Marine Geology 209 (2004) 69–90 www.elsevier.com/locate/margeo

Holocene sediment deposition on the Poverty-slope margin by the muddy , East Coast New Zealand

A.R. Orpin*

National Institute of Water and Atmospheric Research (NIWA), Private Bag 14-901 Kilbirnie, Wellington, New Zealand

Received 20 May 2003; received in revised form 1 June 2004; accepted 1 June 2004

Abstract

The margin off the Raukumara Peninsula, East Coast of New Zealand, is characterised by high terrigenous sediment flux, the dramatic effects of historic land-use change, the rapid uplift of Mesozoic and soft Tertiary sediments, and by complex sediment-tectonic interactions on a steep and unstable continental slope. The Poverty Bay shelf and slope indentation are adjacent to the Waipaoa River, which delivers f 15 Mt yearÀ 1 of mud. This study presents new data from the Poverty slope, the seaward extension of the Waipaoa dispersal system. A 3-km3 lobe of postglacial sediment emerges from the Poverty Gap and feathers down onto the upper slope. Downslope, the Poverty indentation has received a significant Holocene flux of terrigenous sediment, accumulating in two mid-slope basins: the Paritu Trough and a smaller lower-slope basin below the South Paritu ridge. Hemipelagic sediment has accumulated on the mid-slope at around 0.05 g cmÀ 2 yearÀ 1 (0.06 cm yearÀ 1) since the mid–late Holocene. Seismic-reflection echo type and backscatter imagery indicate mud deposits on the mid-slope, debris at the base of regularly spaced upper-slope gullies and a large avalanche deposit in the upper Paritu Trough. The consistency of the tephrostratigraphy indicates that despite slope instability, hemipelagic sedimentation is the dominant background process operating at millennial time scales, at least since the mid-Holocene. The Poverty slope should be included in any budget calculation of post-colonisation increased sediment yield for the margin. The Poverty Canyon has probably been largely inactive during the Holocene highstand. D 2004 Elsevier B.V. All rights reserved.

Keywords: Waipaoa River; Poverty Bay; Hikurangi margin; sediment dispersal; Holocene; slope sedimentation

1. Introduction Raukumara Ranges on the East Coast of New Zealand have deposited thick postglacial sequences on the Typical of high-sediment yield rivers with moun- adjacent continental shelf (e.g., Pantin, 1966; Foster tainous small catchments globally, the muddy fluvial and Carter, 1997). In addition to high-sediment load systems that drain the steep and unstable terrain of the rivers, this region of New Zealand combines the geological complexity of an active tectonic margin with millennial-scale chronological control provided by petrologically distinct volcanic tephra which occur * Present address: Natural Resources Canada, Geological Survey of Canada (Atlantic), PO Box 1006, Dartmouth, NS, throughout the Holocene. This study focuses on the Canada B2Y 4A2. Tel.: +1-902-426-8124; fax: +1-902-426-4104. continental slope adjacent to the Waipaoa River, an E-mail address: [email protected] (A.R. Orpin). area of the Hikurangi margin where large-scale tec-

0025-3227/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2004.06.001 70 A.R. Orpin / Marine Geology 209 (2004) 69–90 tonic–sediment interactions have resulted in the struc- margin and forearc basin (e.g., Lewis, 1980; Lewis turally complex Poverty indentation. Despite a small and Pettinga, 1993). Subduction-related underplating catchment of 2205 km2, today, the Waipaoa River beneath the Raukumara Peninsula is uplifting the axial delivers 15 Mt yearÀ 1 of suspended sediment (1% ranges at an estimated rate of 3 mm yearÀ 1, actively bedload) to coastal Poverty Bay (Hicks et al., 2000), uplifting an allochthonous Palaeogene slab and over- with sediment yields per unit area of catchment lying Neogene cover sequence (e.g., Reyners and among the highest recorded on Earth (Walling and McGinty, 1999). The region encompasses the 2200 Webb, 1996). To date, many studies have focussed on km2 Waipaoa River basin, which drains the eastern the postglacial terrestrial record of fluvial processes flanks of the axial Raukumara Range. The regional and the effects of deforestation since colonisation geology is tectonically and stratigraphically complex. (e.g., Gomez et al., 1999; Marutani et al., 1999; The rocks and sediments range in age from Creta- Berryman et al., 2000). Offshore, postglacial shelf ceous to Recent and the dominant lithologies are sedimentation was investigated by Foster and Carter sandstone, argillite and mudstone (Mazengarb and (1997) using 3.5 kHz seismic profiles and cores. Their Speeden, 2000). The northern part of the catchment, study noted the potential significance of Waipaoa close to the axial ranges, contains a complex suite of sediment dispersal via hypopycnal and flood-driven Cretaceous and early Tertiary sediments including hyperpycnal plumes, the effective capture of Holo- jointed sandstone and argillite, siliceous argillite, cene sediment on the shelf and an estimated five-fold smectitic mudstone, marl and limestone. The strongly increase in sedimentation associated with post-Euro- jointed and clay-rich lithology results in highly unsta- pean deforestation. Key to testing the hypothesis of ble landforms, manifest as slumps, landslides and Holocene shelf-sediment capture, and critical to estab- extensive gully erosion (e.g., Berryman et al., 2000; lishing a margin-wide sediment budget, is an under- DeRose et al., 1998). Elsewhere, the catchment is standing of sedimentation on the continental slope, the dominated by a suite of Miocene–Pliocene sand- seaward extension of the Waipaoa dispersal system. stones and mudstones. These rocks are generally more This study presents new data from the adjacent slope competent and support steeper slopes, but widespread indentation using multibeam bathymetry, seismic gully erosion still occurs in the upper and middle profiles, acoustic backscatter and tephra chrono- reaches (e.g., Trustrum et al., 1999). Quaternary stratigraphic techniques. It outlines evidence for con- lacustrine, fluvial and lagoonal deposits in the middle tinuous terrigenous sedimentation on the Poverty and lower reaches make up a lithologically distinct, slope during the Holocene, and comments on the but volumetrically small suite of cover sediments activity of the Poverty Canyon system in glacial and (Mazengarb and Speeden, 2000). In the headwaters, postglacial sediment dispersal. Accordingly, the Wai- there has been an estimated 120 m of downcutting in paoa depositional system serves as a key reference for the last 15 ky, much of which was accomplished by Holocene sedimentation adjacent to very high-yield the early Holocene (Eden et al., 2001). The alluvium rivers on active tectonic margins. base along the Waipaoa profile has an exponential form until f 25 km from the coast, where regional neotectonism causes a transition from uplift to subsi- 2. Regional setting dence (Brown, 1995; Berryman et al., 2000).

2.1. Waipaoa catchment geology and morphology 2.2. Offshore structure and sedimentation

The Poverty Bay continental shelf and slope are For several million years, regional structure and located on the tectonically active northern Hikurangi basin sedimentation along the northern part of the margin of New Zealand, where oceanic crust of the Hikurangi margin has been strongly affected by a Pacific Plate is being subducted obliquely beneath the succession of subducting seamounts and the massive Raukumara Peninsula and its eastern margin (Fig. 1). avalanche deposits that collapse from the margin in The margin lies within a zone of active deformation their wake (Lewis et al., 1998). The scars of tectonic that straddles the top of the Neogene imbricated erosion by seamount impacts are manifest as slope A.R. Orpin / Marine Geology 209 (2004) 69–90 71

Fig. 1. Locality of Waipaoa river catchment, Poverty Bay, and Poverty indentation, Hikurangi subduction margin, northeastern New Zealand. 72 A.R. Orpin / Marine Geology 209 (2004) 69–90 indentations and structural re-entrants in the deforma- and Gibb, 1968; Miller, 1981). Volumetrically, the tion front, such as those features observed on the bulk of these deposits are derived from the Waipaoa Poverty margin (Lewis et al., 1998). As a result, the River (Miller, 1981; Kensington, 1990; Sanders, geology along the inboard portion of the margin is 1993; Wood, 1993), which acts as point-source of structurally and stratigraphically complex. Accretion- sediment at the coast, and its impact is manifested by ary tectonics and plate convergence have imbricate- an elongate, shelf-parallel lobe of mud that extends thrust and folded Neogene slope sediments as a cross-shelf, seawards from the Waipaoa River mouth deforming backstop, with only a narrow accretionary (Foster and Carter, 1997). Outside of the confines of prism locally forming in places at the toe of the slope Poverty Bay, the distribution of surficial sediments (Lewis and Pettinga, 1993; Collot et al., 1996). Along follows the classic pattern of a wave-graded open the upper margin, postglacial deposition has occurred shelf seen elsewhere around New Zealand (e.g., predominantly in tectonically subsiding, mid-shelf Carter, 1975), where sand dominates the inner shelf basins, bordered along their seaward flank by emer- and fines seaward to be predominately muddy at 30– gent anticlinal barriers (e.g., Foster and Carter, 1997; 40 m depth. The modern mud blanket extends to the Orpin et al., 2002a). The southern shoal bordering the shelf edge, except in the vicinity of the anticlines on Poverty shelf is unnamed, but is considered to repre- the outer shelf where exposures of Neogene sedimen- sent the northern extension of the Lachlan Ridge tary rocks (Lewis, 1973; Katz, 1975) are surrounded anticline (Lewis, 1973; Barnes et al., 2002).The by aprons of relict gravel and sand (Foster and Carter, northern shoal is Ariel Bank, which is the sea floor 1997). expression of the Ariel Anticline (Foster and Carter, Previously published studies of the sedimentology 1997). Ariel Bank and Lachlan Ridge are separated by of the Poverty upper slope and indentation are limited a 13-km wide gap in the anticlinal barrier, named the to a regional 1:200,000 sediment chart (Pantin and ‘‘Poverty Gap’’ in this study (Fig. 2). Early seismic Gibb, 1968) and detailed outer-shelf studies of the interpretation of Quaternary stratigraphic sequences in northern Lachlan Ridge anticline (Barnes et al., 2002) a mid-shelf basin in Hawke’s Bay demonstrated the and Ariel Banks (Katz, 1975). interplay of eustatic sea level, sedimentation rate, and rates of tectonic uplift and subsidence (Lewis, 1973). 2.4. Oceanography This framework has been expanded and many of the regionally extensive reflectors are now widely char- Regional oceanographic features play a major role acterised and some are dated (e.g., Barnes et al., in the offshore dispersal of surface flood plumes from 2002). Foster and Carter (1997) estimated that post- the Waipaoa River, as evidenced by satellite remote- glacial subsidence on the Poverty Bay mid-shelf is sensing images (Foster and Carter, 1997). Beyond the high, around 2–4 mm yearÀ 1. shelf break, circulation is most strongly influenced by the Current (e.g., Stanton et al., 1997; 2.3. Sediments Chiswell and Roemmich, 1998). This relatively warm, saline flow passes south over the outer shelf and upper The Waipaoa River discharges into the southern continental slope between East Cape (37jS) and sector of Poverty Bay onto a shallow submarine Wairarapa (41jS) before turning eastward along Chat- platform. Poverty Bay opens to the southeast, has an ham Rise. In general, the East Cape Current is entrance that is 8.5 km wide, and in the centre of the offshore from the 1000 m isobath, with a mean entrance the water depth is 25–30 m (Fig. 2). Poverty southeast-directed velocity of f 30 cm sÀ 1 near the Bay is fringed by long sandy beaches and sea cliffs on surface and < 10 cm sÀ 1 near the sea floor along the the northern and southern tip of the bay. The Neogene Hikurangi Trough (Chiswell and Roemmich, 1998). rocks beneath Poverty Bay are mantled by postglacial The East Cape Current transports suspended sediment sand and mud that fines seaward (Foster and Carter, to the south. At the coast, the Waipaoa surface flood 1997). Sediments range from coarse sand near the plume moves anticlockwise around the southern river mouth and beaches, to fine sand, silt, and mud in shores of Poverty Bay and out onto the open shelf, the middle of the bay at water depths >20 m (Pantin consistent with the baroclinic circulation (Foster and A.R. Orpin / Marine Geology 209 (2004) 69–90 73

Fig. 2. Detailed bathymetry of the Poverty margin from EM12D multibeam collected during the GeodyNZ project (e.g., Collot et al., 1996). The EM12D is a deep water multibeam system that measures depth and seabed acoustic reflectivity using a 12.6–13 kHz sound source, operating with a swath width approximately seven times the water depth, and with a vertical precision of 0.2% of the water depth, i.e., 4 m at 2000 m. The bathymetry resolution along-track and across-track at depths < 2000 m is 80 and 100 m, respectively, falling to 100 and 200 m, respectively, between 2000 and 4000 m water depth. Swath data were corrected and processed for sound velocity in the water column from expendable bathythermographs collected during the survey. NIWA cruise tracks (2011, 3021, 3044 and TAN0005), piston core and dredge locations are indicated, along with the Calypso piston core MD97-2122 collected on the Poverty Bay mid-shelf. 74 A.R. Orpin / Marine Geology 209 (2004) 69–90

Carter, 1997). However, linear flows are complicated 14C years BP (McGlone et al., 1994), and forest by the incursion of coastal promontories, such as clearing accelerated with European colonisation in Young Nicks Head and Mahia Peninsula, causing the mid-18th century (Pullar, 1962). By 1880, most eddies in the circulation pattern. Once on the mid- of the hinterland had been cleared, and by 1920, all shelf, beyond the confines of the bay, surface flood but a few percent of the land had been converted to plumes on the inner shelf move longshore in either a pasture. An intense phase of landscape erosion was northeast or southwest direction, driven by the pre- initiated in the upper reaches of Waipaoa catchment vailing wind (Foster and Carter, 1997). High-resolu- around the turn of the 20th century, following tion time-series data are limited, but a month-long deforestation (Allsop, 1973). Since then the river current meter record at 193 m on the outer shelf has been aggrading in response to the increased adjacent to the Raukumara Peninsula shows that shelf sediment yield (Gage and Black, 1979; Gomez et flows are topographically controlled along the 200 m al., 1999). Offshore, Foster and Carter (1997) in- isobath (Chiswell and Roemmich, 1998). Flows are ferred from a shelf-sediment budget that deforesta- highly variable but with a weak mean southerly flow tion caused a five-fold increase in sediment of f 2cmsÀ 1 (standard deviation of 14 cm sÀ 1) and accumulation in the Poverty mid-shelf depocentre. maximum flows up to f 20 cm sÀ 1 in either north or More recent stratigraphic data from a 16-m Calypso south directions (Chiswell and Roemmich, 1998). piston core (MD97-2122) from the mid-shelf (Fig. 2) Preliminary analysis suggests fluctuations in current suggest that European deforestation was the major strength on the shelf are wind-driven (pers. comm. S. factor leading to a four-fold increase in sediment Chiswell, NIWA). Measurements at similar depths off accumulation, estimated from changes in vertical East Cape show a consistent northeast current and accumulation rate (Gomez et al., 2001). Palynologi- Chiswell and Roemmich (1998) speculate that this cal indicators of European forest disturbance have could be evidence of persistent clockwise circulation been described in the top 50 cm of MD97-2122 and inshore of the East Cape Current. In summary, the NIWA core W396 from the Poverty Bay mid-shelf, transport direction for surface flood plumes that but evidence of Polynesian land-disturbance might traverse the Poverty shelf is uncertain, but is likely occur at >100 cm depth in MD97-2122 (cf. Wilm- to be variable in either north or south direction and shurst et al., 1999; Gomez et al., 2001; M.B. Elliot, wind driven. The volumetric significance and shelf NIWA, pers comm., 2002). trajectories of bottom-hugging density flows await further investigation. Local tides are diurnal and weak; at the entrance to 3. Methods and data Poverty Bay the peak current speed at 10 m depth is 2–4 cm sÀ 1 (e.g., Healy et al., 1998). Within Poverty The multibeam bathymetric data from the Poverty Bay, the average wave height is f 1 m, but local indentation and Hikurangi Trough largely were col- storm waves reach 4 m, and the ocean swell can lected during the 1993 GeodyNZ Project, using a hull- exceed 6 m. On the open shelf, the persistent, long mounted, SIMRAD EM12D swath mapping system period (8–12 s), southerly swell is typically 1–2 m in operated from the R.V. l’Atalante. Acoustic backscat- height (Foster and Carter, 1997). Poverty Bay near- ter intensity data were also collected concurrently. shore waters are stratified by freshwater discharged Limited areas of the Hikurangi margin were resur- from the Waipaoa River mouth into the southern veyed in 1994 using MR1 sidescan, operated from the sector of the bay, which generates a persistent anti- R.V. Giljanes. EM12D swath bathymetry, backscatter clockwise gyre in baroclinic circulation within the bay imagery (both EM12D and MR1), and additional (Healy et al., 1998; Stephens et al., 2002). multichannel seismic data collected off the East Coast of the North Island are summarised at a regional scale 2.5. Anthropogenic impact on landscape erosion in Collot et al. (1994, 1995, 1996). Lewis et al. (1998) have published EM12 imagery and a tectonic-sedi- Polynesian Maori settlers started to clear the ment interpretation of the Poverty indentation and landscape of thick temperate rain-forest 800–500 Hikurangi Trough region, augmented by data from A.R. Orpin / Marine Geology 209 (2004) 69–90 75 earlier seismic surveys by Lewis and Pettinga (1993) the measured thickness between discrete Taupo (1718 and Lewis (1994). cal. years BP) and Waimihia (3472 cal. years BP) The 3.5-kHz seismic-reflection echo characteristics eruption tephra layers. Using the thickness between for the Poverty slope reported here were obtained tephra beds minimises inherent sampling problems using a hull-mounted ORE 140 profiler that incorpo- associated with disruption of the water-saturated rates a 16-element transducer array. Approximately uppermost core. Published long-sediment cores from 1200 line kilometres are summarised in this study waters east of New Zealand, namely cores ODP 181- from data collected on NIWA research cruises 2011, 1124 (Carter et al., 1999) and MD97-2121 (Carter et 3021, 3044 and TAN0005 (Fig. 2). Navigation was al., 2002) show that there is a robust, linear correla- provided by GPS and Transit Satellite data. Line tion between AMS-radiocarbon and tephra dates to spacing on the upper slope and outer shelf is gener- core depth. All calendar ages for eruptions used in ally < 8 km and is less constrained on the lower slope this paper are from Carter et al. (2002). To comple- and structural trench, but coverage is sufficient to ment the tephrochronology, nannofossil ages were adequately characterise the major echo types across also determined from selected dredge and core sam- the continental margin. However, significant changes ples, undertaken by A.R. Edwards (Stratigraphic of gradient and side-swipe from steep canyon walls Solutions, Waikanae, pers. comm., 2002). precludes fine-scale mapping in some areas of the slope immediately adjacent to the main Poverty Can- yon. Echo classification follows the scheme pioneered 4. Results by Damuth (1975, 1980), and more recently modified for specific New Zealand sea floor studies by Barnes 4.1. Continental slope bathymetry (1992) and Carter (1992). Eleven piston cores were collected across the The 1500-km2 Poverty indentation is a major Poverty indentation at water depths of 78–3405 m, continental margin depression extending from a re- respectively, during NIWA cruises TAN0005 and entrant in the deformation front at the Hikurangi TAN0106 (Fig. 2). These cores range in length from Trough to the continental shelf (Collot et al., 1996). 40 to 298 cm and all contain macroscopic tephra The bathymetry of the Poverty indentation is com- beds. Core lithologies were used to help distinguish plex, but can be distinguished as comprising six basic the acoustic characteristics determined from 3.5-kHz morphologic components (in order of increasing water profiles and to provide an overview of the sediment depth): (i) a heavily gullied upper slope; (ii) a gently facies present on the slope. Cores were also visually sloping mid-slope trough (Paritu Trough); (iii) the logged for lithology and sedimentary structures, and Poverty Canyon system; (iv) margin-parallel lower profiles of volume magnetic susceptibility and spec- slope ridges (North and South Paritu Ridges); (v) a V- tral reflectance were obtained routinely using a Bar- shaped structural re-entrant at the deformation front in tington MS2F probe and a hand-held Minolta the canyon mouth; and (vi) the largely flat expanse of Spectrophotometer CM-508d, respectively. These the Hikurangi Trough, seamounts (e.g., Gisborne physical properties assisted in the identification of Knolls), and Hikurangi Channel seawards of the fine-grained tephra layers. Where appropriate, volca- deformation front (Figs. 1 and 2). nic tephra beds were analysed for glass-shard chem- The upper part of the indentation is a 40 km-wide istry (using electron microprobe) and heavy mineral depression which lies at 1000–1500 m water depth, assemblage to identify the eruption source, undertak- bounded along its landward flank by a 500–600 m en by P.A. Shane (Auckland Uniservices, University high scarp which is incised by regularly spaced, upper of Auckland, pers. comm., 2001). In accordance with slope gullies with V-shaped cross-sections (Fig. 2). standard laboratory service, tephra ages were These gullies incise into the shelf break at around 160 obtained by correlation with stratigraphically estab- m, but the four largest have tributary canyon heads that lished, petrologically characterised and dated counter- shoal to 80 m. These gullies collectively form the parts onshore (e.g., Froggatt and Lowe, 1990; Carter Poverty Canyon head (Arron and Lewis, 1992). The et al., 1995). Sedimentation rates were calculated by tributaries upslope of the confluence of the main 76 A.R. Orpin / Marine Geology 209 (2004) 69–90 canyon incise the upper slope and reach a maximum depth in the Calypso core MD97-2122 (Gomez et al., incision depth of around 250 m. They have a V-shaped 2001) from the Poverty Bay mid-shelf basin, confirm- cross-section, and the tributary walls are generally ing that the underlying major reflector is probably <10j. In contrast, the three large tributaries of the early Holocene in age. canyon incise 600–1200 m into the lower slope and Approximately 140 km2 of shelf sediment lies the canyon walls reach a maximum dip of 37j. The seaward of Lachlan Ridge and Ariel Banks, extending Poverty indentation has a stepped-like profile with two the terrigenous sediment basin mapped by Foster and major slope troughs (Fig. 2). The shallower Paritu Carter (1997), emerging from the Poverty Gap which Trough is the most expansive, occupying f 900 km2, separates the shoals, as a sediment lobe that feathers between 1250 and 1500 m water depth, immediately down onto the upper slope to 160 m water depth (Fig. down slope of the upper-slope gullies. The Paritu 3). As on the shelf, the two major internal reflectors Trough slopes gently southeastward at f 1j but is evident are a basal erosion surface assumed to be last- deeply incised to the south by three main tributaries of glacial in age, and a strong internal reflector that the main Poverty Canyon axis. The seaward limit of overlies a transparent unit of variable thickness, as- the Paritu Trough is the North and South Paritu Ridges, sumed to be early Holocene in age. The outer-shelf which mark the limit between the upper and lower sediment lobe contributes f 3km3 to the total slopes. The Paritu Trough laps down into a small 2-km volume of the postglacial prism. wide gully that incises between the North and South Where visible, there is only local evidence that the Paritu Ridges, called the ‘‘Paritu Gap’’ in this study last-glacial erosion surface is incised by channels. One (Fig. 2). Collot et al. (1994, 1996), Lewis et al. (1998) profile examined in this study showed a broad channel and Lewis and Orpin (2002) recognised this gully as a approximately 300 m wide and 30 m in depth, with potential pathway for active turbidity currents. A several stacked internal reflectors in an otherwise deeper trough, but smaller in size, occurs immediately acoustically transparent section. The internal reflec- downslope of this gully, some 15 km north of the tors are vertically spaced 5–10 m apart. There is also Poverty Canyon mouth, and is perched between 2250 weak development of stepped, semi-horizontal seg- and 2350 m, below a 1000 m high scarp. This lower- ments on the last-glacial surface at the shelf break, slope trough covers only 12 km2 and has fully which could be old wave-cut platforms. Up to three enclosed contours at 2300 and 2350 m, which suggest platforms can be tentatively identified in shore-normal a gentle bowl-shape. profiles, but given the tectonic uplift and deformation associated with the anticlinal ridges, it is unlikely that 4.2. Postglacial seismic architecture of the Poverty the depths of these platforms define original depths of Bay outer shelf wave erosion during periods of lowered sea level (cf. Barnes et al., 2002). Regionally, the postglacial shelf architecture can be broadly characterised by two major seismic reflectors, 4.3. Seismic reflection echo types of poverty slope and namely the last-glacial transgressive erosion surface indentation and a conformable strong reflector in the top 15 m of the prism (Foster and Carter, 1997). The last-glacial The classification adopted for the 3.5-kHz system in erosion surface crops out on both the landward and this study uses acoustic classes based mainly on the seaward flanks of the emergent Lachlan and Ariel form and signature of the echo from the sea bed, and the ridge anticlines, where it defines the unconformity depth of sea bed penetration. There are 3 broad classes, between unconsolidated postglacial mud and the un- which consist of irregular, wavy and smooth echoes, derlying deformed and indurated Neogene strata (Fos- with 11 subdivisions based on character and environ- ter and Carter, 1997). The conformable strong mental setting (Fig. 3). Spatially, the shelf, slope and reflector in the top 15 m of the prism has been trough are represented by the following echo types: (1) tentatively applied by numerous authors to be early the shelf has a sharp near horizontal echo type with Holocene in age ( f 8–10 ka BP) after Pantin (1966). stratified weak internal reflectors (type 1B); (2) the Tuhua tehpra (6970 cal. years BP) occurs at 14.13 m slope has a complex array of highly variable echo A.R. Orpin / Marine Geology 209 (2004) 69–90 77

Fig. 3. Seismic echo classes on the Poverty margin based on 3.5 kHz soundings. Note the Poverty shelf basin and outer shelf lobe of postglacial sediment, and the large area of irregular echo class on the Paritu Trough interpreted to be blocky gully debris. The patchy nature of the echo classes is largely a function of limited coverage by seismic soundings. 78 A.R. Orpin / Marine Geology 209 (2004) 69–90 character from highly irregular (type 3B) to smooth ular bottom reflector, lumpy surface and weak internal return (type 1A); (3) the Hikurangi Trough has a wavy reflectors suggest this could be a localised sediment echo type with stacked internal reflectors and deep apron at the base of the slope fed by the sediments penetration (type 2B). The range of echo types are cascading off the lower-slope basin, as opposed to an largely influenced by steep bathymetric slopes. apron from hemipelagic sedimentation. A 2.98-m long Small hyperbolae of irregular echo type 3C and 3D piston core collected from the Trough (U1746) shows outline a lobate area approximately 240 km2 that stacked sandy mud turbidities and tephra beds (Fig. extends down slope onto the Paritu Trough (Fig. 3). 4). Petrological fingerprinting indicates that five indi- In this area echo type 3D has smaller hyperbolae than vidual tephra beds from 72 to 228 cm depth are type 3C and occurs immediately down slope of 3C. sourced from the Waimihia eruption, dated at 3472 There is also a broad area northeast of Tuaheni Ridge calendar years BP from well established chronostra- where echo type 3D occurs unflanked by 3C on the tigraphy on land. These repeated tephra beds are upper slope. These seismic echo types are interpreted interpreted to represent turbidite deposits. as evidence of hummocky and irregular seafloor bathymetry. In the Paritu Trough, this morphology 4.4. Acoustic backscatter imagery of the Poverty probably results from gully debris derived from the slope and canyon system steep slopes below the shelf break at 160 m. The greatest relief occurs higher on the upper flanks of the The classification scheme used to interpret the trough, presumably the result of large blocks of rock acoustic backscatter imagery (Fig. 5) consists of three debris, and the relief becomes progressively more classes of acoustic backscatter signal from EM12D gentle and less blocky down slope. Northeast of multi-beam data: strong, medium, and weak (Fig. 6). Tuaheni Ridge, the source of blocky debris is unclear The classifications follow the generalised scheme as the upper slope is not heavily gullied, but material adopted by Lewis et al. (1998) and Collot et al. could be shed from the seaward flank of Ariel Bank or (1996) along the Hikurangi Margin, which used the sediment slides on the upper slope. same multibeam data. However, the subdivisions of Much of the slope gives a hyperbolic echo with backscatter signal strength and interpreted environ- varying vertex height (type 3B), which is directly mental setting have been modified and expanded related to steep slopes and dramatic changes in specifically for the Poverty indentation. bathymetry. Elsewhere on the mid-slope, wavy echo The backscatter imagery reveals a low to medium type 2A and smooth echo type 1A show strong, intensity acoustic response from hemipelagic sedi- continuous stacked reflectors representing persistent ments, which cover much of the gently sloping areas deposition. Piston cores U1748 and U1749 were of the mid-slope. Superimposed on this low-medium collected from the lower Paritu Trough and fall within signal are elongate dendritic strips of strong backscat- the area of echo type 1A. They reveal a mud-domi- ter extending from a network of small upper-slope nated lithology, inter-layered with discrete sandy gullies (Figs. 5 and 6).BothEM12DandMR1 volcanic beds (Fig. 4). Piston core W695 from the backscatter data indicate that areas < 8 km2 of strong lower slope basin (echo type 2A) has a similar mud- backscatter occur at the base of these gullies at around dominated lithology, with several volcanic sand beds 1250 m, but do not appear to extend to the head of the (Fig. 4). The regional extent of these facies is limited main Poverty Canyon. However, the southernmost to gently sloping terrain, such as the lower-slope limb of the Poverty Canyon does shoal to within 50 basin, and the Tuahine Ridge on the northern flank m depth of an upper-slope gully (Fig. 2). The stratig- of the Poverty indentation. raphy from piston core W703 shows that the sediments The seismic echo types of the Hikurangi Trough at base of one of these gullies is composed of uncon- are typified by a wavy echo (type 2C) with stacked solidated sandy mud underlain by stiff mudstone that internal reflectors that drape the bathymetry. Abutting contains a nannofossil assemblage of mid-Miocene the base of the slope leading up to the northern Paritu age (Fig. 4). A thin sandy-mud bed immediately above Ridge is f 70 km2 of low relief which has an the stiff mudstone contained abundant detrital tephra irregular echo character (type 3E). The diffuse irreg- grains with glass-shard chemistries compatible with ..Opn/Mrn elg 0 20)69–90 (2004) 209 Geology Marine / Orpin A.R.

Fig. 4. Lithological and magnetic susceptibility profiles of 11 piston cores from across the Poverty indentation (arranged approximately landward to seaward). NIWA core identification numbers, water depths, approximate location, and seismic and backscatter classification (refer to Figs. 2, 3 and 6) are indicated. Interpreted depositional environments are also shown. Large values of magnetic susceptibility generally indicate tephra-rich horizons but small concentrations of pyrite also influence the values. Logged and petrologically fingerprinted tephra beds are indicated with their correlated eruption age. Sedimentation rates are shown in brackets and were estimated from the thickness of sediment between identified tephras. 79 80 A.R. Orpin / Marine Geology 209 (2004) 69–90

Fig. 5. Montage of EM12 backscatter and MR1 side scan imagery for the Poverty margin, originally derived from the GeodyNZ project (cf. Collot et al., 1996). The EM12D acoustic imagery at water depths >1000 m has an along-track and across-track resolution of 100 and 2.4 m, respectively. The backscatter imagery data were partially corrected for grazing angle and bathymetric effects onboard R.V. l’Atalante. In some circumstances, the EM12D’s low operating frequency of 12.6–13 kHz will result in additional acoustic response from highly reflective layers that are buried up to a few metres, similar to behaviours described for the Gloria and Sea Marc II systems by Kenyon (1992). MR1 data shown in this study have not been reprocessed. the Kaharoa eruption ash, f 600 calendar years BP. draped over the hummocky seafloor. Additionally, The strong backscatter signal observed along the gully given the low frequency of the EM12D multibeam axes is likely to be the acoustic response of the system, the sub-surface sandy tephra beds might also underlying stiff mid-Miocene mudstone. be contributing to the backscatter signal. Further Downslope of these gullies, but to the north of the downslope, this mottled zone merges into a more even, main Poverty Canyon system, there is an oblate area of medium-backscatter zone that ultimately funnels into around 100 km2 of mottled backscatter that stretches the head of a steep gully (Figs. 5 and 6). Piston core eastwards down slope from 1300 to 1500 m across the U1747 reveals a similar mud-dominated lithology to Paritu Trough. Piston core W696 taken from the W696, but only one sandy tephra bed (Fig. 4). The middle of this mottled zone reveals f 2 m of mud- reduction in backscatter response probably reflect a dominated sediment, with well defined sandy tephra less hummocky seafloor. This gully feeds into the beds (Fig. 4). The likely cause of the mottled back- lower-slope basin at around 2250 m, which has a scatter signal could be uneven thicknesses of mud strong backscatter signature that is confined to the A.R. Orpin / Marine Geology 209 (2004) 69–90 81

Fig. 6. Map of variations in backscatter echo strength across the Poverty margin. Note the area of strong backscatter marking the large area of gully debris in the Paritu Trough, the axes of upper slope gullies, and the Poverty Canyon mouth. gully walls and floor. This zone of strong backscatter At the mouth of Poverty Canyon, an area of strong does not extend onto the nearby lower-slope basin, backscatter extends eastwards from the canyon con- which exhibits weak backscatter. Piston core W695 fines across the margin of the Hikurangi Trough at from the lower slope basin reveals a mud-dominated around 3350 m depth. There is a mottled signal at the lithology and several sandy tephra beds. This relatively canyon mouth as well as two sub-circular zones ap- thick, unconsolidated mud sequence is likely to give proximately 1 km in diameter, of weak-medium back- rise to the low backscatter signal strength. scatter. These features are broadly coincident with 82 A.R. Orpin / Marine Geology 209 (2004) 69–90 seabed depressions, the most prominent being a broad- and Waimihia (3472 cal. years BP) eruptions, and core ly circular depression between 50 and 100 m deep and U1749 also contained Whakatane (5580 cal. years around 1 km in diameter. The northern limit of this BP) eruption tephra (Fig. 4). The accumulation rate strong backscatter is the base of the slope and the between these tephra beds is 0.04–0.05 cm yearÀ 1 for easternmost margin is diffuse and convoluted. Piston cores W696 and U1749 (verified using Whakatane core W693B from 2404 m depth in the lower reaches of tephra also), and f 0.7 cm yearÀ 1 for W695 (Fig. 4). the Poverty Canyon contains stiff sandy-mudstone The highest rates occur in the lower slope basin, but overlain by f 10 cm of unconsolidated mud, with a rates are in general consistent. These data give an sharp erosional contact between the units. The basal average accumulation rate across the indentation of section of the core contained stiff mudstone with a f 0.06 cm yearÀ 1 ( + 20% assuming linear core nannofossil assemblage of Late-Pleistocene age. Two compaction). Accumulation rates are more variable other attempts to core at the canyon mouth (W694 and using only the depth to Waimihia tephra, due in part to U1751, Fig. 2) recovered very short ( < 10 cm) and the poor preservation of the uppermost, less consoli- disrupted core records but did retrieve plugs of litho- dated parts of the piston cores (Fig. 4). Using an logically similar, stiff mudstone that contained a nan- approximate dry bulk density of f 0.8 g cmÀ 3 for nofossil assemblage of early–late Pleistocene age. In surficial slope sediments (from Carter et al., 2002), the contrast, core U1752 collected from 2264 m depth in average sediment mass accumulation on the slope is the axis of a southern branch of the Poverty Canyon estimated to be around 0.05 g cmÀ 2 yearÀ 1 or 500 t contained f 40 cm of mud and has medium backscat- kmÀ 2 yearÀ 1. These Holocene rates are compatible ter strength. Elsewhere on the flat-lying expanse of the with 210Pb accumulation profiles determined from Hikurangi trough, the backscatter is of low intensity three precision short-cores taken from the Paritu except for a seamount at the base of the accretionary Trough that show accumulation rates of 0.1–0.04 g slope to the north, and the crests and troughs of the cmÀ 2 yearÀ 1 (C.R. Alexander, Skidaway Institute of sediment waves, 2–4 km in wavelength, adjacent to the Oceanography, pers. comm., 2001). Applying this bend in the Hikurangi Channel (cf. Lewis et al., 1998). accumulation rate only over 450 km2 of the lower At water depths around 2000 m two sub-parallel Paritu Trough and the sub-basin below the South lineations of strong backscatter following the canyon Paritu ridge (identified as zones of deposition from outline are interpreted to be the result of steep canyon backscatter imagery, seismic echo and core data) gives walls and outcropping strata. Reconaissance dredging amid–lateHolocenesedimentfluxof f 0.2 Mt of the northern Poverty Canyon wall (Fig. 2),at yearÀ 1. Given this relatively slow rate of sedimenta- depths of 1572–2176 m, recovered indurated mud- tion on the slope, evidence of the last century of stone (W692A) with a nannofossil assemblage of landscape disturbance and higher sediment yields due early Pleistocene age. A similar age of early Pleisto- to European deforestation is likely to be restricted to cene was also yielded from canyon wall mudstone the top 10 cm of the slope sediments. retrieved from dredging at depths of 2750–3214 m The estimated vertical sedimentation rate of 0.06 (W691). These sediments are considerably more con- cm yearÀ 1 for the Poverty indentation is 1.5–6 times solidated than other hemipelagic sediments found greater than that determined from Holocene tephra elsewhere on gently inclined areas of the slope and stratigraphy from cores collected in slope-depressions trough, and presumably would cause the strong back- off southeastern Hawke’s Bay by Lewis and Khon scatter signal from both EM12D and MR1 systems. (1973). From a similar location at 2314 m water depth off southeastern Hawke’s Bay, Carter et al. (2002) also determined early Holocene mass accumulation 5. Discussion rates, between 0.02 and 0.03 g cmÀ 2 yearÀ 1, using radiocarbon and tephra chronologies from a giant 5.1. Holocene sedimentation on the Poverty slope piston core (MD97-2121). In general, the higher sedimentation rates on the Poverty indentation reflect NIWA piston cores W696, U1749 and W695 its closer proximity to a sediment source, the Waipaoa contain tephras from the Taupo (1718 cal. years BP) River. A comparison of current riverine inputs reveals A.R. Orpin / Marine Geology 209 (2004) 69–90 83 that regionally the very muddy East Coast rivers 5.3. The possible significance of intraslope sediment deliver f 6 times more suspended sediment than sources rivers entering Hawke’s Bay, and the Waipaoa alone supplying approximately 40% more suspended sedi- Lewis et al. (1998) speculated that the upper slope ment than all Hawke’s Bay rivers (Hicks and Shankar, gullies and associated zone of strong backscatter on 2003). the Poverty margin resulted from elevated pore pres- Vertical accumulation rates on the uppermost slope sures, where gullying is initiated by a reduction in are problematic. Core U1741 from the outer-shelf shear strength at seep sites, which propagate headward mud lobe that extends from the Poverty Gap, seaward by fluid sapping (the focusing of fluid flow at slope of the emergent anticlinal barriers, shows clear evi- incisions, e.g., Orange and Breen, 1992), and down- dence of a thick accumulation of mid-Holocene sed- slopebyfailureanderosion.Lewis et al. (1998) iment. Waimihia eruption tephra occurs at f 90 cm suggest that these mechanisms probably produce rock depth and the lower core is composed of massive mud debris, resulting in strong backscatter along the gully with little evidence of layers of higher magnetic axes (Fig. 5). A case for fluid sapping is made more susceptibility (Fig. 4). Further down slope at 477 m compelling by the occurrence of methane-rich fluid depth, core U1742 is weakly bedded and does not seeps at 1200 m depth on Calyptogena Bank (named contain any distinct tephra beds, and the magnetic after the discovery of the vent-diagnostic mollusc susceptibility trace shows little variation. However, at Calyptogena; Baldwin and Lewis, 1991) on the north- the base of the upper slope, detrital grains of Kaharoa ern extent of the Ritchie Ridge (summarised in Lewis tephra ( f 600 years BP) at f 30 cm depth in core and Marshall, 1996) east of Mahia Peninsula, some 30 W703 indicates that down-slope sediment dispersal km south of Poverty indentation. In addition, gas occurs at sub-millennial times scales. The implication masking is clearly visible on seismic records from is that Waipaoa sediment dispersal extends beyond the the Poverty continental shelf (Foster and Carter, confines of the Poverty shelf. 1997). It is clear, therefore, that gas and fluids are common on much of the shelf and upper slope of the 5.2. Implications to post-colonisation sedimentation margin. The >100 km2 of gully debris visible on 3.5- kHz seismic and EM12D backscatter records across Viewed as margin-wide deposition system, data the Paritu Trough add weight to the hypothesis that from this study shows that the Poverty slope also fluids play a significant role both in the development received a significant Holocene flux of terrigenous of the gully incisions and the generation of sediment sediment, and should, therefore, be included in any debris from slope instability. budget calculation of post-colonisation increased Quantification of intraslope sediment sources is yield. If this assertion is correct, then the overall problematic. The piston cores described in this study increase in post-European yield is likely to be higher show a consistent tephra chronology for much of the than the four and five times increase estimated from late Holocene, and yet there is clear evidence of debris shelf studies by Gomez et al. (2001) and Foster and deposits at the base of the upper-slope gully complex. Carter (1997), respectively. A larger increase offshore It is possible that the hummocky avalanche debris is supported by terrigenous flux estimates from un- material observed in the upper Paritu Trough (Figs. 3 disturbed late-Holocene lake sequences. Data from and 6) pre-dates the mid-Holocene tephra (Waimihia) Lake Tutira in the Hawke’s Bay hinterland suggest an recorded in core UW696 (Fig. 4) A thin ( < 50 cm) 8–17 times increase in accumulation rate under pas- sediment drape could not be determined from 3.5 kHz toral land use versus pre-colonisation indigenous records over the irregular debris material. Further forest (Page and Trustrum, 1997). Similarly, from a down slope, seismic strata are clearly discernable study of the thickness of alluvial formations in the suggesting the avalanche over lies continuous hemi- Gisborne Plains basin, Pullar and Penhale (1970) pelagic sediments (Fig. 3). Additional cores would be calculated that the rate of infilling since 1932 has required to fully investigate the relative timing of the been an estimated 5–10 times greater than at any time avalanche. Elsewhere on slope at the base of Tuahine during the last ca. 3500 years. Ridge there is evidence from seismic echo character 84 A.R. Orpin / Marine Geology 209 (2004) 69–90 of minor slope instability. These deposits are inter- occurrence of other muddy rivers north of the Poverty preted to be debris material similar in origin to the Bay—the largest being the Uawa-Hikuwai River larger avalanche debris seen in the Paritu Trough. around 50 km to the north which delivers f 7Mt Again, the sediment source appears to be the upper- yearÀ 1 of suspended sediment (Hicks et al., in press). most slope, associated with narrow gullies. With Flood-derived sediment from the Uawa-Hikuwai Riv- respect to background sedimentation processes, the er must be transported seaward of the 1000 m isobath consistency of the tephrostratigraphy indicates that to become entrained in the southward-flowing East despite slope instability, hemipelagic sedimentation Cape current. Across-shelf sediment transport is com- is an under-riding process operating at millennial time pounded by highly variable, wind-driven shelf cur- scales, at least since the mid-Holocene. Nonetheless, rents. Moreover, preliminary seismic investigations over longer time scales, given the unstable nature of suggest that, like the Poverty shelf, a significant the Hikurangi margin, clearly slope failure can volu- component of the sediment output from the Uawa- metrically overwhelm any other background sedimen- Hikuwai River is trapped in the adjacent mid-shelf tation processes, as evidenced by the 3000 km3 basin, boarded at its seaward edge an outcropping avalanche to the north (Lewis et al., 1998; anticlinal ridge of Neogene mudstone (Orpin et al., Collot et al., 2001). 2002a; L. Carter, NIWA, unpublished data). On the One of the triggers for mass failure along the slope, the Tuaheni Ridge along the northern boarder of margin is earthquakes. Neotectonic evidence from the Paritu Trough forms a 250–500 m high obstacle to uplifted terraces on Mahia Peninsula suggests that southward-directed currents that flow along the mid- earthquakes with moment magnitude (Mw) 7.3–8.0 slope between 1000 and 1500 m water depth (Fig. 2). have occurred often over the late Holocene, associated More oceanographic and core data is required to better with localised steep reverse faults (Berryman, quantify along-slope sediment transport. 1993a,b); during the last 2500 years, there have been major periods of earthquake activity of 100–200 5.4. Poverty Canyon: an inactive conduit? years duration, followed by quiescence for perhaps 200–500 years (Berryman et al., 1989; Berryman, Lewis (1980) and Lewis et al. (1998) suggest 1993a,b). Paleoseismic records reveal only rupturing canyons that incise the upper slope supply little events of large magnitude, but in zones of high sediment to the Hikurangi Trough because their load seismicity such as the Hikurangi margin, slope insta- is trapped in structural ‘‘baffles’’ on the slope. These bility triggered by earthquakes might be a frequently baffles include mid-slope basins, fault scarps, and occurring process. However, the consistency of the slump blocks. This hypothesis is supported by sedi- tephrostratigraphy across the Paritu Trough suggests ment flux estimates on the lower slope off Hawke’s that large avalanche deposits probably occur only Bay by Carter et al. (2002). Lewis et al. (1998) every few thousand years or longer. Perhaps at sub- highlight the contrast with the Poverty Canyon sys- millennial time scales, it seems feasible that the gully tem, which incises the slope, bypassing slope basins, debris observed on the upper slope could have been and links to the Hikurangi Trough. Seismic evidence triggered by an earthquake, enhanced by elevated of buried, infilled channel axes 0.4–1.0 km beneath fluid pressures that occur along the upper margin. In the trough suggest that the Poverty Canyon might comparison with other marine records along the have once supplied sediment directly to the Hikurangi Hikurangi margin, MD97-2121 off southern Hawke’s Channel, which lies only 30 km seaward of the mouth Bay does not reveal discernable pulses in sediment of Poverty Canyon Lewis et al. (1998). flux that are coincident with seismic events back to Sediment fans on active accretionary margins the mid-Holocene (Carter et al., 2002).However, might only be recognised if their rate of accumulation pulses recorded at base of the slope and trough could exceeds their rate of destruction by subduction. Using be buffered by sediment capture on the shelf and inter- a plate convergence rate between the Pacific and slope basins. Australian plates at Poverty Bay, it is possible to Along-slope sediment sources transported by the estimate the approximate preservation potential of a East Cape Current appear to be unlikely despite the submarine fan at the Poverty Canyon mouth. Apply- A.R. Orpin / Marine Geology 209 (2004) 69–90 85 ing a convergence rate of 5 cm yearÀ 1 (Lewis et al., sedimentation processes during the late Holocene in 1998) since the last glacial maximum at 18 ka BP this part of the blocked canyon system are domi- suggests there has been ca. 900 m of E–W conver- nantly hemipelagic, and provide a consistent chro- gence at the base of the Poverty Bay slope. Hence, if a nostratigraphy (Fig. 4). fan of lowstand riverine-derived sediment had dimen- Acoustic backscatter, seismic and sediment core sions at least as great as the 4000-m wide mouth of data suggests that the gullies on the upper slope Poverty canyon, it is unlikely that it would have been contain acoustically reflective, consolidated sedi- completely destroyed. An alternative explanation for ments. The heads of these gullies at f 140 m water the lack of a fan is active erosion by turbidity currents depth could tap into sediment-laden coastal waters down the canyon. A slope of 3–4j is maintained during glacial lowstands, potentially acting as con- along the final 22 km reach of the canyon axis. Piston duits for sediment dispersal onto the mid-slope. core W693B indicates that the sea floor at the canyon Similarly, the heads of the larger tributaries of the mouth has little sediment cover and is largely eroded Poverty Canyon shoal to 80–140 m, which, during Late Pleistocene mudstone (Fig. 4). From an analysis the peak of last glacial, could have funnelled near- by Lewis and Pantin (2002) of turbidity flow behav- shore sediment directly into the Poverty Canyon iour along the Hikurangi Channel, such a slope within system. The likely sediment sources include sand Poverty Canyon might induce turbulent hydraulic and silt entrained in longshore drift and earthquake- flow conditions at the canyon mouth, which could induced upper slope failure. A modern analogue for conceivably resuspend or erode the seafloor (K.B. such a sedimentary system could be Kaikoura Can- Lewis, NIWA, pers. comm., 2002). Irrespective, there yon of the eastern South Island of New Zealand. At is no obvious link to a Holocene fluvial source for Kaikoura, the canyon acts a conduit for more than Poverty Canyon as its upper reaches were disconnect- 90% of the riverine sediment delivered to the coast, ed from the shelf by coeval tectonic uplift along the feeding it directly into the head of the 1500 km long northern extension of the Lachlan Ridge anticline (cf. Hikurangi Channel (Lewis and Barnes, 1999). The Barnes et al., 2002). Today, the Poverty Gap lies f 10 Kaikoura Canyon shoals to within 500 m of the km north of the northernmost branch of the Poverty shore and 20 m water depth, tapping into mobile Canyon system (Fig. 2). Hence, at least during the zones of gravel, sand and mud migrating northwards current sea level highstand, the main canyon is pre- under the influence of waves and currents. The sumably being bypassed as the primary conduit for conduit into the Poverty Canyon system might have Waipaoa sediment dispersal and is presumably a relict progressively closed and become disconnected from feature. the coastline with transgression and uplift of the The southern branch of the Poverty Canyon con- Lachlan Ridge anticline (Barnes et al., 2002).On tains unconsolidated mud and a distinct bed of the southern flanks of Lachlan Ridge, the last glacial Taupo eruption tephra (core U1752, Fig. 4). In surface shows erosion and planation of underlying contrast, the lower reaches of the Poverty canyon strata now at 140–150 m below sea level, and contains stiff sandy-mudstone, sometimes overlain Barnes et al. (2002) speculate that this erosion took by a few centimetres of unconsolidated mud (core place close to the last glacial-maximum lowstand. W693B, W694 and U1751, Figs. 2 and 4). This These data suggest that the landward southern rea- contrast implies different sedimentation processes ches of the Poverty Canyon system might have occur at the southern branch of the canyon. An tapped into locally derived sediment during the last examination of the bathymetry indicates that the glacial lowstand. canyon branch narrows down slope, suggesting that Whether Poverty Canyon was also a conduit for its mouth could be partially blocked, and sediment fine-grained suspended sediment to the Hikurangi has ponded upslope as a result. The source of this Trough during the last-glacial maximum and early hemipelagic sediment is likely to be intraslope, as transgression is uncertain. One of the major influen- the canyon branch does not incise landward to tap ces on offshore sediment transport dispersal is the into an outer-shelf or upper-slope sediment source. palaeoceanography. High-resolution sediment records Like the other mid-slope cores, it would appear of postglacial sedimentation on the continental slope 86 A.R. Orpin / Marine Geology 209 (2004) 69–90 of the Hikurangi margin are limited. Off southern off-shelf sediment dispersal on the Eel River margin Hawke’s Bay, terrigenous sediment flux to the Hikur- (e.g., Ogston et al., 2000; Puig et al., 2003). In angi Trough was variable and buffered by mid-slope addition, it highlights that sediment transfer from basins, but generally decreased from ca. 18 ka BP the coast to the continental slope in tectonically active through to the early Holocene, after which a weak- environments can occur rapidly, focussed in high ening northerly wind component and the domination turbidity events lasting perhaps a day or less. Here, of the East Cape Current suggest southward sediment tectonics are not as influential as the slope and current transport (Carter et al., 2002). The effect of shelf regime in the offshore dispersal of sediment. The Eel retention of terrigenous sediment was also suggested and Waipaoa Rivers each deliver an average of 15 Mt by Carter et al. (2002) to be an important influence in yearÀ 1 of mud, have similar flood delivery, and the reducing the sediment flux to the trough since the mid sedimentation rates on the Eel and Poverty shelves Holocene. However, the reverse effect might also are comparable, but sedimentation rates on the Eel occur. Given the typical geometry observed along continental slope are up to six times higher (Table 1), the East Coast margin of a subsiding mid-shelf basin probably a function of its narrower shelf width. The and ridge barrier seaward of the basin, it is possible fate of 60% of the Eel sediment discharge is undoc- that high fluxes during the early-postglacial transgres- umented, but a recent sediment transport study sug- sion are the result of reworking of lowstand deposits, gests that some may be dispersed beyond the slope i.e., significant lowstand aggradation might have via the Eel Canyon (Puig et al., 2003). Such a style of occurred on the shelf. Without sufficient age control sediment dispersal, that supplies the slope and can- such a hypothesis for the Poverty shelf is tenuous, but yons directly during a high-stand of sea level, is on low gradient shelves elsewhere, reworking of contrary to traditional sequence stratigraphic models lowstand deposits has been inferred to contribute that would favour shelf deposition (e.g., Van Wagoner significantly to the slope and trough sediment flux et al., 1988). Moreover, it supports the notion that a during the early transgression (e.g., Dunbar et al., major fraction of terrigenous sediment supplied to the 2000). ocean by flood-prone mountainous rivers bypasses narrow shelves on tectonically active continental 5.5. Comparison to another muddy river on an active margins (e.g., Milliman and Syvitski, 1992). Similar margin and possible implications to sequence strati- to the Eel, data from the Poverty margin suggests that graphic models both the shelf and slope receive significant volumes of terrigenous sediment, but the Poverty Canyon Data from the tectonically active Eel margin in system today appears to be inactive as a major California infer a combination of hyperpycnal and sediment conduit. The seaward barrier provided by resuspension-induced density flows are significant to the Lachlan Ridge precludes the direct dispersal of

Table 1 Compilation of sediment output from the Waipaoa River, and the approximate dimensions of the resulting postglacial sediment prism on the continental shelf from this study and data modified after Foster and Carter (1997) River Mean sediment Specific sediment 100-year Max postglacial Postglacial volume Modern 210Pb mass output yield flood delivery thickness on shelf of shelf wedge accumulation rate (Mt yrÀ 1) (t kmÀ 2 yrÀ 1) (Mt dayÀ 1) (m) (km3) (cm yrÀ 1) Waipaoa 15 6800 15 f 45 18 (0–0.9) 0.5 av. (shelf ) 0.1 (slope) Eel 15 1620 16 z 75 unknown (0.2–0.8) 0.4 av. (shelf ) 0.1–0.6 (slope) 210Pb sedimentation rates from the Poverty shelf are from Orpin et al. (2002b), and S.A. Kuehl (VIMS, pers. comm., 2001) and C.A. Alexander (Skidaway Institute of Oceanography, pers. comm., 2001). Sediment output and yield data for the Waipaoa are from Hicks et al. (2000). Data from the Eel Margin are summarised from Alexander and Simmoneau (1999), Sommerfield and Nittrouer (1999) and Burger et al. (2001). Estimates of the 100-year flood sediment delivery were calculated using the Eel sediment rating curve of Syvitski and Morehead (1999) and Waipaoa gauging data described in Hicks et al. (2000). A.R. Orpin / Marine Geology 209 (2004) 69–90 87

Waipaoa sediment into Poverty Canyon. If this asser- depositional process operating at sub-millennial tion is correct, the Waipaoa is a prime example of a time scales, at least since the mid-Holocene. system that possesses a high sediment yield, high 6. No vestige of a sediment fan occurs at the mouth of annual sediment discharge, and a propensity to flood, Poverty Canyon, rather the seafloor is eroded and and yet Holocene sedimentation does not extend has only a thin cover of hemipelagic mud. The significantly seaward of the slope. In contrast to the seaward barrier provided by the Lachlan Ridge Eel margin, tectonics appears to exert a greater precludes the direct dispersal of Waipaoa sediment influence on offshore sediment pathways for the into the upper-slope tributaries of Poverty Canyon. Waipaoa dispersal system. Further research using The canyon system has probably been largely high-resolution geophysical surveys in conjunction inactive during the Holocene. with geochemical tracers (e.g., 210Pb, 7Be) are re- quired to provide a more complete mass balance which will better evaluate the degree of sediment Acknowledgements capture and transport tragectories, both historically and today. Early drafts benefited from helpful comments by NIWA colleagues Lionel Carter, Keith Lewis and Scott Nodder. The final manuscript was significantly 6. Summary improved by the constructive criticism of Peter 210 2 3 Harris and an anonymous reviewer. Pb geochro- 1. Approximately 140 km and 3 km of postglacial nological data was kindly provided by Clark sediment lies seaward of Lachlan Ridge and Ariel Alexander (Skidaway Institute of Oceanography) Banks, emerging from the Poverty Gap, which and Steve Kuehl (Virginia Institute of Marine separates the shoals, as a sediment lobe that Science). Miles Dunkin and Richard Garlick offered feathers down onto the upper slope. skilful assistance with bathymetric and backscatter 2. Seismic reflection echo type and backscatter signal data. This work is funded by the New Zealand strength indicate gully debris at the base of Foundation for Research Science and Technology regularly spaced upper slope gullies, possibly grants C0X0013, C01X0037 and C01X0038, and caused by earthquakes and fluid sapping along AO was supported by a NZST Postdoctoral Fellow- the upperslope. Blocky debris occurs on the upper ship NIWX0003. flanks of the Paritu Trough and there is evidence of a large avalanche deposit. 3. The Poverty slope has received a significant flux of terrigenous sediment since the mid-Holocene, References accumulating on the slope predominantly in two mid-slope basins, the Paritu Trough and a smaller Alexander, C.R., Simmoneau, A.M., 1999. Spatial variability in sedimentary processes on the Eel continental slope. Mar. Geol. sub-basin below the South Paritu ridge. 154, 243–254. 4. The average sediment mass accumulation for the Allsop, F., 1973. The Story of Mangatu. Government Printer, Wel- mid–late Holocene on the slope is estimated to be lington, New Zealand. 100 pp. around 0.05 g cmÀ 2 yearÀ 1 (0.06 cm yearÀ 1). Arron, E.S., Lewis, K.B., 1992. Mahia Bathymetry, 2nd edition. This rate is higher than that recorded on the slope- New Zealand Oceanographic Institute Chart, Coastal Series 1:200,000. depressions and the lower slope off southeastern Baldwin, R.P., Lewis, K.B., 1991. Cook bathymetry, 2nd edition. Hawke’s Bay, but is sufficiently slow that the last New Zealand Oceanographic Institute Chart, Oceanic Series century of higher post-European sediment yields is 1:100,000. likely to be restricted to the top 10 cm of the Barnes, P.M., 1992. Mid-bathyal current scours and sediment drifts seabed. adjacent to the Hikurangi deep sea channel, eastern New Zea- land: evidence from echo character mapping. Mar. Geol. 106, 5. The consistency of the tephrostratigraphy across the 169–187. Paritu Trough indicates that despite slope instabil- Barnes, P.M., Nicol, A., Harrison, T., 2002. Late Cenozoic evolu- ity, hemipelagic sedimentation is the under-riding tion and earthquake potential of an active listric thrust complex 88 A.R. Orpin / Marine Geology 209 (2004) 69–90

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