 1036, 1139, 2019,

and 2034 (Fig. 2). Line spacings average about 5 km and

provide markedly better coverage than the broad scatter of

bottom sampling stations (see Lewis & M itchell 1980). Up to

1985,

records were obtained with an Edo-Western 248C

system and more recently with an ORE 140 profiler incorp-

orating a 16-element transducer array.

The different acoustical responses from the various

substrates in Coo k S trait are classified according to a scheme

outlined in the next section. Th e resultant acoustical types are

mapped with boundaries dictated by the 3.5 kHz lines and,

where line coverage is sparse, by bathymetry and sediment

data. These same data are used to verify interpretation of

acoustical facies and include unpublished and published

information on surficial sediments (Reed & Leopard 1954;

Lewis & Eade 1974; Lewis & Mitchel l 1980), bot tom

photographs (Hurley 1959; Estcourt 1968; Carter 1983; Black

1986), and cores (Carter 1983).

Whereas the acoustical response from the seabed is

primarily a function of sedimen t type, physical properties, and

morphology (e.g., Smith & Li 1966; Damuth 1978, 1980;

Pratson & Laine 1989), it may a lso be influenced by v ariables

unrelated to the seabed (e.g., King 1965; Damuth 1975)

including: (1) instrum ent settings, particularly overall gain and

time variable gain adjustments; (2) pitch an d roll of the survey

vessel; and (3) orientation of survey lines relative to linear

morphologic elements.


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Carter—Sedimentary processes in Cook Stra i t

29 1

Fig. 2 Tracks for 3.5 kH z profil-

ing runs, with locations for profiles

shown in Fig 7A-C .

4 1 ° S -

41°20 -

similar frequency sounders usually provided little information

about sediments below the seabed and they have been largely

superseded by 3.5 kH z systems. A pioneer of 3.5 kH z based

classifications is Damuth (1975) who presented a scheme that

was subsequently modified by Damuth (1978, 1980) and

Pratson & Laine (1989). The main classes within these

classifications are based on either the strength or shape of the

echo (e.g. , dist inct versus indist inct and planar versus

hyperbolic ech oes). Further subdivision is based on the degree

of seismic penetration and character of subbottom reflectors.

A Damuth-type classificatory scheme has been retained

for Cook Strait, but it has been modified to accommodate

substrate types peculiar to that depositional setting. Such

changes are necessary as existing classifications pertain

mainly to continental slope and deeper water environments

(e.g., Damuth 1975, 1980; Embley & Langseth 1977). In

contrast, much of Cook Strait is within continental shelf

depths and is therefore subject to different sedimentary

regimes that yield substrates with their own characteristic

acoustical responses.

The classification used here has four classes based mainly

on the form and signature of the echo from the seabed surface.

Classes are subdivided into types that form the basic charting

units of this study. These types, together with their respective

substrate character, environmental setting, and formational

process are summarised in Fig. 3.

Class I encompasses distinct, planar seabed echoes with or

without subbottom penetration. Nonpenetrating varieties

include smooth reflections from flat to undulating seafloors

(Type IA) and smooth, steeply inclined reflections from

prominent subm arine slopes (Type IB). Responses exhibiting

subbottom penetration have distinct, closely spaced and

continuous internal reflectors that can be conformable w ith the

seafloor or intercept the seafloor because of deformation and/

or erosion (Type IC).

Class II echoes are indistinct planar responses that may be

accompanied by diffuse, discontinuous subbottom reflectors

(Type IIA) or may be devoid of obvious internal structure

(Type IIB). The two types are gradational with one another.

Class HI includes irregular, distinct seabed echoes which in

Cook Strai t are principal ly nonpenetrat ive, i rregular

reflections from a rough substrate with relief typically but not

invariably 10 m relief) individual or small groups of non-

overlapping hyperbolae (Type IVA); zones of irregular-sized

overlapping hyperbolae (Type IVB); and small (< c. 2 m

relief), regular-sized, overlapping hyperbolae (Type IVC).

DISTRIBUTION AND SIGNIFICANCE OF

EC HO-TYPES

The significance of the various echo-types identified in Cook

Strait was determined by the correlation of the echo-character


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New Zealand Journal of Geology and Geophysics, 1992, Vol. 35

Type

I

1. Centra l s t ra i t and exposed

cont inental shelf.

2. Relict gravel and m o d e r n /

pa l imps es t s and .

3.

Tidal winnow ing

of

centra l s t ra i t

gravel ; wave/current agi ta t ion

on shelf.

Fig. 3 Echo-type classification

together with (1) occurrence, (2)

lithology, and (3) main sedi-

mentary process for each type.

Type

IB

1. Canyon and channel walls.

2. Rock or semiconsolidated

sediment usually with sand/mud

veneer.

3. Tidal and/or turbidity current

erosion.

Type IC

1.

2 .

3.

Shelf,

north of Narrows and

marginal to canyons and holes.

Late Pleistocene, semiconsoli-

dated mud and fine sand.

Shallow marine deposition during

low sea level, reduced tidal flow.

Type II

1. Semiprotected bays and sounds.

2.

Muddy fine sand to sandy mud.

3. Rapid deposition during high

sediment supply interspersed by

periods of wave/current action.

Type

IIB

1. Exposed inner-middle cont i -

nen ta l shelf.

2 . Sand somet ime s with minor

gravel or mud componen t s .

3.

Depos i t ion un der a wave and

s torm-dr iven currents .

chart (Fig. 4) with surficial sediments (Fig. 5), bottom

photographs (Fig. 6), side-scan sonographs, and cores (e.g.,

Carter 1983; Black 1986; Carter et al. 1991). The use of cores

was especially relevant because

an

echo-type sometimes

failed to equate with the surficial sediment cover. Such a

shortcoming is common in areas where the cover is too thin

(


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Carter—Sedimentary processes in Cook Strait

Fig. 3

continued).

293

Type III

1. Co ntin enta l shelf off Wellington,

Cape Campbell and northern

Narrows.

2. Rock with veneer of sand /gra vel.

3.

Strong tidal an d/ or wave-

induced current action.

Type IVA

1. Canyon walls.

2.

Rock and semiconsolidated

sediment usually with san d/m ud

veneer.

3. Erosion, large-scale m ass

wasting, also may be structu ral

control.

Type IVB

1. Base of slope s, canyo n floor.

2.

Not samp led.

3.

Gravitational m ass movement.

Type IVC

1. Floor of Narro ws Ba sin.

2. Surficial sed im ents are rippled

s a n d s .

3. Origin unc er ta in, may be sand

wa ve s .

a strong reflector at or close to the seafloor p roduces a distinct

single echo.

In places, the sediment co ver is sufficiently thick to exhibit

some internal reflectors. Cores from the canyon w alls indicate

that the principal reflector is a semiconsolidated, blue-grey

mud and fine sand (NZOI Stns Q821, Q822) of Pleistocene

age,

as identified from nannoplankton (A. R. Edwards pers.

comm.).

The distinctive seismic sequences of subparallel, con-

tinuous reflectors belonging to Type IC dominate northern

Cook Strait as well as forming prominent patches about the

Cook Strait Canyon system and a series of holes off the

Marlborough Sounds (Fig. 4, 6C, 7B). Erosion of Type IC

increases towards the Narrows, where deposits are either

absent or are masked acoustically by lag gravels. This trend is

accompanied by increased deformation of Type IC as it

extends into the northeast-trending axial tectonic belt (e.g.,

Carter et al. 1988). Accordingly, IC sediments south of the

Narrow s are usually tilted, faulted, and/or folded (Fig. IC ).

Cores from Type IC (Q 815, Q818, Q82 1, Q824, Q839) are

semiconsolidated blue-grey mud or muddy fine sand with

local interbeds of shell-rich gravelly sand. The associated

nannofossils yield a late Pleistocene age . Bottom samples and

photograph s indicate the deposits have a man tle of sand that is

usually too thin to be resolved on the 3.5 kHz profiles. One

prominent exception is the shelf off Cape Jackson, where a

possible sand wave-field partially mask s the Type IC substrate

to yield a Type IIB response.

Type IIA echoes characterise the sediment wedges

extending seaward from the semisheltered embayments of

Cloudy B ay, Marlborough Soun ds, and Palliser Bay as well as

a shore-parallel belt off Po rirua Harbour (Fig. 4) . The w edges

contain up to 37 m of mud and fine sand that rest uncon-

formably on what is presumed to be the last postglacial

transgressive surface (Fig. 6D). The acoustic response of

semidistinct surface echoes and a few discontinuous internal

reflectors vary within a we dge, and between the three wedges,

implying variability in sedimentary properties. For instance,


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New Z ealand Journal of Geology and Geophysics, 1992, Vol. 35

175°E

ECHO - TYPES

1.

Planar,

sharp may or may not

be penetrative

IA - gravel/sand beds

IB-canyon

walls

IC - Pleistocene s ands-silts

2.

Planar, indistinct, penetrative

IIA - weakly bedded muddy

sandy sediment

IIB - massive sand;

Possible

mass movement

3.

Irregular,

sharp

non-penetrative

I ./.,] I l l R o c k

4.

Hyperbolae

IVA - pinnacles ridges on

canyon slopes

IVB - slump deposits

IVC - bedforms?

Fig . 4 Distribution of echo-types

in Cook Strait.

seismic penetration through the Cloudy Bay wedge decreases

as sediments coarsen towards the main tidal stream. The

Palliser Bay wedge is more readily penetrated to the basal

transgressive surface by virtue of its higher mud content

compared to Cloudy Bay.

Type IIB echoes are associated mainly with deposits off

northern Cook Strait , Cape Campbell, and Wellington

Harbour (Hg . 4). Sediments are mainly sand and silt, with the

Wellington deposit containing a subordinate gravel com-

ponent (Fig. 6E). Some Type IIB sediments are quite

transparent, especially in northern Cook Strait, where they

form a southward-thinning mantle over Pleistocene sands and

silts (Fig. 7B).

The highly reflective and rough topography characterising

Type III reflections, dom inates the continental shelf and upper

slope off the Wellington Peninsula, with additional, small

outliers dotting the perimeter of the Narrows Basin and the

shelf off Cape Campbell (Fig. 4). Side-scan sonographs,

dredge hauls, and divers' observations from the W ellington-

Narrows Basin area indicate a predominantly rocky substrate

of greywacke with a thin, patchy cover of granule gravel and/

or fine sand (e.g., Carter 1987; Carter eta l. 1991). The acoustic

reflections off Cape Campbell are probably also from a rocky

seafloor that is tentatively correlated with Miocene and

Pliocene sedimentary rocks onshore (Fig. 6F).

Large, irregular, hyperbolic returns of T ype IVA m ainly

flank intercanyon areas of the Cook Strait Canyon system

(Fig. 4). Bathymetric and seismic profiles suggest these

returns are from ridges and pinnacles isolated by a com-

bination of structural and erosional processes. The echoes may

be sharp and nonpenetrating, indicating a rock or gravel

covered substrate such as Types IA and III, or they may

prograde downwards to closely spaced, parallel, subbottom

reflectors characteristic of Pleistocene deposits (Type IC).

The small, irregular, overlapping hyperbolae of Type IVB

are located within canyons, occupying the base of walls and

the adjacent canyon floor (Fig. 7C). These locations, together

with the irregular topography and a paucity of coherent

internal structure, are all consistent with a slump origin for

these deposits (e.g., Embley 1980).

Type IVC hyperbolic reflections cover two zones in the

Narrows Basin at 250- 330 m depth (Fig. 4). The small, closely

spaced, overlapping hyperbolae equate with an area of shelly

gravelly sand, east of the main gravel belt (Fig. 5). Bottom

photographs (Q836, Q839) confirm the sandy substrate and

further reveal the presence of small ripples, c. 20-4 0 mm high

and 100-150 mm wavelength, with crest orientations ranging

from northeast-southwest to east-west (Fig. 6H). Because

these current-induced bedforms are small, it is unlikely that

they are the cause of the much larger hyperbolic reflectors.

Similar-sized ripples on the Cloudy Bay wedge invoke a

simple planar response of Type IIA (Fig. 6D). It is also

unlikely that the regular hyperbolae are generated by the

Pleistocene substrate, which tends to have irregular morph-

ology induced by erosion. In the absence of other evidence, it

is suggested the hyperbolae represent large bedforms such as

sand waves developed in response to the strong flow in this

area (e.g., Carter et al. 1991).


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Carter—Sedimentary processes

in

Cook S t rait

295

Fig.

5

Distribution

of

surficial

sediments modified from Le wis

Mitchell (1980). Alphanumeric

notations refer

to

samples

mentioned

in the

text.

41

°S-

Gravel+sandy gravel

Shell gravel

Sand+muddy sand

Mud

41°20 ' -

SEDIMENTARY P RO CES S ES

IN

C O O K STRAIT

When echo characteristics are com bined w ith available

geological

and

physical oceanographic data,

it

becomes

possible

to

identify different sedimentary processes operating

in Cook Strait,

and

gain

an

impression

of

their relative

importance.

Erosion

and

bedload transport

Much of northern and central Cook Strait are eroded by the

strong, tide-dominated flow, even

in

water depths

of 350 m.

The degree

of

erosion

is

largely

a

function

of

tidal strength

and

seafloor geology. Thus, on the eastern m argin of the Narrows,

where tidal currents

are

strongest

and the

substrate

is

erosion-

resistant greywacke,

the

seafloor

is

mainly

a

rough, rocky

pavement (Type III) with a thin, patchy cover of gravel and

fine sand. Components finer than pebble size are transported

over

the

pavement under

the

influence

of

tides that

are

periodically reinforced by the mea n flow, eddies, storm -

induced currents, and swell (Black 1986; Carter 1987; Carter

et

al.

1991).

The

only depocentres

are

within small embay-

ments outside

the

main tidal streams (e.g., Carter

et al.

1991).

In the central reaches of Cook Strait, where the flow is

slightly weaker,

it is

still sufficiently strong

to

erode

semiconsolidated Pleistocene sediments (Type

IC). The

degree

of

erosion increases from

the

northern approaches

to

the Narrows

in

accord with increasing current strength

(Bowman etal . 1980; Proctor

Carter 1989).

The

erosion rate

is estimated there at c. 1.1 m/1000 yr (Lewis et al. in press).

However, erosion

has

been retarded

by the

development

of a

pebble-cobble armour represented

by the

belt

of

Type

IA

echoes extending from the Marlborough Sounds to Nicholson

Bank (Fig. 4; Carter 1987). Wh ere cored, the gravel armou r is

d m th ic k,

yet

this

is

apparently sufficient protection

to

maintain a gently undulating seabed in contrast to the

dissected topography of unprotected Pleistocene sediments.

The

lag

itself serves

as a

pavement

for

mobile sand

entering

the

general vicinity

of

the Narrows Basin

and

western

Terawhiti Sill. Bottom photographs and cores indicate at least

par t

of

this mobile sand

is

patchy

and

thin

(Fig. 6B).

Consequently,

it is not

readily identified

on 3.5 kHz

profiles,

an d its precise distribution is uncertain. The problem is

compounded

by

thick sand deposits having

an

acoustic signal

similar

to the lag

gravel.

Sand may be transported into the Narrow s B asin from the

north

via the

west coasts

of the

North

and

South Islands (e.g.,

Carter

Mitchell

1987) and

from

the

south

via

Cloudy

Bay

(Carter 19 83; Black 1986). Depiction of these transport paths

by

3.5 kHz

data alone

is

difficult because

of the

similarity

of

sand

and

gravel ech oes,

and the

thinness

of

some sand bodies.

However, the profiles provide an insight into bedload

transport when combined with other information.

1. Active bedload transport over

the

shelf between

the

Marlborough Sounds

and

Kapiti Island

is

evident from

patches with Type IA and Type IIB echoes, which bottom

samples

and

photographs show

to be

composed

of

rippled


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t


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297

Fig. 7 3.5 kHz profiles from the

nor th Narrows (A) , cent ral

Narrows (B), and Cook Strai t

Canyon (C), as examples of the

distributions of various echotyp es.

The profiles are located on Fig. 2;

the solid bar represents 2 km.

1 5 0

1 5 0

300

Ic

v

l l b

la

W

B

1 5 0

300

W.

sand. In at least one case, these small bedforms appear to be

superimposed on larger features th at are tentatively identified

as sand waves. Such features have amplitudes of 2-8 m,

wavelengths of up to 2 km, and asymm etric profiles. Although

only two 3.5 kHz profiles, at 90° to one another, cross these

bedforms, it seems that the waves are aligned approximately

east-west with the direction of travel to the north. Such a

transport direction is at odds with known current patterns

which have tidal residuals, peak bottom stress, and m ean flow

to the south in this part of the strait (Bowman et al. 1980;

Heath 1986; Proctor & Carter 1989). The contradictory

Fig. 6

opposite)

Bottom photographs from various echo-type

zones including: (A) Type IA lag gravel in Narrows, Station Q832;

(B) Type IA with sand veneer above gravels in Narrows, Q839; (C)

Type IC Pleistocene sediment w ith sand m antle, Q847; (D) Type IIA

wave-stirred silty sand of Cloudy Bay, Q 828; (E) Type IIB sand on

Nicholson Bank, Q816; (F) Type IA-III gravel veneer above rock

substrate off O teranga Bay, P I; (G) Type IVA gravel-encrusted wall

of Nicholson Canyon, Q815; (H) Type IVC rippled sand from

Narrows, Q836. The circular compass is 50 m m in diameter.

northwards transport inferred from the bedforms may simply

be a temporary response to the ebb tide reinforced by

meteorologically induced motions from the south: the survey

was carried out immediately after high water under southerly

gale conditions.

2.

The prevailing direction of sand movemen t over the inner

shelf,

west of the lower North Island, is south towards Cook

Strait at least as far a s Kapiti Island (Lewis 1979; Gibb 1979).

The distribution of Type IA patterns infers such movement

may co ntinue down the west side of Kapiti Island, although it

is likely transport will also occur along the North Island coast,

judging from sediment distribution (Lewis & Mitchell 1980).

3. Sand may also be moved towards the Narrows from the

south, at least along the northwestern extremities of Cloudy

Bay. Such a transport direction is not evident in the regional

numerical m odels of Cook Strait (Bowm an et al. 1980; Proctor

& Carter 1989). Nevertheless, near-bottom current measure-

ments from the Narrows suggest a net transport of sand to the

northeast (Black 1986). This contention is given limited

support by the presence of northeastward-moving sand/gravel

waves, and northeast-tapering, linear sediment ridges (Carter

1983).


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298


The ridges appear on 3.5 kHz records as transitional

between Types IA and IIA, implying they have both gravel

and sand components — an analysis supported by bottom

photographs and side-scan sonographs.

Although the site of converging sediment transport paths,

the Narrows retains little sand, as manifest by the exten t of lag

gravel (Type IA ) and rocky (Ty pe HI) substrates. Instead, sand

is moved north and south by the ebb and flood tidal phases,

respectively. Current-meter data empirically indicate net

transport is northwards (B lack 1986), but a reverse transport is

likely in view of the proximity of the Narrows to the Cook

Strait Canyon system. Sand moved by flood tides would be

irretrievably lost to the deep canyons so that actual net

transport is southeastward. Certainly, Nicholson and Cook

Strait Canyons have sandier and more reflective sediments

(Type

I A

at their heads.

Deposition

Type IIA echoes are associated with sedimentary bodies

composed of mud and sand, whose accumulation is favoured

by a semisheltered environment and an adequate sediment

supply. The largest zone of Type IIA is Cloudy Bay, which lies

west of the main tidal stream. It is partially protected from

southerly swell by Cape Cam pbell and is fully protected from

west-no rthwest winds and waves. These factors, together with

a significant sediment supply from the Wairau River (4.69 x

10

6

t/yr; Griffiths & Glasby 1985) and Awatere River (1.5 x

10

6

t/yr), have produced a prominent wedge of muddy sand

that grades into coarser sediment at the seaward limit of the

wedge as a consequence of w innowing (Type IA echoes), as

the wedge extends into the main tidal stream near the shelf

edge. The southward coarsening of the wedge (Type IIB

echoes) near Cape Campbell probably reflects greater

exposure to southerly swell.

A depocentre in Palliser Bay also lies outside the main

tidal flow and the influence of northwesterly winds, but it is

exposed to southerly swell. Nevertheless, a small wedge-like

body of mainly sandy silt has been deposited seaward of the

Ruamahanga River (0.85 x 10

6

t/yr; Griffiths & Glasby 1985).

Com pared to Cloudy Bay, the fluviatile supply to Palliser Bay

is lower and finer grained; both a spects are evident in 3.5 kHz

profiles, which reveal a wedge of more acoustically trans-

parent sediment with a maximum recorded thickness of 16 m

compared to 37 m in Cloudy Bay.

Type IIA echoes northeast of Marlborough Sounds are

mainly from muddy sands deposited as a consequence of

sheltered hydraulic conditions provided by the peninsulas and

islands of the outer sounds. There is no major fluviatile supply

and the sediments are probably d erived from local streams and

distal sources, the sediments of the latter being transported by

the wind-driven mean flow in conjunction with the tides (e.g.,

Carter & Heath 1975; Carter 1976).

A depocentre off Porirua Harbour appears to be the

southern terminus of a narrow, discontinuous band of sandy

silt that runs along the midd le shelf from prominent fluviatile

sources to the north (see Lewis & Mitchell 1980). Sediments

have accumulated here because the Wellington Peninsula

provides limited shelter from southerly swells, and tidal flows

are low to moderate, especially during the flood phase

(Bowman etal. 1980).

A small depositional body with Type IIB characteristics

has formed at the mouth of the broad embayment leading into

Wellington Harbour. Although 3.5 kHz tracks terminate well

seaward of the harbour, other seismic data and side-scan

sonographs suggest the lobe is continuous to the harbour

entrance itself (Arron & Lewis in press). The embayment

favours deposition because of its location outside the main

tidal stream, especially during the southward flood phase

when the Wellington Peninsula deflects the stream seaward of

the embayment m outh (Bowman et al. 1980). Nevertheless,

current and sw ell action are sufficiently strong to produce a

gravelly sand deposit in contrast to the muddy sandy deposits

elsewhere. In the absence of provenance studies, we can only

surmise that the deposit receives fluviatile sedim ent from the

southeast (Matthews 1980a, b) and inner shelf-coastal

sediment from the Wellington Peninsula and Kapiti coast

(e.g., Carter et al. 1991). It is unlikely that the lobe collects

sand transported from Wellington Harbour during the flood

phase, as bedload transport in the harbour mouth is mainly in

the opposite direction under the influence of southerly gales

and storms (Carter 1977). Once through the entrance, sand is

irretrievably trapped within the harbour basin. Thus, in this

regional context, the shelf depositional lobe may act as a

reservoir for sediments ultimately destined for the harbour

sink.

Gravitational mass m ovement

Conclusive evidence of gravitational mass movem ent is found

within the axial reaches of the submarine canyons and

associated re-entrants (Fig. 4 and 7C). Here a number of

localised slumps and slides have b een identified on the basis of

their: (1) irregular morphology manifest as Type IVB

hyperbolae; (2) chaotic or absent internal reflectors; and (3)

position near slope bases (e.g., Embley 1980).

A prominent slump occurs in Cook Strait Canyon, above

its confluence with Nicholson Canyon (Fig. 4). The deposit

has partially blocked the canyon and appears to extend down-

canyon as a slide. Up-canyon of the slump, the Type IVB

hyperbolae merge to Type IA echoes, which bottom photo-

graphs indicate is a response from a ripp led sandy fill.

It may be argued that because of the high seismic activity

of central New Zealand (e.g., Hatherton 1980) there should be

more widespread evidence of mass failure outside the Cook

Strait Canyons system. Certainly, in other seismic areas,

failures of continental shelf and slope sediments have been

recorded on the seafloor with inclinations as low as 0.4°

(Lewis 19 71; Herzer & L ewis 1979; Carter & Carter 1985). In

Cook Strait, evidence of failure outside canyons is unclear,

probably because widespread seafloor erosion has either

modified or removed such dep osits. Probable slump and slide

deposits may occupy the flanks of the Narrows Basin in the

vicinity of branches of the active Wairau F ault (Fig. 4; Carter

eta l. 1988).

CONCLUSIONS

High-resolution 3.5 kHz profiles, supported by other oceano-

graphic data from Cook Strait, highlight a complex sedi-

mentary regime that has resulted from the interplay of

sediment supply, bathymetry, tides, and storm-driven water

motions.

Semisheltered, river-fed embayments that lie outside the

main tidal flow, are depositional sites for fine-grained

sediment wed ges. On more exposed sectors of the continental

shelf, but still outside the main tidal stream, sand deposition

and transport prevails in response to swell and wind-induced


12/13


299

currents. However, once in the main tidal stream, sedi-

mentation changes dramatically; erosion becomes pervasive.

In the central and northern strait, the tides erode into

semiconsolidated Pleistocene sands and muds, thus reducing

the overlying Holocene sediment cover to a patchy veneer

of mobile sand. Between Marlborough Sounds and the

Wellington Peninsula, the Pleistocene deposits are protected

from further erosion by a layer of coarse gravel that has

formed in response to tidal winnowing of presumed fluviatile

sediment deposited during a lower stand of sea level. Such

strong current effects are also felt on the continental shelf and

slope off the Wellington Peninsula where the combination of

tides and grounding southerly swell have created a thin

discontinuous cover of gravel and coarse sand above a rough

basement of grey wacke.

In southern Cook Strait, the deeply incised topography of

the Cook Strait Canyon system has created its own sedi-

mentary environment with mobile sands in the tide-influenced

canyon heads grading to finer deposits in deep, calmer waters.

The seismic profiles also reveal the canyon sediments are

subject to gravitational mass movement that is probably a

consequence of high seismicity of the region. Mass movement

of the seabed has also occurred in other parts of Cook Strait,

but the resultant deposits have been modified by tidal action,

so that the full extent of such redeposition cannot be realised.

ACKNOWLEDGMENTS

The assistance

of

technical staff

of the

New Zealand Oceanographic

Institute and officers and crew of R.V.

Tangaroa

and R.V.

Rapuhia

is greatly appreciated. The draft m anuscript was read critically by

Keith Lewis

and

Phil Barnes whose pert inent comments have

improved the final product. Word processing was by Rose-Marie

Thompson,

and

graphics w ere provided

by

Karl M ajorhazi

and his

computer.

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acoustic characterisation of seafloor sediments and its relationship to active sedimentary processes...

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