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Location Maps: Waipaoa and Eastern South Island Sedimentary Systems.
In presenting New Zealand as a focus area to the MARGINS Source-to-Sink community, we are mindful of the fact that any sediment dispersal system is influenced spatially by physiography, as well as local climatic and oceanographic regimes, and also varies with time in response to changes in base level (relative sea level). To accommodate this variability the proposed New Zealand focus area comprises two complementary sub-areas, the Waipaoa (East coast, North Island) and Eastern South Island sedimentary systems. In specifying two sub-areas we accept that the interval between time horizons associated with specific events varies, that budgets will need to be closed at a variety of timescales, and that reservoirs that operate over long timescales force lower resolution measurements. Both systems contain a variety of high-resolution terrestrial and marine sediment archives and ALL the major sources and sinks of sediment are open to investigation. There is, therefore, no operational constraint on our ability to close sediment budgets over appropriate timescales. Anthropogenic activity in the historic period (post-1850) has affected the terrestrial sediment dispersal system of both proposed sub-areas, but to an extent that is well-documented and the magnitude of the resultant change in sediment fluxes has been quantified. For example, in the Waipaoa Sedimentary System (by comparison with pre-settlement levels) sedimentation rates, as computed from lake and marine cores, doubled after the arrival of Polynesian settlers, and increased by an order of magnitude after the arrival of European colonists.The Waipaoa Sedimentary System is positioned in an active margin tectonic setting. The primary focus is on contemporary (Recent through Holocene) processes, and on the conditions under which specific erosion events in the hinterland are translated to the plain and shelf systems, though the shelf-indenting canyon and basin floor fan that were major features of the low-stand system are also open to investigation. As in other meso-scale river basins that are too small to modulate varying rainfalls in the way larger basins do, high-intensity storms play an important role in the sediment transport regime. Shallow landsliding, triggered by such storms is a dominant erosion process (however, on an annual basis, the contribution gully erosion makes to the catchment sediment yield supercedes that made by landsliding). The system is compact (~2570 km2source, ~900 km2 sink), with point-source discharge to the ocean, and virtually closed under present highstand conditions (i.e., sediment budgets can be balanced from upland to shelf though, as in most systems, the contemporary highstand conditions may prevent specific erosion events from extending their influence to the deep marine basin).The Eastern South Island Sedimentary System, comprising the Clutha, Waitaki and Rangitata river basins discharges onto a passive margin with a broad continental shelf. The major rivers constitute a line source and northward flowing, along-strike, currents influence sediment dispersal patterns offshore. Both onshore and offshore, the system is more complex than that of the Waipaoa. Thus, balancing the modern highstand sediment budgets is more difficult, but still feasible. At lowstand the system discharges into the Bounty Trough and Fan, and is virtually closed. The strength of this subarea lies in the rich record of terrestrial events (preserved in contiguous sedimentary deposits contained in terraces, lakes, shelf edge clinoforms, canyons and fans) for an entire glacioeustatic sea-level cycle, or longer. The focus here is, therefore, on larger scales, both spatial (~35 000 km2 source, ~250 000 km2 sink) and temporal (~105 kyr).The Waipaoa and Eastern South Island sedimentary systems thus present a range of research opportunities that should encourage participation from the entire spectrum of the MARGINS Source-to-Sink community. Logistical and infrastructural considerations are equally propitious. Ease of access is an essential field requirement and transient weather conditions provide the only constraint on accessibility to the major sources and sinks of sediment within the two systems. A range of data provide information about sediment fluxes throughout the historic period and insight into the stratigraphic and sedimentologic conditions that characterize each system. There are baseline data and time series for many of the fundamental processes and summary information on the type, range and quality of the information available on each dispersal system is presented as part of the site descriptions. The quality of the background information should encourage theoretical as well as experimental and observational research and serve, from the outset, to concentrate attention on the processes relevant to the goal of developing a better understanding of landscape evolution, sedimentary processes and stratigraphy. Scientific priorities and financial resources will benefit from active research programs funded by NSF (e.g., the South Island Geophysical Transect and Southern Alps Passive Seismic Experiment which are components of the Continental Dynamics Program - South Island: PI's T. Henyey and D. Okaya, USC), the ODP (Leg 181), and from ongoing research being undertaken by New Zealand scientists. Letters of support from Crown Research Institutes with complementary research objectives confirm the interest that New Zealand scientists have in forging international linkages, leveraging resources and conducting collaborative research.
Present-day earthquake activity (1991-1995, Magnitude >3.0) and crustral deformation (1990-1998) measured by GPS (high strain rates relate to high uplift rates) delineate the magnitude and extent of tectonic forcing within the (Australasian/Pacific) plate boundary zone.
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Waipaoa Sedimentary System
Physiography of the Waipaoa River Basin and adjacent seafloor.
Geology of the Waipaoa Sedimentary System: source; mid-shelf depocenter (core region shaded for emphasis, tranagle marks location of MD22122); and adjacent ocean floor.
Precipitation distribution and hydrometeorological network in the Waipaoa River Basin and adjacent areas. Reflection seismic tracks, including 3.5 kHz profiles (aqua tone delimits area covered by detailed hydrographic soundings and yellow tone area covered by Simrad EM12 swath mapping).
Seismic profiles (3.5 kHz) across the mid-shelf depocenter (W1 marks the unconformity at the base of the postglacial transgressive sequence, H is a Holocene reflector of c. 8000 BP, MD = mud diapir,and GMZ is gaseous mud zone). Post-glacial terraces of the Waipaoa River. Earthquakes on theRaukumara Peninsula (symbol sized is scaled to magnitude, depth sections lie to the northwest andsoutheast of line A-A', and curved lines on depth sections delimit approximate location of plate interface).
The ~2200 km2 Waipaoa River Basin, together with the smaller (~370 km2), neigboring Waimata River Basin which also drains into Poverty Bay, comprise the terrestrial portion of the Waipaoa Sedimentary System. Located on the East Coast of New Zealand's North Island, the Waipaoa River Basin is positioned above a major tectonic plate boundary and is underlain by deformed Cretaceous and Early Tertiary mudstone and argillite, Early Cretaceous greywacke, and a thick forearc sequence of Miocene-Pliocene mudstone and sandstone. Active fault traces and growing folds are present within the greater catchment. Subduction induces variable uplift rates around the Raukumura Peninsula, locally >4mm yr-1 (as determined from coastal terraces). Local subsidence is recorded near the river mouth and there is up to 300 m of Quaternary sediment beneath the Poverty Bay Flats. In the middle reaches of the Waipaoa River basin, uplift rates estimated from fluvial terrace sets are ~1 mm yr-1. Uplift rates in the head of the basin are much greater, but remain to be constrained. Mean annual rainfall averages 1470 mm above the mainstem gauging station on the Waipaoa River (at Kanakanaia: 48 km upstream from the coast), and the basin has a maritime climate that is periodically disturbed by intense cyclonic and more localized storms. The largest historic (Cyclone Bola) storm occurred in March 1988 (peak flow ~5000 m3 s-1; suspended sediment yield 26 Mt). Rainfall variation across the basin is primarily controlled by topography, although it also varies with the source-direction of individual weather systems.
Today sediment and nutrient fluxes in the Waipaoa Sedimentary System are dominated by anthropogenic influences. However, humans have only been present in detectable numbers since c. 700 BP, and extensive landscape transformation has only taken place since the arrival of European colonists. The well drained lowland areas of the East Coast region were originally covered with dense podocarp/ hardwood forest, dominated by Prumnopitys taxifolia and Dacrydium cupressiunum emergent over a canopy of mixed hardwood trees. Fires following volcanic eruptions and lightning strikes periodically disturbed the natural vegetation, which was also disrupted by the Maori settlers. Widespread clearing of the indigenous forest did not commence until after the arrival of European colonists in the late 1820's. By 1880, most of the lower reaches of the greater catchment had been cleared, and the headwaters had been cleared by 1920. Today, ?3% of the basin remains under primary indigenous forest. Reforestation of headwater areas with exotic species, such as Pinus radiata, began in 1960, and commercial timber harvesting commenced in 1990.
European colonization destabilized the landscape and initiated a phase of severe hillslope erosion in the Waipaoa River Basin's headwaters, where amphitheater-like gully complexes (up to 0.2 km2 in area) developed in the highly sheared rocks. In the lower reaches of the greater catchment the hills underlain by the Miocene-Pliocene cover sequence are prone to shallow landsliding when storm rainfall exceeds about 120 to 200 mm in a 24 hr period (established gullies, by contrast, are activated by small, frequent rainstorms). Large volumes of fine sediment are delivered to stream channels by gully erosion and shallow landsliding, and the Waipaoa River has a mean suspended sediment concentration of ~1700 mg l-1. The annual average suspended sediment yield to Poverty Bay of 15 million tonnes (5900 t km2 yr-1) ranks amongst the highest measured in New Zealand for a basin of comparable size, and is also very high by global standards. New Zealand is a major source of terrigenous sediment, supplying nearly 1% of the suspended load to the world ocean and the Waipaoa River ranks as one of the largest North and South Island sources. Moderate flows (less than the bankfull discharge of ~1800 m3 s-1) transport ~75% of the suspended sediment load. The suspended sediment load of the Waimata River amounts to ?5% of that generated by the Waipaoa River.
Main gauging site Kanakanaia Basin area (km2) 2200 Basin mean annual rainfall (mm) 1471 Years of flow record 1960 - present Mean discharge (m3 s-1) 35 Mean annual flood (m3 s-1) 1346 Current suspended load to coast (mt yr-1) 15 Bedload (percent of suspended load) 1% Years of suspended sediment gaugings 1962 - present Number of suspended sediment gaugings 301 Maximum gauged sediment concentration (mg l-1 ) 36814
At the coast, sediment from the Waipaoa River is dispersed mainly as hypopycnal plumes with a net southward transport; though the dispersal pattern is subject to periodic reversals under wind-assisted currents from the south. During high inflow conditions (> 2000 m3 s-1) the fluvial discharge density (~ 40 kg m3) may exceed the density of coastal waters forming hyperpycnal flows. The combination of a broad crescentic coast and actively growing anticlines (Lachlan and Ariel) on the outer shelf effectively captures sediment on the middle shelf. Subsidence of the middle shelf at 2 mm yr-1 further enhances retention. Post-glacial accumulation rates are high (~ 1.9m ka-1). A 13 km-wide gap between Ariel and Lachlan anticlines provides an escape route for some sediment but it is estimated that the off-shelf loss of modern mud is small. Sediment escaping onto the continental slope is likely to be retained within a large amphitheatre of an old (Pliocene?) collapse structure formed by collision of the accretionary prism with a seamount transported on the subducting Pacific Plate. Given the shelf and slope entrapment of sediment, conditions must be considered favorable for closing the modern Waipaoa budget. The only uncertainty relates to sediment transported into the area by the prevailing along-margin circulation. On the shelf, transport is weak and ephemeral but on the outermost shelf and upper slope sediments come into contact with the southwest flowing East Cape Current. This flow mainly affects sediment escaping the shelf depocentre and transport rates have yet to be resolved.
During lowstands, the Waipaoa River presumably led towards the shelf edge, although the exact point of discharge is not obvious as the course of the ancestral river has not been traced entirely across the shelf. If discharge was into Poverty Canyon, south of the Ariel-Lachlan gap, sediment within the canyon would be guided to a shallow basin at 3000 to 3200 m depth that has formed between the base of slope and the left bank levee of Hikurangi Channel. If the lowstand river discharged at a gully near the central part of the gap, then sediment would accumulate mainly within the amphitheatre of the collapse structure. Whichever is the case, the slope conduits lead into basins where there is a reasonable expectation of closing the budget. The primary caveats relate to uncertainties about sediment transported along the slope by the ancestral East Cape Current and the aeolian contribution to the terrigenous budget which may be substantial (~ 20%) under glacial conditions. At this timescale, sequences of fluvial terraces provide a punctuated chronology of erosion since Oxygen Isotope Stage 5 that may, in principle, be correlated with the marine record. regional timeline correlation and intercorrelation between terrestrial and marine sequences is facilitated by a well-developed tephra-stratigraphy. Four aggradational terrace sets in the middle reaches of the catchment (dated primarily by tephra stratigraphy) appear to be correlated with cold/cool climate episodes. Down-cutting rates between aggradational phases are dramatic, and >120m of incision since c. 15 000 yr BP is recorded in the upper catchment.
Rainfall records in the headwaters of the Waipaoa River Basin date from 1946, over 70 years of record exist for several sites in the lower catchment, and there is a 100 year record for a site on the coast. Hydrological records have been collected from the gauging station at Kanakanaia since 1960. River flow, rainfall, and suspended sediment data are electronically archived by Gisborne District Council (HYDSYS software system). Suspended sediment has also been gauged in many tributaries. Landcare Research maintains the 1:50,000 scale Land Resources Inventory and the Institute for Geological and Nuclear Sciences 1:250,000 coverage of basin lithology and structural geology. A 20 m DEM covers the entire basin, and 2 m DEMs are available for selected portions of the headwaters. Sequential aerial photography is available from 1939. Well log data for the Poverty Bay Flats areas is good and Landcare Research have drilled at 10 sites on the Waipaoa River floodplain and in 2 landslide-dammed lakes. Four sites have yielded a high-resolution record of sedimentation spanning the past 5 to 10 kyr.
Continental shelf bathymetry is covered by closely spaced, hydrographic sounding lines (0.5 - 2.0 km apart), supplemented by bathymetric data from the National Institute for Water and Atmosphere (NIWA). In contrast, the rest of the margin and adjacent Hikurangi Plateau are mapped by Simrad EM12 swath bathymetry and side-scan sonar. These data have been processed and archived at NIWA. In addition, the shelf is covered by good quality 3.5 kHz lines (4.5 km apart) which provide a high resolution record to a maximum sub-seabed depth of 37 m. At that depth, further seismic penetration is impeded by an acoustical masking layer assumed to be a zone of gaseous sediments. Deeper penetrating airgun data are also available and include multi-channel industry records lodged at NIWA. Surface sediment samples and cores are concentrated on the shelf and include a complete 16 m-long core (MD2122), recently acquired from the Marion Du Fresne IPHIS cruise of 1997. This core yielded a high resolution record of sedimentation since the late deglaciation. The region is well served by remotely sensed data. Sea surface temperatures are collected daily by satellite receiver. Data are processed to form a complete archive covering the past 6 years thus providing a substantial timeseries of the surface circulation including major changes that accompanied the last major ENSO event. Additional information on the surface circulation comes from TOPEX Poseidon, ERS 2, and SeaWyfs.
The East Coast region is a focus for several on-going Public Good Science Funded (PGSF) research programs (e.g., Ecosystem Processes for Catchment Management and Ocean Variability of Current and Water Masses) that are managed and directed by Landcare Research, the National Institute of Water and Atmospheric Research (NIWA), and the Institute for Geological and Nuclear Sciences (IGNS).
Sediment generated by the recent (post-1850) change from forest to pasture is currently being redistributed on the landscape and in the marine environment. Hydrometeorological data provide information about sediment and nutrient fluxes under existing conditions, but the rate at which erosion occurs and sediments and nutrients accumulate (or are depleted) is far from constant over time. It responds to climatic and tectonic, as well as anthropogenic forcing, and the impact of these changes, that occur over time scales of decades to millennia, must be resolved before the magnitude of the change in sediment and nutrient fluxes caused by human activity can be quantified. However, even for the relatively recent geologic past (late Quaternary and Holocene) there are uncertainties about the levels of operation surficial processes and their natural variability. Research conducted in the Waipaoa sedimentary system has the potential to answer such key questions as: at what rate, over what time scale, and to what level does the landscape respond and recover from naturally and anthropogenically induced change?
Landslides in the Waimata catchment generated by the March 1988, Cyclone
Bola storm. The Waipaoa River floodplain at Te Karaka and overbank
sedimentation after the Cyclone Bola storm. The Tarndale gully complex in
the Waipaoa River Basin headwaters. Photography: N.A. Trustrum.
Realtime image: Poverty Bay foreshore (Waikanae Beach), Gisborne
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Eastern South Island Sedimentary System (ESISS)
Physiography of the Clutha, Waitaki and Rangitata basins and the marine sector of the Eastern South Island Sedimentary System (+ marks location of ODP 181 - 1122), and detailed physiography of the Eastern South Island Sedimentary System continental margin outlining the main lowstand conduits to the Bounty Trough and Fan.
Geology, precipitation distribution and the hydrometerological network in the terrestrial sector of the Eastern South Island Sedimentary System.
Generalized outlines of the main physiographic features and depocenters of the Eastern South Island Sedimentary System. Sediment isopachs for ~9 ka, highlighting the main depocenter of the Clutha River sand.
Interpreted surficial sediment cover for the Otago shelf. Reflection seismic section across the Bounty fan with 400 m thick levee sequence that is the main distal sink.
Reflection seismic tracks (including 3.5 kHz lines) for the Eastern South Island Sedimentary System. Waitaki River and lake sediment traps of the upper catchment.
Seismic profile across Lake Pukaki (Waitaki catchment) showing post ~10 ka fill which has accumulated at an average rate of 15 m ka-1. Lake Pukaki, formed at the end of the last glaciation and is surrounded by prominent moraines of four major advances.
The three primary catchments of ESISS belong to the Clutha, Waitaki and Rangitata rivers which drain 33,800 km2 of the eastern flanks of the Southern Alps. All three catchments lie within the broad zone of crustal deformation associated with the boundary between the Australian and Pacific plates. Developing since the early Miocene, the plate boundary has strongly influenced the geology and modern landscape. The Alpine Fault defines the western margin of the Southern Alps, and is the dominant tectonic feature. Deformation is most intense near this major fault, where uplift rates can exceed 10 mm yr-1, basement rocks of high metamorphic grade have been raised and exposed, and geothermal gradients are high. Recent precise GPS surveys through the Waitaki and Rangitata catchments graphically show that plate boundary deformation decays from the Alpine fault, south-eastwards to the coast, where it is low.
The Clutha, Waitaki and Rangitata catchments are underlain by a series of tectonostratigraphic terrains of Permian to Jurassic age which form the Eastern Province basement rocks. These rocks have been strongly affected by later Cenozoic faulting and folding and, in many places, they are overlain by Cenozoic sediments located within remnant basins. The upper reaches of the catchments have also been glaciated in Quaternary times. Basement rocks are predominantly greywacke/argillite and schist, all of which were metamorphosed from the same suite of sands, silts and gravels, deposited 260 to 195 Ma. Metamorphism is timed at 130 to 110 Ma. The basement rocks are the principal sediment sources in the catchments. Marine, terrestrial, and fresh-water lacustrine sediments, ranging in age from late Cretaceous to Miocene (100 – 5 Ma), were deposited discontinuously on a Cretaceous peneplain cut into the greywacke/schist terrain. Some of these sediments are preserved in fault-bounded synclines and fault-angle depressions. They include mudstones, sandstones, conglomerates, and occasional limestones. Late Quaternary sediments include tills, moraines and terrace gravels (predominately glacial outwash deposits), loess, alluvium, and colluvium. Their chronology is based on thermoluminesence, radiocarbon and other isotopic techniques, in combinate with indirect methods, such as weathering profile developemt and lichenometry, and they have been correlated with dated sequences elsewhere.
Active tectonics, climate, lithology and topograhy constitute the principal forcing agents. An example of the influence lithology and topography have on slope stability can be seen in the Upper Clutha catchment, about the Shotover River where extensive creeping landslides occur mainly on dip slopes formed in predominantly pelitic schist and green schist. The rock strength of these units is significantly lower than in other schist units because of their high mica content and abundant foliation-parallel planar defects. The upper reaches of the catchments are characterized by moderate to high earthquake activity with deep and shallow events. Earthquakes, with magnitudes ~M8 are plausible, though none have occurred in historic time, and active faulting is widespread.
Over the last few million years, particularly within in the upper Clutha and Waitaki catchments, the river systems have been repeatedly reformatted by glaciers. Such changes have substantially affected some drainage patterns and sediment supply. For instance, during the last glaciation, lakes formed along the Waitaki and Clutha rivers were occupied by glaciers. Glacio-fluvial sediment was transferred directly to the lowstand shoreline near the shelf edge. However, glacial retreat about 10 ka recreated the lakes which became effective traps as manifested by average accumulation rates of 10 to15 m ka-1 and, not surprisingly, river input to the shelf declined dramatically. In the case of the Waitaki River, an estimated glacial load of 22 x 106 t yr-1 declined to ?1 x 106 t yr-1 (before modern hydro-electric dam construction). Similarly, the modern input for the Clutha River is a five-fold reduction from glacial values. Far from being a disadvantage, the lake sediments present a quantifiable record of post-glacial deposition, as shown by high resolution seismic records from Lakes Pukaki and Tekapo. Moreover, some lakes are varved and thereby provide a year-by-year account of deposition which should enable both short- and long-term climatic influences on sediment supply to be evaluated (e.g., ENSO; Pacific Decadal Oscillations).
The Clutha Basin covers over 20,000 km2 in the schist terrain of Otago. Precipitation occurs all year round but the greatest runoff occurs in late spring, in conjunction witth rain-assisted snowmelt. Three large natural freshwater lakes (Wakatipu, Wanaka, and Hawea) cover 607 km2 and intercept runoff from an area of 6700 km2. Roxburgh Dam was built across the Clutha mainstem in the early 1950s. The resultant Lake Roxburgh, covering 4.5 km2, has been subject to a rigorous monitoring programme including regular cross-section surveys, daily suspended load sampling of inflows and outflows, and particle size analysis of the suspended load and sediment deposited along the lake bed. As a result, the sediment budget of the lake is accurately known: it traps all of the bedload entering it and 80% of the suspended load. In effect, Lake Roxburgh has served as a sediment sampler for the last 40 years, allowing the long-term average load of the upper Clutha, and its size grading, to be accurately defined. Tributaries downstream of Roxburgh, in the region of gentler topography and lower rainfall, contribute only a small fraction of the Clutha sediment load. In the late 1980's, the Clyde Dam was built upstream from Lake Roxburgh. The small, non-glaciated coastal basins, between the Clutha and Waitaki Rivers contribute an estimated 0.9 x 106 t yr-1 of sediment to the ocean.
The Waitaki River drains over 12,000 km2 in the mainly greywacke/argillite terrain of north Otago and Canterbury. As with the Clutha, the mountainous western margin experiences high orographic precipitation, much as winter snow. The eastern tributaries experience a rain-shadow effect and receive much less rain. Highest runoff is in late spring with the rain-assisted snowmelt. Three lakes of glacial origin (Ohau, Pukaki, and Tekapo) cover 240 km2, and intercept runoff from 2900 km2. The Ahuriri River supplies the largest tributary load downstream of the natural lakes (0.11 x 106 t yr-1). The highest sediment-yielding tributary is the Hooker, in the northwest (0.3 x 106 t yr-1), but this flows into Lake Pukaki. Three hydro-dams (Waitaki, Aviemore, Benmore) have been built across the lower Waitaki since 1934. The total area of hydro-lake is approximately 100 km2. As in the Clutha, the Waitaki hydro-reservoirs also serve as large-scale sediment traps.
The Rangitata River drains 1800 km2 of greywacke/argillite terrain, flowing eastward from the main divide of the Southern Alps where the annual precipitation exceeds 9000 mm. Like the Clutha and Waitaki, runoff in the Rangitata is greatest in the late spring. With no large lakes upstream, discharges in the Rangitata are relatively flashy, rising to flood levels over periods of several hours following heavy rainfalls in the basin headwaters. Flood recessions typically last from a few days to a week. There is a strong precipitation gradient down the basin, with annual rainfall reducing to 600 mm at the coast. Lakes cover less than 10 km2 in the Rangitata Basin, and there is no hydro-electric development. The non-glaciated coastal river basins between the Waitaki and Rangitata Basins produce an estimated total sediment load of 3.5 x 106 t yr-1.
Conventional geological data have been collected by mapping (1:250,000, 1:50,000 and at larger scales for the hydroelectric projects). This database is augmented by drilling (e.g., the 20,000 water bores and four petroleum wells in the Canterbury Plains), seismic refraction/ reflection surveys and landslide databases. These data, together with limited regional gravity surveys, have been largely integrated with surface mapping and a more extensive offshore seismic reflection and drill hole database. Recent industry seismics have been acquired throughout the Canterbury Plains but this information is currently confidential. Open file seismic reflection coverage in the catchments is restricted to a few lines in the Rangitata catchment. The latter profiles extend to 0.6 sec (TWTT). They outline the general thickness of the Quaternary sediments, and delineate their basal contact with the underlying Pliocene marine mudstones.
All the significant tributaries to the mainstem Clutha have suspended sediment gauging stations, though the time span of sediment gaugings varies among sites; ranging up to 26 years for the Shotover tributary. Most gaugings were conducted from the 1970’s to mid 1980’s, in conjunction with hydro-electric power developemnt. Suspended sediment particle-size data are available for seven sites. Bedload has been sampled on the Shotover River. Suspended sediment has been gauged at eight tributary sites within the Waitaki River Basin. On the Rangitata River suspended sediment is gauged at the main flow-recording site at Klondyke, River flow, rainfall, and suspended sediment data are archived on the Water Resources Archive, a national electronic database (TIDEDA software system) administered by NIWA.
River Clutha Waitaki Rangitata Main gauging site Balclutha Kurow Klondyke Basin area (km2) 20582 12118 1775 Years of flow record 1954 - present 1963 - present 1967 - present Mean discharge (m3 s-1) 563 369 98 Mean annual flood (m3 s-1) 1730 1105 939 Current suspended load to coast (mt yr-1) 0.6 0.35 1.7 Suspended load to coast before dams (mt yr-1) 2.9 0.99 1.7 Bedload (% of suspended load) 14% 18% 13% Main sediment-supplying tributary Shotover Ahuriri Rangitata Basin area (km2) 1088 537 1461 Mean annual rainfall (mm) 2007 1775 2278 Mean discharge (m3 s-1) 37 23 98 Years of suspended sediment gaugings 1965 - 1991 1965 - 1995 1978 - 1996 Number of suspended sediment gaugings 124 69 7 Maximum gauged sediment concentration (mg l-1) 11336 3591 4269 Suspended sediment load (mt yr-1) 1.3 0.11 1.7
Although within sight of the transcurrent Alpine Fault section of the Australian/Pacific plate boundary, the eastern South Island margin to the south of Banks Peninsula, is a stable passive margin. The broad shelf experiences high sediment input from the actively rising New Zealand Alps, a strong imprint from eustatic changes in sea level, and a moderately vigorous along-shelf circulation system. The shelf between the Clutha and Rangitata is typically 30 to 80 km wide but reduces to 10 km off Otago Peninsula. The shelf break is at 125 to 165 m water depth and is locally indented by the heads of submarine canyons feeding the channel system in Bounty Trough. Shelf morphology is variable with zones of featureless seabed interspersed with ridge and swale topography, terraces and changes in slope that represent palaeoshorelines formed at previous stillstands of sea level.
The shelf sediment cover is a mixture of modern, relict and palimpsest deposits. Sand is confined mainly to the inner shelf where it is subject to along-shelf transport under the Southland Current reinforced by southerly swell and storm-forced currents. Satellite imagery shows river plumes directed to the northeast along the inner shelf. Tides are generally weak except off peninsulas. Most Clutha sand is captured in a nearshore sand prism formed in the lee of a peninsula, near the river mouth. In contrast, the Waitaki and Rangitata rivers discharge onto an exposed coast and modern sediment is swept northeast with only a minimal development of a nearshore prism. Where the nearshore cover is thin, relict gravel from the feather edge of the Canterbury gravel plains, is evident. On the middle shelf, the dominant sediment is sand with varying mixtures of mud. Cleaner sands are probably palimpsest; a contention supported by the presence of active bedforms. Terrigenous sand prevails to the shelf edge off Canterbury (North ESISS), but the Otago shelf cover (South ESISS) is mainly calcareous biogenic sand and gravel of modern and relict origins. This southern sector lies under the main path of currents associated with the Southland Front (a part of the Subtropical Convergence). Further north, the front diverges from the shelf edge, perhaps explaining the finer grained deposits on the Canterbury shelf. A lack of major sediment injection points to the south of the Clutha implies the ESISS receives little external sediment. Within ESISS, bedload from the Clutha River is retained. However, some bedload from the Waitaki and Rangitata may be transported outside ESISS along with the suspended load, as shown by the plume pathways. Thus, to rationalise the modern sediment budget it is important to determine the volume of material leaving ESISS.
Stratigraphy of the post-transgressive shelf cover has been evaluated using high-resolution seismic profiles in conjunction with radiocarbon-dated piston cores. Additional information from the shelf benthic fauna has allowed an analysis of the palaeoenvironmental changes accompanying the last post-glacial transgression and subsequent stabilisation of sea level.
Effects of eustatic changes on shelf sedimentation have been profound. At the last glacial maximum, sea level was ~ -118 m and rivers extended to canyon heads, and discharged directly into the head of Bounty Trough, where turbiditic and hemipelagic sedimentation dominated. The subsequent transgression was a series of rapid rises punctuated by stillstands at -89 m (17 ka), -75 m (15 ka) and -55 m (~12 ka). The palaeoshoreline and associated longshore transport system were by then well landward of canyon heads. Bounty Trough was bypassed by the terrigenous supply, which switched from an ESE to NE route in accord with the developing shelf circulation system. Thus, the trough changed abruptly from a terrigenous to calcareous biopelagic depocentre.
The outer shelf and upper slope are bathed by northeastward currents associated with the Southland Front, which affects depths to 400 m or more. Thus, sediment escaping over shelf break is shifted NE along the margin. Seismic profiles reveal that along-slope transport has created a series of linear sediment drifts that extend back to the late Miocene. Lateral accretion of these slope drifts has provided a mechanism for margin progradation that contrasts with progradation by shelf-normal deposition.
Bounty Trough, a 350 km-wide linear depression between Campbell Plateau and Chatham Rise, is the main depocentre for ESISS during glacial lowstands. Sediment captured by submarine channels was transported by turbidity currents 900 km eastward to feed Bounty Fan. This major depocentre has grown from the trough mouth onto the 4500 m-deep floor of the SW Pacific Basin. En route, overspill from Bounty Channel has created an extensive levee system, laterally constrained by the lower flanks of Bounty Trough. Bounty Fan itself began in the late Pliocene and is now >400 m thick. The lower fan/channel has developed into the path of the northbound Pacific Deep Western Boundary Current (DWBC) which not only receives channel discharge but also has winnowed the lower fan. Thus, some sediment ultimately escapes ESISS to accumulate on drifts formed under the DWBC. The amount of sediment involved is unknown, but it is likely to be small because the fan is not extensively eroded (e.g., Stage 1, 2) sediments are still present on the lower fan. Furthermore, the amount of fan/channel sediment lost to the drifts may be estimated from the amount of metamorphic mica within the drifts; that mineral being a diagnostic tracer for South Island sediments.
In Bounty Trough, the broad scale, post-Cretaceous stratigraphy is based mainly on reflection seismic profiles. Verification of seismic reflectors has relied on widely scattered dredge samples, and correlation with a single DSDP core from Site 594. Further confirmation of the stratigraphy is possible using data from the recently completed ODP Leg 181 (which can be accessed via Texas A?M University's JANUS database). Closely spaced (0.5-2 km apart) hydrographic sounding lines, supplemented by data from NIWA’s extensive archives, have been used to compile into a series of 1:200,000 bathymetric charts of the shelf. Bathymetric information for Bounty Trough is less detailed, but sufficient to define the main morpholgic features such as the Bounty Channel and its tributaries at a scale of 1:1,000,000. High-resolution seismic lines are concentrated on the shelf, in particular, the Otago sector (Uniboom profiles on the inner to middle shelf). Further offshore, localised surveys, such as that of Bounty Fan, are sufficiently closely spaced to give reasonable acoustical facies maps. Deeper reflection seismic profiles, including unprocessed single channel airgun lines and processed multi-channel lines, are scattered throughout Bounty Trough and provide insight into post-Cretaceous stratigraphy and structure. In contrast, deep seismic coverage on the shelf is more extensive, especially off Canterbury, where intensive hydrocarbon exploration has yielded a wealth of data that is now on open file. In addition, the Canterbury shelf will undergo an intensive multichannel survey in January 2000, by the Maurice Ewing.
Confirmation of the seismic stratigraphy comes from 3 long boreholes on the northern flank of Bounty Trough (DSDP Site 594), the sediment drifts along the Canterbury margin (ODP Site 1119) and the Bounty Fan (ODP Site 1122). All sites, provide detailed records of sedimentation back to the main influx of terrigenous sediment commencing in the early Pliocene. Short (? 5 m) piston cores are scattered throughout Bounty Trough and the adjacent continental shelf. Charts of surficial sediments are available at 1:200,000 scale for the shelf and 1:1,000,000 for Bounty Trough.
The physical oceanographic database for the shelf comprises a series of hydrologic transects, and current meter deployments of up to 1 year duration. In addition, considerable use has been made of satellite altimetry and thermal data, the sea surface temperature information being part of the 6-year long record collected, processed and archived at NIWA. In Bounty Trough, time series CTD transects and long term current meter moorings are continuing and are reinforced by SST imagery. Finally, the oceanic circulation has been investigated using the Los Alamos global circulation model (grid size of 0.28o x 0.28o).
Views of the Waitaki Catchment. December 1991 rock avalanche on Tasman Glacier (such events are a major debris source for valley glaciers). Mount Cook, Hooker and Tasman river valleys (Mueller, Hooker and Tasman glaciers). The Waitaki River.
Photography: D.L. Homer ? D. Barrell..
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Criteria for Site Selection
Natural factors:1) Strong forcing that produces strong signals
New Zealand is a major source of terrigneous sediment, supplying ~1% of the suspended load to the Earth's oceans. In the Waipaoa Sedimentary System the major late Quaternary and Holocene shifts in climate were accompanied by changes in vegetation type that profoundly modified the pattern and rate of erosion. Shallow landsliding triggered by high-intensity storms is a dominant erosion process, and periods of enhanced erosional activity were also stimulated by short-term changes in the frequency of large storms and earthquakes. Uplift rates in the headwaters of the Eastern South Island Sedimentary System, which are characterized by moderate to high (deep and shallow) earthquake activity, can exceed 10 mm yr-1. The interglacial-glaicial-interglacial transition provides the dominate climatic forcing mechanism.2) Active sedimentation spanning source to sink environments
In the Eastern South Island Sedimentary System there is a rich record of contiguous sedimentary deposits contained in fluvial terraces, lakes, shelf edge clinoforms, canyon and fans throughout an entire glacioeustatic sea-level cycle. Under highstand conditions sediment shed from the actively rising New Zealand Alps is stored in lakes, terrestrial gravel fans, and on the shelf, where the sediment cover is a mixture of modern, relict and palimpsest deposits. Under lowstand conditions the rivers extend to canyon heads and discharge directly into the Bounty Trough. In the Waipaoa Sedimentary System sediment generated/mobilized from primary hillslope source areas by large magnitude, low frequency storm events is primarily sequestered on the floodplain and shelf.3) Active transfer among environments
Within the Waipaoa Sedimentary System, hinterland to shelf transport may be accomplished in tens to hundreds of hours. This permits attention to be focused on the conditions under which specific erosion events in the hinterland are translated to depositional sites on the floodplain and shelf. The major rivers of the Eastern South Island Sedimentary System constitute a line source and northward flowing, along-strike, currents influence sediment dispersal patterns offshore (sands are deposited on the inner shelf, muds move northward, and the Bounty Fan is inactive), though lakes now trap much of the sediment load of the Clutha and Waitaki rivers. The situation changes at lowstand, when the lakes are effaced by glaciation, the rivers discharge close to the head of Bounty Trough and sediment captured by submarine channels is transported by turbidity currents 900 km eastward to the Bounty Fan.
4) Closed system
A dearth of sediment injection points south of the Clutha River suggests external influences on the Eastern South Island Sedimentary System are minimal, but under contemporary highstand conditions it is open at the northern end. Thus to rationalize the modern sediment budget it is necessary to determine the volume of mud transport north towards the Hikurangi system. The system is essentially closed during lowstands, when the dispersal system can be traced from the hinterland to abyssal plain. The Bounty Fan is influenced by the deep western boundary current, but knowledge of the downcurrent drift provides a means of gauging these losses. The Waipaoa Sedimentary System is virtually closed under present highstand conditions (i.e., sediment budgets can be balanced from upland to shelf). Sediment is transported into the shelf depocenter by the prevailing along-margin circulation, but transport rates have yet to be resolved. The off-shelf loss of modern mud is small, and sediment escaping onto the continental slope is probably be retained within a collapse structure formed by collision of the accretionary prism with a seamount transported on the subducting Pacific Plate. Under lowstand conditions, the course of the Waipaoa River presumably extended to the shelf edge, though it remains to delineate the entire course of the ancestral river.
5) High resolution stratigraphic record
In the Waipaoa Sedimentary System high-resolution terrestrial (floodplain) and marine (shelf) sedimentary archives preserve a ~10 kyr long record of environmental change (periods of enhanced erosional activity and instability) in the Waipaoa River Basin. The well-developed tephra-stratigraphy will facilitate regional timeline correlation. In the Eastern South Island Sedimentary System the highstand trilogy of terrestrial gravel fan, proximal sand fan, distal mud fan is supplemented by a varved lacustrine record. Clinoforms and sediment drifts beneath the shelf provide an exceptional sequence stratigraphic record of global sea-level change during the last glacial cycle, as does the eustatically dominated (lowstand) input to the Bounty Fan.6) Presence of carbonate environments -- Not Applicable
7) Significant differences between the two sites (other focus areas)
The Waipaoa Sedimentary System is positioned in an active margin tectonic setting. The primary focus is on contemporary (Recent through Holocene) processes and on the conditions under which specific erosion events in the hinterland are translated to the plain and shelf systems, though the shelf-indenting canyon and basin floor fan that were major features of the low-stand system are also open to investigation. The Eastern South Island Sedimentary System discharges onto a passive margin with a broad continental shelf. The major rivers constitute a line source and northward flowing, along-strike, currents influence sediment dispersal patterns offshore. The strength of this subarea is the rich record of terrestrial events preserved in contiguous sedimentary deposits contained in fluvial terraces, lakes, shelf edge clinoforms, canyons and fans throughout an entire glacioeustatic sea-level cycle.
Human considerations:1) Background data and scientific infrastructure
New Zealand’s scientific infrastructure is well-developed, and the literature incorporates a plethora of reconnaissance scale studies and background information for both sub areas. The available hydrometeorological records are of similar length and quality to those available for US sites. Telemetry systems and satellite data provide real-time data for key terrestrial and marine sites in both sub-areas. There is complete conventional (topographic and geological) map and air photo (dating from 1939) coverage for both sub-areas, supplemented by a variety of digital data bases (including remotely sensed data), a 400-station GPS network, seismic and swath bathymetry (including 3.5 kHz) surveys, and deep and shallow borehole data. NIWA operates two oceanographic research vessels as well as inshore survey craft. The RV Tangaroa (built 1991, 70 m long, 2280 t) is a purpose built oceanographic vessel that accommodates 26 scientists in single berth cabins. She has full survey facilities including DGPS, 3.5khz, ADCP, three sounders, coring to 7000 m water depth, multichannel seismics, camera-video system, side-scan sonar, onboard labs, computer facilities, etc. The RV Kaharoa (built 1981, 28 m long, 300 t) is a coastal survey craft that accommodates six scientists. The vessel can deploy most acoustical and geophysical gear and is equipped with DGPS, onboard laboratory and computing facilities.2) Manageable logistics
US citizens require neither visas nor vaccinations to enter New Zealand, nor special permission to undertake research once there, and the country is politically stable. There are daily direct flights to Auckland, with frequent internal connections to regional airport in both sub-areas. Towns in both sub-areas offer a range of accommodation, and direct wireless phone/fax/modem access to the US is available via the Omnipoint network. Transient weather conditions represent the only constrain on access to headwater areas reached on foot, and there is easy access to most terrestrial locations by car/4-wheel drive vehicle. There are complete port facilities (e.g., Gisborne and Lyttelton) within or in close proximity to both sub-areas. There are, thus, no structural impediments to research activities that require a rapid response to high magnitude, low frequency events, such as cyclonic storms.
3) Definable anthropogenic influence
The extent to which artificial lakes moderate sediment budgets within the terrestrial portion of the Eastern South Island Sedimentary System is known from gauging and proxy records. In the Waipaoa Sedimentary System (by comparison with pre-settlement levels) sedimentation rates, as computed from lake and marine cores, doubled after the arrival of Polynesian settlers and increased by an order of magnitude after the arrival of European colonists.4) Societal relevance
Humans are postulated to be the most important geomorphic agent currently shaping the surface of the Earth, but is against the background of short-term environmental change that the impact of human activity has to be assessed, and future anthropogenically-induced change will occur. An immediate need is to determine how much sediment fluxes have changed on time scales of decades to millennia during the Holocene. The Waipaoa Sedimentary System possesses the high-resolution sedimentary sequences required to determine the magnitude of and response time to environmental change that occurs within this temporal scale. The dynamics of the land-sea interface are also of fundamental societal importance, and understanding the history of global sea level and its links to climate change are essential for predicting future sea-level trends. The rich record of terrestrial events preserved in contiguous sedimentary deposits within the Eastern South Island Sedimentary System afford a perspective on an entire glacioeustatic sea-level cycle.
5) Potential to leverage resources
Letters of support from Crown Research Institutes with complementary research objectives confirm the interest that New Zealand scientists have in forging linkages with the MARGINS Source-to-Sink community and engaging in collaborative research conducted in the New Zealand focus area. These letters indicate that there is a strong likelihood of significant funding inputs being made by the Crown Research Institutes towards MARGINS objectives. For example, from the marine perspective, two existing long-term, funded NIWA science programs would compliment and could be integrated with MARGINS research. This would provide MARGINS collaborators with some access to ship time and facilities.
Why New Zealand?
The New Zealand focus area provides a range of research opportunities that should encourage participation from the entire spectrum of the MARGINS Source-to-Sink community. Advantageous logistics, a wealth of high-quality background data, and a highly-developed scientific infrastructure mean that, from the outset, peripheral distractions will not diffuse effort and creative activities will be focused on achieving research goals rather than combating logistical obstacles.
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