Bull Volcanol (2002) 64:441–454 DOI 10.1007/s00445-002-0217-2
R E S E A R C H A RT I C L E
Phil Shane · Joy Hoverd
Distal record of multi-sourced tephra in Onepoto Basin, Auckland, New Zealand: implications for volcanic chronology, frequency and hazards
Received: 9 October 2001 / Accepted: 15 March 2002 / Published online: 24 April 2002 © Springer-Verlag 2002
Abstract We have documented 80 tephra beds dating from ca. 9.5 to >50 ka, contained within continuously deposited palaeolake sediments from Onepoto Basin, a volcanic explosion crater in Auckland, New Zealand. The known sources for distal (>190 km from vent) tephra include the rhyolitic Taupo Volcanic Centre (4) and Okataina Volcanic Centre (14), and the andesitic Taranaki volcano (40) and Tongariro Volcanic Centre (3). The record provides evidence for four new events between ca. 50 and 28 ka (Mangaone Subgroup) suggesting Okataina was more active than previously known. The tephra record also greatly extends the known northern dispersal of other Mangaone Subgroup tephra. Ten rhyolitic tephra pre-date the Rotoehu eruption (>ca. 50 ka), and some are chemically dissimilar to post-50 ka rhyolites. Some of these older tephra were produced by large-magnitude events; however, their source remains uncertain. Eight tephra from the local basaltic Auckland Volcanic Field (AVF) are also identified. Interpolation of sedimentation rates allow us to estimate the timing of 12 major explosive eruptions from Taranaki volcano in the 27.5–9.5-ka period. In addition, 28 older events are recognised. The tephra are trachytic to rhyolitic in composition. All have high K2O contents (>3 wt%), and there are no temporal trends. This contrasts with the proximal lava record that shows a trend of increasing K2O with time. By combining the Onepoto tephra record with that of the previously documented Pukaki crater, 15 AVF basaltic fall events are constrained at: 34.6, 30.9, 29.6, 29.6, 25.7, 25.2, 24.2, 23.8, 19.4, 19.4, 15.8 and 14.5 ka, and three pre-50 ka events. This provides some of the best age constraints for the AVF, and the only reliable data for hazard recurrence calculations. The minimum event frequency of both distal and local fall events can Editorial responsibility: J. McPhie P. Shane (✉) · J. Hoverd Department of Geology, University of Auckland, Private Bag 92019, Auckland, New Zealand e-mail:
[email protected] Tel.: +64-7-3737599 ext. 7083, Fax: +64-7-3737435
be estimated, and demonstrates the Auckland City region is frequently impacted by ash fall from many volcanoes. Keywords Auckland Volcanic Field · New Zealand · Taupo Volcanic Zone · Tephrostratigraphy · Tephra
Introduction The study of late Quaternary tephra in New Zealand plays a pivotal role in the chronology of late Quaternary events and sequences, in the reconstruction of the eruptive history of major volcanic centres and the assessment of potential future hazards (Lowe 1988; Froggatt and Lowe 1990; Wilson 1993; Shane 2000). The dispersal and source of most widespread rhyolitic tephra younger than Rotoehu tephra erupted from the Okataina Volcanic Centre (OVC) at about 50–60 ka are well established. However, despite numerous investigations, several problems remain. The age of Rotoehu tephra is subject to debate (Wilson et al. 1992; Lian and Shane 2000), and there are few precise age constraints on widespread rhyolitic tephra beds older than the Kawakawa tephra erupted from the Taupo Volcanic Centre (TVC) at 26.5 ka. Devegetation and erosion during the Last Glacial Maximum (ca. 20 ka) have obliterated many sequences. Previously unrecognised proximal rhyolitic tephra beds erupted from the OVC have recently been documented (Jurado-Chichay and Walker 2000; Smith 2001). Even less is published on the occurrence and geochemical character of rhyolitic tephra that pre-date Rotoehu tephra and are younger than Mamaku ignimbrite (ca. 230 ka; Manning 1996). The record of andesitic tephra from the contemporaneous and frequently active Taranaki volcano and Tongariro Volcanic Centre (TgVC) is less complete than that of the rhyolitic record. Most studies have constructed stratigraphies for proximal deposits in restricted sectors of the volcanoes (e.g. Alloway et al. 1995; Nairn et al. 1998). Subaerial weathering of glass and some ferromagnesian phases has prevented comprehensive fingerprinting, and the fragmentary char-
442
Fig. 1 Map of North Island, New Zealand showing the location of Onepoto Basin and the volcanic centres: Auckland Volcanic Field (AVF), Okataina Volcanic Centre (OVC), Taupo Volcanic Centre (TVC), Tongariro Volcanic Centre (TgVC) and Taranaki volcano (Tk). TVZ Outline of Taupo Volcanic Zone
acter of ring-plain deposits has hindered documentation of older fall deposits. Tephra beds preserved in continuously deposited lake sediments in distal settings provide an opportunity to document the more explosive episodes of volcanoes. Despite numerous such investigations in New Zealand (e.g. Lowe 1988), few high resolution and continuous records of pre-18 ka tephra have been published (e.g. Sandiford et al. 2001; Shane et al. 2002). We document 80 tephra beds within palaeolake sediments dating from ca. 9.5 ka to >50 ka, from a drill core in Onepoto Basin, a volcanic explosion crater in the Auckland city region of northern North Island (Fig. 1). The tephra were erupted from centres more than 190 km distant: TVC, OVC, Taranaki and TgVC, and include tephra that pre-date the Rotoehu eruption. Nearly all known widely dispersed rhyolitic tephra erupted post-50 ka are from either OVC or TVC, and other centres have been relatively quiescent (Froggatt and Lowe 1990). In addition, the Onepoto core contains tephra from local basaltic eruptions of the Auckland Volcanic Field (AVF). We document new eruptive events, and estimate ages for eruptions from the poorly dated Taranaki volcano and AVF.
Onepoto Basin Onepoto Basin is a 700-m-diameter, basaltic phreatomagmatic explosion crater located in North Shore, Auckland (NZMS 260 map grid reference R11/666866). It is one of about 50 known volcanoes in the AVF. No radiometric ages have been presented for Onepoto, and age control for the entire AVF is poor (e.g. McDougall et al. 1969;
Shane and Smith 2000; Sandiford et al. 2001). However, activity is considered to post-date ca. 140 ka, and the most recent eruption occurred at ca. 600 years B.P. (Froggatt and Lowe 1990). A fresh-water lake occupied the crater of Onepoto following phreatomagmatic eruptions that produced a tuff ring. The tuff ring was breached by rising sea level at the end of the last glacial period (ca. 9 ka), producing an estuary that rapidly filled the basin with mud up to present sea level. Human reclamation has occurred in the last about 100 years. Open barrel piston coring at three closely spaced sites (within 10 m) near the middle of the basin in November 2000 and July 2001 recovered 36 m of massive, fossiliferous estuarine mud overlying ca. 25 m of laminated, organic-rich, lacustrine mud interbedded with tephra. No macroscopic tephra was identified in the estuarine mud. Between depths ca. 55.2 and 57.1 m, a poorly sorted, massive, basaltic debris flow deposit was encountered. This consists of weathered, fine ash with occasional lapilli and could represent a slump from the crater wall. The deepest hole terminated at 61.17 m in 19 cm of fresh basaltic ash and lapilli (Fig. 2). The lake mud is dark grey and in places brown-grey, and laminated on a 1–2-mm scale. Their continuity through out the sequence, except for the debris flow deposit at ca. 55.2–57.1 m, suggests no post-depositional bioturbation and no erosional gaps.
Tephra beds We have recognised 80 macroscopic tephra beds within the palaeolake sediments (Fig. 2). They range in thickness from <1-mm-thick discontinuous pods to a 63-cmthick bed (Rotoehu tephra). Only ten beds are thicker than 1 cm and most are 1–2 mm thick (Table 1). Most beds have sharp basal and upper contacts. Some display gradational upper contacts with the overlying mud. The tephra beds are white or dark grey, medium to coarse ash that contrast with the enclosing fine mud. Some of the grey beds have a pinkish hue. Electron microprobe analysis of glass shards revealed that the thickest white beds are rhyolitic, where as andesitic to trachytic beds are commonly dark grey, but some are pale grey and white. The few basaltic beds are dark grey to black and commonly coarse ash. Most tephra are crystal poor, so we did not attempt to quantify the relative proportions of crystals. The rhyolitic tephra consist almost entirely of fresh pumiceous and bubble-wall glass shards, and are crystalpoor, reflecting their great distance from source. These beds generally contained <20 wt% crystals, mostly subhedral plagioclase. Trace amounts of euhedral ferromagnesian phases (Table 1) and Fe–Ti oxides were noted in some beds. The andesitic and trachytic tephra contain significantly more crystals by weight than do rhyolitic tephra. Plagioclase in these beds is in the range 10–90 wt% of bulk sample. It usually occurs as euhedral laths with adhering
443
Fig. 2 Summary stratigraphic column of the palaeolake sediments in Onepoto Basin showing the distribution of macroscopic tephra beds. Basaltic and intermediate composition beds are labelled according to volcanic source (abbreviations as in Fig. 1)
ene. Some beds contain large (1–2 mm) and abundant (5–10 wt%) olivine.
Composition of tephra glass. Hornblende is common in most intermediate composition tephra that contain crystals. It is commonly euhedral and acicular to tabular, and can represent up to about 30 wt% of the bulk sample of some beds. Pyroxene and sometimes olivine occur in some beds, and rare biotite. Glass in most of the intermediate composition tephra consists of highly vesicular, brown to pale yellow shards. The basaltic tephra beds are composed mostly of black to dark brown glass shards. The shards range from highly vesicular pumice to fluidal teardrops, suggesting they are the product of ‘dry’ explosive magmatic activity. These beds also contain plagioclase and very rare pyrox-
Methods Tephra samples were wet sieved at 63 µm, and the coarse fraction was retained for analysis. Glass was concentrated by a Frantz magnetic separator. The glass shards were embedded in epoxy resin, and polished for electron microprobe analysis. We analysed ten glass shards in each of the tephra beds (Table 2). Analytical totals were typically 92–98%. The difference from 100% is considered to represent secondary hydration (e.g. Shane 2000) and, thus, the analyses were recalculated to 100% to aid comparisons. A complete data set is available from the authors on request.
444 Table 1 Summary of tephra beds in the Onepoto core. Ages are inferred from sedimentation rates, except those in bold. OVC Okataina Volcanic Centre; TVC Taupo Volcanic Centre; Tk Taranaki volcano; TgVC Tongariro Volcanic Centre; AVF Auckland Volcanic Field; TVZ unknown source in Taupo Volcanic Zone. hb Hornblende; hy hypersthene; aug augite; bio biotite; cum cummingtonite; ol olivine
Depth (m)
Thickness Sample Name (mm)
Source
Age (ka)
Mineralogy
Composition
36.25 36.31 36.35 36.36 36.62 36.73 36.80 36.87 37.00 37.04 37.09 37.18 37.46 37.47 37.66 37.97 38.02 38.17 38.26 38.29 38.37 38.47 38.52 38.54 38.56 38.62 38.63 38.81 38.92 38.93 39.10 39.63 39.76 39.83 39.90 40.03 40.06 40.19 40.31 40.36 40.37 40.68 40.71 40.72 40.73 40.82 40.87 41.42 41.82 43.48 43.77 44.08 44.39 44.60 47.04 47.44 48.19 48.34 48.34 48.45 48.53 48.64 48.86 49.07 49.11 49.17 49.26 49.28
43 2 <1 2 <1 2 2 <1 2 <1 2 20 4 3 2 <1 2 1 <1 <1 27 <1 2 1 12 2 7 3 2 4 8 12 <1 6 40 1 <1 4 1 <1 1 1 1 <1 2 1 2 630 5 4 1 3 4 1 1 1 3 4 1 <1 1 9 8 4 4 6 2 4
OVC Tk Tk TVC Tk Tk Tk TgVC OVC Tk TgVC OVC Tk OVC Tk
9.5 9.9 10.1 10.2 11.7 12.3 12.7 13.1 13.8 14.2 14.8 15.7 17.6 17.7 19.0 21.0 21.4 23.5 25.0 25.4 26.5 26.8 26.9 27.0 27.1 27.4 27.5 28.8 29.6 29.6 30.9 34.6 35.6 36.1 36.7
cum>hb>>hy hb>>bio
Rhyolite Trachyte Trachyte Rhyolite Trachyte Trachyte Trachyte Andesite Rhyolite Trachyte Andesite Rhyolite Trachyte Rhyolite Trachyte
Rm 144 143 142 141 140 139 138 Wh 136 135 Rr 133 Rw 131 130 Ok 128 Tr 126 Kk 124 123 Po 121 120 O 118 116 115 T3–4 114 113 112 111 110 109 108 107 106 105 104 103 102 101 99 98 96 95 93 JH2 92 90 89 74 73 72 71 JH13 70 69 68 67 65 64 63 61 60
Rotoma Opepe
Waiohau Rotorua Rerewhakaaitu Reworked Okareka Te Rere Reworked Kawakawa Poihipi Okaia Reworked
Unit G? Hauparu Te Mahoe Tahuna Reworked
OVC Tk OVC TVC Tk Tk TVC AVF Tk TVC AVF AVF AVF AVF OVC OVC OVC OVC? OVC? TVZ
hy>>hb hb>>hy hb>>hy hb>>hy hb>>hy hb>>hy hb>>hy hb, bio>>hy hb>>hy hb, bio>>hy hb>>hy bio hb>>hy
Rhyolite Trachyte Rhyolite
hb>>hy hb>>hy hb>>hy
Rhyolite Trachyte Trachyte Rhyolite Basanite Trachyte Rhyolite
hb>>hy hb>>hy
Basanite Basanite Basanite Basanite hy>hb hy>hb>>bio hy, hb
Rhyolite Rhyolite Rhyolite Rhyolite Rhyolite
OVC?
Rhyolite
Tk OVC? Tk Tk Tk Tk OVC Tk Tk Tk Tk AVF Tk Tk TVZ TVZ Tk Tk Tk TVZ Tk Tk Tk Tk Tk Tk Tk
Trachyte Rhyolite Trachyte Trachyte Trachyte Trachyte Rhyolite Trachyte Trachyte Trachyte Trachyte Basanite Trachyte Trachyte Rhyolite Rhyolite Trachyte Trachyte Trachyte Rhyolite Trachyte Trachyte Trachyte Trachyte Trachyte Trachyte Trachyte
Reworked
Rotoehu
hb>>hy hb>>hy cum>>hb hb>>hy hb>hy hb>>hy, aug hb, hy hb>>hy, aug hb>>hy hb>>hy, aug hb>>hy, aug hb>>hy, aug hb>>hy hb>>hy, aug hb>>hy hb>>hy hb>>hy hb>hy, aug hb>hy
445 Table 1 (continued)
Depth (m)
Thickness Sample Name (mm)
Source
49.45 50.39 50.86 51.54 51.59 51.62 51.63 51.82 51.88 51.98 54.87 53.86 59.12 59.56 60.62 60.73 61.17
2 3 1 2 20 1 4 1 2 4 20 2 1 8 20 4 190
Tk Tk Tk TVZ TVZ TVZ TVZ Tk TVZ Tk Tk TgVC Tk AVF TVZ TVZ AVF
59 52 47 Jh32 41 40 39 34 33 30 18 153 9 7 4 3 1
Source Using the total alkali-silica scheme of Le Maitre (1984; Fig. 3), the glasses can be classified as rhyolites, andesites, trachytes and basanites (Table 1). The 85 beds in Table 1 include seven that contain phenocrysts similar to trachyte tephra, but lack glass. Five of the beds are heterogeneous mixtures of rhyolite and trachyte that we do not consider to record eruptions (see below). All of the rhyolites are calc-alkaline and have major oxide abundances typical of Taupo Volcanic Zone deposits (e.g. Shane 2000). Glass composition, mineralogy and stratigraphy allow most post-Rotoehu rhyolites to be matched with known TVC or OVC events. Ten rhyolitic tephra beds occur in the Onepoto sequence, stratigraphically below the Rotoehu tephra (45–64 ka). The tephra beds are compositionally homogeneous, comparable with most post-Rotoehu tephra from the Taupo Volcanic Zone (Shane 2000). Their sources are uncertain. They are compared with rhyolitic tephra of known origin in Fig. 4. The composition of intermediate tephra in the Onepoto sequence is similar to that of Taranaki and TgVC tephra (e.g. Lowe 1988). Shane (2000) has shown that low-K andesitic tephra are compositionally similar to TgVC deposits, whereas high-K andesitic tephra plot on a trend with Taranaki deposits and are trachytic. A similar source identification approach is used here (Fig. 3). On this basis, units 135, 138 and 153 are considered to have a TgVC source, whereas the remaining trachytic tephra are considered to be derived from Taranaki volcano. The eight basanitic tephra are compositionally similar to local AVF tephra (Shane and Smith 2000; Sandiford et al. 2001), and this is consistent with the coarse shard size of some beds. Intermediate tephra Intermediate composition tephra contain glass with SiO2 mostly in the range 57–67 wt%. A few tephra layers con-
Age (ka)
Mineralogy
Composition
hb>>hy hb>>hy
Trachyte Trachyte Trachyte Rhyolite Rhyolite Rhyolite Rhyolite Trachyte Rhyolite Trachyte Trachyte Andesite Trachyte Basanite Rhyolite Rhyolite Basanite
hy>>hb hb>hy hb>>hy hb>hy hb, hy ol, aug hb>>hy hy>>hb ol, aug
tain some shards with SiO2 >70 wt%, and are characterised by high total alkali contents (Fig. 3). We consider these to represent highly differentiated fractions of intermediate magmas. Similar silicic glass has been previously recorded in proximal andesitic tephra (Donoghue et al. 1999). Trachytic glasses have K2O contents in the range 2.5–6 wt%. The andesitic beds (135, 138, 153) are distinguished by lower K2O contents (1–2.5 wt%), and higher FeO and CaO contents. The inter-shard compositional range within these intermediate tephra can be variable (Fig. 5). Some beds are compositionally homogeneous, displaying variability no greater than analytical uncertainty (e.g. SiO2±0.5 wt%). Other beds display compositional trends, especially in SiO2 (up to 7 wt%) and K2O (up to 2 wt%), which presumably represent compositional gradients in the magma (Fig. 5). Simple binary plots of SiO2 vs. K2O, and FeO vs. CaO can be used to distinguish many trachytic tephra (Fig. 5). No compositional trends with time are evident for the entire sequence (Fig. 5). Seven tephra beds that lack volcanic glass (59, 70, 34, 74, 89, JH13, 92) are classified as intermediate because they are composed entirely of euhedral plagioclase and hornblende accompanied by other minor phases such augite and olivine. The composition of the hornblende is distinctly pargasitic compared with those in Taupo Volcanic Zone rhyolites (an alternative source; Fig. 6), and consistent with an intermediate composition volcanic source (e.g. Froggatt and Rogers 1990). Basaltic tephra These units contain glass with SiO2 in the range 42–48 wt%, and generally have K2O contents of ca. 2–3 wt%. The inter-shard compositional range within the basaltic tephra layers is variable (e.g. SiO2 range up to 5 wt%). This reflects compositional gradients in the magma. Similar heterogeneity has been reported for subaerially exposed pyroclastic deposits in the AVF (Shane
446 Table 2 Glass composition of tephra in the Onepoto core. See Table 1 for sample information. Analyses are recalculated to 100% on a volatile free basis and expressed as a mean (top line) and standard deviation (bottom line) in wt%. Total Fe as FeO. Water by difference. Analysed by a Jeol JXA-840 probe fitted with a PST Prism 2000 EDS detector at University of Auckland. Absorbed current 1.5 nA current at 15 kV and a beam defocused to 15 µm
Sample
SiO2
TiO2
Al2O3
FeO
MnO
MgO CaO
Na2O K2O
Cl
Water
3
77.66 0.15 75.63 0.17 73.12 4.03 59.44 0.93 63.64 2.04 63.39 2.06 78.00 0.18 73.27 0.19 73.52 0.15 73.41 0.22 73.51 0.26 65.09 1.16 64.21 1.79 64.07 1.28 62.35 0.58 63.73 2.48 64.56 1.38 66.10 2.06 66.03 0.58 66.03 1.00 77.27 0.53 64.30 1.39 70.36 0.56 77.72 0.20 44.88 0.50 70.59 2.66 72.17 0.84 65.52 1.32 77.84 0.18 63.92 0.76 62.60 1.55 62.08 0.51 61.54 0.92 77.23 0.60
0.14 0.05 0.25 0.06 0.37 0.22 1.04 0.13 0.73 0.14 0.64 0.19 0.21 0.07 0.28 0.09 0.28 0.05 0.28 0.06 0.31 0.05 0.55 0.08 0.65 0.09 0.60 0.09 0.78 0.10 0.54 0.20 0.63 0.19 0.74 0.17 0.59 0.06 0.60 0.08 0.12 0.07 0.68 0.07 0.39 0.11 0.18 0.04 2.93 0.37 0.52 0.23 0.38 0.08 0.69 0.13 0.17 0.04 0.74 0.09 0.84 0.15 0.83 0.07 0.77 0.14 0.24 0.09
11.97 0.11 13.18 0.06 13.95 1.44 16.74 0.74 17.74 1.50 18.42 0.99 12.21 0.13 13.72 0.17 13.63 0.12 13.65 0.17 13.59 0.11 17.46 0.88 19.23 1.33 18.37 1.33 17.81 0.65 18.38 1.25 17.74 0.95 17.47 1.20 17.35 0.83 16.86 0.74 12.27 0.24 17.54 0.96 15.33 0.48 12.24 0.09 15.97 1.50 15.79 1.36 15.00 0.53 16.99 1.01 12.29 0.09 17.36 0.81 17.48 1.00 17.46 0.34 18.11 1.53 12.36 0.32
1.11 0.12 1.53 0.09 1.90 0.94 6.99 0.57 3.46 0.67 3.00 0.98 0.82 0.09 2.91 0.25 2.68 0.10 2.74 0.08 2.78 0.27 2.94 0.62 2.21 0.76 2.71 0.87 3.86 0.71 2.81 0.77 2.93 0.52 2.36 1.20 2.50 0.32 2.83 0.25 1.12 0.22 3.12 0.24 1.77 0.14 1.00 0.10 11.16 0.63 1.35 0.56 0.98 0.17 3.12 0.86 0.87 0.10 3.36 0.33 4.38 0.79 4.63 0.37 4.30 0.70 1.09 0.29
0.05 0.05 0.11 0.05 0.12 0.07 0.12 0.10 0.14 0.07 0.16 0.10 0.08 0.06 0.10 0.06 0.06 0.04 0.07 0.05 0.10 0.08 0.14 0.11 0.12 0.07 0.11 0.08 0.17 0.10 0.12 0.07 0.08 0.07 0.10 0.04 0.14 0.07 0.12 0.06 0.06 0.03 0.12 0.04 0.08 0.05 0.07 0.05 0.23 0.04 0.07 0.04 0.05 0.05 0.13 0.07 0.03 0.02 0.15 0.08 0.18 0.08 0.14 0.06 0.19 0.10 0.07 0.06
0.04 0.05 0.31 0.06 0.36 0.37 3.34 0.43 1.05 0.54 0.97 0.57 0.14 0.05 0.25 0.11 0.24 0.05 0.22 0.09 0.23 0.08 0.89 0.30 0.62 0.22 0.74 0.41 1.34 0.40 0.90 0.48 0.84 0.58 0.52 0.54 0.78 0.14 0.86 0.09 0.07 0.07 1.03 0.29 0.45 0.04 0.14 0.06 4.31 1.45 0.24 0.28 0.08 0.06 0.99 0.47 0.10 0.07 1.10 0.15 1.27 0.46 1.69 0.33 1.62 0.22 0.18 0.09
4.25 0.18 4.02 0.17 4.46 0.51 3.72 0.19 5.36 0.31 5.20 0.16 3.66 0.08 4.23 0.13 4.30 0.09 4.42 0.13 4.27 0.11 5.60 0.16 5.60 0.21 5.79 0.45 5.72 0.16 5.37 0.26 5.43 0.29 5.52 0.43 5.49 0.17 5.55 0.20 4.07 0.10 5.73 0.15 5.15 0.08 4.29 0.13 6.86 1.24 5.12 0.67 4.82 0.23 5.02 0.16 4.34 0.17 5.60 0.19 5.44 0.32 5.36 0.14 5.30 0.12 4.23 0.15
0.21 0.02 0.19 0.03 0.26 0.10 0.16 0.12 0.31 0.12 0.32 0.10 0.13 0.04 0.14 0.04 0.16 0.03 0.12 0.02 0.16 0.04 0.24 0.09 0.15 0.11 0.17 0.09 0.35 0.05 0.28 0.08 0.15 0.11 0.20 0.11 0.30 0.06 0.28 0.06 0.18 0.05 0.30 0.05 0.45 0.02 0.20 0.03 0.10 0.04 0.11 0.06 0.07 0.07 0.23 0.04 0.18 0.01 0.37 0.05 0.24 0.07 0.33 0.04 0.35 0.04 0.26 0.03
5.72 0.83 5.54 1.09 5.83 1.75 2.79 1.11 5.25 0.80 6.18 1.38 5.56 0.63 7.28 0.59 7.56 0.38 6.87 2.00 6.16 1.11 3.51 2.08 5.63 2.35 3.11 1.73 5.22 2.51 4.99 2.31 5.62 2.05 4.53 2.00 5.96 1.73 5.73 2.13 5.94 0.68 6.03 2.89 7.49 0.93 6.15 0.99 1.39 0.35 5.38 2.08 3.47 2.13 7.40 1.32 6.61 0.75 6.32 1.93 3.31 1.93 4.00 2.20 7.28 3.56 6.87 1.32
4 9 153 18 30 33 39 40 41 JH32 47 52 60 61 63 64 65 68 67 69 71 72 73 90 JH2 93 95 96 98 99 101 102 103
0.58 0.04 1.92 0.03 1.71 1.04 6.98 0.34 3.53 1.29 4.50 0.99 0.78 0.08 1.94 0.09 1.94 0.08 1.88 0.06 1.86 0.09 3.10 0.67 3.98 0.95 3.25 0.86 3.27 0.23 3.85 1.11 3.40 0.78 2.70 0.81 3.09 0.55 2.84 0.57 0.73 0.19 2.88 1.01 1.78 0.26 0.85 0.04 10.89 2.42 1.89 0.63 1.18 0.35 3.41 0.71 0.86 0.04 3.22 0.61 3.90 0.77 4.03 0.29 4.55 0.93 1.04 0.17
4.00 0.17 2.87 0.11 3.75 0.75 1.46 0.15 4.05 0.96 3.40 0.45 3.98 0.12 3.15 0.07 3.18 0.04 3.20 0.04 3.20 0.06 4.00 0.38 3.24 0.92 4.18 0.75 4.36 0.15 4.01 0.74 4.24 0.66 4.29 0.83 3.74 0.42 4.03 0.43 4.10 0.20 4.29 0.68 4.23 0.14 3.32 0.10 2.68 0.60 4.32 0.56 5.26 0.38 3.90 0.43 3.32 0.07 4.17 0.43 3.67 0.58 3.45 0.27 3.28 0.53 3.31 0.20
447 Table 2 (continued)
Sample
SiO2
TiO2
Al2O3
FeO
MnO
MgO CaO
Na2O K2O
Cl
Water
104
62.11 0.69 72.46 4.70 77.52 0.44 73.30 5.49 77.31 0.24 75.61 1.45 74.30 1.70 75.16 1.27 75.36 1.15 76.71 1.28 46.88 0.43 44.59 0.66 44.54 0.46 44.60 0.28 72.90 5.94 77.59 0.20 61.00 1.76 43.43 1.34 77.12 0.29 61.54 1.25 64.26 2.07 77.49 0.30 72.85 6.59 77.75 0.19 65.60 0.47 77.83 0.20 75.73 3.93 67.88 2.59 77.74 0.25 60.78 1.36 76.84 0.40 61.99 0.87 62.15 0.63 77.86 0.25
0.81 0.15 0.48 0.26 0.21 0.08 0.39 0.26 0.21 0.07 0.33 0.16 0.37 0.13 0.37 0.07 0.35 0.08 0.26 0.14 3.14 0.20 3.11 0.32 3.43 0.14 3.43 0.12 0.34 0.17 0.19 0.03 0.73 0.02 3.41 0.21 0.19 0.06 0.82 0.11 0.91 0.16 0.18 0.08 0.41 0.26 0.12 0.06 0.79 0.06 0.20 0.06 0.23 0.17 0.65 0.19 0.15 0.07 0.90 0.12 0.27 0.06 0.72 0.09 1.03 0.07 0.18 0.06
18.97 0.51 14.16 1.87 12.32 0.10 13.54 1.74 12.23 0.11 12.96 0.51 13.59 0.85 13.12 0.45 12.95 0.40 12.53 0.48 16.46 0.82 15.83 0.78 16.09 0.41 15.99 0.28 14.90 3.30 12.19 0.13 19.76 2.18 14.59 0.26 12.35 0.20 18.34 0.62 17.63 1.74 12.22 0.12 13.95 2.48 12.15 0.06 16.55 0.43 12.11 0.10 13.24 1.70 16.48 0.95 12.22 0.12 17.46 0.56 12.50 0.18 19.77 0.50 15.81 0.50 12.20 0.17
3.26 0.46 1.89 1.12 1.08 0.16 1.97 1.35 1.01 0.11 1.68 0.43 1.85 0.46 1.73 0.40 1.75 0.31 1.28 0.25 12.27 0.75 11.67 0.52 12.22 0.38 11.92 0.15 1.74 0.95 1.22 0.14 3.74 0.32 13.25 1.46 0.95 0.08 4.07 0.71 3.17 0.94 1.20 0.18 2.16 1.35 1.19 0.12 3.66 0.22 0.89 0.08 1.44 0.66 2.10 1.01 0.92 0.10 4.96 0.67 1.35 0.14 2.56 0.67 6.52 0.43 1.08 0.05
0.16 0.06 0.06 0.07 0.07 0.05 0.10 0.07 0.06 0.04 0.12 0.06 0.08 0.06 0.09 0.05 0.10 0.06 0.06 0.08 0.15 0.06 0.19 0.09 0.18 0.04 0.17 0.09 0.10 0.09 0.07 0.04 0.10 0.09 0.20 0.10 0.08 0.05 0.11 0.04 0.07 0.08 0.08 0.05 0.08 0.05 0.07 0.05 0.15 0.06 0.06 0.06 0.10 0.05 0.11 0.06 0.08 0.06 0.20 0.03 0.07 0.06 0.08 0.05 0.14 0.07 0.06 0.07
0.70 0.11 0.49 0.38 0.15 0.06 0.45 0.42 0.14 0.06 0.23 0.12 0.41 0.15 0.32 0.14 0.31 0.06 0.17 0.13 3.69 0.24 4.04 0.68 4.25 0.31 4.08 0.12 0.22 0.23 0.10 0.06 1.26 0.27 4.70 0.17 0.07 0.06 1.12 0.36 0.58 0.38 0.12 0.11 0.55 0.70 0.10 0.06 1.29 0.16 0.15 0.26 0.18 0.27 0.77 0.48 0.08 0.05 1.80 0.46 0.21 0.11 0.37 0.12 2.70 0.74 0.09 0.08
5.50 0.19 4.11 0.77 3.98 0.32 4.39 0.76 3.68 0.13 4.55 0.27 4.76 0.16 4.76 0.17 4.54 0.23 4.26 0.53 5.59 0.22 6.09 0.76 5.94 0.35 5.55 0.23 4.36 0.59 4.06 0.12 5.32 0.21 5.83 0.18 3.99 0.10 5.34 0.18 4.94 0.45 4.17 0.18 4.34 0.62 4.19 0.17 4.54 0.26 4.02 0.11 4.25 0.46 4.61 0.47 4.13 0.16 4.96 0.35 4.25 0.12 5.63 0.17 3.51 0.12 3.96 0.33
0.29 0.03 0.24 0.10 0.27 0.04 0.28 0.06 0.20 0.03 0.17 0.03 0.25 0.04 0.18 0.03 0.15 0.03 0.24 0.03 0.11 0.02 0.11 0.06 0.14 0.03 0.06 0.02 0.16 0.05 0.13 0.03 0.16 0.04 0.15 0.04 0.26 0.03 0.29 0.04 0.23 0.07 0.16 0.03 0.17 0.03 0.17 0.03 0.16 0.01 0.14 0.03 0.15 0.04 0.32 0.14 0.10 0.03 0.30 0.03 0.14 0.05 0.41 0.09 0.11 0.03 0.15 0.03
6.23 1.95 6.45 1.43 6.31 1.93 6.52 2.65 3.75 1.22 5.18 1.80 4.91 1.64 4.43 1.00 3.52 1.42 5.76 1.80 2.36 0.49 2.00 0.49 3.13 1.18 0.85 0.08 7.25 1.71 4.89 1.15 5.22 2.00 3.47 1.91 6.26 0.62 5.00 3.13 6.61 2.06 6.54 2.04 5.68 1.72 6.12 1.57 2.24 0.79 5.05 1.13 6.43 1.83 3.92 2.03 5.32 0.83 2.44 0.70 6.09 1.25 8.38 1.37 1.61 1.25 5.92 0.80
105 106 107 108 109 110 111 112 113 114 T3-10 115 116 118 O 120 121 Po 123 124 Kk 126 Tr 128 Ok 130 131 Rw 133 Rr 134 135 Wh
4.84 0.46 1.40 0.73 1.02 0.09 1.59 0.90 1.06 0.04 1.58 0.33 1.80 0.32 1.63 0.28 1.75 0.25 1.25 0.35 9.82 0.26 10.71 0.98 10.72 0.55 10.88 0.20 1.74 1.44 1.16 0.09 4.71 0.98 12.13 0.39 0.82 0.09 4.45 0.58 3.71 1.17 1.11 0.08 1.80 1.12 1.01 0.11 3.16 0.22 0.80 0.05 1.36 0.83 2.57 0.72 0.86 0.08 4.31 0.83 1.27 0.12 4.12 0.59 5.87 0.27 0.88 0.05
3.37 0.23 4.72 0.98 3.36 0.13 4.00 0.49 4.10 0.08 2.77 0.28 2.59 0.17 2.64 0.10 2.73 0.20 3.24 0.60 1.90 0.13 2.55 0.37 2.49 0.16 2.46 0.10 3.55 0.48 3.29 0.12 3.23 0.56 2.30 0.08 4.19 0.11 3.92 0.35 4.50 0.90 3.26 0.15 3.70 0.53 3.25 0.17 4.10 0.17 3.79 0.28 3.31 0.36 4.51 1.34 3.71 0.39 4.32 0.41 3.10 0.05 4.35 0.46 2.17 0.14 3.55 0.44
448 Table 2 (continued)
Sample
SiO2
TiO2
Al2O3
FeO
MnO
MgO CaO
Na2O K2O
Cl
Water
138
60.31 2.24 66.85 1.15 63.46 2.36 62.12 1.49 75.98 0.24 72.58 5.71 65.84 1.50 77.80 0.21
0.64 0.12 0.67 0.12 0.87 0.09 0.88 0.03 0.23 0.08 0.39 0.33 0.62 0.09 0.17 0.07
20.10 2.20 16.33 0.64 17.93 2.10 17.33 0.38 12.94 0.09 14.10 2.11 17.60 1.33 12.28 0.13
4.11 0.94 2.78 0.35 3.65 0.39 4.43 0.33 1.75 0.10 2.13 0.89 2.59 0.77 0.88 0.10
0.12 0.04 0.15 0.07 0.12 0.07 0.16 0.07 0.06 0.05 0.09 0.08 0.08 0.05 0.10 0.04
1.33 0.97 0.78 0.17 1.12 0.14 1.44 0.17 0.13 0.05 0.44 0.44 0.73 0.28 0.11 0.05
3.81 0.15 5.00 0.17 4.99 0.20 5.24 0.32 4.22 0.21 4.75 0.62 5.19 0.19 4.31 0.14
0.09 0.02 0.22 0.03 0.17 0.04 0.27 0.06 0.14 0.03 0.20 0.08 0.19 0.06 0.15 0.02
2.13 2.33 3.35 1.51 3.81 1.81 2.48 0.84 5.85 1.66 4.17 2.06 5.23 1.74 4.35 1.63
139 140 141 Op 143 144 Rm
Fig. 3 a Composition of single glass shards in Onepoto tephra beds and likely sources. Compositional fields from Le Maitre (1984). A Andesite; B basalt; BA basaltic andesite; BS basanite; D dacite; F foidite; T trachyte; TA trachyanadesite. b Single glass shards in intermediate-composition tephra beds compared with proximal lavas from Taranaki volcano (Price et al. 1999), and lavas and pyroclastic deposits from TgVC (Graham and Hackett 1987; Nakagawa et al. 1998). Analyses plotted on an anhydrous basis. Lines represent boundaries for high, medium and low K
and Smith 2000), which hinders correlation with the source volcano. The basaltic tephra beds are compositionally distinguishable, and this suggests they were not derived by erosion from crater rim deposits of Onepoto. This is also supported by their glassy character compared with the lithic-rich tuff of the crater rim.
7.83 1.34 2.03 0.54 3.34 1.04 3.56 0.56 1.48 0.06 1.68 0.61 2.96 1.02 0.77 0.05
1.64 0.41 5.20 0.36 4.34 0.70 4.57 0.12 3.07 0.04 3.63 0.75 4.20 0.42 3.44 0.06
Fig. 4 Pre-Rotoehu age rhyolitic tephra compared with a TVC tephra and b OVC tephra (Shane 2000)
Heterogeneous beds Six beds are composed of a mixture of trachytic and rhyolitic shards. These beds could represent secondary reworking events or they could be the product of two closely spaced eruptions that cannot be resolved on our sample spacing. They are not considered products of a compositionally zoned magma because, in each case, the rhyolitic fraction is typical of calc-alkaline rhyolites of the Taupo Volcanic Zone, whereas the intermediate fraction is trachytic typical of the Taranaki volcano. The rhyolitic shard population is also very heterogeneous
449
Fig. 7 Examples of the identification of tephra using glass chemistry. Compositional fields (Shane unpublished data) of TVZ rhyolites Rotoehu (Re), Tahuna (Ta), Okaia (O), Rerewhakaaitu (Rw) and Rotorua (Rr) compared with single shard analyses of correlatives at Onepoto
Tephrostratigraphy
Fig. 5 a Glass composition of intermediate tephra at Onepoto plotted against stratigraphic position. b Composition of single glass shards within intermediate tephra from selected beds at Onepoto
The stratigraphy, mineralogy and glass chemistry of most post-Rotoehu rhyolitic tephra from the TVZ are well established (e.g. Shane 2000). We identified five key tephra beds that display distinctive characteristics and occur in a sequence that allows a unique stratigraphy to be erected. We identified the remaining rhyolitic tephra by comparing their geochemistry and stratigraphic position with known tephra beds. In addition, the post-28 ka record was compared with that published for the nearby Pukaki crater sequence (Sandiford et al. 2001), another crater filled with lake sediment, some 20 km south of Onepoto. Key tephra beds
Fig. 6 Composition of amphibole crystals in crystal-rich beds (samples 59, 70, 34, 74, 89, JH13, 92) and the field of amphiboles found in TVZ rhyolites (Froggatt and Rogers 1990). Amphibole classification scheme from Deer et al. (1966)
compared with that typical of primary tephra. In several of the beds, the part of rhyolitic shard population compositionally matches that of the underlying primary tephra bed, suggesting reworking has occurred. The beds are listed as reworked in Table 1. The occurrence of abundant detritus including quartz and rock fragments in many of these beds also points to reworking. There are numerous sandy beds composed of quartz, K-feldspar, olivine and rock fragments in the Onepoto sequence that lack glass and other juvenile components. They are typically 1–2 mm thick and visually resemble tephra.
The five key tephra beds are ca. 45–64 ka Rotoehu, 26.5 ka Kawakawa, 17.7 ka Rerewhakaaitu, 15 ka Rotorua and 9.5 ka Rotoma (Fig. 2). The Rotoehu tephra is identified at 41.42 m and contains cummingtonite. Only three tephra beds are known to contain this phase in high abundance: Rotoehu, 9.5 ka Rotoma and 6 ka Whakatane, all from the Haroharo complex of OVC. Rotoehu tephra is significantly more widely dispersed and is a larger volume event than Rotoma and Whakatane tephra (Nairn 1989). The stratigraphic position of this tephra beneath numerous rhyolitic beds and its great thickness (63 cm) seemingly preclude correlation with either of the younger two eruptions. Correlation with Rotoehu tephra is further confirmed by glass chemistry (Fig. 7). Kawakawa tephra is identified at 38.37 m by its thickness, coarse grain size and glass chemistry. The Kawakawa tephra is the product of a particularly large-volume eruption from TVC (Froggatt and Lowe 1990). It occupies the same stratigraphic position and displays similar characteristics in Pukaki Crater (Sandiford et al. 2001). Rerewhakaaitu tephra, erupted from OVC (Nairn 1989),
450
Fig. 8 Glass composition of rhyolitic tephra at Onepoto bracketed between Rotoehu and Okaia tephra (samples 103, 106, 108, 109, 110, 111, 112, 113) compared with that of Mangaone Subgroup tephra (units A–G; Smith 2001)
is recognised at 37.47 m. It is distinctive amongst post26 ka OVC tephra in being composed of high-K and low-K subpopulations of glass shards (Fig. 7). These subpopulations are reflected in two pumice types found in proximal deposits (Shane unpublished data). The occurrence of biotite in the Rerewhakaaitu correlative further supports identification. The 15.7 ka Rotorua tephra (Nairn 1989) is found at 37.18 m, stratigraphically above Rerewhakaaitu tephra. Rotorua tephra is particularly distinctive in its relatively high FeO and CaO contents compared with other OVC tephra (e.g. Lowe 1988). The top of the palaeolake sequence is constrained by the occurrence of Rotoma tephra at 36.25 m, which is identified by abundant cummingtonite and glass chemistry. The following tephra were then identified by stratigraphic position and glass chemistry: Te Rere, Okareka and Waiohau from OVC; and Okaia, Poihipi and Opepe from TVC. Mangaone Subgroup tephra At least 14 eruptive events from OVC are identified in proximal deposits bracketed between Rotoehu and Kawakawa tephra beds in central North Island, and are referred to as the Mangaone Subgroup (Howorth 1975; Jurado-Chichay and Walker 2000; Smith 2001). Eight rhyolitic tephra beds occur in the Onepoto sequence between Rotoehu and Okaia tephra (Fig. 2) and, thus, occupy the same period as the Mangaone Subgroup. One of these beds is identified as Tahuna tephra (at 40.19 m) by virtue of its stratigraphic position and glass composition (Fig. 7). This unit has a particularly wide dispersal and is commonly found a short stratigraphic distance above Rotoehu tephra (Pillans and Wright 1992; Shane et al. 2002). It is compositionally distinct from interbedded OVC tephra, and its source is uncertain (Smith and Shane 2002). The remaining tephra in the Onepoto sequence are characterised by a heterogeneous compositional range of glass shards, including lower SiO2 varieties. They have compositions broadly similar to the older Mangaone Subgroup tephra (units A–G; Fig. 8). Two beds (103,
Fig. 9 Interpolated sedimentation rates (mm/year) at Onepoto estimated from the age of rhyolitic tephra beds. Tephra name abbreviations as in Table 1
106) are compositionally distinct, but do not match Waihora and Tihoi tephra that were erupted from TVC (Froggatt and Lowe 1990) in this time interval. Most of the Onepoto tephra are thin (<2 mm). However, two thick units occur above the Tahuna tephra correlative: 111 (40 mm) and 112 (6 mm). These units most likely match two of the larger volume and widely dispersed members of the Mangaone Subgroup originally recognised by Howorth (1975): Maketu, Te Mahoe, Hauparu, Mangaone or Omataroa. Glass chemistry excludes Maketu, which is compositionally homogeneous, and Mangaone and Omataroa that have high SiO2 contents (Smith 2001). We tentatively match 111 and 112 to Te Mahoe and Hauparu tephra, respectively. We note that the distinctive reddish pink fine ash at the top of 111 is similar in colour to beds within proximal exposures of Te Mahoe (Howorth 1975). The absence of Maketu tephra is surprising because it is a large volume eruption with a northerly dispersal (Jurado-Chichay and Walker 2000). It is possible that bed 111 is an amalgamation of Maketu and Te Mahoe because there is no significant palaeosol between the two tephra beds at proximal sites, and the composition of Maketu overlaps that of Te Mahoe (Smith 2001).
Sedimentation rates We have estimated sedimentation rates for part of the Onepoto sequence using the known age of tephra beds.
451
and the uncertainty in the age of Rotoehu tephra (ca. 45–64 ka, Lian and Shane 2000). Poihipi and Okaia tephra have not been directly dated and have estimated ages of 23 and 23.5 14C ka (Froggatt and Lowe 1990), which equate to ca. 27 and 27.5 cal ka using the calibration data of Stuiver et al. (1998). However, calibration at these older times is considered imprecise. Two 14C ages on organic mud have been reported for Hauparu tephra (35.7±1.3 ka; 39±5 ka) with an error weighted mean of 35.9±1.3 14C ka (Froggatt and Lowe 1990). These ages are inconsistent with one age on charcoal of 31.7±0.4 14C ka (=ca. 36.7 cal ka) for underlying Te Mahoe tephra (Jurado-Chichay and Walker 2000). Because the ages for Hauparu tephra are on mud and one is near the common limit of reliable radiocarbon dating (ca. 40 ka), we have tentatively used the Te Mahoe tephra age for sedimentation rate estimates. The sedimentation rate for the interval between Te Mahoe and Rotoehu tephra varies from 0.06 to 0.18 mm/year depending on the choice of Rotoehu tephra age (Fig. 9). We used sedimentation rates to interpolate ages of intermediate composition tephra for the post-30 ka part of the sequence (Table 1). These data, along with the position of tephra beds and their glass chemistry, allow us to correlate four of the layers with tephra in the nearby Pukaki crater sequence (Fig. 10).
Discussion and conclusions Age of Rotoehu tephra
Fig. 10 Correlation of tephra in the Onepoto core with the Pukaki core (Sandiford et al. 2001). Tephra name abbreviations as in Table 1. Rh Uncorrelated rhyolitic tephra (see Sandiford et al. 2001)
Ages are in calibrated years from Lowe et al. (1999) and Sandiford et al. (2001). Sediment accumulation rates are not corrected for compaction. Ages for rhyolitic beds post-dating Kawakawa tephra (26.5 ka) are relatively well constrained. The calculated rates for this part of the sequence are relatively uniform (mostly 0.15–0.18 mm/year; Fig. 9). The estimated rates are slower in the interval ca. 21–26 ka. The calculation of rates for the interval between Poihipi tephra and Rotoehu tephra is problematic because of limited age data for most units in this interval
The uncertainty in the age of the widespread Rotoehu tephra (45–64 ka, Wilson et al. 1992; Lian and Shane 2000) remains a problem in the construction of a robust tephrostratigraphy for New Zealand. In the Onepoto sequence, the estimated sedimentation rate for the interval between Te Mahoe (ca. 36 ka) and Rotoehu tephra varies from 0.06 to 0.18 mm/year depending on the choice of age for Rotoehu tephra (Fig. 9). For much of the interval above Te Mahoe tephra, estimated sedimentation rates are 0.15–0.18 mm/year (Fig. 9). A 64-ka age for Rotoehu tephra would require a very low sedimentation rate for the interval up to Te Mahoe tephra, whereas a 45-ka age would produce a rate of 0.18 mm/year comparable with much of the younger part of the Onepoto sequence. It is currently not possible to gauge whether sedimentation rates varied greatly. However, there are no significant changes in the lithology for much of the sequence. This could point to a younger age for Rotoehu tephra. Older rhyolitic tephra The Onepoto sequence contains ten rhyolitic beds that pre-date the Rotoehu tephra (pre-45–64 ka), an interval poorly documented in the Taupo Volcanic Zone and elsewhere (Shane 2000). Some of these older rhyolitic tephra are compositionally similar to known OVC tephra, and
452
most are dissimilar to TVC tephra (Fig. 4). Many of the beds are characterised by high K2O contents similar to comparably aged beds at Lake Poukawa in southern North Island (Shane et al. 2002). Five of the beds at Onepoto are compositionally distinct from all postRotoehu tephra from the Taupo Volcanic Zone: unit 4 at 60.62 m, and units 39, 40, 41 and J32 clustered at 55.96–55.87 m (Fig. 2). These five beds are characterised by high K2O and CaO contents and low SiO2 contents. Both OVC and TVC are known to have erupted tephra of similar mineralogy and glass chemistry over intervals of 10,000–20,000 years following dramatic changes in composition (e.g. Shane 2000). The cluster of tephra beds at 55.96–55.87 m could represent such an interval. In addition to OVC and TVC, the Maroa Volcanic Centre is also a possible source for these eruptions. The thickness of some of the pre-Rotoehu beds at Onepoto (up to 20 mm) is comparable with that of the Kawakawa (27 mm) and Rotorua (20 mm) beds, both of which were produced by large-volume events. This implies some of these older events were caldera-forming or calderamodifying eruptions. OVC eruptions A higher frequency of tephra beds is found in the postRotoehu tephra part of the Onepoto sequence (Fig. 2). This includes both rhyolitic and intermediate composition beds. Although the older part of the Onepoto sequence lacks age control, the uniformity of the finely laminated muds suggests similar sedimentation rates for most of the sequence. Most OVC eruptions are recorded at Onepoto, including all known 25–9.5-ka events and many of the Mangaone Subgroup (ca. 50–28 ka) eruptions, plus newly recognised events. The scarcity of pre-Rotoehu rhyolitic tephra implies OVC was relatively quiescent for a prolonged period prior to this eruption. Tephra from large explosive events at Taranaki volcano show a similar pattern. However, narrower dispersal from low-elevation columns for the trachytic eruptions could account for their absence at Onepoto. Recent investigations of ca. 50–28 ka proximal deposits of the OVC (Mangaone Subgroup) have revealed small-volume Plinian rhyolitic deposits (Jurado-Chichay and Walker 2000; Smith 2001). Only a few distal records of the Mangaone Subgroup have been documented (e.g. Pillans and Wright 1992). Further events are recognised in the Onepoto sequence. Two tephra beds (109, 110) are bracketed by Tahuna and Te Mahoe tephra, and two beds (103, 106) are bracketed by Rotoehu and Tahuna tephra (Fig. 2). They are characterised by heterogeneous glass compositions that include low-Si glass typical of the older Mangaone Subgroup tephra (Smith 2001). However, they do not match previously recognised units in these intervals. Therefore, OVC was more frequently active than previously established. The presence of such units in the Auckland area demonstrates their wide dispersal.
The occurrence of older Mangaone Subgroup units Te Mahoe, Hauparu and possibly unit G at Auckland greatly extends their known dispersal into northern North Island. However, two of the largest volume and most widely dispersed plinian deposits in the Mangaone Subgroup, Mangaone and Omataroa tephra, are not recognised in the Onepoto sequence. Both of these tephra beds are strongly dispersed east of OVC whereas Hauparu tephra has a more northerly dispersal (Jurado-Chichay and Walker 2000). Taranaki volcano eruptions Estimated sedimentation rates allow interpolation of the age of 12 major explosive events from Taranaki volcano (Table 1). The frequency of Taranaki tephra is not uniform and the volcano appears to have experienced phases of explosive activity or variations in tephra dispersal. In particular, six events at 14.2–9.9 ka, and five events closely spaced and bracketed by Rotoehu and Tahuna tephra. An older undated episode of ten events is recorded at 49.6–48.2 m (Fig. 2). Numerous tephra beds from Taranaki volcano have been documented in the proximal to medial setting (Alloway et al. 1995). However, subaerial weathering has altered or obliterated the glass fraction in many units, preventing geochemical fingerprinting. This has hindered both correlation studies and geochemical investigation of the pyroclastic part of the eruptive record. Geochemical data are restricted to lavas and lava clasts from volcaniclastic deposits on the ring plain (Price et al. 1999). This record suggests the K2O content of magmas has increased with time, and only post-30-ka deposits are characterised by K2O contents >3 wt%. The tephra assigned to Taranaki volcano are similar to the evolved post-30-ka lavas (Fig. 3), but also include more evolved, high-SiO2 rhyolitic glasses of high alkali content. Thus, the distal pyroclastic record is the product of magmas that are poorly represented by deposits near Taranaki volcano. Both pre- and post-Rotoehu tephra Taranaki layers in the Onepoto sequence are characterised by high K2O contents and there is no temporal trend (Fig. 5). This suggests that the high K2O magma type was common throughout the history of Taranaki volcano, but perhaps prevalent in the more explosive episodes. A similar conclusion was reached from the distal tephra record at Lake Poukawa (Shane et al. 2002). AVF chronology Previous attempts to date AVF volcanoes have been hindered by the presence of excess Ar in lavas (McDougall et al. 1969), and the scarcity of samples for radiocarbon dating. In addition, organic material is prone to contamination in high-rainfall environments, especially near the limit of its range. The separate volcanoes are considered monogenetic or short-lived. The combined tephra record from Onepoto and Pukaki (Sandiford et al. 2001) pro-
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vides some of the few reliable age constraints for AVF fall events. Fifteen fall events are well constrained by interpolation of sedimentation rates at 34.6, 30.9, 29.6, 29.6, 25.7, 25.2, 24.2, 23.8, 19.4, 19.4, 15.8 and 14.5, and three pre-50-ka events. The post-Poihipi tephra (ca. 27 ka) basaltic layers occur in the Pukaki sequence and were not observed in the Onepoto sequence, some 20 km to the north. This demonstrates the limited dispersal of AVF fall events, and the likelihood that other eruptions have occurred in this time interval without leaving deposits in either crater. Compositional heterogeneity of single pyroclastic units from AVF volcanoes (Shane and Smith 2000) currently hinders identification of sources. Hazard implications Auckland City is the largest urban area in New Zealand. The work of Sandiford et al. (2001) and this study have demonstrated that eruptions at the distal volcanoes OVC, TVC, Taranaki and TgVC, all frequently impact the region, in addition to local basaltic volcanism. In the interval of overlap (27–9.5 ka), the Pukaki and Onepoto cores each contain tephra that do not occur in both sequences (Fig. 10). In particular, there are significant differences in the records from Taranaki volcano and AVF. Therefore, other fall out events have probably occurred in the Auckland region during this interval, but are not recorded in either core. However, the two cores allow minimum estimates of tephra fall event frequency to be calculated. During this interval, 44 eruptions are recorded with an average recurrence of 400 years. This comprises events from OVC (1 per 2,200 years), TVC (1 per 5,800 years), Taranaki (1 per 830 years), TgVC (1 per 2,900 years) and AVF (1 per 2,900 years). This does not imply that the eruptions were uniformly spaced in time. For example, four eruptions from the AVF occurred in the interval 27–30 ka in the Onepoto sequence. All of these events produced ash layers that are at least 1 mm thick and that would have been thicker prior to compaction. The local AVF tephra would have been considerably thicker near their sources. One millimetre of ash would cause respiratory problems, close airports and disrupt some water supplies. The effects of thicker rhyolitic tephra fallout such as Rotoma (43 mm), Rotorua (20 mm) and Kawakawa (27 mm; compacted thickness) would include reduced visibility, immobilisation of transportation, drainage blockages, crop damage and significant health problems. The more frequently active Taranaki volcano is also capable of depositing ash more than about 10 mm thick. Estimating event frequencies for older parts of the record is difficult because of the uncertainty in the age of Rotoehu tephra (45–64 ka). If an age of 50 ka is used, then 22 eruptions are recorded at Onepoto between Rotoehu tephra and Poihipi tephra (27 ka) with an average recurrence of ca. 1,000 years. This is a substantially lower frequency than in the post-27 ka interval, and highlights the difficulty of assessing longer-term frequencies.
Acknowledgements Ian Smith and Paul Augustinus assisted with the acquisition of the Onepoto core. Funding was provided by University of Auckland Research Committee grants, and through the Foundation for Research Science and Technology via collaboration with Jamie Shulmeister. We thank Brad Pillans and John Westgate for their reviews, and Jocelyn McPhie for editorial assistance.
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