Wanganui Basin Research Articles

New early Miocene species of the cheilostome bryozoan Microporella from the South Island of New Zealand

Since its first appearance in the early Miocene, the cheilostome bryozoan genus Microporella has been cosmopolitan, recorded from most continents. However, Miocene Microporella records in New Zealand are scarce, and currently limited to a single middle Miocene species identified as Microporella hyadesi (a Recent bifoliate erect form originally described from Cape Horn and Tierra del Fuego) from the Mt. Brown ‘E’ Limestone Formation of North Canterbury. Here, we describe and illustrate three new early Miocene (Otaian–Altonian New Zealand stages corresponding to the Aquitanian–Burdigalian) species, namely Microporella incurvata sp. nov., M. gladirostra sp. nov. and M. whiterocki sp. nov., that represent the geologically oldest regional examples of the genus to date. A fourth species is left in open nomenclature because complete ovicells are not preserved in the only recovered specimen. The colonies of Microporella were collected from several rock formations exposed in limestone quarries on the South Island. The three new species share ovicells with a personate structure, but differ in the appearance of the ooecial surface (evenly pseudoporous versus imperforate), shape of the ascopore opening (cribrate versus non-cribrate), number of oral spine bases, and shape of the avicularian rostrum and crossbar. We also illustrate for the first time ovicells of another fossil species, Microporella rusti, from the Pleistocene Nukumaru Limestone Formation of the Wanganui Basin on the North Island. The ovicells of this taxon are rare, being found in only six of several hundred specimens collected to date. The ovicells of M. rusti are also very large, covering the entire orifice of the maternal zooid, similar to those of some other Microporella species all characterized by erect bifoliate colonies contrasting with the encrusting colonies of M. rusti. http://zoobank.org/urn:lsid:zoobank.org:pub:5430969F-B75B-40E7-8F65-74CDC87AA662 Mali H. Ramsfjell [m.h.ramsfjell@nhm.uio.no] Natural History Museum, University of Oslo, Blindern, PO Box 1172, Oslo 0318, Norway; Paul D. Taylor [p.taylor@nhm.ac.uk] Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, UK; Emanuela Di Martino [e.d.martino@nhm.uio.no] Natural History Museum, University of Oslo, Blindern, PO Box 1172, Oslo 0318, Norway.

New bryozoan species from the Pleistocene of the Wanganui Basin, North Island, New Zealand

Three new fossil bryozoan species, a ctenostome and two cheilostomes, are described and figured from Pleistocene strata of the Wanganui Basin, New Zealand. Buskia waiinuensis sp. nov., a soft-body ctenostome preserved as a mould bioimmuration, is the first fossil record of the genus from New Zealand. Microporella rusti sp. nov., which is notable for the lack of ooecia in the large suite of colonies available, is one of the most common bryozoans in the Nukumaru Limestone and Nukumaru Brown Sand shellbeds, forming large encrusting sheet-like colonies, but is uncommon in younger beds. Rare, small-sized colonies of Parkermavella columnaris sp. nov. were found as fossil in two Quaternary beds, the Nukumaru Limestone and the Upper Kai-Iwi Shellbed, and also alive on rocks from the greater Cook Strait area.

Crust and upper-mantle structure of Wanganui Basin and southern Hikurangi margin, North Island, New Zealand as revealed by active source seismic data

Crust and upper-mantle structure of Wanganui Basin and southern Hikurangi margin, North Island, New Zealand as revealed by active source seismic data

Interspecific interactions through 2 million years: are competitive outcomes predictable?

Ecological interactions affect the survival and reproduction of individuals. However, ecological interactions are notoriously difficult to measure in extinct populations, hindering our understanding of how the outcomes of interactions such as competition vary in time and influence long-term evolutionary changes. Here, the outcomes of spatial competition in a temporally continuous community over evolutionary timescales are presented for the first time. Our research domain is encrusting cheilostome bryozoans from the Wanganui Basin of New Zealand over a ca 2 Myr time period (Pleistocene to Recent). We find that a subset of species can be identified as consistent winners, and others as consistent losers, in the sense that they win or lose interspecific competitive encounters statistically more often than the null hypothesis of 50%. Most species do not improve or worsen in their competitive abilities through the 2 Myr period, but a minority of species are winners in some intervals and losers in others. We found that conspecifics tend to cluster spatially and interact more often than expected under a null hypothesis: most of these are stand-off interactions where the two colonies involved stopped growing at edges of encounter. Counterintuitively, competitive ability has no bearing on ecological dominance.

The Moho in Australia and New Zealand

Australia and New Zealand share in part a history in the Gondwana supercontinent. Australia has a long and complex tectonic history with the last major accretion in the early Paleozoic, whereas New Zealand is still undergoing major plate boundary processes. The Australian continent is relatively well covered with both active and passive seismic techniques. Multiple sources of information are therefore available for building a model of Moho depth. Results from on-shore and off-shore refraction experiments are supplemented by receiver functions from a large number of portable stations and the recently augmented set of permanent stations. Moho picks from more than 10 500 km of full-crustal reflection profiles provide valuable additional constraints. The composite data set provides good sampling of much of Australia, though coverage remains low in some remote desert areas. The various datasets provide multiple estimates of the depth to Moho in many regions, and the consistency between the different techniques is high. Some of the thinnest crust lies beneath the Archean craton in the Pilbara, and in the neighbourhood of the Simpson desert. Thick crust is encountered beneath parts of the Proterozoic in Central Australia, and beneath the Paleozoic Lachlan fold belt in southeastern Australia. There are a number of zones of sharp contrast in depth to Moho, notably in the southern part of Central Australia. Despite most of the continental material around New Zealand being submerged, Moho data for this region is mainly onshore concentrating on the Australia–Pacific plate boundary. Two major wide-angle reflection transects provide the bulk of the active source data with just a few traditional reflection profiles offshore. The plate boundary provides an abundance of local earthquakes for tomographic imaging and this data is supplemented with receiver functions from both portable and permanent networks. Onshore the combined coverage is as dense as that of Australia, although it could be argued that a higher spatial resolution is required to capture the nature of the Moho of tectonically active New Zealand. Three regions of thickened crust can be identified, one beneath the Southern Alps, another beneath Fiordland, and below the Wanganui Basin between the North and South Island. Thin crust is identified west of the volcanic arc, with extensive underplating below the back-arc region.

Marine Mollusca of the last 2 million years in New Zealand. Part 5. Summary

Evolution occurred equally through allopatric speciation and anagenesis during Pliocene–Early Pleistocene time. However, no anagenesis is observed in Castlecliffian to Recent Mollusca. Possible explanations include (1) increasing shallowness and energy of the environment (Sheldon's ‘plus ça change’ model); (2) lecithotrophs increased with falling temperatures, and are more susceptible than planktotrophs to allopatric speciation; (3) ‘plus ça change’ implies that anagenesis continued unabated, but unrecorded, offshore from the recorded Pleistocene fauna. Three lineages (Pelicaria vermis, Stiracolpus symmetricus, Aoteadrillia wanganuiensis) became highly variable as a result of their changed developmental mode. Origination continued throughout Castlecliffian–Haweran time at a background rate of 0–2 new species per oxygen isotope stage (OIS), whereas extinction and range extensions of warm-water taxa to Wanganui Basin both reached a peak in OIS 9–7. However, the faunal record results from the interplay of temperature change and the preservation of different depositional environments. Biostratigraphy and temperature change also are reviewed.

Plio-Pleistocene Bryozoan faunas of the Wanganui Basin, New Zealand: stratigraphic distribution and diversity

The New Zealand continental shelf has a rich bryozoan fauna, and Bryozoa are relatively common as fossils in local marine-deposited strata. We record 103 species of Bryozoa occurring in the Wanganui Basin sequence spanning the Plio-Pleistocene (3.0–0.3 Ma), 77 of which belong to the order Cheilostomata and the remainder to the order Cyclostomata. The majority are conspecifics of the modern New Zealand shelf fauna. Palaeoecological characteristics of Wanganui Basin bryozoan faunas including diversity, taxonomic and colony growth-form combinations are identified, defining three biofacies groupings corresponding to depositional sequences (systems tracts). Water depth, sedimentation, substratum characters and taphonomic processes have all influenced the distribution and preservation of bryozoans in the local fossil record. Broader stratigraphic variations in Wanganui Basin faunas are explained by ‘habitat tracking’: a lateral shifting of bryozoan palaeocommunity zones in response to changing environments related to Plio-Pleistocene glacio-eustatic sea-level fluctuations.

Landscape history of the Marlborough Sounds, New Zealand

The Marlborough Sounds is a large and spectacular network of rias or drowned valleys. The Sounds were formed by stream incision and dissection of uplifting rocks followed by subsidence and marine incursion into the valleys. The evolution of the Marlborough Sounds is examined using a combination of seismic-reflection lines from the offshore Wanganui Basin, topographic data and seafloor bathymetry. Drainage incision produced valleys and ridges in the Sounds that were part of a mountainous region which reached maximum altitudes of c. 2000 m and extended northwards across the basin to the Wanganui coast. Sedimentary strata as old as c. 5 Ma infill this palaeotopography in the basin, suggesting that topography in the Sounds formed primarily during the Miocene. This topography started to submerge in the Sounds at c. 1.5 Ma when the Wanganui Basin stepped southwards by about 50 km. At the present outer edge of the Sounds, the average rates of subsidence since this time have been low (c. 0.3 mm a−1) and the resulting accommodation space in the valley system filled by sediments. As a consequence of sedimentation, the absolute altitude of valley floors remained approximately constant relative to a fixed earth datum. Thus, while subsidence was important for bringing strongly dissected topography close to sea level, isostatic sea-level rises are inferred not to have been responsible for submergence of the Sounds region. Instead, marine inundation may have been primarily driven by eustatic sea-level changes which resulted in ephemeral submergence during interglacial periods when sea levels were highest. The present coastline in the Marlborough Sounds formed c. 7 ka ago, and similar highstand sea-level configurations probably occurred about every 100 ka throughout the Late Quaternary.

Marine Mollusca of isotope stages of the last 2 million years in New Zealand. Part 3. Gastropoda (Vetigastropoda - Littorinimorpha)

Three new species: Grandicrepidula hemispherica (Nukumaruan, S Hawke's Bay), Pelicaria granttaylori (Mangapanian-early Nukumaruan, Wanganui – Hawke's bay), Pelicaria arahura (Waipipian-early Mangapanian, Westland and Hawke's Bay). Drawings of marine species in Smith's (1874) three plates of New Zealand molluscan types are republished. Further Australian molluscs in Wanganui Basin: Sabia australis (Lamarck), Clanculus plebejus (Philippi), both early Nukumaruan. Further northern New Zealand mollusc in Wanganui Basin: Stephopoma roseum (Quoy & Gaimard), OIS 13, 9. Distinctive gastropods extinct at end Nukumaruan: Struthiolaria frazeri (Hutton), Taxonia suteri (Marwick). Taniella planisuturalis (Marwick) (Opoitian-Nukumaruan, southern NZ) and Trivia (Ellatrivia) zealandica (Kirk) (Nukumaruan, Hawke's Bay-Wanganui; Castlecliffian, North Canterbury) occur in Castlecliffian (OIS 15?) rocks at Whakatane. Cantharidella tessellata (A. Adams) and Risellopsis varia (Hutton), formerly Haweran, are recorded from Nukumaruan and Castlecliffian rocks, respectively. New fossil late Nukumaruan-early Castlecliffian records listed from Mikonui-1 offshore well, Westland, include Malluvium calcareum (Suter) and 10 other species. Other biostratigraphically useful gastropods: Calliostoma (Maurea) nukumaruense (Laws) (Mangapanian-OIS 17); Argobuccinum pustulosum (Lightfoot), Semicassis labiata (Perry) (both earliest in OIS 7). New synonymy: Zeacumantus perplexus (Marshall & Murdoch) = Z. lutulentus (Kiener); Pelicaria vermis (Martyn) = all named Nukumaruan-Recent forms (other than P. rugosa (Marwick) and P. granttaylori n. sp.); Trivia flora Marwick = T. zealandica Kirk. Taxonomy revised: Zelippistes benhami (Suter) (OIS 13 & 9 at Wanganui), distinguished from Lippistes and Separatista; Stiracolpus species, informally; Maoricrypta profunda (Hutton), Waipipian-early Castlecliffian (- OIS 19); M. radiata (Hutton) (= incurva Zittel, = hochetteriana Woods, = wilckensi Finlay), (Middle Miocene?) Tongaporutuan-Opoitian; Eunaticina cincta (Hutton), holotype illustrated.

Holocene subsidence at the transition between strike-slip and subduction on the Pacific-Australian plate boundary, Marlborough Sounds, New Zealand

This paper provides the first solid evidence in support of a century-old hypothesis that the mountainous Marlborough Sounds region in central New Zealand is subsiding. More recent hypotheses suggest that this may be a result of southward migration of a slab of subducted Pacific Plate causing flexural downwarping of the overlying crust in the vicinity of the transition between subduction and strike-slip on the Pacific-Australian plate boundary. The proxy evidence for gradual Holocene subsidence comes from micropaleontological study of seven intertidal sediment cores from the inner Marlborough Sounds (at Havelock, Mahau Sound and Shakespeare Bay). Quantitative estimates (using Modern Analogue Technique) of former tidal elevations based on fossil foraminiferal faunas provide evidence of tectonic (not compaction-related) subsidence in all cores. Estimates of subsidence rates for individual cores vary within the range 0.2–2.4 m ka −1. The wide variation within subsidence rate estimates are related to a combination of the accuracy limits of radiocarbon dates, elevation estimates, and particularly our poor knowledge of the New Zealand Holocene sea-level curve. The most consistent subsidence rate at all three sites for the mid-late Holocene (last 6–7 ka) is ∼0.7–0.8 m ka −1. This rate is consistent with the average subsidence rate in the adjacent 4-km thick Wanganui sedimentary basin for the last 5 myr. Subsidence is inferred to have migrated southwards from the Wanganui Basin to impinge on the inner Marlborough Sounds in just the last 100–200 ka.

Late Miocene turnover of molluscan faunas, New Zealand: Taxonomic and ecological reassessment of diversity changes at multiple spatial and temporal scales

Previous estimates of molluscan biodiversity during the Neogene in New Zealand have revealed a pattern of significant diversity decline during the latest Miocene. This study uses an occurrence-based dataset (derived from published literature) to examine this pattern in further detail with both traditional synoptic measures and subsequent sampling standardization. Synoptic analyses of regional-scale data reveal diversity trajectories similar to those previously established, and rarefaction techniques demonstrate that the inferred pattern of Late Miocene diversity decline and Early Pliocene rebound is only somewhat biased by variations in sampling intensity. The pattern is also affected by temporal variations in the preservation of shelf paleoenvironments, with representation of highly diverse shallow-water facies varying considerably between biostratigraphic stages. The sampling and corresponding synoptic genus richness of biogeographic areas within the New Zealand region is also uneven. Within a single sedimentary basin sampled at high temporal resolution (Wanganui Basin), new data suggest that Late Miocene faunas were as diverse as those of the Early Pliocene, contrasting with the pattern of diversity decrease and subsequent increase observed at the regional spatial scale in prior studies. However, an ecological analysis of the new data demonstrates that turnover and restructuring of benthic communities did take place at the end of the Late Miocene. These findings suggest that spatial scale and temporal resolution are important considerations in determining the magnitude of diversity and ecological change observed in paleontologic studies. Contemporaneous biodiversity may not be similarly expressed at varying spatial or temporal scales in the fossil record. Rather, regional patterns of diversity change are governed by a complex interplay of not only large-scale environmental factors (e.g. temperature control), but also independent basin-scale processes (e.g. basin subsidence) and local sampling intensity.

Late Pleistocene sea-level oscillations (MIS 10–2) recorded in shallow marine and coastal plain sediments of the southern Wanganui Basin, New Zealand

The northern Wanganui Basin, New Zealand, is one of the key global sites for understanding marine cyclic sedimentation during the Quaternary. This paper presents the first evidence of marine cyclic sedimentation from its central-southern parts. Sedimentological, micropalaeontological and palynological analyses on a 280-m-deep borehole encountered units dating back to MIS 10. The sequence includes four marine cycles spanning MIS 9–5, which are overlain by terrestrial fluvial aggradation surfaces dating from MIS 4–2. Each marine unit represents a progressively shallowing depositional environment from the mid-shelf to coastal plain. This is overlain by a terrestrial sequence of lowstand fluvial terraces. Localized fault movements appear to have influenced the sedimentary character of the sequence during MIS 7a and 5e producing basement highs which provided protection to the shoreline. The cyclothems described in this paper now extend the already extensive, previously described record from MIS 17–10 to produce a combined eustatic record of Quaternary sea level change within the basin to MIS 5. They also provide an excellent example of the sedimentary response of a coastal basin to a progressive loss of sedimentation accommodation space.

Constraints on the amplitude of Mid-Pliocene (3.6–2.4 Ma) eustatic sea-level fluctuations from the New Zealand shallow-marine sediment record

Ice-volume calibrations of the deep-ocean foraminiferal delta18O record imply orbitally influenced sea-level fluctuations of up to 30m amplitude during the Mid-Pliocene, and up to 30 per cent loss of the present-day mass of the East Antarctic Ice Sheet (EAIS) assuming complete deglaciation of the West Antarctic Ice Sheet (WAIS) and Greenland. These sea-level oscillations have driven recurrent transgressions and regressions across the world's continental shelves. Wanganui Basin, New Zealand, contains the most complete shallow-marine Late Neogene stratigraphic record in the form of a continuous cyclostratigraphy representing every 41 and 100ka sea-level cycle since ca 3.6Ma. This paper presents a synthesis of faunally derived palaeobathymetric data for shallow-marine sedimentary cycles corresponding to marine isotope stages M2-100 (ca 3.4-2.4Ma). Our approach estimates the eustatic sea-level contribution to the palaeobathymetry curve by placing constraints on total subsidence and decompacted sediment accumulation. The sea-level estimates are consistent with those from delta18O curves and numerical ice sheet models, and imply a significant sensitivity of the WAIS and the coastal margins of the EAIS to orbital oscillations in insolation during the Mid-Pliocene period of relative global warmth. Sea-level oscillations of 10-30m were paced by obliquity.

Characterizing earthquake recurrence parameters for offshore faults in the low‐strain, compressional Kapiti‐Manawatu Fault System, New Zealand

Seafloor fault scarps and near‐surface deformation of late Quaternary seismic reflectors occur along the eastern margin of the Wanganui Basin, 200 km behind the active Hikurangi subduction front, southern North Island, New Zealand. The offshore scarps are associated with the low‐strain, compressional Kapiti‐Manawatu Fault System (KMFS), which comprises high‐angle (>60°) reactivated reverse and normal faults oriented NE‐SW, highly oblique to the coast. Seafloor scarps range from <10 to 50 km in length with vertical seafloor offsets of 2 to 30 m. The longest structure is the Mascarin Fault, with maximum late Quaternary vertical slip rates of 3 mm a−1 (where a is years). Other faults in the KMFS have typical rates of <1 mm a−1, comparable to long‐term estimates. Three zones of recent deformation are identified: faults in the north and south of the KMFS are characterized by high scarps (>10 m high) and moderate to long fault seafloor rupture lengths, and those in central parts of the fault system are characterized by low scarps (<5 m high), variable slip rates, and short to moderate fault lengths. Empirical equations indicate that KMFS faults may generate earthquakes with moment magnitudes (Mw) of 5.7–7.5 (mean 6.9 ± 0.3, ±1 standard deviation, for sources with Mw ≥ 6.5). Estimated recurrence intervals that are generally >10,000 a, suggest that the seismic hazard of the Kapiti‐Manawatu region is relatively low. Incorporation of these new geological data, however, is likely to increase slightly the expected seismic hazard in southern North Island. The method of determining the earthquake recurrence parameters of offshore faults has potentially wider applications elsewhere.

PALEOECOLOGY OF LATE MIOCENE-EARLY PLIOCENE SIXTH-ORDER GLACIOEUSTATIC SEQUENCES IN THE MANUTAHI-1 CORE, WANGANUI-TARANAKI BASIN, NEW ZEALAND

The Manutahi-1 core of Wanganui Basin, New Zealand, provides a special opportunity to investigate variations in depositional paleoenvironments by way of high-resolution paleoecologic and sedimentologic studies in a succession that accumulated under the influence of late Neogene rapid glacioeustatic sea-level oscillations. Thirty cyclothems of the Matemateaonga Formation (late Miocene–early Pliocene) have been identified through a 960 m interval of the core. Temporal patterns in both lithofacies distribution and macrofaunal paleoecology indicate a close link with environmental shifts that took place over the duration of these sixth-order sea-level cycles (∼41 kyr duration). Because foraminiferal assemblages have proven to be depauperate in the Manutahi-1 core, macrofossils, especially bivalves and gastropods, are used as the primary source of paleoecologic data. The paleoenvironmental analyses presented here have relied extensively on the transfer of ecologic data from the modern fauna to extant or closely related species observed in the core. Most notably, a relative sea-level curve has been constructed for the Matemateaonga Formation from analysis of bathymetric data for appropriate modern taxa. A range of exploratory statistical approaches, including cluster and ordination techniques, allow rapid, objective, and reproducible classification of macrofaunal associations and the elucidation of large-scale paleoenvironmental patterns. Several implications arise from this study. First, sequence boundaries and other key surfaces are confidently identified on the basis of integrated sedimentologic, taphonomic, and paleoecologic investigation. Second, changes in depositional environment and the responses of benthic communities are revealed at a range of temporal scales. The temporal pattern of paleoecologic change appears to be the result of lateral, facies-related shifting of biofacies (habitat tracking) in response to sea level, sediment flux, and other associated paleoenvironmental variables. Temporal patterns of biofacies occurrence and diversity are, however, strongly overprinted by stratigraphic and taphonomic processes.

Marine Mollusca of oxygen isotope stages of the last 2 million years in New Zealand. Part 2. Biostratigraphically useful and new Pliocene to recent bivalves

New Zealand Plio‐Pleistocene bivalves revised: Limopsis, oysters, Pecten, Mactra (Maorimactra), Oxyperas, Lutraria, Austrovenus, Barnea, Myadora. Recent Indo‐West Pacific Lutraria species are reviewed; the Red Sea‐East African species L. (Lutraria) turneri Jousseaume resembles L. grandis (Hutton) (adopted in place of the junior homonym L. solida Hutton). Six new species proposed: Saccella maxwelli (Nukumaruan‐Recent; known previously as S. bellula (A. Adams)); Limopsis turnbulli (Castlecliffian, OIS 15–20, SW Fiordland); Mactra (Maorimactra) marwicki (late Nu‐kumaruan); M. (Maorimactra) carteri (early Castlecliffian, OIS 45?‐19); Paphies delta (early Castlecliffian, OIS 31?‐17); Myadora fortecosta (Kapitean‐Opoitian). Further Indo‐West Pacific warm‐water migrants to New Zealand: Chama ruderalis Lamarck (Kapitean‐Nukumaruan; = C. huttoni Hector, = C. pittensis Marwick); Lutraria (Psam‐mophila) vellai (Beu) (late Nukumaruan); probably the two New Zealand late Neogene Ctenoides species. Further temperate migrant from the Atlantic: Lutraria (Lutraria) grandis. New synonymy: Xenostrobus Wilson = Limnoperna Rochebrune; Pecten no‐vaezelandiae (Reeve) = all other New Zealand Pecten names; Serratina eugonia (Suter) = S. charlottae (E. A. Smith); Oxyperas komakoensis (Carter) = O. elongata (Quoy & Gaimard); Austrovenus stutchburyi (Wood) varies clinally over New Zealand, = A. aucklandica Powell, = A. crassitesta Finlay; Dosinia (Austrodosinia) horrida Marwick = D. anus (Philippi) (Nukumaruan‐Recent); Panopea wanganuica Powell = P. smithae Powell (Nukumaruan‐Recent); Myadora stephaniae Carter = M. waitotarana Powell (Waipipian‐Nukumaruan). Distinctive bivalves extinct at the end of Nukumaruan time: Glycymeris shrimptoni Marwick, Patro undatus (Hutton), Lutraria (Lutraria) grandis, L. (Psammophila) vellai, Paphies crassiformis (Marshall & Murdoch), Eumarcia plana Marwick, Dosinia (Raina) nukumaruensis Marwick, Myadora waitotarana. Ennucula Iredale is used for Cenozoic and Recent Nuculominae; the application of Leionucula Quenstedt is unclear. Saccostrea glomerata (Gould) is used in place of S. circumsuta (Gould). Protothaca crassicosta (Deshayes) (formerly Haweran‐Recent) is recorded from Opoitian rocks. Pecten novaezelandiae appeared below Potaka Tephra (1.0 Ma) during OIS 29 or earlier in eastern New Zealand, but just above the Brunhes‐Matuyama transition (0.78 Ma, early OIS 19) in Wanganui Basin. Oxyperas elongata and Barnea similis (Gray) changed anagenetically through late Miocene‐Pleistocene time; earlier forms are conspecific with Recent ones. Tawera Marwick is restored as a genus. Purpurocardia Maxwell is compared with Glans Megerle von Muhlfeld and Glyptoactis Stewart, and all are retained. Limnoperna huttoni (Suter) became extinct during OIS 13, Barytellina crassidens Marwick during OIS 9 or younger. Pholadidea tridens (Gray) (OIS 23 & 5a) and Ostreola virescens (Angas, 1868) are recorded fossil.

Palaega kakatahi n. sp.: The first record of a marine fossil isopod from the Pliocene of New Zealand

Discovery of a well‐preserved fossil isopod specimen representing the last three pereonites plus the pleonites and pleotelson permits description of Palaega kakatahi n. sp. from the Tangahoe Formation (mid Pliocene), in the Wanganui Basin, New Zealand. This represents the first confirmed representative of the genus from the fossil record in the Southern Hemisphere.

Subduction evolution and mantle dynamics at a continental margin: Central North Island, New Zealand

Central North Island, New Zealand, provides an unusually complete geological and geophysical record of the onset and evolution of subduction at a continental margin. Whereas most subduction zones are innately two‐dimensional, North Island of New Zealand displays a distinct three‐dimensional character in the back‐arc regions. Specifically, we observe “Mariana‐type” subduction in the back‐arc areas of central North Island in the sense of back‐arc extension, high heat flow, prolific volcanism, geothermal activity, and active doming and exhumation of the solid surface. Evidence for emplacement of a significant percent of new lithosphere beneath the central North Island comes from heat flux of 25 MW/km of strike (of volcanic zone) and thinned crust underlain by rocks with a seismic wave speed consistent with underplated new crust. Seismic attenuation (Qp−1) is high (∼240), and rhyolitic and andesitic volcanism are widespread. Almost complete removal of mantle lithosphere is inferred here in Pliocene times on the basis of the rock uplift history and upper mantle seismic velocities as low as 7.4 ± 0.1 km/s. In contrast, southwestern North Island exhibits “Chilean‐type” back‐arc activity in the sense of compressive tectonics, reverse faulting, low‐heat‐flow, thickened lithosphere, and strong coupling between the subducted and overriding plates. This rapid switch from Mariana‐type to Chilean‐type subduction occurs despite the age of the subducted plate being constant under North Island. Moreover, stratigraphic evidence shows that processes that define the extensional back‐arc area (the Central Volcanic Region) are advancing southward into the compressional system (Wanganui Basin) at about 10 mm/yr. We link the progression from one system to another to a gradual and viscous removal of thickened mantle lithosphere in the back‐arc regions. Thickening occurred during the Miocene within the Taranaki Fault Zone. The process of thickening and convective removal is time‐ and temperature‐dependent and has left an imprint in both the geological record and geophysical properties of central North Island, which we document and describe.

Evolution of the southern termination of the Taupo Rift, New Zealand

To understand the tectonic evolution of the southern termination of the Taupo Volcanic Zone (TVZ), New Zealand, we compare the late Quaternary structure and kinematics of the southern part of the Taupo Rift or Taupo Fault Belt (Mt Ruapehu Graben) with central parts of the rift (Ngakuru Graben), and with the South Wanganui Basin. We also investigate the differences between displacements of Pliocene and late Quaternary markers within the southern Taupo Rift. Comparison of fault displacement rates derived from displacements of late Quaternary and Pliocene markers yields a preliminary estimate of <400 000 yr for fault initiation of the majority of faults in the southern Taupo Rift. The timing of the onset of faulting in the southern TVZ appears to be younger than central parts of the TVZ and may indicate a step‐wise southward propagation of the rift. Faulting also appears to be less evolved in the south than in the central parts of the Taupo Rift (thicker seismogenic crust and hence wider fault spacing), reinforcing the impression of a recent incremental propagation of the Taupo Rift to the south. The onset of faulting coincides with independent observations of the initiation of volcanism in the southern TVZ. We propose that the termination of the TVZ, geographically short of the limit of the Hikurangi margin, may only be temporary, and it is most likely to propagate southward in association with future major perturbations in the subduction margin, such as the occurrence of large ignimbrite volcanism.

Cool-water shell bed taphofacies from Miocene-Pliocene shelf sequences in New Zealand: utility of taphofacies in sequence stratigraphic analysis

Abstract An investigation of the taphonomy, palaeoecology and stratigraphy of cool-water skeletal concentrations (shell beds) of the Matemateaonga Formation (Late Miocene-Early Pliocene) of Wanganui Basin, New Zealand, has provided the basis for the classification of taphofacies presented here. Two taphofacies described from transgressive systems tracts include the amalgamated shell bed and sediment starved shell bed taphofacies, representing skeletal concentration dominated by wave and current agitation, and sediment starvation, respectively. A further five taphofacies described from highstand and regressive systems tracts exhibit a gradient of sedimentological, taphonomic and palaeoecological properties that result from variation in storm event and fair-weather wave processes across the palaeoshelf bathymetric gradient. A principal components analysis of semi-quantitative data (53 observations) from sequences in Manutahi-1 well core demonstrates that taphonomic properties may be limited to particular systems tracts in some cases, but can also be repeated in different system tracts where the depositional environments are similar. Taphofacies, which are contained within siliciclastic-dominated portions of sequences (highstand and regressive systems tracts) possess little direct relevance to sequence stratigraphic analyses, but do provide valuable information on environmental conditions, in particular, depth relative to storm and fair-weather wave base, and proximity to shoreline.