An integrated terrestrial paleoenvironmental record from the Mid-Pleistocene transition, eastern North Island, New Zealand

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Quaternary International 178 (2008) 146–166

An integrated terrestrial paleoenvironmental record from the Mid-Pleistocene transition, eastern North Island, New Zealand Elizabeth M. Kennedya,, Brent V. Allowaya, Dallas C. Mildenhalla, Ursula Cochrana, Brad Pillansb a GNS Science, P.O. Box 30-368, Lower Hutt, New Zealand Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia

b

Available online 3 March 2007

Abstract A tephra-bearing lake sequence from near Ormond, New Zealand, provided the opportunity to conduct a multi-proxy paleoenvironmental analysis within the Mid-Pleistocene time period. A 10.5-m-thick section was measured and analysed for pollen, spores, diatoms, macrofossils, and tephra geochemistry. Palynological assemblages in the lower 4 m of the section indicate an ameliorating temperate climate and increased humidity. The upper 6.5 m is dominated by diatomite that did not yield sufficient palynomorphs for study. The source vegetation was distal lowland mixed broadleaf podocarp forest, swamp forest from ca. 2.5–4.0 m, and proximal scrubland with sporadic forest encroachment. Abundant freshwater algae in the samples suggest that any brackish influence in the lower part of the section was minor. Marine dinoflagellates found in the basal pollen samples could be mostly recycled. Diatomite samples were overwhelmingly dominated by a freshwater but mildly salt-tolerant diatom. We interpret the section as representing a shallow coastal lowland lake that intermittently had access to the sea. The chronology of the section is based on a single glass-ITPFT age of 0.6270.09 Ma from a tephra interbed (T8) in the lower part of the section. In addition, the lowermost tephra in the sequence (T1) is geochemically correlated to AT-485, preserved in the marine record east of New Zealand, with an astronomically tuned age of 0.7163 Ma. This chronology places the basal portion of the Ormond section within the Brunhes normal Chron with sequence deposition occurring during either the OIS 18/17 or 16/15 transitions. However, paleomagnetic data indicate a clear reverse polarity from an equivalent stratigraphic position and at this stage it is difficult to reconcile this age discrepancy. r 2007 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction The Mid-Pleistocene transition (MPT) is a significant period of climate change between ca. 1.2 and 0.5 Ma when global climate cycles underwent a major shift in periodicity. Prior to this shift, climate fluctuated on a broad ca. 41 ka cycle, but the major climate cycles lengthened to ca. 100 ka during the MPT. The MPT is the focus of numerous recent studies (e.g. Mildenhall et al., 2004; Head and Gibbard, 2005) which are trying to establish rates of climate change Corresponding author. Tel.: +64 4 5704838; fax: +64 4 5704600.

E-mail addresses: [email protected] (E.M. Kennedy), [email protected] (B.V. Alloway), [email protected] (D.C. Mildenhall), [email protected] (U. Cochran), [email protected] (B. Pillans).

and to discover what happened to climate and biota during these cycle changes. Most previous MPT studies in New Zealand have focussed on marine sections (e.g. ODP Leg 181, Site 1123 record, and Wanganui Basin), including study of terrestrial palynomorphs (Mildenhall, 2003; Mildenhall et al., 2004). Although the onshore terrestrial record of the MPT has received relatively little detailed attention, some short sequences, incorporating part or all of the MPT, have been investigated for their palynological content. Such sequences include the top of the Okariha and Oruarangi sections (Nelson et al., 1989), Patiki (Byrami, 2003; Byrami et al., 2005), and Transmission Gully (Mildenhall and Alloway, this volume). Pollen from extinct plants occur within these sediments—some are certainly in situ and represent last appearances, some are definitely recycled

1040-6182/$ - see front matter r 2007 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2007.02.011

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from older sediments, and some, such as Haloragacidites harrisii (Casuarina), could be recycled, in situ, or even the result of long distance dispersal from Australia. Wellpreserved pollen of Casuarina is found in Holocene sequences and recent deposits (Moar, 1969; Mildenhall, 1995). These onshore sequences often have the disadvantage of representing short periods of time, of having missing time of unknown duration, and of having no alternative means of confirming dating where tephras are absent. Because many terrestrial sequences derive from environments such as swamps, the vegetation changes may be seral rather than climatic and the regional changes in the vegetation may be poorly represented. In addition, other published but poorly dated sequences in New Zealand could well fall within this time frame, and those previously considered to be older than 1 Ma may instead be much younger. This is because recent research has shown that many plant taxa previously believed to become extinct at ca. 2 Ma actually became extinct within the last 1 Ma (Mildenhall and Byrami, 2003), especially in northern North Island. Lacustrine deposits provide potentially important archives, particularly if they represent relatively long periods of time because they contain a regional pollen rain and hence may record climate changes. A section of lake sediments of Mid-Pleistocene age from two outcrops in the Ormond Valley near Gisborne, North Island, New Zealand, was investigated in detail (Fig. 1). The occurrence of numerous tephra horizons provided the opportunity to directly date the section to allow correlation and comparison with other sequences. This paper discusses the paleoenvironmental record preserved in this section and shows how tephrochronology was essential for interpreting the section in a broader context. Ormond Valley is a small rural valley surrounded by low hills and bounded to the north by a hill scarp of the Mangatuna Formation (Neef et al., 1996; Mazengarb and Speden, 2000). The predominantly lacustrine Mangatuna Formation is up to 150 m thick in places (Mazengarb and Speden, 2000) and holds the potential for a detailed terrestrial record of biota and paleoenvironment through the MPT. Lake sediments in the Gisborne region have been investigated intermittently for over 100 years with a focus on the fossil flora and fauna (e.g. Hill, 1888, 1889; Oliver, 1928; McQueen, 1954; Kennedy and Alloway, 2004a, b). There have been few studies on these fossils, and the age and stratigraphy of the lake sediments are not well known. Early investigations reported leaf, fish, and bivalve impressions as well as other plant debris. A report of feather impressions was also made by Hill (1889); these specimens have not been observed during this study and no new evidence of feather impressions was found. Oliver (1928) described 31 plant species from collections made by Hill (in 1910) and Ongley (in ca. 1914–1916) from Ormond Valley, of which 24 were angiosperm species. Small leaf collections were made by Phil Huber, Len Brown, and

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Colin Mazengarb in 1996 as part of mapping and landscape change studies in the Waipaoa region (Mazengarb and Speden, 2000). Two masterate projects (Charleston, 1999; Coleman, 1999) were undertaken in the Waipaoa catchment area and included investigation of Pleistocene lake sediments. Charleston (1999) collected and discussed some fossil leaf material from Ormond Valley and his site is almost certainly the same as site GF03/4 investigated briefly in this study. The Ormond section was measured as part of a project investigating the application of quantitative leaf morphology-based paleoclimate methods to the New Zealand Pleistocene (Kennedy and Alloway, 2004a, b). An attempt was made to re-investigate the same locality from which Oliver described fossil leaves and to use leaf fossil data to examine the application of leaf-based paleoclimate methods to New Zealand Pleistocene assemblages. However, although we believe we recollected from the same general locality as the researchers of the 20th century, the original, and comparatively richly fossiliferous, horizons described by Oliver (1928) were not rediscovered. We investigated a series of exposures cropping out along a hill scarp on the north side of Ormond Valley. An initial climate analysis of previously collected leaf material held by GNS Science was made by Kennedy and Alloway (2004a). 2. Methods The section was cleared and measured on a cm to dm scale (Fig. 2) and samples were collected for palynology, tephrochronology, paleomagnetic analysis, and studies of diatoms and macrofossils. Most of the section was measured at site GF03/1 (E2941009 N6283498), but the upper 2 m was measured from a nearby outcrop, site GF03/ 2 (E2940815 N6283442). Grid references are for the NZMS 260 1:50 000 topographic map series and were taken by hand-held GPS. Palynological samples were collected from the section on two separate occasions and processed using standard palynological techniques by the GNS Science laboratory. All pollen samples are catalogued in the New Zealand Fossil Record File system (Y17/f1628–f1630). Initially three pollen samples were analysed from this section (Y17/f1628–30: L21127–29), and an additional higher resolution suite of 10 samples (Y17/f1628a: L22159–68) was collected later from only the lower 4 m of the section because the initial samples showed that the diatomite higher in the section did not yield sufficient palynomorphs. The uppermost sample (L21127), which came from the leaf and fish fossil-bearing diatomaceous silt, did not contain any identifiable pollen grains, except for significant modern contamination. Pollen slides and residues are lodged in the GNS Science palynological collection. Each sample has been given a unique slide number prefixed with the letter ‘L’. Counts of between 300 and 500 grains were made for each productive sample but absolute abundances were not obtained.

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Fig. 1. (a) Map of Gisborne area of eastern North Island showing the distribution of Plio-Pleistocene sedimentary deposits (shaded) and the locations of the Ormond section and nearby sites where sequences (including tephra) have been described. (b) and (c) are modified from Alloway et al. (2005). (b) New Zealand regional setting with the Taupo Volcanic Zone (TVZ), ODP Leg 181 drillhole sites, and DSDP site 594, where possible offshore correlatives have been identified. (c) North Island plate tectonic setting, the main axial ranges (black), and main tephra source volcanoes including rhyolitic calderas in the TVZ, andesitic stratovolcanoes of Tongariro Volcanic Centre (TgVC) and Egmont Volcanic Centre (EgVC), and basaltic volcanic field of Auckland (AkVF). Location of Wanganui Basin and Ormond Valley also shown.

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Fig. 2. Composite stratigraphy of two sections exposed at Gallagher’s Farm, Ormond Valley Road, at Sites GF03/1 (Y17/410834*) and GF03/2 (Y17/ 408834*). *Grid references of 1:50 000 New Zealand Map Series NZMS 260.

Plant macrofossils required minimal preparation using an airscribe and hand tools. The new Ormond plant fossil collection is catalogued at GNS Science under the ‘B’ number series as B1356–60 (plant material was collected

from five outcrops). Fossil leaf specimens from an existing collection (GNS Science—B195) from Ormond were also re-investigated. The fish impressions were sent to Dr. R. McDowall at the National Institute of Water and

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Atmosphere (NIWA) for identification (McDowall et al., 2006a, b) and are lodged at GNS Science with individual vertebrate collection numbers CD597–98, 607–612 and associated ‘B’ numbers. Six diatom samples were physically and chemically processed to concentrate diatom valves, and mounted permanently on microscope slides. Three additional samples were examined as temporary wet slides to determine if diatoms were present. Residues and slides from the processed samples are stored at GNS Science under a unique ‘D’ number. Diatom species were identified at  1000 magnification with reference to standard floras. For samples on temporary mounts, or those with rare diatoms, full counts were not attempted. For all other samples at least 300 diatom valves were counted. Samples from 18 tephra beds (T) and 5 hyperconcentrated flow deposits (hcf) (Fig. 2) recorded in the Ormond Valley Road composite section were described and collected for geochemical analysis and isothermal plateau fission-track (ITPFT) dating. Fresh glass shards were subsampled from all tephra beds for electron microprobe analysis. The resultant glass major oxide and chlorine contents were used to characterise tephra beds and assist in their possible correlation to ODP-Sites (Table 1). Direct dating using the glass-ITPFT technique (Westgate, 1989) was conducted on one comparatively coarse-grained tephra bed (T8) (Table 2). Most of the Ormond Valley Road tephra beds were unsuitable for glass-ITPFT dating because of their fine grain size. Freshly excavated vertical field exposures at the Ormond Section were sampled for paleomagnetic analysis using 6cm3 plastic boxes, which were carefully pushed into the soft sediment, or fitted over cube-shaped pedestals that were carved with a sharp knife. Samples from field exposures were oriented with a compass corrected for local declination. Samples were stored in magnetically shielded containers to minimise acquisition of any modern magnetic field components. Care was taken to keep samples moist until paleomagnetic measurements were made at Black Mountain Paleomagnetic Laboratory, Canberra. Stepwise thermal demagnetisations were performed using a Schonstedt TSD-1 oven and magnetic remanence directions were determined after each heating step using a ScT 2-axis cryogenic magnetometer. Magnetic susceptibilities were measured on a Digico bulk susceptibility bridge to monitor possible mineralogical changes with increasing demagnetisation temperature. Characteristic remanent magnetisations (ChRMs) were identified by principal component analysis (PCA) (Kirschvink, 1980). 3. Results 3.1. Stratigraphy Outcrops of the dominantly lacustrine Mangatuna Formation exposed along a scarp in the Ormond Valley

were investigated (Fig. 1). Although outcrop along the scarp is discontinuous, we were able to measure in detail a 10.5-m-thick composite section, mostly from one outcrop (GF03/1) (Fig. 2). Lithologies in the section include carbonaceous to non-organic silts, diatomite, silicic tephrafall, and hyperconcentrated to flood-flow siliciclastic horizons (Fig. 3). The basal 4 m of section is dominated by fine-grained (silt to sand size), mm- to cm-thick planar beds of varying carbonaceous content. The upper 6.5 m of section consists of pale yellow to light yellowish brown, finely laminated diatomaceous lake sediments. From ca. 4 to 6.5 m the laminated lake sediments alternate with siliciclastic flow deposits (Fig. 2). Macrofossils were found sparsely distributed within diatomite and silt horizons and were collected mostly from the laminated diatomaceous lake sediments. Leaves are mostly preserved as impressions, some of which have well-preserved fine detail, but others are faint and poorly preserved. Fish impressions and bivalve fragments (cf. Hyridella sp.) were also found. The generally faint leaf impressions, faunal remains that are also only impressions, and poor palynomorph yield suggest that oxidation compromised organic preservation in the diatomite unit. Eighteen dm- to cm-thick tephra beds were found within the section as well as 5-dm-thick siliciclastic hcfs (Figs. 2 and 3). Fifteen of these tephras occur within the lowermost 4 m of section; the other three tephras occur within the diatomaceous-laminated lake sediments. The tephra beds in most instances are laterally continuous, massive to normal-graded, sometimes bioturbated, mm- to cm-thick layers of very fine to medium vitric sand-textured ash. Multiple mm-planar bedding in some beds suggests local redeposition. The lower contacts of the tephra beds are accentuated by strong brown iron and black manganese oxide staining. White to grey, coarser-grained (sand-sized) hcfs punctuated lacustrine deposition. They comprise dm-thick, slightly cemented, irregular, and crudely stratified planar to low-angle cross-bedded, very coarse to fine sandtextured vitric and pumiceous ash with evidence of highenergy deposition in the form of lake sediment rip-up clasts, convolute dewatering structures and irregular scouring of underlying beds (Fig. 3). The massive, white, very fine silt at 558–663 cm was not found to contain diatoms in significant quantities and is probably a vitric siliciclastic deposit. Major oxide compositions of glass shards from the tephras reveal ranges for SiO2 of 70–78 wt% and K2O+Na2O of 6.0–7.9 wt%, when the shard compositions are averaged and recalculated to 100% on a water-free basis (Fig. 4). Using the compositional classification scheme of Le Maitre (1984), all Ormond Valley Road tephra beds plot within the dacitic to rhyolitic fields (Fig. 5). Many tephra beds are homogeneous (e.g. T1, T8, T17) with respect to inter-shard variation and can be

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Table 1 Major element glass compositions of tephra layers and hyperconcentrated flow (hcf) deposits from Ormond Valley Road and potential ODP-181 tephra correlatives SiO2

TiO2

Al2O3

FeO

MnO

MgO

CaO

Na2O3

K2O

Cl

H2O

n

0.22 0.10 0.17 0.08 0.35 0.12 0.16 0.10 0.18 0.09 0.65 0.16 0.41 0.20 0.14 0.09 0.23 0.10 0.37 0.06 0.17 0.06 0.14 0.07 0.19 0.07 0.17 0.11 0.14 0.08 0.12 0.07 0.12 0.08 0.21 0.08 0.28 0.09 0.36 0.06

13.33 0.21 12.65 0.26 13.80 0.53 12.68 0.14 12.78 0.30 14.71 0.30 14.47 1.29 12.67 0.35 13.14 0.27 13.87 0.17 13.21 0.10 13.22 0.17 13.25 0.14 13.28 0.16 12.80 0.11 12.94 0.21 12.72 0.28 13.60 0.18 13.45 0.18 13.20 0.25

1.70 0.19 1.33 0.10 2.10 0.37 1.36 0.05 1.45 0.20 3.06 0.20 2.51 0.81 1.39 0.15 1.65 0.15 2.23 0.06 1.72 0.06 1.72 0.05 1.72 0.07 1.72 0.13 1.53 0.08 1.49 0.14 1.31 0.18 1.94 0.06 1.94 0.21 1.87 0.20

0.05 0.06 0.06 0.05 0.08 0.05 0.06 0.05 0.04 0.04 0.07 0.03 0.08 0.05 0.06 0.06 0.04 0.04 0.07 0.05 0.06 0.05 0.06 0.04 0.07 0.06 0.06 0.04 0.05 0.04 0.06 0.06 0.06 0.04 0.06 0.04 0.26 0.05 0.28 0.05

0.15 0.03 0.11 0.03 0.34 0.16 0.09 0.02 0.13 0.05 0.63 0.11 0.44 0.27 0.12 0.04 0.18 0.03 0.37 0.02 0.14 0.03 0.14 0.02 0.15 0.03 0.14 0.03 0.10 0.03 0.12 0.03 0.11 0.04 0.22 0.03 – – – –

1.10 0.10 0.83 0.08 1.64 0.41 0.86 0.05 0.94 0.10 2.55 0.23 2.05 0.77 0.97 0.15 1.21 0.10 1.91 0.03 1.03 0.03 1.05 0.04 1.04 0.05 1.03 0.06 0.88 0.03 0.97 0.08 0.91 0.14 1.38 0.04 1.29 0.16 1.41 0.13

4.09 0.26 3.92 0.11 3.85 0.38 3.84 0.13 3.77 0.16 4.15 0.22 3.87 0.63 3.53 0.44 3.54 0.75 3.84 0.08 3.93 0.10 4.06 0.12 3.99 0.16 4.04 0.15 3.95 0.13 3.99 0.16 3.90 0.19 4.14 0.12 4.27 0.06 4.75 0.19

3.40 0.16 3.70 0.14 3.26 0.42 3.69 0.13 3.68 0.11 2.78 0.11 3.00 0.46 3.67 0.19 3.42 0.18 3.14 0.06 3.50 0.09 3.52 0.10 3.50 0.14 3.49 0.11 3.80 0.10 3.70 0.10 3.85 0.18 3.45 0.07 3.28 0.12 3.10 0.22

0.16 0.06 0.15 0.04 0.17 0.11 0.16 0.03 0.17 0.04 0.16 0.03 0.14 0.06 0.17 0.05 0.20 0.03 0.15 0.03 0.16 0.04 0.17 0.03 0.18 0.10 0.16 0.06 0.15 0.05 0.16 0.04 0.15 0.04 0.16 0.04 0.08 0.02 0.18 0.04

4.32 1.19 4.37 0.70 5.74 2.09 4.21 0.59 4.62 0.60 4.02 0.69 5.24 1.71 4.95 1.01 5.51 0.94 4.13 0.68 5.05 0.39 4.57 0.80 4.90 1.11 5.23 0.71 5.71 0.35 5.58 0.91 5.25 0.64 6.15 0.98 5.58 1.31 4.77 1.30

17

T8 (AT-575) T7 (AT-574) T6 (AT-573) T5 (AT-572) T4 (AT-571) T3 (AT-570) T2 (AT-569) T1 (AT-568) 1124, 28.37 AT-485 1123, 31.46 AT-337

75.82 0.54 77.12 0.36 74.43 1.37 77.13 0.22 76.91 0.49 71.28 0.70 73.07 2.45 77.32 0.52 76.44 0.67 74.07 0.29 76.11 0.17 75.95 0.17 75.95 0.21 75.95 0.33 76.63 0.26 76.47 0.45 76.90 0.60 74.88 0.29 75.08 0.53 74.80 0.61

hcf-5 (AT-591) hcf-4 (AT-590) hcf-3 (AT-589) hcf-2 (AT-588) hcf-1 (AT-587)

73.26 3.55 70.27 0.94 76.36 0.48 76.42 0.50 76.35 0.25

0.39 0.25 0.58 0.17 0.16 0.09 0.21 0.08 0.14 0.11

13.93 1.04 15.09 0.19 13.03 0.21 13.08 0.20 13.10 0.11

2.59 1.18 3.42 0.29 1.55 0.12 1.60 0.12 1.65 0.06

0.11 0.07 0.08 0.05 0.05 0.05 0.04 0.03 0.08 0.06

0.43 0.44 0.71 0.15 0.14 0.03 0.16 0.03 0.13 0.03

1.82 1.03 2.65 0.28 1.04 0.07 1.18 0.11 0.99 0.03

4.11 0.55 4.46 0.19 4.02 0.15 3.62 0.44 3.92 0.32

3.23 0.67 2.64 0.13 3.52 0.10 3.54 0.30 3.49 0.27

0.15 0.04 0.14 0.04 0.15 0.04 0.19 0.06 0.20 0.04

5.53 0.88 4.25 0.85 4.08 0.67 5.73 1.35 4.86 0.93

T18 (AT-585) T17a (AT-584) T16 (AT-583) T14 (AT-582) T13 (AT-581) T12 (AT-579) T11 (AT-578) T10 (AT-577) T9, pop.1 (AT-576) T9, pop.2

19 19 17 19 15 17 20 4 15 19 20 19 18 16 16 15 14 13 12 16 15 17 19 18

Analyses were made using a CAMEBAX SX-50 microprobe housed at the University of Toronto. The microprobe was operating at 15 kV, beam current at 10 nA, beam focus, and in scanning mode. All elements calculated on a water-free basis, with H2O by difference from 100%. All Fe expressed as FeO. Mean and 71s (in parentheses) based on n analyses. Analyst—B.V. Alloway. All samples normalised against glass standard UA-5831.

readily distinguished on the basis of a few glass major element oxides such as FeO and CaO (wt%). Other tephra beds appear heterogeneous with several discrete populations (e.g. T9) or a near continuum of shard compositions spread out along a compositional trend (e.g. T16). Such

compositional heterogeneity implies either magma mixing, syn- and post-eruption entrainment of xenolithic material, or sedimentary mixing with older unconsolidated tephra (e.g. Shane, 2000). Irrespective of origin, such compositional variability, once recognised, can potentially enhance

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Table 2 Glass isothermal plateau fission-track (ITPFT) age of Ormond Valley Road T8 (AT-575), Ormond Sample number (analyst)

UT2060 (BVA)

Etching conditions HF:temp:time

Ds

Di

Ds/Di

Age

(104 t/cm2)

Track density on muscovite detector over dosimeter glass (105 t/cm2)

(%:1C:s)

(mm)

(mm)

(mm)

(106 yr)

6.8770.09 (5291)

4.89870.04 (15,640)

24:21:150

5.8070.15 (86)

5.8270.09 (320)

1.0070.03

0.6270.09

Spontaneous track density

Induced track density

(102 t/cm2) OVRd-T8 (AT575) 2.7370.02 (99)

The population-subtraction method was used; details are given in Westgate et al. (1997). Correction for partial track fading is done by heating the induced and spontaneous aliquots at 150 1C for 30 days (Westgate, 1989). Ages calculated using the zeta approach and lD ¼ 1.551  1010 yr1. Zeta value is 31873 based on 6 irradiations at the McMaster Nuclear Reactor, Hamilton, Ontario, using the NIST SRM 612 glass dosimeter, Jankov Moldavite (Lhenice locality, Czech Republic) with an 40Ar/39Ar plateau age of 15.2170.15 Ma (Staudacher et al., 1982; Balestrieri et al., 1998) and Roccastrada R2V glass with an 40Ar/39Ar plateau age of 2.44470.015 Ma (Balestrieri et al., 1998). Error (71SD) is calculated by combining the Poisson errors on the spontaneous and induced track counts and on the counts in the muscovite detector covering the dosimeter glass. Ds ¼ mean spontaneous track diameter and Di ¼ mean induced track diameter. Number of tracks counted is given in brackets. Analyst: BVA—B.V. Alloway.

correlation by providing additional fingerprinting criteria (Alloway et al., 2005). 3.2. Paleomagnetic data A summary of paleomagnetic data obtained from the Ormond Valley Road site is presented in Table 3. In general, samples were weakly magnetised, with typical magnetic intensities less than 0.4 mA/m, except in the case of three samples from T7, which were in the range 1.4–2 mA/m. Samples were given a reliability ranking (Class A, B or C), based on their demagnetisation behaviours (cf. Alloway et al., 2004). Class A. Stable magnetic directions, indicated by linear demagnetisation vector plots, with little angular scatter. Linear segments with a mean angular dispersion (MAD) of less than 5 were accepted. Reliable polarity identification of ChRM at intermediate and high temperatures (4250 1C). Class B. Demagnetisation vector plots and PCA do not yield linear components; however, magnetic polarity can be assigned on the basis of consistency of directions (e.g. directions are confined to a single quadrant on the vector plot). Less reliable polarity. (No samples of Class B were encountered in the Ormond section.) Class C. Unstable, weakly magnetised, samples, displaying erratic directions on vector plots, or directions only stable at low temperatures (o150 1C). All low-temperature components are of normal polarity, representing a recent viscous overprint. Identification of ChRM is not possible. The results are quite clear for samples 010, 011, and 012, taken from T7. All three specimens are Class A (unambiguous) reversed polarity ChRM, after removal of a lowtemperature (secondary) normal overprint. Other specimens (004–009) from mudstones beneath T7 have southerly declinations, suggestive of an underlying reverse polarity magnetisation that is not fully resolved. These specimens also show a marked increase in susceptibility above

temperatures of about 300–4001, typical of Plio-Pleistocene mudstones from Wanganui Basin. Such an increase is caused by thermal alteration of the samples and the formation of magnetite during laboratory heating, which means that the results are spurious (cf. Turner, 2001; Turner et al., 2005). Our interpretation is that the section from T7 down possibly predates the Matuyama/Brunhes polarity transition at 0.78 Ma. The rest of the specimens (013–021) are weakly magnetised, with a low-temperature (secondary) normal polarity overprint, and primary polarity cannot be determined. 3.3. Palynology All the identifiable palynological grains were attributed to modern New Zealand taxa; no in situ extinct taxa were identified except for occasional specimens of Haloragacidites harrisii (Casuarina) which could be recycled or in situ. Results are summarised in Table 4 and Fig. 6 and full assemblage lists are available from GNS Science. In most samples the preservation is very uneven with many smaller grains, particularly those lacking surface sculpture, not identified as were numerous psilate algal cysts. These unidentified pollen grains are represented in the pollen diagram by ‘Unknowns’ (Fig. 6). Podocarpus and Prumnopitys were not always distinguished in this sequence because so many grains were disarticulated, badly etched, or diaphanous ‘ghosts’. Many recycled palynomorphs, including abundant marine dinoflagellate cysts and tasmanitids, abundant charcoal, and other black organic material occur in the basal sample (L22164). These dinoflagellates comprise a very mixed assemblage, some appear to be re-worked (e.g. Areosphaeridium sp., Hystrichokolpoma rigaudiae), others are consistent with a Mid-Pleistocene age, but could be reworked from earlier Pleistocene sediments. The Pleistocene species provide a mixture of environmental indicators. The assemblage includes Impagidinium sphaericum and Nemato-

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Fig. 3. (a) Basal sequence of tephras (T1–T8) and overlying hyperconcentrated flow deposit hcf-1; (b) diatomaceous laminated lake sediments (Lls) in the upper part of the section; (c) laminated lake sediments enveloping T9 and hcf-2. Note the rip-up clasts of underlying lake muds as well as convolute bedding which indicates rapid hcf-emplacement.

sphaeropsis labyrinthus which are open marine forms (de Vernal et al., 2001; Marret and Zonneveld, 2003), Operculodinium sp. and Spiniferites sp. which are generally found in shelf environments, and Lingulodinium machaerophorum which is thought to be a coastal species (Marret and Zonneveld, 2003). The mixture of environmental indicators amongst the Pleistocene-consistent dinoflagellate species implies some degree of sediment mixing and/or recycling. Recycled spores and pollen come from both Neogene (mainly) and Paleogene sediments and form at least 10% of the total terrestrial assemblage. The charcoal is probably a combination of recycled material and in situ charcoal formed from local fires, possibly caused by the

periodic volcanic episodes and lightning strikes. Charcoal is most abundant at the very top and base of the section and sparse in between. Coenobia from the freshwater chlorophycean algae Botryococcus and Pediastrum (Fig. 7h and i) are present. Botryococcus is salt-tolerant, often occurring in brackish water (Batten and Grenfell, 1996) whereas Pediastrum is salt-intolerant, often inhabiting eutrophic sites (Batten, 1996). These algae suggest a non-marine, non-turbulent, lacustrine depositional environment, possibly with access to the sea because a poorly preserved pollen grain tentatively identified as Avicennia marina (mangrove) was also located. The vegetation at the time was a lowland

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154

1.6

4

T8,AT-575 T7,AT-574 T6,AT-573 T5,AT-572 T4,AT-571 T3,AT-570 T2,AT-569 T1,AT-568

Ormond Valley Road T1-T8

1.4 T8,AT-575 T7,AT-574 T6,AT-573 T5,AT-572 T4,AT-571 T3,AT-570 T2,AT-569 T1,AT-568

2 T1, AT-568

1

CaO (wt%)

CaO (wt%)

3

1.2 1.0 0.8 0.6

0 1

2

3

0.8

4

1.0

1.2

1.0

1.2

1.4

1.6

1.8

2.0

2.2

1.4

1.6

1.8

2.0

2.2

5

4 Ormond Valley Road T9-T18

2 1

T18,AT-585 T17a,AT-584 T16,AT-583 T14,AT-582

T13,AT-581 T12,AT-579 T11,AT-578 T10,AT-577 T9,AT-576

3

4

0 1

2

FeOt (wt%)

KO2 (wt%)

CaO (wt%)

3 4

3 0.8

FeOt (wt%)

Fig. 4. CaO versus FeOt (wt%) composition of glass shards. (a) T1–T8; (b) T9–T18. Note that shard compositions from all samples are spread out along a compositional trend. CaO versus FeOt (c) and K2O versus FeOt (wt%) (d) compositions of glass shards from the closely clustered T1–T8. Note that T1 occupies a compositionally unique field whereas T8 is identical in composition to T7.

12 T

Na2+KO2 (wtO%)

10

R TA

8

D 6

4

A

Quaternary tephra Ormond Valley Road (T1-T18) Tongariro Volcanic Centre (TgVC) Egmont Volcanic Centre (EgVC)

2 55

60

65 70 SiO2 (wt %)

75

80

Fig. 5. Compositional scheme of Le Maitre (1984) showing the distinct rhyolitic (R)–dacitic (D) composition of tephra from Gallagher’s Farm, Ormond Valley. Glass compositions from the andesitic (A)–dacitic (D) Tongariro Volcanic Centre and the trachyte (T)–rhyolitic (R) Egmont Volcanic Centre are included for comparison (from Sandiford et al., 2001; Shane and Hoverd, 2002).

beech/podocarp forest with a grassland/scrubland immediately surrounding the deposition site. The climate was warm temperate and relatively frost-free. Although the abundance of Nothofagus plus grass pollen, especially in the lower samples, is suggestive of drier conditions (Newnham, 1999), significant moisture must have still been

available because ferns were clearly abundant and diverse. The mixture of freshwater/marine/recycled taxa in the basal sample may be indicative of a catastrophic event, such as a cyclone or tsunami. In higher samples unidentified freshwater algae occur, but there is no convincing evidence of in situ marine dinoflagellates and the samples commonly contain zygospores of the purely freshwater algae Debarya (Zygnemataceae) (Fig. 7j), along with Botryococcus and Pediastrum. The latter two are often in bloom proportions suggesting the existence at the time of constant shallow ponds or lakes with significant eutrophic conditions. Such de-oxygenated, high-nutrient environments may be the result of the frequent volcanic activity in the central North Island. The next six samples (up to and including sample L22159) above the basal sample contain significant grass and shrub pollen and the diversity and abundance of fern spores decreases dramatically. The forest appears to be some distance from the deposition site while near the site open ground is indicated by the presence of diverse spores from the taxa Anthocerotales, Ricciaceae, Lycopodium, and Sphagnum. Drier conditions are indicated by an increase in grass and shrub pollen relative to the lowest sample. Acidic swamps containing Restionaceae (Empodisma, jointed rushes), Cyperaceae, and Gleichenia, with Phyllocladus common, developed adjacent to the lake. A moderate climate shift is detectable within the sampled interval, towards a warmer and more humid climate. Fuscospora, and pollen from herbs and shrubs decrease as

Table 3 Summary of paleomagnetic data from Ormond Valley Road

79.5 71.2 49.4

Indet. Indet. Indet.

C C C

170.8 255.3 186.6

32.6 81.5 45.8

Indet. Indet. Indet.

C C C

1.49

200.7

37.2

GIS011

1.46

189.9

29

GIS012

1.96

194.8

39.6

0.28 0.26 0.18

36.7 299.8 88.7

73.3 86 84.2

NRM Int (mA/m)

NRM Dec (deg)

NRM Inc (deg)

GIS004 GIS005 GIS006

19 cm below T1

0.23 0.29 0.35

245.4 206.5 209.2

GIS007 GIS008 GIS009

7 cm above T4

0.16 0.11 0.14

GIS010

Tephra T7

Component

Temperature range (1C)

Dec (deg)

Inc (deg)

LT IT HT LT IT HT LT IT HT

TH50–TH150 TH150–TH400 TH450–TH550 TH50–TH150 TH150–TH400 TH450–TH550 TH50–TH150 TH200–TH350 TH400–TH550

17.6 193.7 202.5 15.8 189.9 191.4 8.4 194.6 196.7

64.4 44.6 55.7 66.9 39.7 48.9 67.2 47.1 52.6

NOP R R NOP R R NOP R R

A

LT

TH50–TH150

341.9

69.6

LT

NRM–TH100

25.4

65.9

NOP? Indet. NOP?

C C C

A

A

GIS013 GIS014 GIS015

43 cm below T9

GIS016 GIS017 GIS018

87 cm above T9

0.4 0.4 0.38

11 351.3 11.8

63.1 61.9 60

LT LT LT

NRM–TH150 NRM–TH150 TH50–TH150

17.9 15.2 30.7

64.4 66.4 70.1

NOP? NOP? NOP?

C C C

GIS019 GIS020 GIS021

Silts above LK3

0.16 0.18 0.3

206.4 170.4 340.2

28.3 1 74.5

LT

NRM–TH100

9.3

67.8

LT

NRM–TH150

9.7

63.4

NOP? Indet. NOP?

C C C

Notes: N ¼ normal; NOP ¼ normal overprint; R ¼ reverse; Indet. ¼ indeterminate; LT ¼ low temp; IT ¼ intermediate temp; HT ¼ high temp. a Reliability ranking (see text).

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Classa

Description

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Polarity

Specimen

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Table 4 Summary of palynology samples and interpretation of vegetation, climate, and depositional environment based on palynological assemblages Slide number

Section height (above section base)

Source vegetation and climate

Depositional environment

Within leaf-bearing diatomite

No fossil yield, all modern contaminants

No fossil yield, all modern contaminants

Y17/f1628 L22168

393 cm

Lowland broadleaf podocarp swamp forest. Warm climate conditions. Humid

Lowland swamp, possible open sea access

Y17/f1628a L21129

351 cm

Broadleaf-podocarp forest, lake margin scrub. Warm temperate climate. Humid

Lacustrine

Y17/f1630 L22167

326 cm

Lowland broadleaf podocarp forest, probably Kahikatea swamp forest, slowly disappearing from the deposition site. Warm climate conditions. Humid.

Lowland swamp

Y17/f1628a L22166

250 cm

Lowland broadleaf podocarp forest, probably a Kahikatea swamp forest. Pollen assemblage indicates warm climate conditions. Humid.

Lowland swamp, no open water

Y17/f1628a L22165

215 cm

Lowland broadleaf podocarp forest. Warm temperate, humid. Pollen from shrubs show a dramatic decline, suggests forest is encroaching on the depositional site.

Lacustrine, swampy?

Y17/f1628a L22159

134 cm

Lowland broadleaf podocarp forest. Abundance of scrub pollen, a reflection of lake margin deposition with a forest inland of the site. Warm temperate, dry.

Lacustrine

Y17/f1628a L22160

127 cm

Lowland broadleaf podocarp forest. Abundance of scrub pollen, a reflection of lake margin deposition with a forest inland of the site. Warm temperate, dry.

Lacustrine

Y17/f1628a L22161

121 cm

Lowland podocarp forest, abundant scrub pollen. No warm climate indicators (but pollen recovery and diversity poor). Dry.

Lacustrine

Y17/f1628a L22162

117 cm

Lowland broadleaf podocarp forest. Abundant scrub pollen, especially Coprosma, reflection of lake margin deposition with forest inland of the site. Warm temperate, dry.

Lacustrine

Y17/f1628a L21128

113 cm

Beech forest, lake margin scrub and grasslands. Cool temperate climate, dry. Botryococcus dominant.

Lacustrine

Y17/f1629 L22163

112 cm

Lowland broadleaf podocarp forest and a marginal scrubland with acid swamp nearby. Conditions were warm temperate.

Lacustrine or lagoonal

Y17/f1628a L22164

71 cm

Coastal broadleaf podocarp forest. Nothofagus common. Climate was warm temperate, frostfree. Massive recycling including dinoflagellates.

Freshwater coastal estuary or lagoon. Mangroves?

FRF number L21127

Y17/f1628a Note that samples L21127–9 were studied prior to collection of the other 10 samples and have separate FRF numbers.

Cyathea spores, podocarp pollen (including Podocarpus totara, Prumnopitys taxifolia, and Dacrydium cupressinum) increase and plants normally found in frost-free or north-

ern New Zealand sites at the present day appear, especially Ascarina lucida, Dodonaea viscosa (Fig. 7(g), and Nestegis (Fig. 7f). Dacrycarpus dacrydioides increased up to the

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Fig. 6. Pollen diagram for the lower 4 m of the Ormond section. Tephras in this part of the section are indicated by T1–T14.

157

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Fig. 7. Examples of the Ormond fossil flora and fauna. Scale bar for images a–e, m is in centimetres. Scale bar for diatom images (k and l) is 20 mm. (a) Beilschmiedia ovata leaf, (b) Plagianthus betulinus leaf, (c) Clematis obovata leaf, (d) Pittosporum sp., (e) carbonaceous plant debris, (f) Nestegis, (g) Dodonaea viscosa, (h) Botryococcus, (i) Pediastrum, (j) Debarya, (k) Cyclostephanos dubius, (l) diatom assemblage, and (m) Prototroctes (grayling) impression.

middle of this interval where the local forest could well have been a Dacrycarpus swamp forest, and then decreases as full lowland podocarp/broadleaf forest nears the lacustrine deposition site. Agathis australis (kauri) pollen occurs in very small numbers throughout the sampled interval.

3.4. Macroflora and fauna 3.4.1. Leaves Oliver (1928) described a well-preserved leaf flora from lake beds in the Ormond Valley. Some of Oliver’s (1928) nomenclature was subsequently revised (McQueen, 1954;

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Oliver, 1955; Mildenhall, 1970). The flora comprised leaves affiliated to a variety of modern genera including Dryopteris, Blechnum, Pteridium, Thelypteris, Platycerium, Typha, Rhopalostylis, Plagianthus (Fig. 7b), Litsea, Beilschmiedia (Fig. 7a), Clematis (Fig. 7c), Rubus, Pseudopanax, Paratrophis, Knightia, Coriaria, Ceratopetalum, Pittosporum (Fig. 7d), Melicope, Pennantia, Hebe, Coprosma, Melicytus, Kunzea, and Carmichaelia (Oliver, 1928; McQueen, 1954). In this study isolated leaf impressions were found throughout the laminated diatomite part of the section (Fig. 2). Some of the leaf impressions were indifferently preserved and it was difficult to attribute these to either the Oliver (1928) and McQueen (1954) forms or other modern genera. Other specimens had well-preserved venation detail where the impression was in sufficiently fine sediment. Nine different broadleaf angiosperm leaf forms were differentiated in the new collection (including Beilschmiedia, Pittosporum, Metrosideros, possibly Pseudopanax, and several unidentified broadleaf forms), as well as Typha, and occasional fern fragments (Kennedy and Alloway, 2004a). Leaf impressions did not, however, occur in sufficient abundance or diversity to make confident inferences about source vegetation or paleoenvironment. Some horizons yielded disseminated black plant debris (Fig. 7e), several with recognisable fragments of fern, podocarp, and very small (mm-scale) angiosperm leaves. This scattered debris is carbonaceous and is in contrast to the isolated pale or oxidised leaf impressions on other horizons and may represent charcoalified deposits from fires. 3.4.2. Fish and molluscs Several impressions of partly complete fish skeletons were collected from the laminated diatomite at three of the outcrop localities (GF03/1, 2 & 4 (Grid ref for GF03/4: E2941142 N6283560)). Most of the fish specimens belong to the genus Gobiomorphus (McDowall et al., 2006b), a common genus in New Zealand’s freshwater fauna today. However, two specimens were identified as probable Prototroctes, the southern grayling (McDowall et al., 2006a) (Fig. 7m). These are the first described fossil

159

specimens of Prototroctes, a genus present in both Australia and New Zealand until the late 1920s when the New Zealand species (Prototroctes oxyrhynchus) was deemed extinct, the only New Zealand freshwater fish known to have become extinct in post-European times. The Australian species (Prototroctes maraena) lives on today. Two bivalve impressions were found, one in the section at GF03/1 and the other at a nearby outcrop GF03/3 (Grid ref: E2940907 N6283489). This is in contrast to the numerous bivalve impressions reported by Charleston (1999) from the Mangatuna (Te Karaka) Formation at other localities. 3.5. Diatoms Nine samples from the GF03/1-2 section were investigated for diatoms (Table 5). Three of these samples were only examined for diatom presence (D5-7). Samples D457, D456, D458, D323, and D459 are all of the fossiliferous finely laminated lake sediments and consist almost entirely of diatoms with a very small component of silt. Concentrations of diatom valves are in the order of billions per gram of sediment. In comparison, typical diatomaceous lake sediments contain millions of diatoms per gram of sediment. The diatom assemblages are made up of between 94% and 99% Cyclostephanos dubius (Fig. 7k–l). This species is a planktonic, freshwater diatom that has been found in New Zealand in modern and Holocene samples from shallow lakes close to marine influence. It has water chemistry preferences for alkaline waters of pH 47 and slightly brackish waters of 0.5–1.0 g/l salinity (Van Dam et al., 1994). This affinity implies that the depositional environment for the diatomite was a shallow lake not far from an estuary or the sea. In order to characterise the remainder of the assemblages, counts of 100 valves, excluding Cyclostephanos dubius, were carried out for each sample (Fig. 8). Other species present are extant, freshwater, mainly benthic species and some prefer slightly brackish waters. They are consistent with a shallow, coastal lake environment. Most species identified occur in every sample and the small variations in percentage abundance between samples are

Table 5 Diatom assemblages and paleoenvironment Diatom sample no.

Section height (cm)

Lithology

D459 D323 D458 D322 D456 D457 [D7] [D6] [D5]

965 775 735 575 500 440 325 130 70

Laminated Laminated Laminated Silt Laminated Laminated Laminated Laminated Sandy silt

[D4] [D3] [D1] [D2]

D### is the GNS Science sample number and [D#] are field numbers.

diatomite diatomite diatomite diatomite diatomite silt sandy silt

Type of mount

Summary results

Permanent Permanent Permanent Permanent Permanent Permanent Temporary Temporary Temporary

Cyclostephanos dubius Cyclostephanos dubius Cyclostephanos dubius Rare diatoms Cyclostephanos dubius Cyclostephanos dubius Diatoms present Diatoms present Rare diatoms

diatomite diatomite diatomite diatomite diatomite

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Fig. 8. Percentage abundance of diatom species excluding Cyclostephanos dubius.

insignificant because these ‘other’ species make up only ca. 1–6% of the total assemblage. Therefore, paleoenvironmental conditions remained very stable throughout the time of diatomite deposition. Sample D322 from 575 cm (within a massive very fine silt) contains rare diatoms making it hard to determine an environment of deposition. The few valves that were encountered are similar to species in surrounding samples (Cyclostephanos dubius and Cocconeis placentula). One valve showed signs of dissolution so it is possible that a lacustrine diatom assemblage was present originally but has since dissolved out of the sediment. Alternatively, the few valves present may represent contamination. The three samples examined for diatom presence (D5, D6, D7, note these are informal field numbers) all contained diatoms but in low abundance. Further investigation of permanent

mounts would be needed to establish environments of deposition for these samples. 4. Synthesis and discussion 4.1. Age Oliver (1928) reported that the age of these beds was later Pliocene, but our work confirms a broadly MidPleistocene age (Henderson and Ongley, 1920). The glassITPFT age of Ormond Valley Road T8 is 0.6270.09 Ma, suggesting deposition during the Brunhes normal polarity event. Other tephra localities are known in the region (Fig. 9). A single glass-ITPFT age of 0.6270.06 Ma for a tephra bed (AT-157) occurring towards the base of the Rangitira Station section suggests age equivalence with the

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Fig. 9. Stratigraphic columns showing tentative correlation of tephra beds exposed at Ormond Valley Road to tephra sequences at Taengamata Stream, Rangitira Station, and potential correlatives offshore in ODP-1124, mcd—metres composite depth.

Ormond sequence. Although tephra correlation between Ormond, Taengamata Stream, and Rangitira Station sections remains to be firmly established, two tephra beds (AT-156 at Rangitira Station and AT-146 at Taengamata Stream) can be correlated on the basis of glass chemistry and stratigraphic association (B. Alloway, unpublished

data). These two prominent tephra beds are also tentatively correlated with T18 in the uppermost part of the Ormond sequence. The major element glass composition of the Ormond Valley Road T1 tephra is very similar to AT-337 at ODPSite 1123 (1123A 4H 6W 126-128, 31.46 mcd) and AT-485

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162

1.8 1.6

1.8

a

1.6 1.4

1.2 1.0

Onshore record AT-568,OVRd-T1 ODP-record AT-485,1124,28.37mcd AT-337,1123,31.46mcd

0.8 0.6

CaO (wt%)

CaO (wt%)

1.4

ODP-1124 tephra above possible OVRd-T1 correlatives AT-410 AT-413 AT-483 AT-414 AT-263 AT-415 AT-484

1.2 1.0 0.8 0.6

0.4

0.4

5

1.8

c

1.6

d

1.4

4

CaO (wt%)

K2O (wt%)

b

3

1.2 1.0 0.8 0.6

2 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 FeOt (wt%)

ODP-1124 tephra below possible OVRd-T1 correlatives AT-257 AT-486 AT-487 AT-488 AT-320 AT-321 AT-261

0.4 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 FeOt (wt%)

Fig. 10. CaO versus FeOt (a) and K2O versus FeOt (wt%) (b) composition of glass shards from potential Ormond Valley Road T1 (OVRd-T1) correlatives: AT-485 (28.37 mcd) and AT-337 (31.46 mcd) from ODP-1124 and -1123, respectively (Alloway et al., 2005). Potential Ormond Valley Road T1 correlatives are compared with ODP-1124 tephra above (c) and ODP-1124 tephra below (d) to illustrate that no other ODP-tephra either immediately below or above AT-485 has similar glass chemistry.

Fig. 11. Summary of record (ca. 0.5–0.9 Ma) from Sites 1124 and 1123 based on paleomagnetic polarity, reflectance, benthic d18O, and tephra depth/ thickness. The glass-ITPFT age for Ormond Valley Road T8 (71SD) is superimposed on the 1123 benthic d18O record for comparative purposes. Time/ volumes of major caldera-related ignimbrite eruptions from the Taupo Volcanic Zone (Houghton et al., 1995) are included to show broad age-equivalence with the Ormond Valley Road sequence. The TVZ-caldera source of the Ormond Valley Road sequence is presently unknown.

at ODP-Site 1124 (1124C 3H 3W 54-56, 28.37 mcd) which have astronomically tuned (1123) and graphically correlated (1124) ages of 0.8256 and 0.7136 Ma, respectively (Alloway et al., 2005) (Fig. 10). The graphically correlated age for AT-485 is supported by a glass-ITPFT age of 0.7170.08 Ma obtained from AT-257 located ca. 1.5 m (29.86 mcd) beneath AT-485 (Fig. 11) (Carter et al., 2003). Of the two potential ODP-correlatives, AT-485 from Site

1124 is preferred since the compositional positions of the dominant shard population and outlier shards are indistinguishable from T1. The glass chemistry of AT-485 from Site 1124 is distinctive—no ODP-tephra immediately below or above has similar glass chemistry, although the positions of outlier shards are similar to the dominant shard population of an older Site 1124 tephra bed (AT-486) at 30.4 mcd (Fig. 10). It is possible that during the eruption of

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AT-485 and its T1 correlative that older glass material from the earlier AT-486 eruption might have been entrained. None of the Ormond Valley Road tephra beds can be directly correlated to equivalent-aged sequences in proximal TVZ-source areas because of the paucity of nearsource exposure and lack of comparative mineral geochemistry. However, deposition of the closely spaced Ormond tephra beds appear to closely coincide in time with major ignimbrites erupted from the Kapenga Volcanic Centre (e.g. Rahopeka, 0.7770.03 Ma; Waiotapu, 0.7170.06 Ma; Matahana A, 0.6870.04 Ma) (Houghton et al., 1995). Paleomagnetic results indicate that all three samples from T7 have an unambiguous reversed polarity ChRM. This result is at complete odds with evidence (glass-ITPFT and onshore–offshore tephra correlation) indicating that the Ormond sequence was deposited during either the marine oxygen isotope stage (OIS) 18/17 or 16/15 transitions but in either case—during the early part of the Brunhes normal Chron. At this stage it is difficult to reconcile this discrepancy and necessitates further work in the Ormond area to re-examine the chronology. 4.2. Biota, depositional environment, and paleoclimate The palynology samples all suggest temperate conditions, becoming slightly more humid up section from relatively mild and dry conditions at the base. The depositional environment is a freshwater coastal lake or large pond, possibly open to the sea at times, near which a lowland coastal podocarp broadleaf forest gradually encroached, or the coastline gradually moved seaward. At times acid Dacrycarpus swamps were present around the lake. Algal species are predominantly freshwater and are common in many of the samples, indicating that if marine influences occurred then they were minor. Open water at the deposition site was shallow, low energy, relatively warm, poor in oxygen, and high in nutrients, probably supplied by the periodic ash falls. Climate was frost-free, relatively humid, possibly with dry periods. The basal two samples contain dinoflagellates that are clearly marine and are probably a mix of recycled and in situ forms. The palynology of the section suggests a warm temperate climate. The numbers and preservation of new leaf specimens found within the measured section were insufficient to warrant either a taxonomic-based, or a quantitative leaf morphology-based, climate analysis based solely on the new collection. Leaf morphology analysis of the historical leaf collection described by Oliver (1928, 1955) and McQueen (1954) from Ormond Valley suggests a cooler temperate climate (Kennedy and Alloway, 2004a), whereas Oliver (1928) interpreted the collection as representative of a warm temperate climate based on taxonomic composition and modern day species distributions. Uncertainties regarding (1) the stratigraphic position of the historical collection relative to the new measured section

163

and (2) possible methodological limitations related to the young age of this flora make direct comparison between the palynological interpretation from the section and the quantitative results from the historical macrofloral collection difficult (Kennedy and Alloway, 2004a). Deposition of diatomite requires environmental conditions conducive to diatom growth and a very low supply of sediment from the catchment (Harwood, 1999). One of the most important nutrients required for abundant diatom growth is silicon. It is possible that the numerous silicic tephra layers found in the Ormond sections were a source of silica to the catchment and waters of the paleo-lake thereby enabling high rates of diatom growth. A relationship between tephra deposition and diatom abundance and assemblage composition has been noted elsewhere (Harper et al., 1986). The presence of tephra layers is also likely to have contributed to preservation of the diatomite. It appears that the diatom-favourable conditions in the Ormond paleo-lake were specific enough to favour one particular species. There are likely to be numerous physical and chemical variables that combined to enable Cyclostephanos dubius to dominate the diatom flora. Some of these variables are likely to relate to the water chemistry preferences documented for Cyclostephanos dubius from modern samples—slightly brackish water and slightly elevated levels of nutrients such as nitrogen, phosphorus, and silicon (Van Dam et al., 1994). Monospecific concentrations of planktonic diatoms generally occur as seasonal blooms with a major bloom in spring or early summer as light levels and nutrient supplies increase and a secondary bloom in autumn if favourable conditions recur. The laminations in the Ormond diatomite may relate to deposition of such seasonal blooms and, if so, could provide some indication of deposition rates and the time interval represented by the diatomite. Sampling and examination of individual laminae would be required to test the seasonal nature of these layers. On the basis of T1 correlating with AT-485 at Site 1124 and palynological evidence at Ormond indicating ameliorating climatic conditions, the deposition of the T1 tephra appears to have coincided with the transition between cool and warm (interglacial) climate conditions (OIS 18/17) at ca. 0.71 Ma. The glass-ITPFT age of 0.6270.09 Ma for a tephra (T8) closely overlying is indistinguishable (within 1s) (see Table 1). The alternative is that AT-485 at Site 1124 is an older tephra bed of similar glass composition to T1 and that there has been a correlation mismatch. If this second scenario is the case, then based on the glass-ITPFT age for T8 (ca. 0.62 Ma) and palynological evidence, the closely spaced sequence of T1–T8 could have been deposited during the OIS 16/15 transition at ca. 0.62 Ma. 4.3. Section history The measured section represents an increasingly nonmarine, perhaps deepening, coastal lacustrine environment. At its base the carbonaceous muds are consistent with

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estuarine or marine influenced conditions, containing a number of marine dinoflagellates. However, it is possible that all these marine indicators are recycled, both from older and near contemporaneous sediments. Palynology indicates the surrounding vegetation was diverse, and a broadleaf podocarp forest margin was encroaching on the site. The climate was temperate, warm, and slightly increased in warmth and humidity from the base of the section up to 4 m height, above which we do not have suitable palynomorph yield. Conditions become increasingly non-marine up section and at about 4 m from the base become suitable for abundant diatom growth, with very low clastic input into the system. Throughout the section, deposition is interrupted by volcanic episodes, periodic tephra fallout and occasional influx of relatively high-energy siliciclastic flows. Tephrafall is more common in the lower, carbonaceous mud, part of the section. This could indicate more frequent volcanic activity during this time, but it could also reflect lower rates of deposition in the lower part of the section. Within the laminated diatomite, the few tephras are widely spaced, possibly the result of rapid accumulation of diatomite with still relatively regular fallout events. Unfortunately, the poor palynological preservation within the diatomite means that fluctuations in vegetation and climate in the upper ca. 6.5 m of the section cannot be determined. The leaf impressions present in the diatomite are also not common enough to identify changes through this section. Neither can the question of any possible marine influence in the upper part of the section be confidently addressed. The fish impressions in the diatomite and the diatom assemblage may provide some clues. Modern grayling and some Gobiomorphus species have a marine phase to their life-cycle (diadromous), and their presence may suggest the Ormond paleo-lake occasionally had access to the sea (McDowall et al., 2006a, b). The dominant diatom present, Cyclostephanos dubius, is not a fully brackish water form, but it is salttolerant to a degree and supports at least close proximity of the lake to the sea. Although Cyclostephanos dubius constitutes most of the diatom assemblage, the assemblage also contains other mildly salt-tolerant diatom species. 4.4. Paleodrainage routes The occurrence of numerous, mostly compositionally pure hcfs (Table 1) in the vicinity of Ormond indicate that ancestral drainage valleys were frequently inundated by the remobilised products of proximal TVZ-sourced tephrafall and pyroclastic density–current materials. Based on the chronology of the Ormond sequence, it appears that drainage routes from the TVZ through the present mountain ranges, continued to supply volcaniclastic material to a marginal-marine lowland forearc area up to at least to ca. 0.60 Ma. However, based on the occurrence of a prominent vitric-rich channel fill deposit exposed near the top of Kaiti Hill in Gisborne (Y18/478681; Fig. 12) which has been glass-ITPFT dated at 0.2570.025 Ma (B.

Fig. 12. A prominent vitric-rich channel fill deposit exposed near the top of Kaiti Hill in Gisborne (Y18/478681), and dated by ITPFT methods, indicates that this forearc region continued to be inundated by voluminous volcaniclastic sedimentation from the TVZ to at least ca. 0.25 Ma.

Alloway, unpublished data), the supply of water-transported volcaniclastic detritus from the TVZ to this forearc region is known to have continued to at least ca. 0.25 Ma. Since then, uplift and deformation of the axial ranges have disrupted ancestral paleodrainage routes and cut the supply of TVZ-sourced mass- and hyperconcentratedand flood-flow deposits to the region. The occurrence of lacustrine and marginal-marine sequences in the Ormond–Te Karaka area containing numerous well-preserved dacitic to rhyolitic tephra beds indicates a high, and compositionally diverse, eruptive tempo at ca. 0.60 Ma which is not as obviously expressed in areas more proximal to source. This area, therefore, is important for better understanding the history and eruptive dynamics of a poorly known caldera source. Acknowledgements Many thanks to Erica Crouch (dinoflagellate cyst identification) and Colin Mazengarb for their contributions and assistance with this project, and to Roger Tremain for palynological preparation, John Simes for diatom sample preparation, and Philip Carthew for drafting assistance. Many thanks also to the Gallaghers of Ormond Valley Road for access across their farm. This work was funded by an NSOF grant (QCP 2002-3) and the GNS Global Change through Time PGS&T Programme (FRST contract: C05X0202). References Alloway, B., Westgate, J., Pillans, B., Pearce, N., Newnham, R., Byrami, M., Aarburg, S., 2004. Stratigraphy, age and correlation of middle Pleistocene silicic tephras in the Auckland region, New Zealand: a prolific distal record of Taupo Volcanic Zone volcanism. New Zealand Journal of Geology and Geophysics 47, 447–479.

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