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Quaternary shelf structures SE of the South Island,imaged by high-resolution seismic profilingAR Gorman a , MG Hill a , AR Orpin b , PO Koons c , RJ Norris a , CA Landis a , TMH Allan d

, T Johnstone e , FL Gray f , D Wilson g & EC Osterberg ca Department of Geology , University of Otago , Dunedin , New Zealandb National Institute of Water and Atmospheric Research (NIWA) , Wellington , NewZealandc Department of Geological Sciences , University of Maine , Orono , Maine , USAd OMV New Zealand , Wellington , New Zealande TGS , Perth , Western Australia , Australiaf 49 Saxton Road, RD1 , New Plymouth , New Zealandg BH Billiton Petroleum , Houston , Texas , USAPublished online: 23 Apr 2013.

To cite this article: AR Gorman , MG Hill , AR Orpin , PO Koons , RJ Norris , CA Landis , TMH Allan , T Johnstone , FLGray , D Wilson & EC Osterberg (2013) Quaternary shelf structures SE of the South Island, imaged by high-resolutionseismic profiling, New Zealand Journal of Geology and Geophysics, 56:2, 68-82, DOI: 10.1080/00288306.2013.772906

To link to this article: http://dx.doi.org/10.1080/00288306.2013.772906

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RESEARCH ARTICLE

Quaternary shelf structures SE of the South Island, imaged by high-resolution seismic profiling

AR Gormana*, MG Hilla, AR Orpinb, PO Koonsc, RJ Norrisa, CA Landisa, TMH Alland, T Johnstonee, FL Grayf, D Wilsong

and EC Osterbergc

aDepartment of Geology, University of Otago, Dunedin, New Zealand; bNational Institute of Water and Atmospheric Research (NIWA),Wellington, New Zealand; cDepartment of Geological Sciences, University of Maine, Orono, Maine, USA; dOMV New Zealand, Wellington,New Zealand; eTGS, Perth, Western Australia, Australia; f49 Saxton Road, RD1, New Plymouth, New Zealand; gBH Billiton Petroleum,Houston, Texas, USA

(Received 16 August 2012; accepted 25 January 2013)

Along the south-eastern coast of New Zealand’s South Island, observations and characterisations of shelf geology arecomplicated by numerous possibly active faults (e.g. the coast-parallel Akatore and coast-perpendicular Waihemo and CastleHill faults), a Miocene-aged volcanic edifice (i.e. the Dunedin volcano) and incision from an extensive submarine canyon system.Conventional marine seismic data do not adequately image the basin beneath the shallow shelf here. However, six recentlydigitised high-frequency single-channel boomer seismic surveys have enabled the investigation of unique local geologicalstructures and their relationships to the tectonic and sedimentary development of the region. These structures have significantcontrol on active processes such as: (1) the localisation of sedimentation and submarine erosion; (2) the instigation of canyonchannel incision; and (3) the distribution of fluid migration pathways on the shallow shelf. Future data acquisition will furtherconstrain these processes and help to evaluate earthquake risk in this region.

Keywords: active faults; neotectonics; New Zealand; Otago; Quaternary faults; seismic imaging; seismic reflection; shallowcontinental shelf; South Canterbury; sub-bottom profiling

Introduction

The seafloor off the south-eastern coast of New Zealand’s

South Island between the Waitaki and Clutha rivers (Fig. 1)

is characterised by a narrow (15�30 km wide) shallow

(B150 m deep) continental shelf overlying the eastward-

thinning continental crust of the Campbell Plateau. Locally

the shelf has undergone progressive uplift during the

Pleistocene, resulting from convergence of the Pacific and

Australian plates, and in places the coastline is currently

subject to a gentle uplift of B0.1 mm a�1 (Wellman 1979;

Gibb 1986; Litchfield & Lian 2004). A coastal escarpment

has consequently developed, with incised rivers emerging at

the coastline to deliver sandy terrigenous sediments to the

inner shelf.Despite over 150 years of geological mapping in the

provinces of Otago and Canterbury that has constrained the

three-dimensional geometry of coastal rocks and sediments

(e.g. Benson 1968; Bishop & Turnbull 1996; Litchfield &

Norris 2000; Forsyth 2001), their expression and extent on

the adjacent continental shelf and basins is still poorly

understood and is largely limited to petroleum industry

multi-channel seismic (MCS) surveys (e.g. Field & Browne

1989; Cook et al. 1999). Although these MCS surveys have

provided insight into the large-scale basin geology, resolva-

ble with high-energy and low-frequency seismic acquisition

systems, only high-frequency single-channel seismic studies

at specific sites can resolve the fine-scale features required to

interpret Quaternary processes (e.g. Carter et al. 1985;

Carter & Carter 1986; Carter 1986b; Browne & Naish 2003).In the last two decades, a series of high-resolution single-

channel seismic reflection surveys (Fig. 1) has been under-

taken on the shallow shelf and upper slope (B250 m water

depth) off the coasts of Otago and South Canterbury to

address a range of structural, sedimentological, stratigraphic

and hydrogeological aims (Allan 1990; Johnstone 1990;

Orpin 1992; Gray 1993; Orpin 1997; Wilson 1998; Osterberg

2001, 2006). These surveys underpin a substantial body of

mostly unpublished research conducted as part of a series of

postgraduate theses. Regional geological mapping projects

have made use of the results of these surveys (Bishop &

Turnbull 1996; Forsyth 2001); however, the full potential of

the datasets collected for this work has not been realised or

integrated regionally due to the inadequacy of the data

archiving process in the early years of computing. Recent

advances in technology have enabled these analogue data to

be recovered, converted to digital signals and enhanced for

use in further investigations of the shelf. In particular, these

data can be critical in the characterisation of Quaternary

faults, most of which can be regarded as active, on the

shallow shelf off Otago.

*Corresponding author. Email: [email protected]

New Zealand Journal of Geology and Geophysics, 2013

Vol. 56, No. 2, 68�82, http://dx.doi.org/10.1080/00288306.2013.772906

# 2013 The Royal Society of New Zealand

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Figure 1 Position of sub-bottom profile surveys presented in this paper. Numbers refer to MSc theses of (1) Wilson (1998), (2) Allan (1990),

(3) Gray (1993), (4) Osterberg (2001), (5) Orpin (1992) and (6) Johnstone (1990) for which the data were collected. Lines presented within thispaper are identified by dotted lines and labelled with their line numbers; line numbers are unique only within a particular survey. Shadedtopographical relief map on land. Offshore contoured colour bathymetry courtesy of NIWA. Contour interval 100 m. Inset map shows

location of main map in relation to New Zealand and the regional plate boundary.

High-resolution seismic profiling of Quaternary shelf structures 69

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In this paper, we highlight three case studies from thesedata to highlight the control that such geological structureshave had � and continue to have � on sedimentation,submarine erosion, canyon incision and fluid flux. However,the ability of the technique to image evidence of thesegeological processes varies through the region depending onthe modern sediment cover on the seafloor and the nature ofthe underlying bedrock. An additional aim of this study isnot only to assess these earlier data, but to recognise wherenew studies could address outstanding questions for thecontinental shelf SE of the South Island.

Geological setting

The crustal terranes that currently strike NW�SE along theSE coast of the South Island were accreted and regionallymetamorphosed from the Jurassic to Early Cretaceous onthe margin of Gondwana (Mortimer 2004). The LateCretaceous�Early Miocene extensional tectonic regime thatrifted Gondwana apart and thinned and stretched thecontinental crust of New Zealand evolved into the currenttranspressional plate boundary through New Zealand by thelate Miocene (e.g. Carter & Norris 1976; Norris et al. 1990).

Tectonic inversion structures now dominate much ofeastern Otago, with most of these faults trending subparallelto the major plate boundary of the Alpine Fault on the westcoast of the South Island. The easternmost of the onshoremapped Otago inversion structures are the Titri Fault,which extends along the west side of the Taieri Valley SWof Dunedin, and the Akatore Fault, which crosses theshoreline (Litchfield & Norris 2000; Rees-Jones et al. 2000;Litchfield 2001; Litchfield & Lian 2004). Although there hasbeen no historically observed seismicity on the Titri Fault,earthquakes in 1974 (Adams & Kean 1974; Bishop 1974)and 1989 have been attributed to the Akatore Fault. Eventhough there has been no active or palaeo-seismicityconfirmed farther offshore, the basement geology also hasevidence of coast-parallel structures that were possiblyrelated to extensional tectonics (cf. Carter 1988) and arelikely to be susceptible to reactivation under compression.Offshore evidence from Pegasus Bay subsequent to the 2010and 2011 Christchurch earthquakes suggests that suchbasement structures along the Canterbury coast (e.g. Barnes1996) are potential earthquake sources. Such coast-paralleloffshore structures are also the subject of a currentpetroleum industry exploration focus in the deepwaterCanterbury Basin. The area of the shallow shelf immediatelyseawards of the Akatore Fault was identified as being anideal location to explore the offshore expression of coast-parallel faults.

In contrast to the coast-parallel faults, a series ofregional faults lie orthogonal to the SE coast of the SouthIsland. From SW to NE, these include the Livingstone/Castle Hill, Tuapeka, Waihemo and Waitaki fault systems(Bishop & Turnbull 1996; Forsyth 2001). The offshore

coast-parallel faults identified in this work lie between theLivingstone/Castle Hill and Waihemo faults, which respec-tively form the south-western and north-eastern boundariesof the SW�NE-striking Otago fault system. In the NE, theWaihemo Fault is mapped onshore as an inverted normalfault where younger lower-grade textural zone II schists areuplifted to the NE over older higher-grade textural zone IVschists to the SW (Forsyth 2001). This break in thestructural fabric is manifested in the NW-trending KakanuiMountains and Shag River Valley. In the SW, the Living-stone fault zone, with a zone of deformation up to 1 kmwide, is mapped as the tectonic contact between the Maitaiand Caples terranes along the south-western edge of theOtago Schist (Cawood 1986; Bishop & Turnbull 1996).Close to the mouth of the Clutha River it possibly mergeswith the Quaternary active Titri Fault to form the CastleHill Fault and runs offshore (Bishop & Turnbull 1996;Mortimer et al. 2002). Whether these coast-perpendicularfault zones extend offshore remains an outstanding ques-tion, and forms an important motivation for the currentstudy.

The shallow modern continental shelf here lies along theNW (landwards) margin of the Great South (Carter 1988;Cook et al. 1999) and Canterbury (Field & Browne 1989)basins. Late Cretaceous to recent sedimentary units thickengreatly seawards. Although the modern shelf is narrow, theunderlying sedimentary basins on the Campbell Plateau aredeveloped on thinned continental crust that extends severalhundred kilometres offshore beneath typical ocean depths of1000�1500 m.

Superimposed on this underlying geology in the centralpart of the Otago coastline is the Late Miocene (16�10 Ma)Dunedin Volcano (Bishop & Turnbull 1996; Coombs et al.2008) that grew from a base level close to present-day sealevel. With an approximate diameter of 25 km centred onOtago Harbour, the resistant igneous rocks have formed acoastal promontory that has affected coastal current pat-terns and sediment dispersal along the shelf.

Modern sediment transport, and the geometry of theinner shelf Quaternary terrigenous sediment prism, isstrongly influenced by a combination of the persistentsoutherly swell, north-eastwards-travelling longshore cur-rents, winds, tidal flows and the north-eastwards flowingSouthland Current (Carter et al. 1985). Fluvial sources fromthe Clutha, Taieri and Waitaki rivers dominate, supplying0.39, 0.32 and 0.34 Mt of sediment per year, respectively(Hicks & Shankar 2003); note that these values are likely tobe affected by major hydroelectric damming schemes on theWaitaki and Clutha river systems, but these anthropogeniceffects are not considered here. At the shelf edge (approxi-mately 140 m water depth), Carter et al. (1985) suggest thatstorm swells, internal waves and up-canyon currents areprobable mechanisms for sediment transport. They describefour broadly shore-parallel belts of sediment from theshallow shelf off the coast of Otago, and their origin is

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attributed to deposition during episodic post-glacial trans-gressions. These belts consist of: (1) a shore-connectedwedge of highstand terrigenous sand at water depths of0�20 m; (2) transgressive quartz gravel ridges at waterdepths of 20�55 m; (3) sheets and ribbons of relicttransgressive sand at 55�85 m; and (4) thin veneers ofouter-shelf transgressive/lowstand shell hash at water depthsof 85�200 m.

A series of submarine canyons occur off the coast of theOtago Peninsula where the shelf narrows to a width of about10 km, and feeds most sediment from this portion of theshelf to the Bounty Trough (Carter & Carter 1987; Lewis &Barnes 1999). The shelf break at roughly 140 m coincideswith the seaward edge of a terrace that has its inner edge atabout 120 m, in the vicinity of the Last Glacial Maximumshoreline (c. 18 ka). Note that some of the finer, suspendedfractions of sediment bypass these canyons and are poten-tially carried a few hundred kilometres north, beyond BanksPeninsula, where they may descend via the KaikouraCanyon System to the Hikurangi Plateau (Lewis & Barnes1999).

Methods

In comparison to airgun-sourced multi-channel seismic(MCS) methods, boomer sub-bottom profiling techniqueshave limited depth penetration due primarily to the rela-tively low-energy levels of the seismic source. Additionally,boomer surveys are usually conducted with short receiverarrays, which results in low-fold sections that do not havethe noise suppression that high-fold MCS datasets have.However, boomer datasets have several advantages overconventional MCS data. In particular, they generally have amuch higher resolution in both the horizontal and verticaldirections due to the shot firing rate (shot separation is oftenB1 m for boomer surveys compared to �20 m for anairgun survey) and the frequency content of the signal. Also,the scale of the operation is such that it can be undertakenfrom smaller vessels, and therefore has the potential tocollect data economically over targeted regions. As a result,boomer surveys have been widely and effectively used inshallow seas and lakes to make detailed maps of the upper50�200 m of geology (e.g. Dingle 1965; Bastos et al. 2003;Grossman et al. 2006; Upton & Osterberg 2007).

Equipment

Seismic data were collected between 1989 and 1999 on boardthe University of Otago’s RV Munida using a FerrantiOcean Research Equipment (ORE) Geopulse sub-bottomprofiling system (Hill 2007). The acoustic source (boomer)assembly and hydrophone array were towed approximately25 m off the stern, with the vessel travelling at 5�8 km/h.The boomer source signal was controlled by electronics onboard the research vessel (summarised in Fig. 2).

Seismic energy was received by a single-channel array(consisting of 20 piezoelectric hydrophones spaced 15 cmapart) generally deployed 5�10 m to the starboard side ofthe boomer. Analogue signals were amplified, filtered andplotted on board the vessel while simultaneously beingrecorded on a magnetic tape (VCR or DAT) system forlater playback and analysis (see Fig. 2 for more details).

Digitisation of analogue data

Analogue data from the surveys presented here, combinedwith their associated geographical and geometrical refer-ences, have been converted to SEG-Y (Barry et al. 1975)formatted digital files. This enables digital processingmethods such as deconvolution, spatial filtering and migra-tion which, in addition to archiving, facilitates presentationof the data at a range of scales for interpretation anddisplay.

Conversion to SEG-Y format involved several steps.First, the tapes of the analogue signal were played backthrough the original recording system (Fig. 2) and digitisedinto a 16-bit stereo WAVE file, with a sample rate set to10 000 samples/sec using the open-source Audacity program(http://audacity.sourceforge.net). The WAVE files weremanually separated into files where the boomer firing ratewas constant. Each of these files was then converted toSEG-Y format using a python script (Hill 2007). Datasamples were written as 4-point binary IEEE floating pointnumbers. The trigger pulse (as recorded on one of the stereochannels) can be reliably used to indicate the start of eachrecord trace (recorded on the other stereo channel.) Thelength of each trace in a particular file was generally set to beslightly shorter than the shot interval in order to allow for aconsistent trace length given slight variations in the shotinterval.

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Figure 2 Schematic summary of the onboard Ferranti OREGeopulse acquisition setup. Specifics of equipment include the

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High-resolution seismic profiling of Quaternary shelf structures 71

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The header information for each trace in the SEG-Yformatted data contains co-ordinate information interpo-lated from original navigation files (Hill 2007). Theseoriginal navigation files consist of tables of x and y co-ordinates, recording times and reference IDs that enabledthe linking of recorded data to the navigation data. For anygiven trace recorded at an arbitrary time t the algorithminterrogated the navigation file, found the co-ordinates thatcome before and after time t and then interpolated new co-ordinates for the trace. The interpolation algorithm assumesthat the vessel maintained a constant speed and headingbetween navigational fixes.

The navigation data stored within trace headers vary inquality, both in terms of the method used to measure the co-ordinate positions and in the number of points used todescribe a particular survey track. Early methods involvedradar triangulation, which could provide co-ordinates thatwere accurate to within 290 m (Allan 1990; Johnstone 1990;Orpin 1992). These were superseded by GPS measurementswhich in the early 1990s were quoted as being accurate towithin 50 m (Gray 1993) and, more recently, are expected tobe accurate to within 5 m (Wilson 1998; Osterberg 2006).

Seismic processing and analysis

All data processing was undertaken using the GlobeClaritas seismic processing package (Ravens 2001). Theforemost of the processes applied was one to remove theeffect of ocean swell on the data. Boomer data collected onthe open sea are affected by ocean swell that can often havean amplitude of as much as 3�4 m and a period of severalseconds (note that if the average swell was above 2 m, dataacquisition was usually suspended).

Data presented here have had a simple swell filterapplied through a static correction to the data as follows.The seafloor reflection was manually picked for all lines.This was then smoothed by mixing the picks from 30adjacent traces. The smoothed picks were then subtractedfrom the unsmoothed picks and the result was assumed to bethe component resulting from ocean swell. This result wasthen applied as a static shift to the appropriate trace. Notethat this assumes that the seafloor will have few features onthe scale of the swell. Some editing is required at locationson the seafloor where there are significant features that donot meet this assumption.

A Butterworth filter with trapezoidal cutoffs defined at100, 180, 1500 and 1900 Hz has been applied to all data. Anautomatic gain correction (AGC) with a short time window(dependent on the water depth of the particular dataset) wasused.

Seismic imaging of shallow shelf structures

The geology of the shallow shelf offshore from Otago andSouth Canterbury varies considerably along the coast due to

the underlying crustal-scale structures and patterns ofongoing sedimentation. Two of the most obvious differencesobserved along the coast are the rate of crustal subsidence oruplift and the corresponding sediment accumulation rates.Off South Canterbury, in the Canterbury Basin, accumula-tion has been rapid through the Quaternary (Field &Browne 1989; Browne & Naish 2003). This has led to thepreservation of numerous prograding sedimentary se-quences as the shelf has built outwards (e.g. Fig. 3). OffOtago, basin subsidence and sediment accumulation hasbeen considerably slower. In places, incised channels andcanyons have been eroded into the shelf at sea levellowstands during the Pleistocene and partially infilledduring periods of higher sea level (Fig. 3). For much ofthis south-western part of the shelf, especially closer to theshore, a net uplift has occurred that has resulted in theerosion of underlying Tertiary (or older) bedrock that can beclearly imaged with boomer seismic methods at severallocations (e.g. Figs 4�7).

Boomer seismic surveys in the Otago region show that asmooth, hard, fluid-saturated and well-cemented seafloorwill generally result in deeper penetration and higherresolution than one that is particularly soft, rugged, gascharged or organic rich. The reason for this is that the lattergroup of conditions tends to scatter or absorb seismic energymuch more than the former group of conditions. Forexample, regionally limited deposits of post-glacial sandand gravel that occur on the modern shelf off Otago (Carteret al. 1985) can obscure images of the underlying Quaternarylithological units and geological structures below. However,for much of the inner and middle shelf (B60 m water depth)off the coast of Otago, little sediment is deposited on thescoured bedrock. This is particularly so along the coastbetween Taieri Mouth and Brighton, off the coast ofKaritane and offshore from Shag Point (Fig. 1), permittinghigh-resolution imaging of the underlying sedimentaryrocks. The barren seafloor off Brighton and Karitaneappears to be affected by the interplay between the SW�NE-running longshore drift and promontories in the coast-line (i.e. the Akatore block and Otago Peninsula, respec-tively). Basement rocks off the coast of Shag Point are notdraped by sediment because they are locally higher than thesurrounding shelf, probably as a result of active uplift on thenorth side of the Waihemo Fault.

Boomer surveys have successfully been used to char-acterise three contrasting types of structural systems in thestudy area: (1) coast-parallel faulting associated with theoutboard edge of the Otago fault-fold belt is apparent on anoffshore portion of the Akatore Fault, along the Brightoncoast south of the Dunedin Volcanic complex (survey 6 inFig. 1); (2) an orthogonally striking fault system is seen inthe Shag Point region where the Waihemo Fault Systemruns offshore from the Kakanui Mountains (survey 2 inFig. 1); and (3) shelf edge faulting possibly linked tolowstand hydrogeological fluid flow systems is seen near

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the present-day shelf break off the coast of the OtagoPeninsula (survey 5 in Fig. 1). Details of these structuralsystems are described in the following sections.

Offshore continuation of the Akatore Fault System

The coastline of South Otago is roughly parallel to the strikeof the tectonically inverted range and basin system foundthrough the interior of the province. The onshore faultclosest to the coast involved with this system is the AkatoreFault, which runs between the villages of Taieri Mouth inthe north and Toko Mouth in the south and extendsoffshore to the NE and SW (Bishop & Turnbull 1996;Litchfield & Norris 2000). Detailed boomer data from theadjacent shelf (Johnstone 1990), acquired within about12 km of shore and in water depths of 15�60 m, imagesthe upper 50 ms (roughly 50 m) of sub-seafloor sedimentsand sedimentary rocks (Fig. 4). Most of the shelf has littleQuaternary cover in this region (cf. Carter et al. 1985).Seismic profiles are characterised by numerous gentlySE-dipping reflections that are distinct and continuous.They correlate to the Late Cretaceous�Tertiary sedimentarysequences that outcrop onshore, but no direct correlation(by sampling or mapping) has been undertaken between

specific reflections and onshore formations. The single-channel boomer dataset recorded in this region has remark-ably good penetration (i.e. �100 m), which in many caseswas limited first by the occurrence of the seafloor multiple.In a few locations (e.g. as indicated by white arrows inFig. 4), more rugged seafloor outcrops are indicated bystrong seabed reflections and poor penetration. These out-crops are most likely metamorphic basement rocks, volcanicextrusions, eroded igneous intrusions or organic commu-nities that have developed on outcropping hard substrates.

Besides the very regular reflections from sedimentaryunits, the most striking features identified in these data arefaults (Figs 4 & 5) with distinctive folding and deformationon the hanging walls and footwalls (e.g. Line 1, Fig. 4).Unfortunately, the landward extent of the Akatore Faultcould not be imaged in water depths B5 m due tooperational limitations with the vessel. Previous suggestionshave had the fault coming back onshore at the approximatelocation of the Kaikorai Stream estuary (Fig. 5) and perhapsrunning up the length of Otago Harbour. However, onmultiple profiles, at least two other significant faults areinterpreted to run roughly parallel to the Akatore Fault,approximately 3 and 10 km farther offshore (Faults A and Cin Figs 4 & 5). These faults appear to displace post-glacial

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Figure 3 Sections of two boomer lines highlighting Quaternary sedimentation from the shelf north of the Otago Peninsula. Wilson Line 9(Wilson 1998) in water depths of 95�150 m off the coast of the Waitaki River) is interpreted to show sequence boundaries and idealised facies

distributions (darker shading corresponding to finer grain sizes). Seven fifth- (c. 120 ka) or sixth-order (c. 41 ka) glacio-eustatic sedimentarysequences are annotated. Vertical exaggeration is about 11:1. Gray Line 6 (Gray 1993), crossing the landward edge of the Karitane Canyonon the shallow shelf, highlights the stratigraphy and erosional surfaces related to canyon development. The partially eroded present-day

seafloor is underlain by the earlier somewhat-chaotic canyon fill sediments and a former erosional surface. Strata below the lower erosionalsurface are flat-lying and laterally coherent. Likely positions of sequence boundaries that have been cut off by (1) present and (2) ancienterosion are indicated. Vertical exaggeration is about 1.5:1.

High-resolution seismic profiling of Quaternary shelf structures 73

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Figure 4 Boomer profiles 1, 2, 5 and 4 from the nearshore shelf south of Dunedin showing reflective units of the Cretateous�Tertiary sedimentary sequence (Johnstone 1990).In particular, coast-parallel faults and related deformation is imaged well in the upper 50 ms (roughly 50 m) of the seafloor. Faulting is indicated using black lines (solid anddashed, with solid having a higher degree of confidence). Labelled faults A, B and C are mapped in Figure 5. Fault displacement is indicated where apparent. Note possible

seafloor displacement by faults on Line 1 (trace 3200) and Line 2 (trace 2800). Non-penetrative seafloor (regions indicated by white double-arrowed bars) is rare on thismargin. Line positions as indicated in Figure 5.

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sediment cover in places, suggesting that they have been

active in the Holocene. Given their proximity to each other,

either of these faults could be considered alternative sources

of the 1974 (Adams & Kean 1974; Bishop 1974) and 1989

earthquakes attributed to the Akatore Fault. Anticlinal

features on the SE and synclinal features on the NW sides

of the fault support uplift to the SE, but the seismic images

do not convincingly provide information on the dips of these

faults. In some cases, deformed strata are suggestive of

a system of NW-dipping normal faults or, alternatively,

SE-dipping reverse faults.The latter interpretation is preferred due to observations

of the fault onshore through the Akatore block (Litchfield &

Norris 2000). Offshore, measurements of throw on the faults

are not possible because chronostratigraphic correlations

cannot be convincingly made across the faults.

Offshore continuation of the Waihemo Fault System

The Shag River, approximately 50 km north of Dunedin,

delivers 0.060 Mt a�1 of sediment to the coast (Hicks &

Shankar 2003). At the river mouth, the NW�SE-trendingWaihemo Fault System heads offshore where a series of

boomer lines running parallel to the coast has helped to

constrain the offshore extent of this fault system and its

associated deformation (Allan 1990). Three coast-parallel

lines shot in water depths of 35�45 m image the seafloor

sediments of, primarily, the hanging wall of the Waihemo

Fault. At least three fault traces within the Waihemo Fault

System are imaged seismically in this region (Fig. 6).The main Waihemo No. 2 Fault trace interpreted at the

SW end of the three lines in Fig. 6 shows the reflective

Cretaceous�Tertiary hanging-wall sequences to the NE

juxtaposed against seismically transparent sediments to the

0 30km

170˚30’E170˚E169˚45’E 170˚15’E 170˚45’E

-46˚S

-46˚15’S

100 m 500 m

Clutha River

Akatore

Fault

Taieri

Green IslandBrighton

U

U

U

U

U

U

U

D

D

D

D

D

D

DCastle Hill Fault Zone

Kaikorai Stream

DUNEDIN

BALCLUTHA

TAIERI PLAINS

10

11

12

13

16

14

15

4

5

7

6

32

1

Fault

C

Fault

BFa

ult A

Tuapeka Fault Zone

River

Titri F

ault Syste

m

Figure 5 Offshore faults observed (solid lines) and inferred (dashed lines) along the South Otago margin (after Johnstone 1990). Seismic linesused in the fault interpretation are indicated by thin annotated lines; only lines 1, 2, 5 and 4 are shown in Figure 4. Onshore fault positions

are indicated by white lines (after Bishop & Turnbull 1996). Onshore region is indicated by shaded topographical relief. Shelf marginbathymetry is indicated by grey contours.

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SW, interpreted to be the Holocene deposits of the Shag

River. To the south, signal penetration was not sufficient toimage the base of the channel at the mouth of the Shag

River. In contrast, north of Waihemo No. 2 Fault, very little

modern sediment overlies the Cretaceous�Tertiary sequence.The seafloor expression of the fault coincides with thetransition from strong irregular seafloor reflections to the

north (on the hanging wall) to a flat-lying planar seafloor to

the south. The rough seafloor reflections are interpreted to

correspond to indurated rock outcropping as strike ridges(associated with Waihemo No. 1 and Waihemo No. 2 faults

in Fig. 7) that extend approximately 8 km offshore (between

Lines 3 and 4). However, the underlying seismic character

in the deep profile (Line 4, Fig. 6) shows that the faultsystem continues seawards as a blind fault.

Identification and correlation of particular units within

the offshore seismic sequence has been attempted by Allan

(1990), but is tenuous because the survey line spacing is toowide and lacks a tie line (Figs 6 & 7). However, strong

contrasts in the internal reflectivity characteristics of the

units are consistent with the sedimentary formations (pri-

marily clastic nearshore units) observed in outcrop on shore.For example, a hanging-wall anticline is clearly observed in

sea cliffs along the coast at Shag Point, within the fault zone.

A hanging-wall anticlinal feature is also observed on all

three seismic lines, with the crest of the anticline converging

with the main fault trace as the shelf deepens. Sedimentaryunits on the NE limb of the anticline are observed to dip tothe NE at 8�168.

Shelf edge faulting and fluid flow

On the uppermost part of the slope off the coast of the OtagoPeninsula, between the Papanui and Saunders canyons, theotherwise regular margin is interrupted by a B1 km2 bench ata water depth of roughly 200 m (Figs 1 & 8), about 60 m indepth below the regional shelf break. Fishermen dredging forscallops on this bench discovered a collection of chimneys andirregularly-shaped carbonate concretions (Orpin 1992, 1997)that have been linked to distinctive vent phenomena observedelsewhere around New Zealand (Lewis & Marshall 1996).Radiocarbon dating has determined that the chimneys arerelict features with amaximumage of 33 0009550 years and astable oxygen and carbon isotope composition consistentwith a predominantly marine origin for the fluid that wouldhave flowed through shallow organic-rich sediments at a timewhen sea level was considerably lower than present (Orpin1997).

Two high-frequency single-channel seismic profiles(survey 5 in Fig. 1) image up to 150 m of sedimentarysection within the lowered bench feature on which thechimneys are found (Orpin 1992, 1997). Line 1 (Fig. 8)

0.10

0.06

0.12

0.08

0.04

Tim

e (s

)

8000 7000 6000 5000 4000

0.10

0.06

0.12

0.08

Tim

e (s

)

6000 7000 8000 9000 10000 11000

0.10

0.06

0.12

0.08

0.04

Tim

e (s

)

6000 5000 4000 3000 2000 1000 0Allan Line 2

Allan Line 3

Allan Line 4

seafloor multiple

SW NE

SP

SP

SP

Anticline crest

Anticline crest

Anticline crest

2

2

2

buttressunconformity

1

1

1 7

7

7

VE ~ 18:1

SW NE

SW NE400 m

Figure 6 Coast-parallel boomer profiles 2, 3 and 4 located offshore from Shag Point (Allan 1990). The upper part of the water column has

been removed from the plots. The profiles are aligned on the offshore Waihemo Fault (SW/left side of each plot). Quaternary sedimentssourced from the Shag River to the south contrast strongly with the folded and faulted Cretaceous�Tertiary sequence NE of the fault.Numbered faults correspond to those interpreted in Figure 7. Vertical exaggeration is about 18:1.

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shows a series of reflections that generally dip gentlyseawards (maximum 88 dip), representing the progradingsystem of Late Cretaceous�Quaternary sediments seen alongthe outer shelf of the SE South Island. As shown by thetruncated east-dipping beds at the seafloor (e.g. reflection Ain Fig. 8), the present-day outer part of the shallow shelf isstarved of modern sediment (Carter et al. 1985; Orpin et al.1998).

A geological cause for the bench and the focusing ofchimney features in this region has not been determined, butshelf-edge faulting or large-scale slumping of the shelfmargin is a possible mechanism. Potential evidence forsmall-scale slumps is supported by a cemented ridge

observed in close proximity to the zone of carbonatecementation and chimneys by a camera mounted on aremotely operated vehicle (Orpin 1992, 1997). However, alarger-scale slump mechanism for the overall formation ofthe B1 km2 bench is complicated by the observation ofrelatively continuous subsurface reflections (e.g. reflectionsB�E in Fig. 8) that broadly mirror the modern steppedgeometry of the shelf break. Evidence of erosional surfacesthat truncate dipping reflections is common, especially onshallower portions of reflections B, C and E. Reflectionsimmediately above the erosional surfaces appear to infillchannels. In addition, Reflection B is conspicuous because ithas an apparent landwards dip below the bench containing

Otepopo Greensand

Matakaea Gp.

Caversham/Goodwood Fm.

Kaitiki / Herbert Fm.

Shag Point Gp.

19

20

12

8

16

15

16

19

Key

strike/dip ofsediments crest of anticline

Allan L

ine

1

Allan

Line

2Alla

n Li

ne 3

?

?

0 5 10

km

45.48°S

45.52°S

45.56°S

170.80°E 170.84°E 170.88°E 170.92°E170.76°E

Waihemo No 7 Fault

Waihem

o No 1 Fault

Waihem

o No 2 Fault

Shag Point

Shag River

Titri

Fault

?

DU

Allan

Line

4

Figure 7 Schematic illustration of nearshore shelf geology in the vicinity of Shag Point seismic survey (survey 2 in Fig. 1). Strands of theWaihemo Fault System are identified and labelled; dashed lines are inferred faults that link faults on land to interpreted faults on seismic dataoffshore. Weakly constrained interpretations of seafloor sedimentary unit outcrops are shown north of Waihemo No. 2 Fault. South of

Waihemo No. 2 Fault, unmapped seafloor sedimentary units are assumed to be Pleistocene sequences overlain by modern Holocenesediments from the mouth of the Shag River. Seismic lines are indicated by light grey lines. Dips of contacts are annotated where they weredetermined from seismic sections.

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the chimneys. Weaker reflections that drape over ReflectionB appear to mirror this attitude. The repetition of thisgeometric pattern is consistent with glacio-eustatically con-trolled sedimentation on the outer shelf throughout the LateTertiary (cf. Osterberg 2006), suggesting that the bench hasbeen a stable geomorphic feature through successive cycles,perhaps augmented by longstanding fluid-flow processes. Ifslumping did control the initial geometry of the bench, thebase of the slump is now deeper than can be readily imagedin the seismic data.

Discussion and implications

Active structures on the shallow shelf off the coast of Otagoaffect a wide range of geological and hydrodynamicprocesses. These include the localisation of sediment accu-mulation and erosion, the establishment and maintenance offluid migration pathways through the shelf and possibly theinitiation and focusing of shelf-margin canyons. Each ofthese is addressed in the following sections.

Structural controls on sedimentation

Seismic data collected off Shag Point (Allan 1990) show thestrong role that active structures can have on sedimentation.At this location, the seafloor geology changes significantlyacross the NE-dipping Waihemo Fault System of reversefaults. However, the seismic data support the possibletermination of the fault zone just a short distance offshore.The surface expression of this faulting ceases about 8 km

from the coast, and basement faulting is not visible in

conventional marine seismic data a further 7 km offshore(Allan 1990; Mortimer et al. 2002). Immediately NE of the

seafloor expression of the Waihemo Fault System, the

structure consists of the SE-plunging Shag Point Anticline.The juxtaposition of basement units against Tertiary

sedimentary rocks in the footwall of the offshore Waihemo

Fault presents a geometric conundrum. To address thisproblem, Allan (1990) inferred an extension of the coast-

parallel Titri�Akatore Fault System NE of the OtagoPeninsula, trending subparallel and close to the present

coastline, terminating at the Waihemo Fault (Fig. 7).

Southeast of this hypothetical junction of the Waihemoand Titri fault systems, displacement across the Waihemo

Fault System would be relatively small. The termination ofthese structures can be accommodated by linking the motion

of the blocks on the south-eastern side of the Titri Fault

System and the north-eastern side of the Waihemo FaultSystem. These structures are presently insufficiently imaged

to allow a more detailed and quantitative analysis, but thisstyle of fault termination is consistent with several other

central Otago faults such as the Taieri Ridge, Rock and

Pillar, Rough Ridge and Raggedy Faults (Norris 2004;Norris & Nicolls 2004), which all have north-eastern

terminations in the Waihemo Fault System. These offshoredata suggest that the major tectonic boundary between the

SW�NE-striking range and basin inversion structures of

Otago and the NW�SE-striking faults of South Canterburyhas an equivalent expression on the adjacent shelf where it is

overlain by Quaternary sedimentation.

0.3

Tim

e (s

)

0.4

0.2

regional shelf break

500 mEW

Orpin Line 1

seafloor multiple

A

BC

DE

Approximate locationof carbonate chimneys

and cemented ridge

VE ~5:1

Figure 8 Shelf-perpendicular Line 1 (Orpin 1992) located between the Papanui and Saunders submarine canyons. The upper plot isuninterpreted. The lower plot shows an interpretation of earlier erosional surfaces (reflections A�E). The regional shelf break at a depth ofapproximately 140 m is interrupted by a B1 km2 bench at about 200 m. Here, fishermen dredging for scallops discovered a suite of carbonate

concretions and chimneys (location indicated in lower plot) with some similarities to other methane vents observed elsewhere along the eastcoast of New Zealand. The surface expression of the ridge suggests a moderately east-dipping structure of north�south extent. This area canbe seen as a small ‘bump’ on the seismic profile B2 m in height, below which Orpin (1997) interpreted an eastwards-dipping slump-like

feature, which is interpreted in the present study as a widening zone of disturbance below the chimney site (Line 1). Vertical exaggeration isabout 5:1.

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Structures within the Tertiary and older sedimentaryunits on the Otago margin, and especially those that traversethe coastline and adjacent continental shelf, have thepotential to influence the position of river and drainagechannels. For example, on land, the Shag River follows theline of the Waihemo Fault System. Offshore, the Shag Pointanticline and extension of the Waihemo Fault Systemdeform Late Cretaceous�Miocene deposits, exposing themas strike ridges and reefs on the seafloor that can beobserved in the seismic data (Fig. 6) and sidescan sonar(Allan 1990). These units were sub-aerially exposed duringQuaternary glacial periods, and scarps would have influ-enced topographical drainage in a similar way to the on-landsection. For example, evidence of a buttress unconformity isseen in seismic section (e.g. Line 4, Fig. 6) with Holocenemarine sediment deposited against deformed Tertiary sedi-mentary rocks. Furthermore, buried channels are observedto traverse the present-day inner shelf with braided ormeandering cut-and-fill channel patterns (Gray 1993) andlink with other tributaries further towards the shelf break.These fluvial networks presumably drained into an ancientmanifestation of the Karitane Canyon.

Structural influence on fluid flow within and on the shelf

Fluid migration pathways on the Otago margin are poorlyunderstood, but the carbonate chimneys between thePapanui and Saunders canyons provide conclusive evidenceof Late Quaternary fluid flow in the system (Orpin 1997). Atactive marine settings elsewhere along the eastern margin ofNew Zealand, migration pathways can be linked to large-scale contractional faulting associated with a basal decolle-ment (e.g. Barnes et al. 2010), consistent with occurrenceselsewhere in the world (Moore & Vrolijk 1992). Even thoughthe Otago Margin is classified as passive, the coast-parallelNE-trending thrust faults in the region (Beanland & Berry-man 1989; Norris et al. 1990; Jackson et al. 1996) might beexpected to provide similar migration pathways for deepfluids. Offshore, such features have been proposed farthersouth in the Waipounamu Fault System (Johnstone 1990).However, no large-scale imbricate thrust systems originatingfrom a deep master fault have been confirmed along theshallow shelf east or north of the Otago Peninsula (Orpin1992; Gray 1993; Osterberg 2006).

An examination of the shelf bathymetry (Carter 1986a)suggests that the peninsula impacts the shelf gradient out toaround the 60 m isobath, inferring that post-glacial coastalflows might have been steered eastwards since the EarlyHolocene and during at least the highstand phases of marineisotope stages 5 (70�130 ka) and 7 (190�240 ka) (e.g.Martinson et al. 1987). Seismic evidence also supports theoccurrence of enhanced longshore currents during lowstandperiods on the shelf. The lowered bench feature on theuppermost slope between Papanui and Saunders canyonsshows stacked, draped internal reflections that lap down

onto the upper slope (Fig. 8). This regular unbroken patternis indicative of depositional processes. Truncated or dis-rupted reflections consistent with a slump are not visible.The bench therefore appears to be a longstanding geo-morphic feature that has persisted over several eustaticcycles and been maintained by the hydraulic regime.

Structural controls on canyon development

The underlying reason for the concentration of five largecanyons adjacent to the Otago Peninsula (Fig. 1) remains anoutstanding question. As a first-order geomorphic inter-pretation, the creation of submarine canyons has tradition-ally been linked to locations where river mouths likely inciseinto the shelf break during sea level lowstand (Shepard1981). The Waitaki Canyon along the south-eastern marginof the South Island is perhaps the most compelling exampleof this, where the canyon head lies seawards of the modernWaitaki River (0.34 Mt a�1, Hicks & Shankar 2003; Fig. 1).

Underlying coast-perpendicular crustal faulting such asthe Waihemo Fault or basement structures such as theHyde-Macreas Shear Zone or the basement arch (Mortimeret al. 2002; Mortimer 2003) may have focused channelincision across the shelf and led to canyon development,most likely at the location of the Karitane Canyon which liesdirectly offshore from the Waihemo Fault System. However,there are several other factors to consider with respect to thepositions of the Otago canyons.

(1) During highstands, longshore currents would be con-stricted on the shelf by the promontory of the peninsula,accelerating flows and potentially deflecting watermasses, and transported sediment offshore (e.g. Herzer1979) without the need to correlate to existing coast-perpendicular river drainage systems.

(2) The isostatic and thermal effects of the adjacentDunedin Volcanic Complex on the shelf-canyon se-quence architecture are poorly known.

(3) The relative balance of lowstand-driven geomorphologyon highstand bathymetric features is poorly understood.For example, what is the role of geomorphic inheritanceat terrestrial�marine transitions, such as the mid andouter shelf?

(4) Finally, fluid flow (with or without structural controls)at the shelf edge could facilitate headwards incision andcanyon development at zones of elevated fluid pressure,perhaps as a result of slumping or the development ofmore easily eroded surface material (Orpin 1997).

However, as stated earlier, there is no direct evidence forthese processes in the data collected.

Further to point (4) above, a strong correlation appearsto exist between the location of the chimneys on a bench atthe edge of the continental shelf and the heads of nearbysubmarine canyons. The association of canyon heads, rapid

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changes in slope and chimney fields has been described in

detail for the Cascadia convergent margin off the west coast

of North America (e.g. Moore et al. 1990; Orange & Breen

1992; Orange et al. 1997). This suggests that fluid-induced

slope failure � possibly related to coast-parallel faulting �could also influence the formation and ongoing maintenance

of the Otago canyon system.

Future work

Sea conditions and the shallow water of the shelf create

seismic imaging challenges (e.g. reduced signal penetration

due to multiple and soft sediments, access limitations due to

ship operations in shallow water or areas of commercial

fishing and inability to collect data near surf or low-tide

zones). Smaller vessels such as the one used to collect the

data presented in this paper provide a suitable platform for

working in this environment. However, all of the topics

discussed in this paper could benefit from increased data

coverage both laterally (i.e. more seismic lines) and in-depth

penetration. Real-time acquisition of digital seismic data at

sea combined with location (e.g. Global Positioning System)

information is greatly improving our capabilities to collect

more detailed lateral datasets. For example, the collection

and analysis of new digital datasets covering the fault

systems off Shag Point (Allan 1990) and along the south

Otago coastline (Johnstone 1990) in detail � at a resolution

suitable for palaeoseismic and earthquake hazard analysis �is currently underway.

In the future, higher-energy sources and multi-channel

systems (e.g. Missiaen et al. 2002; Bell et al. 2008) will enable

the reduction of contaminating multiple energy in seismic

sections and thereby greatly improve both resolution and

depth penetration. Such techniques are critical to improving

our understanding of shallow Quaternary processes along

the Otago margin and for characterising local seismic

hazard.

Acknowledgements

Acquisition of much of the data presented in this paper was

conducted as part of the MSc thesis research of Allan, Johnstone,

Orpin, Gray, Wilson and Osterberg. The authors gratefully

acknowledge all those who were involved with funding and data

collection for those theses. In particular, Chris Spiers, the master of

the RV Munida, acted as a great source of inspiration, information

and advice concerning all aspects of conducting science at sea; his

enthusiasm for our work has been infectious. We also thank the

various crewmembers of the RV Munida, especially Colin Adam,

Phil Heseltine and Keith Murphy. Mike Trinder kept the geophy-

sical gear in fine working order in a range of environments that

were often hard on the equipment’s and on Mike’s health! Funding

for the synthesis of the datasets presented in this paper � including

digitally archiving the seismic boomer data � was obtained from a

University of Otago Research Grant to Gorman.

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