Gillian M. Turner

Diagenetic formation of ferrimagnetic iron sulphide minerals in rapidly deposited marine sediments, South Island, New Zealand

Abstract Detailed magnetostratigraphic studies of Late Neogene siliciclastic sediments of the Awatere Group, South Island, New Zealand (41°45′S, 174°05′E) have revealed a wide range of palaeomagnetic behaviour. Examination of rock magnetic properties was undertaken using conventional palaeomagnetic techniques and thermomagnetic, X-ray diffraction and electron microprobe analyses. These analyses indicate that the ferrimagnetic iron sulphide minerals, greigite and pyrrhotite, are responsible for a stable and intense magnetic remanence in fine-grained sediments, whereas titanomagnetite is the only remanence-bearing mineral identified in coarser-grained sediments, which are less strongly and less stably magnetised than the fine-grained sediments. Detrital titanomagnetite grains are likely to have undergone dissolution during early diagenesis as a result of iron sulphide formation, which occurs commonly in rapidly deposited, anoxic sediments that support active sulphate reduction and H 2 S formation. Preservation of greigite and pyrrhotite is inferred to result from the arrest of the pyritisation process, probably due to the low permeability of the fine-grained sediments and consumption of available H 2 S before full reaction to pyrite occurred. Relative palaeomagnetic instability and weak remanence intensities in coarser grained sediments is likely to be due to low titanomagnetite concentrations resulting from titanomagnetite dissolution.

Pleistocene unconformity-bounded shelf sequences (Wanganui Basin, New Zealand) correlated with global isotope record

Abstract Eleven successive unconformity-bound sequences in the Castlecliff section (Wanganui Basin), each typically composed of a lower, variably conglomeratic, 0.1–2 m thick shell bed and an upper 5–20 m thick barren siltstone, are identified in the stratotype of the local (New Zealand) middle Pleistocene Castlecliffian Stage. Based on magnetostratigraphy, the section is correlated with the published benthonic oxygen isotope curve for DSDP core 552A from the North Atlantic. Each of the sequences accumulated during one of the odd-numbered interglacial isotope stages in the interval 31 to 11, inclusive; the unconformities developed during intervening glacial ages when the shelf was exposed through glacio-eustatic lowering. The correlation provides a geochronologic framework for assessment of the eustatic contribution to the generation of the sequences in a situation where the paleoshorelines and related shelf sequences are preserved, and for which an independent approximation of the sea-level history is available in the form of an oxygen isotope curve.

Palaeomagnetic location of the Jaramillo Subchron and the Matuyama-Brunhes transition in the Castlecliffian stratotype section, Wanganui Basin, New Zealand

Abstract Magnetostratigraphy is described for the lower part of the Castlecliff section in Wanganui Basin, New Zealand, based on analysis of multiple samples from 37 horizons. The Castlecliff section is the stratotype of the middle Pleistocene Castlecliffian Stage and comprises eleven unconformity-bound sequences which accumulated at inner shelf depths, during interglacials. A range of rock magnetic experiments undertaken on representative lithologies within the section show that the bulk of the remanence in all specimens is carried by ferrimagnetic mineral close to magnetite in composition, in the stable single domain or pseudo single domain size range. However, at some sites post depositional chemical changes appear to have produced small amounts of another ferrimagnetic mineral, chiefly within the superparamagnetic-single domain size range, resulting in hard secondary components of magnetization. Nevertheless, at most sites the primary component of remanence was clearly identified by stepwise demagnetization. Four magnetozones are identified and these can be correlated to the magnetic polarity time scale using existing biostratigraphic constraints. The section sampled spans the interval from below the Jaramillo Subchron to above the Matuyama-Brunhes boundary. The Matuyama-Brunhes transition extends over 10.5 m of section within the Upper Westmere Siltstone. Virtual geomagnetic pole positions describe an equatorial excursion in an otherwise far sided transition path. These represent the first published data on the occurrence of the Matuyama-Brunhes boundary on-land in New Zealand.

Revision of the Piripauan and Haumurian local stages and correlation of the Santonian-Maastrichtian (Late Cretaceous) in New Zealand

Abstract The uppermost Cretaceous in New Zealand is represented by the local Mata Series, composed of the Piripauan and overlying Haumurian Stages. The existing definitions and subdivision of these stages, and the type section at Haumuri Bluff, southeastern Marlborough, are wholly inadequate. To address these problems, three key sections in southeastern Marlborough have been studied in detail: a tributary of Ben More Stream, the headwaters of Kekerengu River, and a railway cutting at the mouth of Conway River. All three sections have yielded rich palynomorph assemblages and the first two contain inoceramid bivalves and foraminiferal faunas. In addition, magnetostratigraphic data have been obtained from the Ben More Stream section. We propose to revise the Piripauan and Haumurian Stages from new boundary stratotypes in the Ben More Stream section. The Piripauan is defined at the lowest occurrence of the inoceramid /. pacificus. The base of the Haumurian Stage is defined at the lowest occurrence of the dinoflagellate Nelsoniella aceras. In addition, we propose to subdivide the Haumurian into formal Lower and Upper substages; the base of the Upper Haumurian is defined at the lowest occurrence of the dinoflagellate Isabelidinium pellucidum in a boundary stratotype in the Conway River railway cutting. These boundary criteria can be correlated widely within New Zealand across a broad range of marine facies. The Piripauan contains two inoceramid and three dinoflagellate zones. The Haumurian contains six dinoflagellate zones and five subzones. Based on biostratigraphic and magnetostratigraphic data, the Piripauan/Haumurian boundary is correlated with the C34‐C33 magnetochron boundary and with the middle‐upper Santonian boundary. The Piripauan and Haumurian Stages thus have durations of c. 1.7 and 19.5 m.y., respectively. In terms of content, these revisions largely preserve the stages as used previously in New Zealand and require no significant changes to existing geological maps.

A palaeomagnetic secular variation record for New Zealand during the past 2500 years

Abstract A record of the secular variation of the geomagnetic field over New Zealand for the past 2500 years has been compiled by measuring and averaging logs of the primary magnetization of five cores of lacustrine sediment from Lake Pounui. A time-scale is provided by four radiocarbon age determinations. The record is constrained by historical observations for the last 350 years and is in good agreement with instantaneous palaeomagnetic directions from lava flows. This is the first continuous record of secular variation from New Zealand and one of only a few from the southern hemisphere. Together with existing Australian records, the results show significant evidence of westward drift of the geomagnetic field over the past 2500 years. It contains new information for modelling the geomagnetic secular variation in the southern hemisphere and will also be valuable for dating applications within New Zealand.

Focusing of relative plate motion at a continental transform fault: Cenozoic dextral displacement >700 km on New Zealand's Alpine Fault, reversing >225 km of Late Cretaceous sinistral motion

The widely accepted ∼450 km Cenozoic dextral strike-slip displacement on New Zealands Alpine Fault is large for continental strike-slip faults, but it is still less than 60% of the Cenozoic relative plate motion between the Australian and Pacific plates through Zealandia, with the remaining motion assumed to be taken up by rotation and displacement on other faults in a zone up to 300 km wide. We show here that the 450 km total displacement across the Alpine Fault is an artifact of assumptions about the geometry of New Zealands basement terranes in the Eocene, and the actual Cenozoic dextral displacement across the active trace is greater than 665 km, with more than 700 km (and 94%) of the relative plate motion in the last 25 Ma at an average rate in excess of 28 mm/yr. It reverses more than 225 km (and <300 km) of sinistral shear through Zealandia in the Late Cretaceous, when Zealandia lay on the margin of Gondwana, providing a direct constraint on the kinematics of extension between East and West Antarctica at this time.

Environmental magnetism and magnetic correlation of high resolution lake sediment records from Northern Hawke's Bay, New Zealand

Abstract Magnetic measurements provide a rapid and flexible means of studying a wide range of environmental processes. Here they are applied to a suite of lacustrine sediment cores from Lakes Tutira and Waikopiro, Northern Hawkes Bay. The cores span the period from just before the settlement of Europeans in the area to the present day. They document the impact of vegetation clearance and land use practices in a region of steep hill country that is prone to recurring major rainstorms. Before European settlement, the lake catchment was covered in bracken fern, which stabilised the soil. The major changes in the magnetic properties of the sediments reflect fluctuations in the concentration of magnetic minerals, rather than composition or grain‐size variations. Since the European arrival, however, each major storm has produced an identifiable pulse of minerogenic sediment, which gives rise to a peak in magnetic susceptibility. These peaks are readily recognised and can be laterally correlated on downcore log...

A coherent middle Pliocene magnetostratigraphy, Wanganui Basin, New Zealand

Abstract We document magnetostratigraphies for three river sections (Turakina, Ran‐gitikei, Wanganui) in Wanganui Basin and interpret them as corresponding to the Upper Gilbert, the Gauss and lower Matuyama Chrons of the Geomagnetic Polarity Timescale, in agreement with foraminiferal biostratigraphic datums. The Gauss‐Gilbert transition (3.58 Ma) is located in both the Turakina and Wanganui River sections, while the Gauss‐Matuyama transition (2.58 Ma) is located in all three sections, as are the lower and upper boundaries of the Mammoth (3.33–3.22 Ma) and Kaena (3.11–3.04 Ma) Subchrons. Our interpretations are based in part on the re‐analysis of existing datasets and in part on the acquisition and analysis of new data, particularly for the Wanganui River section. The palaeomagnetic dates of these six horizons provide the only numerical age control for a thick (up to 2000 m) mudstone succession (Tangahoe Mudstone) that accumulated chiefly in upper bathyal and outer neritic palaeoenvironments. In the Wanganui River section the mean sediment accumulation rate is estimated to have been about 1.8 m/k.y., in the Turakina section it was about 1.5 m/k.y., and in the Rangitikei section, the mean rate from the beginning of the Mammoth Subchron to the Hautawa Shellbed was about 1.1 m/k.y. The high rates may be associated with the progradation of slope clinoforms northward through the basin. This new palaeomagnetic timescale allows revised correlations to be made between cyclothems in the Rangitikei River section and the global Oxygen Isotope Stages (OIS) as represented in Ocean Drilling Program (ODP) Site 846. The 16 depositional sequences between the end of the Mammoth Subchron and the Gauss‐Matuyama Boundary are correlated with OIS MG2 to 100. The cyclothems average 39 k.y. in duration in our age model, which is close to the 41 k.y. duration of the orbital obliquity cycles. We support the arguments advanced recently in defence of the need for local New Zealand stages as a means of classifying New Zealand sedimentary successions, and strongly oppose the proposal to move stage boundaries to selected geomagnetic polarity transitions. The primary magnetisation of New Zealand mudstone is frequently overprinted with secondary components of diagenetic origin, and hence it is often difficult to obtain reliable magnetostratigraphic records. We suggest specific approaches, analytical methods, and criteria to help ensure robustness and coherency in the palaeomagnetic identification of chron boundaries in typical New Zealand Cenozoic mudstone successions.

Palaeosecular variation for 0.1-21 Ka from the Okataina Volcanic Centre, New Zealand

Studies of palaeodirections and palaeointensities were carried out on mainly rhyolitic lavas and pyroclastics from the Okataina Volcanic Centre, New Zealand, which has erupted during the past 32 kyr. Of the 17 sites studied, 14, spanning the period 0.1-21 Ka yielded good mean palaeodirections, while three carried unstable natural remanent magnetizations. Of 49 specimens from 7 sites, on which Thellier palaeointensity experiments were carried out, 21 specimens gave successful results, yielding 3 site mean palaeointensities for 1886 AD, 5 Ka, and 7.5 Ka. When the new palaeodirections, together with previously reported archaeo and volcanic data, were compared with sedimentary records from New Zealand and eastern Australia for the last 10 kyr, good agreement was obtained in inclination but discrepancies were observed in declination. The new 7.5 Ka and 5 Ka palaeointensities are moderately high and relatively low, respectively, and are concordant with the global trend. The mean palaeointensity obtained for 1886 AD is, 11% higher than the IGRF1900. The difference is scarcely significant, but might indicate a small bias toward high values. Although the sedimentary directional curves show excellent agreement with the prediction from CALS7K, the fit of the palaeointensity data to model values was relatively poor over the wider Pacific region. Further reliable palaeointensity data are needed to solve the discrepancy.

Deriving a long paleoseismic record from a shallow-water Holocene basin next to the Alpine fault, New Zealand

A sedimentary sequence that was highly sensitive to fault rupture–driven changes in water level and sediment supply has been used to extract a continuous record of 22 large earthquakes on the Alpine fault, the fastest-slipping fault in New Zealand. At Hokuri Creek, in South Westland, an 18 m thickness of Holocene sediments accumulated against the Alpine fault scarp from ca. A.D. 800 to 6000 B.C. We used geomorphological mapping, sedimentology, and paleoenvironmental reconstruction to investigate the relationship between these sediments and Alpine fault rupture. We found that repeated fault rupture is the most convincing mechanism for explaining all the features of the alternating peat and silt sedimentary sequence. Climate has contributed to sedimentation but is unlikely to be the driver of these cyclical changes in sediment type and paleoenvironment. Other nontectonic causes for the sedimentary alternations do not produce the incremental increase in basin accommodation space necessary to maintain the shallow-water environment for 6800 yr. Our detailed documentation of this near-fault sedimentary basin sequence highlights the advantages of extracting paleoearthquake records from such sites—the continuity of sedimentation, abundance of dateable material, and pristine preservation of older events.