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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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New Zealand Journal of Geology and Geophysics, 1992, Vol. 35
t
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Carter—Sedimentary processes in Cook Strait
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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New Zealand Journal of Geology and Geophysics, 1992, Vol. 35
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
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Carter—Sedimentary processes in Cook Strait
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.
REFERENCES
Arron,
E. S.;
Lewis,
K. B. in
press: W ellington south coast substrates
(1:15 000). New Zealand Oceanographic Institute
miscellaneous chart series.
Black, K. P. 1986: Sediment-transport rates in Cook Strait.
New
Zealand O ceanog raphic Institute field report 24: 14 p.
Bowman, M. J.; Kibblewhite, A. C; Ash, D. E. 1980: M
2
tidal
effects in greater Cook Strait, New Zealand.
Journal of
geophysical research 85:
2728-2742.
Bye, J. A. T.; Heath, R. A. 1975: The New Zealand semi-diurnal
tide.
Journal of marine research 33:
423-442.
Carter, L. 1976: Seston transport and deposition in Pelorus Sound,
South Island, New Zealand.
New Zealand journal of marine
and reshwater research 10:
263-282.
1977:
Sand transport, Wellington Harbour entrance,
New
Zealand. New Zealand journal of geology and geophysics
20:335-351.
1983: Sediments, shallow seismic strat igraphy, and
structure
in the
vicinity
of
the submarine pow er cables, Cook
Strait, New Zealand. New Z ealand Oceanographic Institute
field report 19:
14 p.
-1987: Geological hazards in Cook Strait. Proceedings of
the
8th
Australasian Conference
on
Coastal
and
Ocean
Engineering 87/17:
410-414.
Carter,
L.;
Carter,
R. M. 1985:
Current modification
of a
mass
failure deposit on the continental shelf, North Canterbury,
New Zealand.
Marine geology 62:
193-211.
Carter, L.; Heath, R. A. 1975: Role of mean circulation, tides and
waves
in the
transport
of
bottom sediment
on the New
Zealand continental shelf. New
Zealand journal
of
marine
and reshwater research 9:
423-448.
Carter, L.; Lewis, K. B.; Davey, F. 1988: Faults in Cook Strait and
their bearing
on the
structure
of
central
New
Zealand.
New
Zealand journal of geology a nd geophysics 31: 431—446.
Carter, L.; W right, I. C; Collin, N.; Mitchell, J. S.; Win, G. 1991:
Seafloor stabil i ty along the Cook Strai t power cable
corridor.
Proceedings of the 10th Australasian Conference
on Coastal
and
Ocean Engineering:
565-570.
Damuth, J. E. 1975: Echo-character of the western equatorial
Atlantic floor and its relat ionship to the dispersal and
distribution of terrigenous sediments.
Marine geology 18:
17-46.
1978: Echo-character of the Norwegian-Greenland sea:
relationship to Quaternary sedimentation.
Marine geology
28 : 1-36.
1980:
Use of
high-frequency (3.5-12
kHz)
echograms
in
the study of near bottom sed imentation processes in the deep
sea: a review.
Marine geology 38:
51-75.
Embley,
R. W.
1980:
The
role
of
mass transport
in the
distribution
and character of deep-ocean sed iments wi th special
reference to the North Atlantic.
Marine geology 38:
23-50.
Embley, R. W.; Langseth, M. G. 1977: Sedimentation processes on
the continental rise of northeastern South America.
Marine
geology 25:
279-297.
Estcourt, I. N. 1968: A note on the fauna of a ripple-marked sandy
sediment in western Cook Strai t , New Zealand.
New
Zealand journal of marine and freshwater research 2:
654-658.
Gibb, J. G. 1979:
Late Quaternary shoreline movements
in New
Zealand. Unpublished Ph.D. thesis, lodged in the Library,
Victoria Un iversity of Wellington. 217 p.
Griffiths,
G. A.;
Glasby,
G. P.
1985: Input
of
river-derived sediment
to the New Zealand continental shelf: 1. Mass.
Estuarine,
coastal and shelf science 21:
773-787.
Hatherton, T. 1980: Shallow seismicity in New Zealand 195 6-1975.
Journal of the Royal Society of New Zealand 10:
19-25.
Heath, R. A. 1986: In which direction is the mean flow through Cook
Strait, New Zealand — evidence of 1-4 week variability.
New Zealand ournal of marine andfreshwater research 20:
119-137.
Herzer, R. H.; Lewis, D. W. 1979: Growth and burial of a submarine
canyon
off
Motunau, North Canterbury,
New
Zealand.
Sedimentary geology 24:
69-83.
Hurley,
D. E. 1959:
Some features
of the
benthic environment
in
Cook Strait.
New Zealand journal of science 2:
137-147.
King, L. H. 1965: Use of a conventional echo-sounder and textural
analyses
in
delineating se dimentary facies
—
Scotian
Shelf.
Bedford Institute
of
ceanography report 65-14:
27 p.
Lewis, K. B. 19 71: Slumping on a continental slope inclined at
l°-4°. Sedimentology 16:
97-110.
1979: A
storm-dominated inner
shelf,
western Cook
Strait,
New
Zealand.
M arine geology 31:
31—43.
Lewis, K. B.; Eade, J. V. 1974: Sedimentation in the vicinity of the
Maui
gas
field.
New Zealand Oceanographic Institute
oceanographic summary 6:
8 p.
Lewis, K. B.; Mitchell, J. S. 1980: Cook Strait sediments.
New
Zealand Oceanographic Institute chart coastal series
1:200
000.
Matthews,
E. R.
1980a: Coastal sediment dynam ics, Turakirae Head
to Eastbourne, Wellington. New
Zealand Oceanographic
Institute oceanographic summary 17:
21 p.
-
8/19/2019 Acoustic Characterisation of Seafloor Sediments and Its Relationship to Active Sedimentary Processes in Cook Stra…
13/13
300
New Zealand Journal of Geology and Geop hysics, 1992, Vol. 35
1980b: Observat ions of beach gravel t ranspor t ,
Wellington Harbour entrance, New Zealand.
New Zealand
journal of geology and geophysics 23:
209-222.
Mitchell, J. S.; Lew is, K. B. 1980: Cook S trait bathymetry. 2nd ed .
New Zealand O ceanographic Institute chart coastal series
1:200 000.
Pantin, H. M. 1961: Magn etic concrete as an artificial tracer m ineral.
New Zealand ournal of geology and geophysics
4: 424-433.
Pickrill, R. A.; Mitchell, J. S. 1979: Ocean wave characteristics
around New Z ealand.
New Zealand journal of m arine and
freshwater research 13:
501-520.
Pratson, L. F.; Laine, E. P. 1989: The relative im portance of gravity-
induced versus current-induced sedimentation during the
Quaternary along the mideast U.S . outer continental margin
revealed by 3.5 kHz echo-character.
Marine geology 89:
87-126.
Proctor, R.; Carter, L. 1989: Tidal and sedimentary response to the
late Quaternary closure and opening of Cook Strait, New
Zealand: results from numerical modelling.
Paleocean-
ography 4:
167-180.
Reed, J. J.; Leopard, A. E. 1954: Sediments of Cook Strait.
N ew
Zealand journal of science and technology 36 B): 14-24.
Robinson, R. 1986: Seismicity, structure and tectonics of the
Wellington region, New Zealand.
Geophysical journal of
the Royal Astronomical Society 87:
379-409.
Smith, D. T.; Li , W. N. 1966: Echo-sounding and sea-floor
sed i men t s .
Mar ine Science Laboratories, University
College North Wales, geological report 66-1:
18 p.