Southern Hawke's Bay Research Articles

Holocene marine terraces as recorders of earthquake uplift: Insights from a rocky coast in southern Hawke's Bay, New Zealand

AbstractOn rocky tectonic coasts, data from Holocene marine terraces may constrain the timing of coseismic uplift and help identify the causative faults. Challenges in marine terrace investigations include: (1) identifying the uplift datums; (2) obtaining ages that tightly constrain the timing of uplift; (3) distinguishing tsunami deposits from beach deposits on terraces; and (4) identifying missing terraces and hence earthquakes. We address some of these challenges through comparing modern beach sediments and radiocarbon ages with those from a trench excavated across three terraces at Aramoana, central Hikurangi Subduction Margin, New Zealand. Sedimentary analyses identified beach and dune deposits on terraces but could not differentiate specific environments within them. Modern beach shells yielded modern radiocarbon ages, regardless of position or species, showing age inheritance and habitat is likely not an issue when dating shells on these terraces. By integrating terrace mapping, stratigraphy, morphology, and radiocarbon ages we develop a conceptual model of coastal uplift and terrace formation following at least two, possibly three, earthquakes at 5490–5070, 2620–2180, and 950–650 cal. yr bp. A high step and time gap between the upper two terraces raises the possibility that at least one intervening terrace is completely eroded. The trench exposure also showed that terrace stratigraphy may differ from that inferred from surface geomorphology, with apparent beach ridges being of composite origin and draping of younger beach deposits on the outer edge of a previous terrace. Dislocation modelling and comparison of marine terrace and earthquake ages from ~4 km south and ≤ 73 km north confirms that the most likely earthquake source is the nearshore, landward‐dipping, Kairakau Fault. Alternative sources, such as multi‐fault ruptures of the Kairakau‐Waimārama faults or Hikurangi subduction earthquakes, and/or a combination of the two are also possible and should be examined in future studies.

Soil organic carbon stocks in hill country pastures under contrasting phosphorus fertiliser and sheep stocking regimes, and topographical features

Temporal and spatial measurements of soil organic carbon (C) under grazed pastures are needed to quantify the effects of different grazing management regimes on C stocks. We examined soil organic C stocks under permanent pastures at the Ballantrae Hill Country Research Station in southern Hawke's Bay, New Zealand. Soils were sampled to three depths (0–75, 75–150, 150–300 mm) in 2003 and to the two upper depths in 2014, in three farmlets under different annual phosphorus (P) fertiliser inputs and stocked with sheep to maintain similar grazing pressure (i.e., stock units per unit of pasture production) across farmlets since 1975. The farmlets examined were NF = no annual P applied, LF = 125 kg single superphosphate (SSP) ha−1, and HF = 375 kg SSP ha−1, on an annual basis since 1980. The permanent sites included three slope classes [low slope (LS; 1–12○), medium slope (MS; 13–25○), high slope (HS; >25○)], on three different aspect locations grouped relative to the true north [east (E; 35–155○), southwest (SW; 275–35○), northwest (NW: 155–275○)]. A year-by-farmlet interaction trend on soil C stocks in the upper depth (0–75 mm) was associated with linear numerical differences in soil C stocks in 2003 (30.9, 32.5 and 35.1 Mg C ha−1 on the NF, LF, and HF farmlets, respectively) but not in 2014. This trend was not seen in the deeper soil layers (75–150 and 150–300 mm). In contrast, slope and aspect had major effects on soil C stocks. Overall, soil samples collected on the steepest slope class (>25○) resulted in higher soil bulk densities (BD) and carbon-to‑nitrogen (C:N) ratios, and lower N and C concentration, and soil C stocks at all soil depths, compared with samples collected at the other two slope classes. Soil samples collected on the NW-facing slopes resulted in higher BD, and lower N and C concentration, and soil C stocks at all soil depths, compared with samples collected at the other two aspect locations. Both of these topographic features need to be considered in soil sampling regimes of hill grazing lands to obtain an accurate estimate of organic C stocks. Data from this long-term study provide science, policy and industry with invaluable insights on soil organic C stocks in grazing hill-country soils and highlight the value of long-term structured experiments for monitoring soil C stocks. ImplicationsSequestering of organic carbon (C) in soil offers an option for offsetting C in atmospheric emissions. We examined soil C accumulation under a long-term phosphorus (P) application and sheep stocking regime grazing experiment. The hill country experiment has been running since 1975, with three distinct farmlets that received either no P, an intermediate amount or an amount that exceeds annual maintenance. Farmlet per se had a minimal impact on soil C accumulation, but slopes and aspects had a substantial impact and need to be considered in the design of soil sampling regimes that monitor soil organic carbon over space and time.

Modelling gravel barrier response to storms and sudden relative sea-level change using XBeach-G

This paper investigates the response of a mixed sand and gravel (MSG) barrier coastline to storms and sudden relative sea-level change using the numerical model XBeach-G. The barrier beaches of southern Hawke's Bay, New Zealand provide a natural laboratory for the work owing to spatial variability in instantaneous land deformation that occurred during the 1931 Hawke's Bay earthquake. Historic (1931) and contemporary (2017) morphodynamics of one tectonically uplifted (+1.5 m) and one subsided (−0.78 m) barrier were modelled under energetic waves (Hs 1–6 m). XBeach-G was designed to simulate pure gravel beach morphodynamics, but despite this, the model demonstrated high quantitative skill when calibrated for the MSG coast. Results reveal the storm impact regimes of each profile and potential linkages to decadal-scale barrier development observed in historical beach profile data. Storm-swash processes at the uplifted profile indicate long-term barrier stability due to a lack of change at the barrier crest, and ridge building observed in model simulations provides a potential formative mechanism for decadal-scale beach progradation. At the subsided barrier, modelled historic crest build-up (up to 0.3 m) and narrowing (over 5 m) indicate an overtopping mode of response to sudden relative sea-level rise. Simulation results are consistent with historical beach profile observations that also show chronic narrowing of the subsided barrier through time. Simulations of the modern barrier draw attention to the possibility that it may be on a trajectory toward catastrophic breakdown under extreme conditions, but further work is needed to distinguish the relative importance of historical sudden sea-level rise versus deficits in sediment supply to this shoreline.

Microsite effects on abundance of sown species on uncultivable slopes

Hill country has a vast array of microsites that may influence the success and uniformity of establishment of oversown pasture species and their persistence. In spring 2014, studies were conducted at Woodville (southern Hawke's Bay; summer-wet) and Cheviot (North Canterbury; summer-dry) to determine the effect of gentle (14-21o) and steep (32-40o) slopes on the presence and contribution to sward dry matter (DM) of oversown species in rotationally grazed swards aged 30 or 36 months. Gentle slopes had greater soil water content (29 versus 25%) and Olsen P (34 versus 26 μg/ ml) than nearby steep slopes. Perennial ryegrass (94 versus 80%) and phalaris (6 versus 2%) were present in a greater proportion of plots on gentle than on steep slopes, respectively, whereas subterranean clover had a greater presence on steep than on gentle slopes (6 versus 1%). Perennial ryegrass comprised a higher percentage of DM in swards on gentle than on steep slopes (27 versus 11%). The percentage of DM contributed by other individual species did not differ significantly between slope classes. The persistence of most sown species was unaffected by slope; microsite variation was not a major influence on species persistence. Keywords: hill country, microsites, pasture species, plant-microsite matches

Responses of grazed New Zealand hill pastures to rates of superphosphate application

Effects of superphosphate application rates on pasture production were measured between 1975 and 1988 within 12 grazed farmlets on steep hill country at the Ballantrae Hill Country Research Station in southern Hawke's Bay. The low fertiliser (LF) treatment received 125 kg ha−1 superphosphate pa, and the high fertiliser (HF) an average 625 kg ha−1 pa from 1975 to 1979, then 375 kg ha−1 pa subsequently. No fertiliser was applied to one LF (LFNF) and one HF (HFNF) farmlet after 1980. Soil Olsen phosphorus (P) level declined slightly for the LF treatment as maintenance requirements exceeded P inputs; rose in response to increased fertiliser inputs in the HF treatment but more slowly than anticipated maybe because of the poor fertiliser application history and low starting Olsen P; and declined in the HFNF treatment after cessation of fertiliser inputs. Pasture production was 19% greater in HF than LF in year 1, and on average 39% greater in years 2 to 13. Size of earthworm populations measured up to 30 years after treatments commenced was highly related to level of pasture production, and there was no evidence of deleterious effects of superphosphate application on earthworm populations. Pasture production declined by 3% pa in HFNF after fertiliser application ceased. Variability of annual pasture production was large, year-to-year variation about the mean being 35% for HF and 47% for LF (CV = 8.5% and 12.7%, respectively). This suggests comprehensive farm system redesign will not be required in the near or medium term to cope with the gradual onset of climate change, as farmers already manage large year-to-year variability with existing systems and their careful implementation should suffice. Estimated maximum annual production with non-limiting P supply for low-slope (1–12°), medium-slope (13–25°) and steep (>25°) sites was estimated to be 14.8, 11.5 and 9.4 t DM ha−1 from curves derived using Bayesian statistics. The Olsen P level for 97% maximum pasture production was 18 µg g−1; converted to a volumetric basis this equates to about 15 µg mL−1 which is lower than the commonly accepted industry guideline for near maximum pasture production of 20 µg mL−1.

Longshore transport of cobbles on a mixed sand and gravel beach, southern Hawke Bay, New Zealand

Few large-scale field measurements of longshore sediment transport have been undertaken on beaches composed of coarse sediments, in part owing to difficulties associated with measurement in energetic swash zones in which large clasts are moving. Here we present results from a field experiment in which Passive Integrated Transponder tags were used to investigate patterns of cobble transport over 8 months on a mixed sand and gravel beach on the east coast of New Zealand. The study objectives were to document rates of alongshore transport, and measure rates of cobble abrasion under field conditions. Cobble recovery rates were highly variable over the study period with 30–60% recovery at site 1 compared to much lower (0–20%) recovery rates at sites 2 and 3. Consistent uni-directional patterns of net alongshore sediment transport were observed. The median cobble transport rate was approximately 500 m over 207 days, indicating long-term net northward transport rates on the order of 2–2.5 m/day. Results highlight a number of factors, in addition to longshore energy flux, that are important for understanding the observed patterns. In particular, across-shore sediment transport during storms may have been smaller at site 1 than the other sites owing to lesser wave exposure. This is partly responsible for higher cobble recovery rates at this site and also an increased amount of time during which cobbles were able to be transported alongshore on the lower beachface. Unique to this study, results demonstrate the use of PIT tags to provide direct measurement of cobble abrasion under natural processes. Repeat weight measurement on individually tracked cobbles show that after 207 days the median loss was 11.3 g, or 1.8% of total weight. There was a general trend of abrasion increasing with net transport distance, but longshore cobble movement was highly variable ranging from 130 to 2500 m.

Micropaleontological evidence of large earthquakes in the past 7200 years in southern Hawke's Bay, New Zealand

Foraminiferal and diatom assemblages in 11 cores (3–7.5 m deep) of Holocene sediment from brackish marine Ahuriri Inlet in southern Hawke's Bay, New Zealand, provide a record of 8.5 m of subsidence followed by 1.5 m of uplift in the last 7200 cal years, in a region overlying the subduction zone between the Australian and Pacific Plates. Modern Analogue Technique was used to estimate paleotidal elevation of the 97 richest foraminiferal assemblages. The most precise estimates are for high-tidal salt marsh assemblages cored in marginal settings in the north and south of the former inlet. The least precise estimates are from low-tidal and subtidal assemblages from cores in the middle of the inlet. These paleoelevation estimates combined with sediment thicknesses, age determinations (from tephrostratigraphy and radiocarbon dates), the New Zealand Holocene sea level curve, and estimates of compaction, identify the Holocene land elevation changes and earthquake-displacement events in each core. The following major, earthquake-related displacements are inferred: ca 7000 cal yr BP (>−0.6 m displacement); ca 5800 cal yr BP (>−0.5 m; ca 4200 cal yr BP (ca −1.5 m); ca 3000 cal yr BP (−1.4 to −1.8 m); ca 1600 cal yr BP (ca −1.7 m); ca 600 cal yr BP (ca −1 m); 1931AD Napier Earthquake (+1.5 m). Further smaller events involving regional subsidence or earthquake-shake compaction are indicated during the 7000–3000 yr BP interval, but cannot be identified precisely. The six (possibly subduction interface) subsidence events in the last 7000 years have had a return time of 1000–1400 years. Identified displacement events have a range of sedimentary expressions, from an eroded and burrowed hiatus surface, to an abrupt lithologic switch from mud to sand, or peat to shelly mud, or in some places no change in sediment character whatsoever.

Three‐stage massive gravitational collapse of the emergent imbricate frontal wedge, Hikurangi Subduction Zone, New Zealand

A large‐scale east‐facing Quaternary gravitational collapse is affecting the otherwise thrust‐faulted terrain of coastal southern Hawke's Bay on the east coast of North Island of New Zealand, involving an area >30 km long and up to 6 km inland from the coast, but also extending offshore. The western limit of the collapse structure onshore is represented by a footwall escarpment forming the eastern margin of the Maraetotara Plateau (av. elevation 500 m a.s.l.). To the east of this escarpment, the disposition of the Pliocene Te Aute Limestone within a complex extensional zone reflects numerous northeast‐southwest‐trending normal faults (some Holocene active) with the development of horst and graben, fault‐angle depressions, synclinal folds, fissured zones, and large parasitic landslides. Listric normal faulting is reflected by the back‐tilting of large blocks up to several kilometres in length, near the centre of the gravitational collapse. Horizontal extension across the gravitational collapse domain is calculated at between 15 and 20%. West of the footwall escarpment, the Maraetotara Plateau defines a separate structural domain with horizontal extension of between 2 and 5%, inferred to be related to bending moment faults driven by regional arching accompanying uplift. A fundamental contrast exists between active extensional structures onshore and the mainly contractional structures (asymmetrical anticlines, reverse and thrust faults) offshore. A tectonically driven model involving underplating of trench fill best explains the large‐scale gravitational collapse of this part of the emergent imbricate frontal wedge of the Hikurangi Subduction Zone margin.

Paleoenvironmental and tectonic changes across the Cretaceous/Tertiary boundary at Tora, southeast Wairarapa, New Zealand: A link between Marlborough and Hawke's Bay

The Late Cretaceous‐Paleocene succession exposed on the Tora coast, near the southeastern tip of the North Island, is distinguished by an unusual lithofacies of the Whangai Formation, and by an apparently unique formation, Manurewa Formation, which spans the Cretaceous/Tertiary (K/T) boundary. The Late Cretaceous siliceous Whangai Formation at Tora includes zones of slumps and olistostromes, containing megaclasts of limestone up to 3 m long. The olistostromal deposits suggest steep submarine topography with a high rate of erosion, and imply tectonic activity. The common occurrence of hummocky cross‐stratification suggests deposition in shelf depths above storm wave base. The sharply overlying Manurewa Formation is interpreted as the infill of a major shallow channel complex, perhaps >9 km wide and spanning the K/T boundary in time. The older of two channelled units is of latest Cretaceous (latest Haumurian/late Maastrichtian) age, and consists of bioturbated alternating thin sandstone and mudstone with thin conglomerate lenses and limestone beds. It is likely to have been deposited in a low‐energy environment, probably deeper than that of the Whangai. The younger channel system, of early Paleocene (early Teurian) age, erodes into the older in the northeast, and into the underlying Whangai Formation in the southwest. Basal deposits consist predominantly of medium to coarse, thick‐bedded, glauconitic sandstone, with local low‐angle cross‐stratification and microflora typical of low salinity conditions, suggesting deposition in shallow shelf depths. These deposits contain olistrostromes with megaclasts up to 1 m long of limestone and rarer dark grey siltstone or very fine sandstone clasts typical of Whangai Formation. The inclusion of megaclasts of Whangai Formation indicates that local emergence and erosion of older strata was occurring. Deposits grade upward into well‐sorted bioturbated sandstones of the Awhea Formation, with prominent low‐angle cross‐stratification, interpreted as very shallow marine, probably nearshore deposits. The channel system represented by the Manurewa Formation records an initial relative sea‐level rise, followed by an abrupt sea‐level fall at, or close to, the K/T boundary. New Zealand was in a passive margin tectonic setting at the time, but the widespread presence of olistostromes, some including clasts derived from older strata, suggest that local tectonic activity and uplift was occurring. The effects may have been enhanced by a climatic shift in storm tracks and intensity in the latest Cretaceous, which is supported by the evidence of strong wave activity. By contrast, to the south in Marlborough, the K/T boundary succession is commonly characterised by an apparently conformable lithologic change from limestone to chert, although with local hiatus. To the north, in southern Hawke's Bay, the coeval succession is characterised by a disconformity separating greensand from underlying light grey, slightly calcareous mudstone of the Whangai Formation. The Tora sequence may provide the link between two distinctly different lithologic successions.

Mud volcano eruption within the emergent accretionary Hikurangi margin, southern Hawke's Bay, New Zealand

In September 1994 a mud volcano eruption and associated mudflow occurred adjacent to Brookby gas seep near Waimarama, southern Hawke's Bay, New Zealand. The eruption cycle lasted several weeks, although the bulk of the estimated 8–10 000 m3 of mud was erupted within hours of the beginning of activity. Other minor gas seeps within a 250 m radius also showed signs of increased mud and fluid flow activity. The main eruption vent and nearby permanent gas seep lie within a 900 m wide northeast‐trending tectonic melange zone, with incorporated floater blocks of Late Cretaceous and Paleogene formations. Structural complexity is related to late Cenozoic thrust‐fault deformation of the emergent highest accretionary ridge of the Hikurangi margin frontal prism. The interpreted eruption cycle commenced with the rapid development (<1 h) of a mud dome c. 35 m in diameter and estimated 2–3 m maximum height. The dome was symmetrical and positioned over a central vent. The downslope facing sector of the dome formed by the highly cohesive, fluid montmorillonitic mud collapsed and initiated a mudflow, ultimately reaching c. 180 m downslope, blocking Kaikopu Stream. The bulk of the extruded mud flowed directly from the vent through the breached dome wall. This process appears to have taken place over several hours after collapse of the dome wall. The expelled mud is inferred to have been driven upwards by a trapped pocket of escaping gas. Effectively, the expelled mud appears to have formed a plug in the near‐surface part of the vent pipe, and this is interpreted to extend to a depth of several hundred metres, trapping migrating gas, which then episodically vents to the surface. There is evidence for similar type mud eruptions in the past at the Brookby site. Once the gas pocket reached the surface, it rapidly escaped, generating a minor mud blast, as evidenced by the presence of radial mud splatters extending up to c. 80 m from the main vent. The adjacent pre‐existing and permanent Brookby gas seep also showed evidence of increased activity, with more vigorous expulsion of gas and water bubbling into an enlarged pool. In the days after the main eruption phase, a smaller parasitic dome, with an associated minor mud blast aureole, developed on the margin of the breached ring‐cone of the main eruption dome. Methane gas continued to “burp” from this at intervals of 15–30 s during the following weeks.

Management effects on soil carbon storage in New Zealand pastures

The amount of soil carbon is the net effect of carbon inputs and decomposition and is influenced by many factors including the level of herbage utilisation, root production, litter quality, soil fertility and soil moisture status. Long term experiments at Winchmore, in mid- Canterbury, show soil carbon levels increased after initial development, but after 50 years fertiliser input has had no net effect on soil carbon, while increasing the frequency of irrigation has actually resulted in lower soil carbon levels. Pasture development at Ballantrae , in the southern Hawke's Bay, has been associated with a decline in soil C levels, with the high fertility treatment having only slightly higher soil C than the treatment receiving a lower rate of fertiliser. At Tara Hills, in South Island semi-arid high country, increased stocking rate is associated with a decrease in soil C. Contrary to common perceptions increases in pasture and farm productivity do not necessarily result in increased soil carbon storage. Keywords: carbon, fer tiliser, grazing management, irrigation, soil organic matter

Influence of hydrologic exchange patterns on water chemistry and hyporheic invertebrate communities in three gravel‐bed rivers

The influence of hydraulic exchange patterns on hyporheic water chemistry and invertebrate community composition was measured in three gravel‐bed rivers of southern Hawke's Bay, North Island, New Zealand. Based on patterns reported overseas we predict that down‐welling waters will be similar to surface waters in chemical characteristics and epigean animals will be more common in down‐welling waters than in up‐wellings, which would have a different chemical signature to the surface waters and a fauna dominated by hypogean invertebrates. Conductivity, pH, and temperature of hyporheic water were least like those of surface waters at up‐wellings in the groundwater‐fed Makaretu River. Only 6% of the invertebrate fauna of these up‐wellings comprised epigean animals. In contrast, in the groundwater‐recharging Tukituki and Waipawa Rivers, hyporheic water chemistry was similar to that of surface water in down‐wellings and 40% of the community comprised epigean animals by abundance. Significant increases (P < 0.05) in species diversity and taxonomic richness was also identified in down‐welling zones compared with up‐wellings. Species diversity and taxonomic richness were lowest in up‐wellings in the Makaretu where hypogean animals dominated the hyporheic fauna. Species diversity and taxonomic richness were higher in down‐wellings than up‐wellings because more epigean taxa were present.

Relative importance of surface and subsurface movement on benthic community recovery in the Makaretu River, North Island, New Zealand

Colonisation of macroinvertebrates following changes to the stream environment occurs rapidly through drift, oviposition, lateral movement, and vertical migration. To assess the influence of different sources of colonists on benthic macroinvertebrate community composition, a colonisation experiment using artificial substrate baskets was conducted in the braided Makaretu River in the southern Hawke's Bay (North Island, New Zealand). Both the numbers of taxa and individuals rapidly increased through time in baskets only allowing drifting and surface crawling invertebrates (surface), baskets only allowing invertebrates migrating from the hyporheic zone (hyporheic), and baskets that allowed both (controls). The numbers of taxa and density of individuals indicated that the artificial substrate baskets were similar to the benthos after 14 and 28 days, respectively. However, overall benthic community composition was not similar to the surrounding benthos until Day 42 in all three basket treatments. The colonisation rates identified from artificial baskets in the Makaretu River are consistent with other colonisation experiments undertaken in New Zealand and overseas. Elmidae (Coleoptera) was most abundant within surface treatments until Day 28 after which Deleatidium spp. (Ephemeroptera) became dominant. The abundance of Elmidae and Deleatidium in the control treatments was consistent with colonisation patterns in the surface baskets. Deleatidium dominated baskets allowing access from the hyporheic zone throughout the experiment. It appears that Elmidae colonise quickly in greater numbers by drifting and/or crawling and Deleatidium colonises more slowly from the hyporheic zone, eventually replacing Elmidae as the most common animal. This indicates that the hyporheic zone provides a source of invertebrate colonists later during community recovery, whereas drifting and/or crawling provides colonists early during recovery.

Aerial archaeology of the southern Hawke's Bay coast, New Zealand

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Identification of a Waipawa Formation equivalent in the upper Te Uri Member of the Whangai Formation ‐ implications for depositional history and age

Stable isotopes and biomarkers have identified a unit with similar organic geochemistry to the Waipawa Formation, in the upper Te Uri Member of the Whangai Formation, exposed in the Akitio River, at Tawanui, southern Hawke's Bay, New Zealand. At Tawanui, the uppermost greensand of the Te Uri Member contains a large positive 813C isotopic excursion from ‐27.0%o to ‐20%e and an increase in total organic carbon from 0.1% to 1.0%. Biomarker analyses demonstrate a similar C30 sterane fingerprint to other deposits of the Waipawa Formation. We propose that the uppermost greensand of the Te Uri Member at Tawanui is a condensed stratigraphic equivalent of the Waipawa Formation at nearby Angora Stream and other East Coast Basin localities. This correlation demonstrates that Waipawa Formation is middle Teurian (middle Paleocene) and precedes the late Paleocene thermal maximum event by c. 5 m.y. The likely upwelling event that resulted in deposition of the Waipawa Formation was geographically widespread but probably restricted to the outer shelf/upper slope. In places, biogenic activity prevented the preservation of organic carbon in equivalent condensed stratigraphic intervals. Localised restriction of upwelling and black shale deposition may be demonstrated by the occurrence of a thick black shale at Angora Stream only c. 10 km from the coeval greensands at Tawanui. Alternatively, Oligocene‐Miocene east‐west shortening and structural reorganisation in the East Coast Basin may have juxtaposed facies that were originally many tens of kilometres apart. Our correlation also implies that the Te Uri Member is diachronous. It may have been on the outermost shelf to upper slope during lowstand conditions, where it is oldest, to higher on the shelf during transgression and highstand conditions.

Effects of fertiliser application on nutrient status and organic matter content of hill soils

Effects of two fertiliser treatments on soil characteristics were measured during 1972–87, within 10 grazed permanent‐pasture “farmlets”, on steep hill country in southern Hawke's Bay, New Zealand. The low fertiliser (LF) treatment received 125 kg ha‐1 superphosphate p.a., and the high fertiliser (HF) an average 625 kg ha‐1 p.a. for 5 years then 375 kg ha‐1 p.a. subsequently. Ground limestone was applied to HF in 1975 and 1979. Grazing pressure was the same across treatments. Soil fertility decreased with increasing slope of measurement site, and aspect had a less marked effect. Phosphorus initially accumulated mainly in inorganic forms, with organic P accumulating at a slower rate similar to that for organic S. Olsen P status reflected P application regime, although the responsiveness was lower than anticipated. Soil pH in the non‐limed LF treatment increased slightly over time, probably as a consequence of the decline in soil organic C. Soil total N (0–75 mm depth) remained constant in LF, but increased by 19 kg ha‐1 yr‐1 in HF soils. Soil carbon decreased in LF and HF by 200 kg C ha‐1 yr‐1 This suggests that soil organic matter may be decreasing in some New Zealand pastoral environments.

Strong motion modelling of the 1993 Tikokino earthquake, southern Hawke's Bay, New Zealand

The 1993 Tikokino, New Zealand earthquake (ML 6.1) is modelled as a unilateral rupture, exhibiting clear source directivity to the south. The earthquake was recorded by four strong motion stations within 30 km: Waipawa to the south, and three sites in Napier and Hastings to the northeast. The shorter duration and greater amplitudes (by a factor of 10) observed at Waipawa with respect to the other stations provide clear evidence for the southward rupture direction. The Tikokino earthquake occurred on a shallow dipping, oblique reverse fault, and probably represents movement at the plate interface. A high rupture velocity is required to match the distribution of observed ground shaking, and the rupture area is constrained to be c. 7 × 2 km2. The moment of the preferred model is 1.1 × 1018 Nm (Mw 6.0) and the stress drop about 35 MPa. This high average stress drop is consistent with the rupture being confined to an isolated asperity. The model used consists of a finite, rectangular fault rupturing with prescribed velocity and direction, and with uniform slip. The fault is embedded in a planar layered seismic velocity structure. The ability of the model to match the principal features of the observed seismograms suggests that it will be a useful tool in the prediction of strong ground motion for seismic hazard studies in the region.

Stratigraphy, paleontology, and tectonics of lower Miocene rocks in the Waipatiki/Mangatuna area, southern Hawke's Bay, New Zealand

Three lower Miocene units occur in the Waipatiki/Mangatuna area: Mangapuku Mudstone (Otaian), Westcott Sandstone (Otaian‐Altonian), and Tunakore Mudstone (Altonian). They are regarded as members of the Ihungia Formation and crop out mostly in the axis of the Waipatiki Syncline. Major faults near Toi Flat trend west‐east and, with an associated zone of minor faults, appear to thicken Westcott Sandstone in this area. Rocks showing gradual deepening from initially shallow water (Westcott Sandstone and Tunakore Mudstone) overlie those of deep‐water origin (Mangapuku Mudstone). Mangapuku Mudstone is a dark‐grey, calcareous, sandy mudstone with rare macrofaunas and diverse and abundant foraminiferal faunas of abyssal to shelf break origin. It is separated from older units by a deep‐water unconformity and was uplifted at rates of up to 1.1 km/m.y. to shallow‐water depths, with submarine erosion preceding deposition of Westcott Sandstone. Westcott Sandstone is dominated by grey, calcareous, fossiliferous muddy sandstones to sandy siltstones with intercalated unfossiliferous turbidite sandstones, deposited in a forearc basin subsiding from inner shelf to shelf break depths. Westcott Sandstone has diverse and abundant shallow molluscan faunas along with rich non‐molluscan macrofaunas and foraminiferal faunas. Tunakore Mudstone is a dark grey, calcareous, sandy mudstone with minor thin sandy turbidites, deposited at shelf break to mid‐bathyal depths, and grades up from Westcott Sandstone from continued subsidence. It contains rare macrofaunas and highly diverse foraminiferal faunas. Sedimentation in Westcott Sandstone and Tunakore Mudstone was >225 m/m.y. in combination with an average basement subsidence rate of 880 m/m.y.

Pasture responses to phosphorus and nitrogen fertilisers on dry hill country

In order to evaluate the relative responsiveness of summer-dry hill pasture to phosphate (P) and nitrogen (N) fertilisers, a large scale field trial was established in southern Hawke's Bay in 1995. Pasture growth rates and species, and soil moisture levels were measured on easy and steep slopes of generally north- and south-facing aspects in two pairs of farmlets with either a Low or High soil P status, one of each pair also receiving 30 kg N/ha annually in winter. Soil moisture levels were always higher on south- than on north-facing slopes, the difference being greatest from autumn to spring. However, total annual pasture growth was higher on north aspects because of better winter production, whereas the colder, south aspects produced little pasture at this time. In other seasons, pasture growth was similar on both aspects. Pasture growth was higher on easy than on steep slopes at all times. P responses occurred from spring to autumn especially on south aspects, and on easy north-facing slopes, which showed a large increase in clover growth. N responses were best in winter and autumn especially on steep, north aspects and at Low P conditions. N application depressed clover production, especially in the High P+N fertiliser treatment, where there was no net increase in production compared with the High P treatment. Only tentative conclusions can be made at this time since the pasture is considered to be still in a development phase. However, results suggest that application of P only to south aspects and moist north slopes, and for steep, north-facing slopes to receive N, plus only limited P, will be the most efficient fertiliser policy. Keywords: dry hill country, phosphate response, nitrogen response, pasture production, slope, aspect

The effect of poisoning on the movement of rooksCorvus frugilegusL. in southern Hawke's Bay, New Zealand

Abstract The effect of poison bait at a rookery on the movement of rooks (Corvus frugilegus L.) was studied using marking and radiotelemetry at five rookeries in southern Hawke's Bay, New Zealand. In November 1987 and 1988, before poisoning, rooks were retrapped near rookeries other than those near where they were marked, showing that feeding areas overlapped. Birds marked in 1987 were located north and north-east of original rookeries in 1988. In spite of extensive searches throughout the study area, no southward movement was seen. Between July and November 1988, 12 marked rooks moved to rookeries in a south-east direction. Poisoning at the rookery in November 1988, using DRC-1339, killed 38 rooks, including 10 birds marked near that rookery and 4 marked near other rookeries. At least six marked birds and one with a transmitter survived, as well as three birds with transmitters released after the poisoning. These 10 rooks were seen at other rookeries in 1989, including 2 marked rooks that moved 3 km south-east. The surviving rook with a transmitter moved 4km south-west. A search of rookeries south of Dannevirke failed to reveal marked rooks, so there was insufficient evidence to suggest that poisoning was responsible for the recent southward extension of their range.