boron isotopic compositions in growing corals from the south china sea abstract in order to determine incorporation of boron species boron isotopic fractionation and influence of trace elements on isotopic compositions of boron in corals 11bcoral concentrations of mg sr na b and 11bcoral in growing corals from the south china sea were measured relative to seawater sr enriched while mg depleted in corals in the south china sea although the 11bcoral values were different from various species and were not closely correlated with the element concentrations in corals in the south china sea mg oh 2 existed in corals can result in high 11bcoral thus it is necessary to examine the existence of mg oh 2 and to choose the same species when 11bcoral is used in the 11b ph proxy based on the measured 11b values of corals and coexisting seawater as well as the seawater ph in the south china sea a new isotopic fractionation factor a4 3 between b oh 4 and b oh 3 was determined to be 0 979 besides b oh 4 into corals our results showed that b oh 3 may also be incorporated into corals with variable proportions the incorporation of b oh 3 into corals may challenge the hypothesis of 11bcoral 11b4 resulting in increasing uncertainty to the calculated seawater ph values to the 11b ph proxy we suggested that a best fit empirical equation between 11b of bio carbonates and seawater ph needs to be established by the precipitation experiments of inorganic carbonates or culture experiments of corals or foraminifera

oceanographic and climatic changes over the past 160 000 years at deep sea drilling project site 594 off southeastern new zealand southwest pacific ocean high resolution textural carbonate microfossil and oxygen 18o and carbon 13c stable isotopic analyses are presented for the late quaternary isotopic stages 1 to 6 interval of a core at deep sea drilling project site 594 situated just south of the present subtropical convergence in northernmost subantarctic surface waters on the southern flank of chatham rise in the southwest pacific downcore alternations of pelagic and hemipelagic oozes correspond to interglacial and glacial episodes respectively interglacial oozes contain a northern subantarctic assemblage of planktonic foraminifera with rare cool subtropical species while glacial oozes are characterized by species typical of southern most subantarctic waters and include radiolaria with affinity for antarctic waters the planktonic 18o record for the site supports a 3 6 c temperature change in near surface waters between interglacial and glacial stages and indicates that during stage 5e the near surface waters were about 1 c warmer than at present a pronounced cooling during stage 5d matches that of the vostok ice core d record and negative excursions at the end of stages 2 and 6 support a southern latitude warming preceding northern hemisphere deglaciation benthic foraminifera typical of cold intermediate to deep waters can increase dramatically in abundance in the hemipelagic ooze intervals when 18o results suggest a temperature drop in glacial stage bottom waters of 2 4 c possibly a result of upward displacement of antarctic intermediate water by circumpolar deep water at the site the foraminiferal 13c records support a reduced influence of north atlantic deep water in the southwest pacific during glacial stages when nutrient enhancement occurred in both bottom and surface waters despite the pronounced changes between interglacial and glacial conditions inferred at site 594 contributed to by a substantial northward shift by at least 5 of latitude in the position of the antarctic convergence south of new zealand during glacial episodes we conclude that the subtropical convergence remained locked to chatham rise approximately 44 s throughout stages 1 to 6 major compression of the intervening belt of subantarctic water during glacial episodes and the associated very steep thermal gradients and intensified atmospheric and oceanic circulation patterns that developed in this part of the southwest pacific account for the harsh frigid environment reported for on land southern new zealand at these times

geostatistical analysis of seafloor depressions on the southeast margin of new zealand s south island investigating the impact of dynamic near seafloor processes on geomorphology abstract seafloor depressions are widespread on the present day continental slope along the southeast coast of new zealand s south island the depressions appear to be bathymetrically constrained to depths below 500 m correlating to the top of the gas hydrate stability zone and above 1100 m similar depressions observed on the chatham rise are interpreted to have formed as a result of gas hydrate dissociation leading to the hypothesis that a similar origin can be applied for the depressions investigated in this study our investigation however has found limited geophysical or geochemical evidence to support this hypothesis the objective of this paper is to examine whether a causal relationship can be established between potential mechanisms of depression formation and the present day seafloor geomorphology geostatistical analysis methods applied to multibeam bathymetry and interpretation of 3d seismic data have been used to empirically describe the geomorphology of the seafloor depressions and investigate potential correlations between geomorphology and other processes such as current flow along the shelf and slope in this region and underlying polygonal fault systems although the results of our analysis do not preclude that the seafloor depressions formed as a result of gas hydrate dissociation neither does our geophysical or geochemical evidence support the theory therefore we propose an alternative mechanism that may have been responsible for the formation of these structures based on the evidence presented in this study the most likely mechanism responsible for the formation of these seafloor depressions is groundwater flux related to the interaction of current systems and the complex geomorphology of submarine canyons on the southeast coast of the south island

plate boundary tectonics global sea level changes and the development of the eastern south island continental margin new zealand southwest pacific the great south basin is a mid cretaceous continental margin rift related to the break up of the new zealand sector of gondwanaland syn rift cretaceous paleogene sediment accumulated up to a thickness of 8 km followed by post rift subsidence of the northwest and southeast basin margins to form the canterbury basin and campbell plateau respectively the sedimentary evolution of the eastern south island continental margin is described within the context of four megasequences containing eight regionally recognizable seismic sequences syn rift phases 1 2 non marine graben fill post rift phases 3 6 epeiric marine transgression paraconformity phase 7 mid oligocene maximum transgression and syn orogenic phase 8 plate boundary driven terrigenoclastic regression the successive phases of this overall transgressive regressive cycle can be equated with the natural and progressive development of the sedimentary sequence within known regional factors the eastern south island margin sequence forms an appropriate southern hemisphere test of the global sea level curves of vail et al 1977 and haq et al 1986 although eustatic sea level cycles may have played some part in the differentiation of transgressive phases 3 7 the stratigraphic sequence is adequately interpreted in terms of intrinsic basin evolution within a local tectonic context

geological controls on seismic sequence resolution a seismic sequence analysis of the canterbury basin eastern south island of new zealand has illustrated the roles of subsidence sediment supply and current activity as controls on sequence resolution and architecture during basin evolution the rates of sediment supply and subsidence determine the background depositional regime transgressive or regressive and effectively determine the frequency response of the continental margin sedimentary section to input signals with a broad range of frequencies including eustasy a regressive progradational depositional regime and minimal current erosion favor the preservation of high frequency sequences particularly at fourth order level under less favorable conditions the record of sequences is incomplete or ambiguous such frequency response characteristics must be considered when inverting sequence records to derive the frequency of the input cyclicity and when making global comparisons of regional sequence stratigraphic studies clastic basins with simple subsidence histories and uniform or increasing rates of sediment supply develop from a transgressive phase characterized by ramplike major sequence boundaries to a mature progradational shelf phase with clinoform sequences and optimum sequence resolution the mature phase constitutes the preferred setting for sequence stratigraphic analyses

a salt oscillator in the glacial atlantic 1 the concept as shown by the work of dansgaard and his colleagues climate oscillations of one or so millennia duration punctuate much of glacial section of the greenland ice cores these oscillations are characterized by 5 c air temperature changes severalfold dust content changes and 50 ppm co2 changes both the temperature and co2 change are best explained by changes in the mode of operation of the ocean in this paper we provide evidence which suggests that oscillations in surface water conditions of similar duration are present in the record from a deep sea core at 50 n based on this finding we suggest that the greenland climate changes are driven by oscillations in the salinity of the atlantic ocean which modulate the strength of the atlantic s conveyor circulation

current controlled sediment deposition from the shelf to the deep ocean the cenozoic evolution of circulation through the sw pacific gateway the circulation of cold deep water is one of the controlling factors of the earth s climate forty percent of this water enters the world ocean through the southwest pacific as a deep western boundary current dwbc flowing northwards at bathyal to abyssal depths east of the new zealand microcontinent south of latitude 50 s the dwbc is intimately linked with the antarctic circumpolar current acc which is the prominent force for the shallow water circulation the pacific dwbc is presently the largest single contributor of deep ocean water and deciphering its evolution is of fundamental importance to understanding ocean and climate history and global ocean hydrography the evolution of the dwbc system and of related circum antarctic currents has taken place since 30 25 ma when plate movements created the first oceanic gaps south of australia and south america the stratigraphic record preserved in sediment drifts of the southwest pacific in eastern new zealand is the best available for deciphering the neogene history of southern ocean water masses and of the circulation of the acc dwbc and their precursor systems major current activity commenced on the new zealand margin in the late eocene or early oligocene hoiho drift early acc and was widespread by the mid late oligocene marshall paraconformity and weka pass limestone drift acc during the neogene the eastern south island continental shelf built seawards by accretion at its outer edge of large miocene current drifts up to tens of kilometres long and hundreds of metres thick canterbury drifts also commencing in the mid cenozoic but in depths 2000 m the dwbc emplaced large deep water sediment drifts rates of drift deposition accelerated considerably in the late neogene when climatic change and particularly glacial sea level falls caused the delivery of large volumes of turbiditic sediment into the path of the dwbc via the bounty and hikurangi channels

late quaternary development of left bank dominant levees in the bounty trough new zealand abstract the bounty trough encompasses an extensive submarine channel system that arises on the otago fan complex off the south island new zealand and extends 1000 km eastward to feed a large fan on the 4500 m deep pacific ocean floor channels are bordered by prominent levees which on the steep otago fans are mainly narrow sandy ridges by comparison the gently sloping floor of the trough supports broad low relief muddy levees in most cases levees display preferential development on the left side of the channel facing down channel we argue this preference results not only from the coriolis deflection of channel full turbidity currents but also from bottom currents and centrifugal forces operating at channel bends basement relief and locally high sedimentation rates at channel confluences may also influence levee height piston cores reveal two distinct phases of levee growth related to late quaternary climatic cycles during glacial times rivers discharged directly into the bounty trough turbidity currents travelled and overspilled the entire length of the bounty channels thereby producing the main phase of asymmetric levee growth and terrigenous deposition such turbidite deposition was punctuated by bouts of hemipelagic sedimentation in interglacial periods river borne sediment was diverted away from the trough which became the site of predominantly pelagic sedimentation represented by a drape of foraminiferal nannoplankton ooze over channels and levees alike however levees on the otago fan complex still received terrigenous sediment presumably reworked from the adjacent continental shelf

origin signature and palaeoclimatic influence of the antarctic circumpolar current the antarctic circumpolar current acc is today the strongest current in the world s ocean with a significant influence on global climate its assumed history and influence on palaeoclimate while almost certainly equally profound are here called into question in this paper we review 30 years of accumulated data interpretation and speculation about the acc deriving mainly from dsdp and odp drilling in the southern ocean for most of this time a conventional view of acc development signature and influence has held sway among palaeoceanographers and marine geologists in this view the acc began at about 34 ma close to the eocene oligocene boundary the time of onset of significant antarctic glaciation and the time of creation of a deep water gap tasmanian seaway between australia and antarctica as the south tasman rise separated from north victoria land this is the smoking gun of synchroneity the southern ocean sediment record shows a latest eocene development and subsequent geographic expansion of a siliceous biofacies its northern limit taken to indicate the palaeoposition of the acc axis in addition the acc was considered to have caused antarctic glaciation by isolating the continent within a cold water annulus reducing north south heat transport a different and later date for antarctic south american opening drake passage was proposed but the timing of acc onset there was disputed and the simple story survived recent developments however call it into question modern physical oceanography shows that all or most of present day acc transport is confined to narrow jets within deep reaching circumpolar fronts and numerical modelling has suggested that a steady reduction in greenhouse gas concentration through the cenozoic could cause antarctic glaciation with or without a contribution from ocean circulation change the rapidity of antarctic glacial onset at the eocene oligocene boundary and coeval creation of a deep water gap south of tasmania both survive but in light of the new information the presence of a siliceous biofacies cannot be claimed as evidence of the existence of a continuous deep reaching oceanic front and therefore of an acc and the possibility arises that cool and cold sea surface temperatures were effects of antarctic glaciation rather than evidence of a major contributor to its cause in considering future work we emphasise the importance of additional information from ancillary fields better definition of the necessary and sufficient properties of oceanic fronts additional determinations of cenozoic atmospheric pco2 and further developments in models of antarctic glaciation but also suggest the way forward in marine geology our knowledge of the development and palaeoclimatic significance of the acc will be best served by grainsize studies of bottom current strength at selected locations and geochemical or mineralogical studies of clays and ird as a way of examining provenance and therefore surface and bottom current directions and the existence of interocean connections

late quaternary sediment pathways through the deep ocean east of new zealand late quaternary sediment pathways through the deep ocean east of new zealand were determined using sand sized mica tracers in conjunction with physical oceanographic seismic and stratigraphic data results indicate that in glacial times micaceous sediment derived mainly from low grade metamorphic rocks of the eastern south island was delivered by rivers directly to bounty trough here turbidity currents transferred the sediment 1000 km eastward to the bounty fan on the 4500 m deep southwest pacific basin a western boundary current wbc flowing over the fan redirected the sediment 1000 km to the north and northwest where it eventually accumulated as a drift deposit near hikurangi trough the drift also received mica that had been transported 700 km along hikurangi trough from mainly plutonic sources on the northwestern south island interglacial episodes witnessed a major change in transport paths river borne sediment was diverted away from bounty trough which became a site of predominantly calcareous pelagic sedimentation denied the lateral injection of terrigenous sediment the wbc nevertheless continued to supply small quantities of mica to the drift through erosion of existing glacial deposits on the bounty fan the paucity of interglacial mica on the drift indicates it is also receiving little terrigenous sediment by way of hikurangi trough

continental shelf progradation by sediment drift accretion multi channel seismic profiles from the canterbury basin on the eastern margin of the south island of new zealand reveal the importance of current activity in shaping a neogene shelf sediment prism the shelf prism prograded across a broad near horizontal platform in water depths of 1 000 to 1 750 m the platform was formed above a condensed section of late eocene to late oligocene limestones which overlie cretaceous to paleogene rift fill and transgressive sediments the neogene sediment prism contains sediment drifts which are as much as 25 km long and 15 km wide and extend up to 1 600 m uncompacted vertically individual drifts migrated westward and can be traced between dip profiles revealing that the long axes of most are subparallel to the present coastline and shelf edge channel like features at the landward edges of the drifts correspond to residual space left between the landward prograding off shelf sediment drift and the adjacent shelf foreslope erosion or slow deposition characterized the foreslope progradation of the shelf was by the accretion of successive sediment drifts before ca 11 5 ma pink horizon the shelf edge parallel drifts were distributed across the central part of the basin whereas subsequently they were concentrated to the northeast the seismic architecture of the neogene sediment prism results from the interplay of an abundant western sediment source and an offshore boundary current system present day ocean circulation involves northward flow along the east coast of the south island the basin may have been subjected to a middle miocene to late pliocene phase of intensified flow caused by local topographic enhancement and or global paleoceanographic events current activity has played a crucial role in the sedimentary evolution of the canterbury basin neogene shelf prism

geography and history of the louisville hotspot chain in the southwest pacific the louisville ridge is a 4300 km long late cretaceous and cenozoic hotspot chain the south pacific equivalent of the hawaiian emperor chain its northwestern end is being consumed by the tonga kermadec subduction zone but seabeam and magnetic mapping shows that the chain still includes more than 60 major volcanoes of both normal and reversed polarity distributed along a 75 km wide band that can be fitted to small circles about three successive poles of pacific plate hotspot rotation this band obliquely crosses fracture zones of the eltanin system but there is little interaction and no evidence of any genetic connection between the two structures forty of the louisville volcanoes grew above sea level and are preserved as high latitude i e coral free guyots they are spaced less than 100 km apart along most of the chain but there are none within 750 km of the inferred present location of the hotspot beneath a swell at the southeast end of the chain the rate of volcano building by the louisville hotspot declined sharply about 20 m y ago after being remarkably constant at 3 4 103 km3 m y for the previous 50 m y and none of the louisville volcanoes built during the past 10 12 m y the time of most profuse hawaiian volcanism has reached sea level however a seamount from which pleistocene lavas were dredged rises to within 540 m of the sea surface from the crest of the hotspot swell at 50 5 s 139 2 w guyot heights demonstrate that a hotspot swell several hundred meters high has persisted throughout the known life of the chain and sometimes had an isostatic depression on its loaded crest the depths of guyot shelf breaks increase systematically to the northwest from 530 to 2100 m proportional to the square root of volcano age with some scatter and tilting caused by plate boundary tectonism and local isostatic subsidence and uplift

late quaternary paleoclimatic and paleoceanographic change over northern chatham rise new zealand four cores from the lower northwestern flank 3010 3726 m water depth of the chatham rise east of new zealand penetrated a post glacial to last glacial sequence of mainly hemipelagic muds an accurate time frame for this study is provided by well dated tephra layers and additional 14c dates a hiatus is detected at the core tops of all cores studied with the last 900 to 1000 years missing sedimentological and micropaleontological analyses reveal that 1 1 isotope stage 2 ended ca 15 000 yr b p after a transitional period with a fluctuating composition of planktonic foraminifera assemblages and fluctuating isotopic values more stable post glacial conditions were established at ca 10 000 yr b p 2 2 from the end of isotope stage 2 to isotope stage 1 accumulation rates of diatoms aeolian quartz and calcareous microfossils decreased stepwise by a factor of 10 4 and ca 1 5 respectively in response to decreasing wind intensities and upwelling 3 3 in spite of significant climatic and oceanographic changes from the last glacial to the present the chatham rise has remained the northern distributional boundary for endemic subantarctic diatoms this implies that east of new zealand the subtropical convergence has remained bound or close to the chatham rise during both glacial and post glacial times 4 4 sediments are slight influences of contour currents or turbidity currents are recognised only in sediments younger than 16 000 years

spatiotemporal analyses of earthquake productivity and size distribution observations and simulations we use relocated catalogs of microearthquakes to investigate earthquake interaction along sections of the sargent calaveras and san andreas faults in cali fornia we examine the stress dependence of seismicity rate change along the three fault segments and find that the seismicity rate following a mainshock decays ap proximately as 1 time the duration of the aftershock activity seems to be independent of distance from the mainshock and the seismicity rate at lag times of up to about 100 sec is nearly constant in the san andreas and the calaveras catalogs where the return of the seismicity rate to the background level is well resolved we find that the return to the background in the distance range of 1 2 rupture radii from a previous earthquake is preceded by a period during which the seismicity rate falls about 30 below the background rate we also examine the effect of a stress step on earthquake size distribution along these faults and find that the exponent of the power law dis tribution of earthquake magnitudes within 10 4 sec of a previous earthquake is sig nificantly lower than that of the long term while the 1 time decay of seismicity rate the independence of aftershock duration from distance from the mainshock and the constant seismicity rate at short lag times are predicted by dieterich s 1994 model the decrease of seismicity rate below the background level and the changes in earth quake size distribution are not for comparison with our observations we simulate earthquake activity on an in herently discrete fault model that is governed by an approximate constitutive friction law similar to the one used by dieterich 1994 we find that the observed response of earthquake productivity and size distribution to a stress step are produced by these simulations the effect of the mainshock is not only to raise the local seismicity rate but also to systematically modify the earthquake size distribution this is because fault patches that are near failure at the time of the stress step are strengthened whereas fault patches that at the same time were far from failure are weakened as a result similar to what is observed for the calaveras and the san andreas segments late during the aftershock sequence the seismicity rate may decrease below the back ground rate we suggest that the time dependent modification of the earthquake size distribution by a stress step can explain observations of lower b values immediately following a stress step

development of sediment drifts approaching an active plate margin under the sw pacific deep western boundary current the abyssal pacific ocean is fed by a 1000 km wide deep western boundary current dwbc that flows northward along the continental margin east of new zealand between the passive margin of chatham rise and the subduction zone of kermadec trench a distance of 1200 km the dwbc has formed a suite of sediment drifts over a depth range of 2200 5700 m airgun and 3 5 khz profiles record a variety of drift types that reflect regional variations in bathymetry sediment supply and the tectonic volcanic framework on chatham rise the dwbc has deposited a sinuous linear body along the south flank 3000 m an extensive apronlike drift on the north flank 2200 4500 m and a ridgelike drift about the rise base 4500 5200 m the flow has also deposited a body of sediment over 400 km long within a moat at the base of the nearby louisville seamount chain further downcurrent the 250 km long rekohu drift 3600 4190 m has developed northward to 39 s south of this latitude drifts comprise mainly reworked pelagic hemipelagic material and sediment transported from distant southerly sources in contrast drifts north of 39 s have received a major injection of terrigenous sediment from hikurangi channel which runs 1400 km from new zealand eastward across the hikurangi plateau to disgorge on to the abyssal floor at the plateau edge en route turbidity current overspill from the channel has moved north under the influence of the shallow dwbc to contribute to a series of small ridge and patch drifts among the numerous seamounts on the plateau at 3500 4200 m off hikurangi channel mouth a large fan has accumulated the dwbc has extended the fan into a drift running over 250 km along the base of hikurangi plateau 5150 5770 m toward kermadec trench here drift sediment becomes increasingly disrupted by mass wasting associated with the active subduction in this area the seismic stratigraphy reveals the drifts to rest mainly on a widespread erosional surface that is interpreted to mark the inception of the dwbc in the region with the late oligocene opening of the australian antarctic seaway drift construction commenced during the miocene but was punctuated in the late miocene by another period of erosion that coincided with increased bottom water production in antarctica deposition resumed in plio pleistocene times when large quantities of sediment from the rapidly rising landmass of new zealand were injected into the boundary current the modern flow continues to affect drift deposition as manifest by an active boundary channel along the foot of hikurangi plateau and widespread scour zones and sediment wave fields

unlocking the ice house oligocene miocene oxygen isotopes eustasy and margin erosion oxygen isotope records and glaciomarine sediments indicate at least an intermittent presence of large continental ice sheets on antarctica since the earliest oligocene circa 35 ma the growth and decay of ice sheets during the oligocene to modern ice house world caused glacioeustatic sea level changes the early eocene was an ice free greenhouse world but it is not clear if ice sheets existed during the middle to late eocene doubt house world benthic foraminiferal 18o records place limits on the history of glaciation suggesting the presence of ice sheets at least intermittently since the earliest oligocene the best indicator of ice growth is a coeval increase in global benthic and western equatorial planktonic 18o records although planktonic isotope records from the western equatorial regions are limited subtropical planktonic foraminifera may also record such ice volume changes it is difficult to apply these established principles to the cenozoic 18o record because of the lack of adequate data and problems in stratigraphic correlations that obscure isotope events we improved oligocene to miocene correlations of 18o records and erected eight oxygen isotope zones oi1 oi2 mi1 mi6 benthic foraminiferal 18o increases which are associated with the bases of zones oil circa 35 8 ma oi2 circa 32 5 ma and mil circa 23 5 ma can be linked with 18o increases in subtropical planktonic foraminifera and with intervals of glacial sedimentation on or near antarctica our new correlations of middle miocene benthic and western equatorial planktonic 18o records show remarkable agreement in timing and amplitude we interpret benthic planktonic covariance to reflect substantial ice volume increases near the bases of zones mi2 circa 16 1 ma mi3 circa 13 6 ma and possibly mi5 circa 11 3 ma possible glacioeustatic lowerings are associated with the 18o increases which culminated with the bases of zone mi4 circa 12 6 ma and mi6 circa 9 6 ma although low latitude planktonic 18o records are required to test this these inferred glacioeustatic lowerings can be linked to seismic and rock disconformities for example we link 12 oligocene early late miocene inferred glacioeustatic lowerings with 12 of the sequence boundaries inferred eustatic lowerings of haq et al 1987

regional sediment recycling in the abyssal southwest pacific ocean an active plate boundary with a high sediment output three major submarine channels and the world9s largest deep western boundary current dwbc make up an extensive recycling system along the 4500 km continental margin east of new zealand seismic reflection sedimentary and oceanographic data demonstrate that detritus from the rising mountains of the new zealand plate boundary is transferred to the southwest pacific abyssal floor by turbidity currents flowing along solander 450 km long bounty 950 km and hikurangi 1400 km channels these conduits discharge directly into the dwbc which transports material north to form a series of sediment drifts the northernmost drift containing sediment from hikurangi channel and eroded drifts to the south is now subducting into kermadec trench geochemical data suggest that sediment is recycled through the mantle to re emerge in the arc volcanic rocks thus one cycle is completed and a new one begins

the mid oligocene marshall paraconformity new zealand coincidence with global eustatic sea level fall or rise the marshall paraconformity is a regional mid oligocene unconformity that occurs widely in shelf sequences from the eastern new zealand passive continental margin the paraconformity represents a break in sedimentation of at least 3 m y and coincided with a regional sea level highstand in contrast to the prediction of global lowstand at 29 m y b p similar paraconformities are widespread on other southern continents deep sea hiatuses of similar age occurred in the adjacent southwest pacific ocean basins they may or may not have a common origin with the hiatus associated with the marshall paraconformity possible causes for the paraconformity and its associated glauconitic and bioclastic sediments are sea level fluctuations including those related to local geoidal effects or erosive bottom waters stemming from the development of antarctic glaciation or from splitting of the southern continents

marshall paraconformity a mid oligocene record of inception of the antarctic circumpolar current and coeval glacio eustatic lowstand abstract the sedimentary fill of the canterbury basin new zealand is the product of a long term 80 ma tectonically controlled relative sea level cycle with a megasequence geometry analogous to the sequence stratigraphic model of vail am assoc petrol geol stud geol no 27 1 1 10 1987 the condensed section of the megasequence resolvable in detail in outcrop and on seismic profiles comprises a basin wide pelagic to hemipelagic limestone interval a regional mid oligocene unconformity the marshall paraconformity lies within the limestone interval onshore and correlates with hiatuses in at least two and possibly three offshore exploration wells and with a temporary lithological change from limestone to quartz sand at a fourth strontium isotopic age estimates confirm that a 2 4 ma hiatus is associated with onshore outcrops of the marshall paraconformity between 32 and 29 ma which correlates with the opening of the pacific sector of the southern ocean and the postulated mid oligocene sea level fall of haq et al science235 1156 1167 1987 spec publ soc econ paleonotol mineral no 42 71 108 1988 lowering of base level coupled with cooling and enhancement of current activity may have caused the temporary cessation of limestone deposition and a regional hiatus this hypothesis reconciles the apparently contradictory palaeogeographical evidence for a regional highstand the marshall paraconformity may exemplify the signature by which similar glacio eustatic events can be recognized in offshore platform facies

evolution of pliocene to recent abyssal sediment waves on bounty channel levees new zealand abstract levees bordering bounty channel 900 km east of new zealand accommodate a 400 m thick sequence maximum of sediment waves that have formed since pliocene times these bedforms with amplitudes of 2 17 m and wavelengths of 0 6 6 km occur in 4100 4900 m of water and were formed by turbidity currents as indicated by their restriction to levee backslopes the frequent occurrence of turbidites in cores and the preferential but not exclusive development of waves on the left bank levee in accord with the southern hemisphere coriolis deflection the wave field was instigated in the late pliocene when glacially lowered sea level allowed rivers draining the southern alps of south island to discharge directly into bounty channel and its attendant canyons the field grew vertically through the coalescence of small waves into larger bedforms that continually migrated across and up levee backslopes at an average rate of 5 6m 100 yrs wave growth decreased into the late pleistocene probably in response to progressive containment of turbidity currents as the relief of bounty channel increased to 200 m or more the glacial periods of wave growth were interrupted by interglacial interludes of quiescence when the field was draped mainly by pelagic calcareous ooze