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Tephrostratigraphic studies at Tongariro Volcanic Centre, New Zealand: An overview
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QuaternaryInternational, Vols 34-36, pp. 13-20, 1996.
i Pergamon 1040--6182(95)00065-8
Copyright © 1996 INQUA/Elsevier Science Ltd Printed in Great Britain. All rights reserved. 1040-6182/96 $32.00
TEPHROSTRATIGRAPHIC STUDIES AT TONGARIRO VOLCANIC CENTRE, NEW ZEALAND: AN OVERVIEW S. L. Donoghue*# and V. E. Neall* *Department of Soil Science, Massey University, Private Bag 11222, Palmerston North, New Zealand (Received 20 September 1994; accepted in revised form 9 May 1995)
Detailed tephra studies at Tongariro Volcanic Centre (TgVC), North Island, New Zealand, have followed several decades of tephrostratigraphic research, principally at the more northern rhyolitic Taupo and Okataina volcanic centres. The development of reliable fingerprinting techniques for correlating rhyolitic tephras sourced from these northern centres has permitted a volcanic stratigraphic framework to be established at TgVC, where distal rhyolitic tephras are found interbedded with local andesitic tephras and volcanic sediments. Field studies at TgVC in recent years have established a detailed late Quaternary stratigraphy (dating back to 22,600 BP) for both andesitic and rhyolitic tephra cover beds, and laharic deposits, of the Ruapehu and Tongariro ring plains. The eruptive history of TgVC in late Quaternary time is recorded in nine andesitic tephra formations. In order of increasing age these are Tufa Trig Formation, Ngaumhoe Formation, dated ca. 1850 BP to present; Mangatawai Tephra, 2500 + 200 BP; Papakai Formation, between ca. 9700 and 2500 BP; Mangamate Tephra, between ca. 9950 and 9700 BP; Pahoka Tephra, ca. 10,000 BP; Okupata Tephra, comprising units erupted between ca. 13,000 and 10,000 BP; Rotoaira Lapilli, ca. 13,800 BP; and Bullot Formation, between ca. 22,600 and 10,000 BP. Several of these formations (Mangamate Tephra, Pahoka Tephra, Okupata Tephra) are sufficiently distinct and widespread to be defined as regional marker beds, useful for the correlation and dating of local tephras and ring-plain-forming debris flow deposits of the Tongariro and Ruapehu ring plains, with which they are interbedded. Similarly, they are potentially important marker beds at sites distal to source where they are found interbedded with distal silicic tephra layers in non-volcanic Quaternary sediments. The potential value of TgVC tephras as chronostratigraphic marker beds, and thus a tool in Quaternary geomorphological studies in distal environments, is perhaps not fully appreciated, given the traditional focus on using rhyolitic tephras in correlation studies and the wealth of information now available on these. Copyright © 1996 INQUA/Elsevier Science Ltd
graphy and chronology for tephras erupted from the Taupo and Okataina volcanic centres, set the scene for continued tephrostratigraphic study in New Zealand. Through their work, not only was a framework stratigraphy and chronology established, but, equally importantly, methods for correlating tephras were tested and proven. Their studies equipped workers with sufficient knowledge of central North Island rhyolitic tephras, and methods of field correlation, to set about using tephras to interpret volcanic histories, and to date non-volcanic sediments and landforms in a range of geomorphological environments beyond the volcanic regions (Lowe, 1990). By the early 1970s significant interest had developed in the North Island andesitic Egmont and Tongariro volcanic centres. Studies were undertaken by Neall (1972) and Topping (1973) to elucidate the volcanic history of these regions, and framework stratigraphies were established. The establishment of a stratigraphy and chronology of central North Island rhyolitic tephras from the Taupo and Okataina volcanic centres has been crucial to the establishment of a volcanic history at TgVC. The introduction of new laboratory-based tephra correlation techniques in the 1980s, specifically electron microprobe analysis introduced by Froggatt (1983) for correlating rhyolitic tephras, has permitted more detailed stratigraphies to be established in distal regions such as at TgVC.
INTRODUCTION Tongariro Volcanic Centre (TgVC) is a centre of predominantly andesitic volcanism located at the southwestern end of the Taupo Volcanic Zone - - a zone of active volcanism extending 250 km northeast across the central portion of North Island and comprising four major andesitic massifs: Kakaramea-Tihia, Pihanga, Tongariro, and Ruapehu (Cole and Naim, 1975; Cole, 1978). This paper provides an overview of tephra studies at TgVC and the late Quaternary volcanic stratigraphy of Mounts Ruapehu and Tongariro (including Mt Ngauruhoe, an active parasitic cone of the Tongariro massif) (Fig. 1), determined by Topping (1973) and Donoghue et al. (1995).
Tephra studies at Tongariro Volcanic Centre The impetus for detailed tephra studies at TgVC stems from several decades of tephrostratigraphic research at rhyolitic volcanic centres located to the north, specifically Tanpo and Okataina volcanic centres. The pioneering work of Healy (1964) and Vucetich and Pullar (1964, 1969, 1973) during the 1950s1970s, in establishing a late Quaternary regional stratitCurrent address: Department of Geography and Geology, The University of Hong Kong, Pokfulam Road, Hong Kong.
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S.L. Donoghue and V.E. Neall SUMMARY OF TEPHROSTRATIGRAPHY Recent studies at TgVC (1980s-1990s) have led to the establishment of a more detailed record of events at TgVC and an integrated late Quaternary (ca. 22,600 radiocarbon years BP to present) volcanic history. The following section provides a summary of the stratigraphy (post-dating the deposition of the widespread Kawakawa Tephra Formation ca. 22,590 ~- 230 BP) determined from geological mapping and correlation studies on the Tongariro and Ruapehu ring plains (Topping, 1973: Donoghue et al., 1995). A related study on pre-22,600 BP ring-plain deposits at Ruapehu is reported by Cronin et al. (1996).
Distal tephras
FIG. 1. Location of Tongariro Volcanic Centre (TgVC), New Zealand, and the principal andesitic massifs of TgVC (adapted from Cole et aL, 1986).
Tongariro Volcanic Centre tephras
Fourteen distal rhyolitic tephras erupted from the Taupo and Okataina volcanic centres are identified from their stratigraphic position, field appearance, ferromagnesian mineral assemblage, and major element composition of glass shards as determined by electron microprobe analysis (Fig. 2). Recently, Wilson (1993) undertook a detailed re-examination of the post-Kawakawa eruption stratigraphy of Taupo volcano, identifying numerous new
Rhyolitic tephras from Taupo and Okataina volcanic centres
Volcanological name a f t e r Wilson (1993)
FORMATION x 1000 years B P
0 2
KaharoaTephra (770-+20)
Tufa Trig Fm Ngauruhoe Fm Mangatawai Tephra
Taupo Tephra (1850_+10) Mapara Tephra (2160_+25)
4
q 0
Papakai Fm 6
i
Unit Y Unit X
Waimihia Tephra (3280_+20)
Unit S
Hinemaiaia Tephra (4510+-20) Whakatane Tephra 4830-+20) Motutere Tephra (5430+-60
Units R-[
PoronuiTephra (9810-+50) KarapitiTephra (9820_+80)
Units C-D Unit B
Units H-G
8
10
14
£
Mangamate /" Tephra ~N Pahoka Tephra •"-- Okupata Tephra Rotoaira Lapilli
Waiohau Tephra (11850_+60) Rotorua Tephra (13080+_50) Rerewhakaaitu Tephra (14700-+110)
16 Okareka Tephra (c.18000)
18
Bullot Fm
2O 22
Kawakawa Tephra Formation (22590_+230)
FIG. 2. Summary stratigraphy and chronology (ca. 22,600 BP to present) of Tongariro Volcanic Centre (TgVC) tephras, and interbedded distal rhyolitic tephras from Taupo and Okataina volcanic centres which are used to establish a chronology of local TgVC tephras. Okupata Tephra is time transgressive with units erupted between ca. 13,000 and 10,000 BP (Lowe, 1988). Ages for rhyolitic tephras, in radiocarbon years BP, are from Froggatt and Lowe (1990).
Tephrostratigraphic Studies at Tongariro Volcanic Centre deposits, each representing smaller eruptions within previously defined formations. In most cases these new deposits, designated by letters, correspond to tephra formations defined by Vucetich and Puller (Fig. 2). However, distal tephras identified at TgVC and correlated to Hinemaiaia Tephra, Motutere Tephra, and Poronui Tephra possibly represent the deposits of one or more of Wilson's (1993) eruption events.
Local andesitic tephras
The eruptive history of TgVC in late Quaternary time is recorded in nine andesitic tephra formations (Donoghue et al., 1995; Fig. 2). For stratigraphic definitions of formations readers are referred to Topping (1973) and Donoghue et al. (1995); ages are reported in these references and in Fergusson and Rafter (1959) and Lowe (1988). In order of increasing age the formations are Tufa Trig Formation (eruptive source Mt Ruapehu), Ngauruhoe Formation (Mt Ngauruhoe), Mangatawai Tephra (Mt Ngauruhoe), Papakai Formation (principally Mt Tongariro), Mangamate Tephra (Mt Tongariro), Pahoka Tephra (Mr Tongariro), Okupata Tephra (Mt Ruapehu), Rotoaira Lapilli (Mt Tongariro) and Bullot Formation (Mt Ruapehu) (Donoghue et al., 1995). During the period ca. 22,600-10,000 BP volcanic activity (i.e. tephra-producing eruptions) within TgVC was centred principally at Mt Ruapehu. Tephras erupted during this period are grouped into the Bullot Formation, with 23 members presently defined (Donoghue et al., 1995). This formation is dated from interbedded distal rhyolitic tephras (Kawakawa Tephra ca. 22,600 BP, Okareka Tephra ca. 18,000 BP, and Rerewhakaaitu Tephra ca. 14,700 BP). Most of these tephras have accumulated to the east of Ruapehu volcano under the influence of prevailing westerly winds, where they form thick sequences of multiple-bedded pumiceous and lithic lapilli, and ash units. These tephras represent a period of active and widespread tephra deposition from subplinian eruptions, with an average eruption frequency of one event every 200 years. With the exception of Okupata Tephra (0.2 km3), volumes for all these Mt Ruapehu tephras are each < 0.1 km 3. A short period of quiescence at Mt Ruapehu followed the deposition of the Bullot Formation tephras. During this time (ca. 10,000-9800 BP) Pahoka Tephra and Mangamate Tephra were erupted from Mt Tongariro (Topping, 1973). The Holocene tephra record on the volcanic ring plains is dominated by tephras erupted from Mt Tongariro. The lapilli units are particularly distinctive and are more widely dispersed (tephra volumes ranging between 0.2 and 2.0 km 3) than the Holocene Ruapehu eruptives. A less 'intense' period of eruptive activity occurred between ca. 9700 and 2500 BP, during which time Papakai Formation, derived principally from eruptions at Mt Tongariro, was deposited. The Papakai Formation is nonetheless an important unit in that it contains several
15
interbedded distal rhyolitic tephras which are useful time planes. The Tufa Trig Formation tephras, dated between ca. 1850 BP and the present, represent the second main period of activity at Ruapehu volcano. The first eruptions of these tephras began approximately 8000 years after the deposition of Bullot Formation tephras (i.e. ca. 2000 BP), and followed approximately 700 years of activity from Mt Ngauruhoe, during which time incremental units, grouped as the Mangatawai Tephra (Topping, 1973), were deposited. Tufa Trig Formation tephras are very different in character to the older Bullot Formation eruptives and the lapilli units from Tongariro. They are vitric-rich tephras interpreted to be the products of small hydrovolcanic (phreatomagmatic) eruptions through Crater Lake on Mt Ruapehu. Although erupted more frequently (one event approximately every 100 years), Tufa Trig Formation tephras have contributed very little tephra (< 0.1 km 3) to the ring plain. They are, however, useful marker beds for correlating local Onetapu Formation (Donoghue, 1991) debris flow deposits of the Mt Ruapehu ring plain and in establishing a chronology of events where the tephras can be dated. For example, Tufa Trig Formation Member Tf6, dated ca. 650 BP (Donoghue, 1991), is interbedded with laharic sediments deposited along Whangaehu River east of Mt Ruapehu, confirming that at least five lahar events have been channelled down Whangaehu River in recent times.
TEPHRA CORRELATION AT TONGARIRO
VOLCANIC CENTRE Although rhyolitic tephras are found interbedded with local TgVC andesitic tephras in many sections, andesitic tephras are considerably more numerous and are often found as the basal tephra cover beds overlying debris flow and fluvial deposits, and lava flows. Where dated, andesitic tephras are potentially useful chronostratigraphic marker beds, not only locally, but also distally where they are found intercalated with non-volcanic Quaternary sediments, such as gytta and loess. Andesitic tephras derived from both Mts Tongariro and Ruapehu have therefore been fingerprinted using both field and laboratory methods in an attempt to identify diagnostic features by which they may be reliably correlated (Donoghue, 1991). Distinctive ferromagnesian assemblages containing olivine, hornblende (calcic amphiboles), schist xenoliths, accretionary lapilli, and pumiceous clasts showing magma-mingling have all proved helpful in tracing members of Mangamate Tephra, Pahoka Tephra, and members of Bullot Formation (Shawcroft Tephra and Member L3). Olivine, which occurs in two distinct morphologies (non-skeletal and skeletal), is a particularly useful mineral in tephra correlations (Donoghue et al., 1991). The Mt Tongariro-sourced Mangamate Tephra (Poutu Lapilli Member, Waihohonu Lapilli Member, and Te Rato Lapilli Member) and Pahoka Tephra are usually
S.L. Donoghue and V.E. Neall
16
TABLE 1. Diagnosticcharacteristicsof regional marker bed tephras Tephra
Field characteristics
Composition
Ferromagnesian mineral assemblage*
Poutu Lapilli
Angular dark greyish-brown fine lithic lapilli and poorly vesicular, fine, strong brown pumice lapilli; tephra is strongly iron-stained Angular very dark grey medium lithic lapilli and poorly vesicular, fine, yellowish-brown pumice lapilli Angular bluish-grey fine lithic lapilli, and vesicular white, and grey and white colour-banded fine pumice lapilli Grey fine lithic lapilli, and white, and grey and white colour-banded medium pumice lapilli
Scoria, pumice, and lithic lapilli
Cpx + Opx + O1 + Hb (tr) (with forsteritic skeletal olivine)
Scoria. lithic, and pumice lapilli
O1 + Cpx + Opx (with forsteritic skeletal olivine)
Scoria, pumice, and lithic lapilli; lithic fraction contains abundant schist xenoliths Scoria, pumice, and lithic lapilli; lithic fraction contains abundant schist xenolitbs
Hb + Opx + Cpx + Ol (tr)
Waihohonu Lapilli Te Ram Lapilli
Pahoka Tephra
Hb + Cpx + Opx + O1 (tr)
*Cpx = clinopyroxene;Opx = orthopyroxene;Hb = hornblende:OI = olivine; tr = trace. The total mineral assemblageof each tephra also contains plagioclase feldsparand Fe-Tioxides.
sufficiently widespread and distinct to be defined as regional marker beds (Table 1; Figs 2 and 3). They are readily distinguished through a combination of field and mineralogical properties. Isopachs (Fig. 3) indicate that Mangamate Tephra members are unlikely to be preserved as macroscopic tephra layers further south than presently identified, but are very likely preserved in distal environments north of TgVC. Indeed, Mangamate Tephra occurs interbedded with paleosols in the Taupo region, and with medial andesitic and rhyolitic tephra in the Hawke's Bay region; it is also found in fault trenches of the Ruahine and Mohaka faults in Hawke's Bay and is a particularly useful chronostratigraphic marker bed for dating earthquake events (A. Hammond, pers. commun.. 1995). Within TgVC, these tephras are widely used to establish relative ages of local debris flow deposits and of tephras erupted from Mt Ruapehu. The great majority of tephras from Mt Ruapehu are, however, difficult to correlate. Lack of diagnostic field characteristics, restricted distributions, inconsistent exposure, multiplicity of units, similarities in grain size, colour, and composition, and similar ferromagnesian mineral assemblages, make distinction difficult. Detailed examination of the tephras using single-grain techniques, such as electron microprobe analysis of ferromagnesian minerals and volcanic glass, is therefore necessary if individual members are to be correlated proximal to source.
Distinguishing tephra source centres
In stratigraphic studies it is important that the source of tephras be distinguished, especially when working with distal tephras. In working with TgVC tephras there are two important considerations. Firstly, the ability to distinguish distal TgVC tephras from distal andesitic tephras derived from other sources, e.g. Egmont Volcanic Centre (EVC); and, secondly, to distinguish eruptives from the same source, i.e. within TgVC. Studies of tephras at source (Kohn and Neall,
1973; Donoghue et al., 1995), and of distal andesitic tephras (Lowe et al., 1980; Lowe, 1988; Froggatt and Rodgers, 1990), show that it is possible to distinguish TgVC-derived tephras from EVC-derived tephras by (1) comparison of their ferromagnesian mineral assemblages - - TgVC tephras characteristically contain orthopyroxene, often dominant, whilst EVC tephras rarely contain this mineral (Kohn and Neall, 1973; Wallace, 1987); (2) the composition of titanomagnetites (Cr203 and MnO contents) as determined by Kohn and Neall (1973), Lowe (1988), and Donoghue (1991) (Fig. 4); and (3) glass major element composition, specifically the SiO2 and alkali contents, in some instances. Of significance is the ability to distinguish TgVC and EVC tephras by the major element chemistry of phenocryst hornblendes (using magnesium numbers and K20 contents) as determined by electron microprobe (Wallace, 1987; Donoghue, 1991) (Fig. 5; Table 2). The presence of significant amounts of hornblende in TgVC tephras necessarily precludes simple distinction between TgVC and EVC tephras on this basis, since tephras found to contain significant levels of hornblende have previously been attributed an EVC source. The data (Fig. 5), however, further demonstrate that hornblende compositions do not permit distinction between TgVC tephras and those derived from the more northern rhyolitic Taupo Volcanic Centre. Thus, in distal environments where TgVC and Taupo Volcanic Centre tephras are found interbedded, determination of glass composition is necessary to distinguish source. Within TgVC several tephras are readily distinguished on the basis of differences in their field characteristics or ferromagnesian mineral assemblages. Preliminary investigations (Donoghue, 1991) also show that it is possible to distinguish some tephra members, and indeed formations, by their major element glass chemistries. Tufa Trig Formation tephras, for example, have dacitic glass chemistries (Sit2 = 63%; Na20 + K20 = 6.5%), whilst Bullot Formation members L3 and Pourahu have andesitic and rhyolitic glass chemistries, respectively. Thus, in cases where tephras cannot be correlated solely
Tephrostratigraphic Studies at Tongariro Volcanic Centre
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on field characteristics, ferromagnesian mineral assemblages and mineral proportions, or ferromagnesian mineral chemistry, the investigation of glass shard composition may prove useful.
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Problems with correlation of TgVC andesitic tephras iI
The correlation of units at source is complicated by the multiplicity of units in some formations, the advanced weathering of units in other areas (e.g. to the west of Ruapehu volcano, reflecting lower accretionary rates in soil-forming environments), incomplete stratigraphies, and the absence of interbedded datable material within most formations. Few tephras have been dated directly, and stratigraphers remain reliant upon interbedded distal rhyolitic tephras to build up a chronology, with limited opportunity to date individual tephras and formations. Distally, correlation with either formations or members is also problematic, due to incomplete stratigraphies (especially member representation), winnowing of ferromagnesian minerals, mixing of tephras with sedimentary materials (of similar mineralogy) so that ferromagnesian mineral assemblages are an unreliable means of correlation, and the typically high weathering rates of andesitic volcanic glass and microlite contents which together limit the application of single-grain fingerprinting techniques. What are needed are recognized and reliable means of identifying and correlating andesitic tephras at source and distally, as has been established for central North Island rhyolitic tephras.
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FIG. 3. Isopachs of regional tephra marker beds: A, Pahoka Tephra; B, Poutu Lapilli; C, Waihohonu Lapiili. Isopachs of Te Rato Lapilli and Okupata Tephra are given in Topping (1973). Contours are in millimetres. Stars indicate measured sections for isopach data.
Historically, tephra studies in New Zealand have focused on central North Island rhyolitic tephras. Following several decades of research we now have clear "modal concepts" of Taupo and Okataina-derived tephras, in terms of their mineralogy and chemistry. At present, the same cannot be said of TgVC tephras, since very little is known about the mineralogy and chemistry of TgVC eruptives, and much of what is already known has come from studies of distal tephras rather than units at source. Consequently, the potential value of TgVC andesitic tephras as stratigraphic marker beds, and thus a tool in Quaternary geomorphological studies, is not fully appreciated. There are many reasons why andesitic tephras may never be as useful as rhyolitic tephras in New Zealand stratigraphic studies. Their typically small volumes and dispersal areas, the high weatherability of andesitic glasses, and the limited variation in mineral assemblages and compositions of tephras from the same source, restrict their application. It seems appropriate, therefore, that future studies at TgVC should focus on correlation techniques, assessing both the suitability of existing methods employed in rhyolitic tephra correlation to andesitic eruptives, and investigating new approaches in correlation. Equally important is the need to establish detailed member chronologies for TgVC eruptives if TgVC tephras
18
S.L. Donoghue and V.E. Nealt MnO wt. % 1.0
0.8
0.6
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EGMONT I TEPHRA8 I I I ; i
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0.3
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0.4
0.5
0.6
0.7
Cr203 wt. %
FIG. 4. Comparison of phenocryst titanomagnetite composition of tephras from Tongariro and Egmont volcanic centres. Compositional fields (dashed boxes) for Tongariro-derived and Egmont-derived tephras are based on data in Lowe (1988). Labelled data show titanomagnetite compositions for tephras of the Bullot Formation (Donoghue, 1991). Bars show standard deviation from the mean.
are to be used as regional markerbeds. TgVC tephras are known to be preserved distally, having been identified in alpine peat bogs and in lake sediments in regions several hundred kilometres from source (Lowe, 1988; Froggatt and Rodgers, 1990; Eden et al., 1993). Preservation in such
1.4~
K20 wt. % .
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.
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distal environments offers opportunities for the dating of TgVC tephras, and, importantly, providing that the tephras can be correlated to tephra formations at source, an opportunity to refine the proximal member tephrostratigraphy and tephrochronology (Fig. 2).
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A
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Egmont
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FIG. 5. Comparison of the major element composition (K20 weight % v s Magnesium Number) of phenocryst amphiboles from andesitic tephras of Tongariro and Egmont volcanic centres, and from rhyolitic tephras of Taupo Volcanic Centre. Taupo and Egmont data are from Howorth (1976) and Wallace (1987).
Tephrostratigraphic Studies at Tongariro Volcanic Centre
19
TABLE 2. Representative mean electron microprobe analyses* of hornblendes in Tongariro Volcanic Centre tephras: Te Rato Lapilli Member of Mangamate Tephra, Pahoka Tephra, and Pourahu Member of Bullot Formation Te Rato Lapilli Mean SiO2 A1203 TiO2
42.87 11.23
FeO t MnO MgO CaO Na20 K20 Total
1.64
42.13 12.02 1.48
14.19
(0.64)
14.56
0.44
(0.12) (0.49) (0. 30) (0. 50)
0.37
12.79 10.92 1.63 0.38 96.06 n=28
1.991 0.186 1.785 0.056 2.868 1.760 0.476 0.073 15.638 61.63
(0. 51 )
Pahoka Tephra Mean
(0.38) (0.15)
Cations on the basis of 23 oxygens Si 6.448 A1 Ti Fe Mn Mg Ca Na K Total Mg No.
Std. Dev.
(0. 04)
(0. 63)
(0. 063) (0. 067) (0.017) (0.083) (0. 016) (0.106) (0. 049) (0.146) (0. 008) (0.102) (1.82)
12.26 11.09 2.07 0.37 96.29 n=17
Std. Dev.
(0. 86) (0.57) (0.34) (0.94) (0.07) (0.68) (0. 35) (0.13) (0. 05) (0. 5 7)
Pourahu Member Mean 42.68 11.88 1.79 12.89 0.20 13.75 11.14
2.16 0.36 96.84 n=3
6.34
(0.104)
6.34
2.13
(0.108) (0.053) (0.122) (0. 009) (0.146) (0. 061) (0.038) (0.010) (0.069) (2.79)
2.07 0.20
0.15 1.83
0.04 2.75 1.79
0.60 0.07 15.73 59.98
1.60 0.02
3.04 1.77 0.62 0.06 15.76 65.50
Std. Dev.
(0. 67) (1.24) (0.26) (1.71) (0. 05) (1.18) (0.11) (0. 08) (0.13) (0. 4 7)
(0.112) (0. 205) (0.031) (0.225) (0. 007) (0. 232) (0. 034) (0.021) (0.025) (0.036) (4.86)
*From Donoghue (1991). Microprobe set up (JEOL-JXA 633): 3 ~tm beam diameter; current 12 na at 15 kV; 3 x 10s (meaned) peak count time; 1 x l0 s background count, n = number of analyses in mean. tTotal Fe as FeO.
ACKNOWLEDGEMENTS The authors especially wish to thank colleagues Drs A.S. Palmer and R.B. Stewart for their involvement and support in the study of the tephrostratigraphy of Tongariro Volcanic Centre. Special thanks are also extended to Drs C.D. Miller and B.V. Alloway for their helpful suggestions and time given in reviewing this manuscript for publication. Several figures were redraughted from originals by Mr H, Kwan, The University of Hong Kong.
REFERENCES Cole, J.W. (1978). Andesites of the Tongariro Volcanic Centre, North Island, New Zealand. Journal of Volcanology and Geothermal Research, 3, 121-153. Cole, J.W. and Nairn, I.A. (1975). Catalogueof the Active Volcanoes of the World Including Solfatara Fields, Part XXII New Zealand. International Association of Volcanology and Chemistry of the Earth's Interior, Rome. Cole, J.W., Graham, LJ., Hackett, W.R. and Houghton, B.F. (1986). Volcanology and petrology of Quaternary composite volcanoes of Tongariro Volcanic Centre, Taupo Volcanic Zone. The Royal Society of New Zealand Bulletin, 23, 224-250. Cronin, S., Neall, V.E. and Palmer, A.S. (1996). Geological history of the north-eastern ring plain of Ruapehu volcano, New Zealand. Quaternary International, 34-36 (this volume). Donoghue, S.L. (1991). Late Quaternary volcanic stratigraphy of the southeastern sector of the Mount Ruapehu Ring Plain, New Zealand. Unpublished Ph.D. thesis, Massey University, Palmerston North.
Donoghue, S.L., Stewart, R.B. and Palmer, A.S. (1991). Morphology and chemistry of olivine phenocrysts of Mangamate Tephra, Tongariro Volcanic Centre, New Zealand. Journal of the Royal Society of New Zealand, 21, 225-236. Donoghue, S.L., Neall, V.E. and Palmer, A.S. (1995). Stratigraphy and chronology of late Quaternary andesitic~ tephra deposits, Tongariro Volcanic Centre, New Zealand. Journal of the Royal Society of New Zealand, 25(2), 115-206. Eden, D.N., Froggatt, P,C., Trustrum, N.A. and Page, M.J. (1993). A multiple-source Holocene tephra sequence from Lake Tutira, Hawke's Bay, New Zealand. New Zealand Journal of Geology and Geophysics, 36, 233-242. Froggatt, P.C. (1983). Toward a comprehensive upper Quaternary tephra and ignimbrite stratigraphy in New Zealand using electron microprobe analysis of glass shards. Quaternary Research, 19, 188-200. Fergusson, G.J. and Rafter, T.A. (1959). New Zealand ~4C age measurements - - 4. New Zealand Journal of Geology and Geophysics, 2, 208-241. Froggatt, P.C. and Lowe, D.J. (1990). A review of late Quaternary silicic and some other tephra formations from New Zealand: their stratigraphy, nomenclature, distribution, volume, and age. New Zealand Journal of Geology and Geophysics, 33, 89-109. Froggatt, P.C. and Rodgers, G.M. (1990). Tephrostratigraphy of highaltitude peat bogs along the axial ranges, North Island, New Zealand. New Zealand Journal of Geology and Geophysics, 33, 111-124. Healy, J. (1964). Dating of the younger volcanic eruptions of the Taupo Region. New Zealand Geological Survey Bulletin, 73, 7-42. Howorth, R. (1976). Late Pleistocene tephras of the Taupo and Bay of Plenty regions. Unpublished Ph.D. thesis, Victoria University of Wellington, Wellington. Kohn, B.P. and Neall, V.E. (1973). Identification of late Quaternary tephras for dating Taranaki lahar deposits. New Zealand Journal of Geology and Geophysics, 16, 781-792.
20
S.L. Donoghue and V.E. Neall
Lowe, D.J. (1988). Stratigraphy, age, composition, and correlation of late Quaternary tephras interbedded with organic sediments in Waikato Lakes, North Island, New Zealand. New Zealand Journal of Geology and Geophysics, 31, 125-165. Lowe, D.J. (1990). Tephra studies in New Zealand: an historical review. Journal of the Royal Society of New Zealand, 20, 119-150. Lowe, D.J., Hogg, A.G., Green, J.D. and Boub6e, J.A.T. (1980). Stratigraphy and chronology of late Quaternary tephras in Lake Maratoto, Hamilton, New Zealand. New Zealand Journal of Geology and Geophysics, 23, 481-485. Neall, V.E. (1972). Tephrochronology and tephrostratigraphy of Western Taranaki (N108-109), New Zealand. New Zealand Journal of Geology and Geophysics, 15, 507-557. Topping, W.W. (1973). Tephrostratigraphy and chronology of late Quaternary eruptives from the Tongariro Volcanic Centre. New Zealand Journal of Geology and Geophysics, 16, 397-423. Vucetich, C.G. and Pullar, W.A. (1964). Stratigraphy of Holocene ash in the Rotorua and Gisborne districts. New Zealand Geological Survey Bulletin, 73, 43~i3.
Vucetich, C.G. and Pullar, W.A. (1969). Stratigraphy and chronology of late Pleistocene volcanic ash beds in central North Island, New Zealand. New Zealand Journal of Geology and Geophysics, 12, 784837. Vucetich, C.G. and Pullar, W.A. (1973). Holocene tephra formations erupted in the Taupo area, and interbedded tephras from other volcanic sources. New Zealand Journal of Geology and Geophysics, 16, 745-780. Wallace, R.C. (1987). The mineralogy of Tokomaru silt loam and the occurrence of cristobalite and tridymite in selected North Island soils. Unpublished Ph.D. thesis, Massey University, Palmerston North. Wilson, C.J.N. (1993). Stratigraphy, chronology, styles and dynamics of late Quaternary eruptions from Taupo Volcano, New Zealand. Philosophical Transactions of the Royal Society London, A343, 205-306.
QuaternaryInternational, Vols 34-36, pp. 13-20, 1996.
i Pergamon 1040--6182(95)00065-8
Copyright © 1996 INQUA/Elsevier Science Ltd Printed in Great Britain. All rights reserved. 1040-6182/96 $32.00
TEPHROSTRATIGRAPHIC STUDIES AT TONGARIRO VOLCANIC CENTRE, NEW ZEALAND: AN OVERVIEW S. L. Donoghue*# and V. E. Neall* *Department of Soil Science, Massey University, Private Bag 11222, Palmerston North, New Zealand (Received 20 September 1994; accepted in revised form 9 May 1995)
Detailed tephra studies at Tongariro Volcanic Centre (TgVC), North Island, New Zealand, have followed several decades of tephrostratigraphic research, principally at the more northern rhyolitic Taupo and Okataina volcanic centres. The development of reliable fingerprinting techniques for correlating rhyolitic tephras sourced from these northern centres has permitted a volcanic stratigraphic framework to be established at TgVC, where distal rhyolitic tephras are found interbedded with local andesitic tephras and volcanic sediments. Field studies at TgVC in recent years have established a detailed late Quaternary stratigraphy (dating back to 22,600 BP) for both andesitic and rhyolitic tephra cover beds, and laharic deposits, of the Ruapehu and Tongariro ring plains. The eruptive history of TgVC in late Quaternary time is recorded in nine andesitic tephra formations. In order of increasing age these are Tufa Trig Formation, Ngaumhoe Formation, dated ca. 1850 BP to present; Mangatawai Tephra, 2500 + 200 BP; Papakai Formation, between ca. 9700 and 2500 BP; Mangamate Tephra, between ca. 9950 and 9700 BP; Pahoka Tephra, ca. 10,000 BP; Okupata Tephra, comprising units erupted between ca. 13,000 and 10,000 BP; Rotoaira Lapilli, ca. 13,800 BP; and Bullot Formation, between ca. 22,600 and 10,000 BP. Several of these formations (Mangamate Tephra, Pahoka Tephra, Okupata Tephra) are sufficiently distinct and widespread to be defined as regional marker beds, useful for the correlation and dating of local tephras and ring-plain-forming debris flow deposits of the Tongariro and Ruapehu ring plains, with which they are interbedded. Similarly, they are potentially important marker beds at sites distal to source where they are found interbedded with distal silicic tephra layers in non-volcanic Quaternary sediments. The potential value of TgVC tephras as chronostratigraphic marker beds, and thus a tool in Quaternary geomorphological studies in distal environments, is perhaps not fully appreciated, given the traditional focus on using rhyolitic tephras in correlation studies and the wealth of information now available on these. Copyright © 1996 INQUA/Elsevier Science Ltd
graphy and chronology for tephras erupted from the Taupo and Okataina volcanic centres, set the scene for continued tephrostratigraphic study in New Zealand. Through their work, not only was a framework stratigraphy and chronology established, but, equally importantly, methods for correlating tephras were tested and proven. Their studies equipped workers with sufficient knowledge of central North Island rhyolitic tephras, and methods of field correlation, to set about using tephras to interpret volcanic histories, and to date non-volcanic sediments and landforms in a range of geomorphological environments beyond the volcanic regions (Lowe, 1990). By the early 1970s significant interest had developed in the North Island andesitic Egmont and Tongariro volcanic centres. Studies were undertaken by Neall (1972) and Topping (1973) to elucidate the volcanic history of these regions, and framework stratigraphies were established. The establishment of a stratigraphy and chronology of central North Island rhyolitic tephras from the Taupo and Okataina volcanic centres has been crucial to the establishment of a volcanic history at TgVC. The introduction of new laboratory-based tephra correlation techniques in the 1980s, specifically electron microprobe analysis introduced by Froggatt (1983) for correlating rhyolitic tephras, has permitted more detailed stratigraphies to be established in distal regions such as at TgVC.
INTRODUCTION Tongariro Volcanic Centre (TgVC) is a centre of predominantly andesitic volcanism located at the southwestern end of the Taupo Volcanic Zone - - a zone of active volcanism extending 250 km northeast across the central portion of North Island and comprising four major andesitic massifs: Kakaramea-Tihia, Pihanga, Tongariro, and Ruapehu (Cole and Naim, 1975; Cole, 1978). This paper provides an overview of tephra studies at TgVC and the late Quaternary volcanic stratigraphy of Mounts Ruapehu and Tongariro (including Mt Ngauruhoe, an active parasitic cone of the Tongariro massif) (Fig. 1), determined by Topping (1973) and Donoghue et al. (1995).
Tephra studies at Tongariro Volcanic Centre The impetus for detailed tephra studies at TgVC stems from several decades of tephrostratigraphic research at rhyolitic volcanic centres located to the north, specifically Tanpo and Okataina volcanic centres. The pioneering work of Healy (1964) and Vucetich and Pullar (1964, 1969, 1973) during the 1950s1970s, in establishing a late Quaternary regional stratitCurrent address: Department of Geography and Geology, The University of Hong Kong, Pokfulam Road, Hong Kong.
]3
14
S.L. Donoghue and V.E. Neall SUMMARY OF TEPHROSTRATIGRAPHY Recent studies at TgVC (1980s-1990s) have led to the establishment of a more detailed record of events at TgVC and an integrated late Quaternary (ca. 22,600 radiocarbon years BP to present) volcanic history. The following section provides a summary of the stratigraphy (post-dating the deposition of the widespread Kawakawa Tephra Formation ca. 22,590 ~- 230 BP) determined from geological mapping and correlation studies on the Tongariro and Ruapehu ring plains (Topping, 1973: Donoghue et al., 1995). A related study on pre-22,600 BP ring-plain deposits at Ruapehu is reported by Cronin et al. (1996).
Distal tephras
FIG. 1. Location of Tongariro Volcanic Centre (TgVC), New Zealand, and the principal andesitic massifs of TgVC (adapted from Cole et aL, 1986).
Tongariro Volcanic Centre tephras
Fourteen distal rhyolitic tephras erupted from the Taupo and Okataina volcanic centres are identified from their stratigraphic position, field appearance, ferromagnesian mineral assemblage, and major element composition of glass shards as determined by electron microprobe analysis (Fig. 2). Recently, Wilson (1993) undertook a detailed re-examination of the post-Kawakawa eruption stratigraphy of Taupo volcano, identifying numerous new
Rhyolitic tephras from Taupo and Okataina volcanic centres
Volcanological name a f t e r Wilson (1993)
FORMATION x 1000 years B P
0 2
KaharoaTephra (770-+20)
Tufa Trig Fm Ngauruhoe Fm Mangatawai Tephra
Taupo Tephra (1850_+10) Mapara Tephra (2160_+25)
4
q 0
Papakai Fm 6
i
Unit Y Unit X
Waimihia Tephra (3280_+20)
Unit S
Hinemaiaia Tephra (4510+-20) Whakatane Tephra 4830-+20) Motutere Tephra (5430+-60
Units R-[
PoronuiTephra (9810-+50) KarapitiTephra (9820_+80)
Units C-D Unit B
Units H-G
8
10
14
£
Mangamate /" Tephra ~N Pahoka Tephra •"-- Okupata Tephra Rotoaira Lapilli
Waiohau Tephra (11850_+60) Rotorua Tephra (13080+_50) Rerewhakaaitu Tephra (14700-+110)
16 Okareka Tephra (c.18000)
18
Bullot Fm
2O 22
Kawakawa Tephra Formation (22590_+230)
FIG. 2. Summary stratigraphy and chronology (ca. 22,600 BP to present) of Tongariro Volcanic Centre (TgVC) tephras, and interbedded distal rhyolitic tephras from Taupo and Okataina volcanic centres which are used to establish a chronology of local TgVC tephras. Okupata Tephra is time transgressive with units erupted between ca. 13,000 and 10,000 BP (Lowe, 1988). Ages for rhyolitic tephras, in radiocarbon years BP, are from Froggatt and Lowe (1990).
Tephrostratigraphic Studies at Tongariro Volcanic Centre deposits, each representing smaller eruptions within previously defined formations. In most cases these new deposits, designated by letters, correspond to tephra formations defined by Vucetich and Puller (Fig. 2). However, distal tephras identified at TgVC and correlated to Hinemaiaia Tephra, Motutere Tephra, and Poronui Tephra possibly represent the deposits of one or more of Wilson's (1993) eruption events.
Local andesitic tephras
The eruptive history of TgVC in late Quaternary time is recorded in nine andesitic tephra formations (Donoghue et al., 1995; Fig. 2). For stratigraphic definitions of formations readers are referred to Topping (1973) and Donoghue et al. (1995); ages are reported in these references and in Fergusson and Rafter (1959) and Lowe (1988). In order of increasing age the formations are Tufa Trig Formation (eruptive source Mt Ruapehu), Ngauruhoe Formation (Mt Ngauruhoe), Mangatawai Tephra (Mt Ngauruhoe), Papakai Formation (principally Mt Tongariro), Mangamate Tephra (Mt Tongariro), Pahoka Tephra (Mr Tongariro), Okupata Tephra (Mt Ruapehu), Rotoaira Lapilli (Mt Tongariro) and Bullot Formation (Mt Ruapehu) (Donoghue et al., 1995). During the period ca. 22,600-10,000 BP volcanic activity (i.e. tephra-producing eruptions) within TgVC was centred principally at Mt Ruapehu. Tephras erupted during this period are grouped into the Bullot Formation, with 23 members presently defined (Donoghue et al., 1995). This formation is dated from interbedded distal rhyolitic tephras (Kawakawa Tephra ca. 22,600 BP, Okareka Tephra ca. 18,000 BP, and Rerewhakaaitu Tephra ca. 14,700 BP). Most of these tephras have accumulated to the east of Ruapehu volcano under the influence of prevailing westerly winds, where they form thick sequences of multiple-bedded pumiceous and lithic lapilli, and ash units. These tephras represent a period of active and widespread tephra deposition from subplinian eruptions, with an average eruption frequency of one event every 200 years. With the exception of Okupata Tephra (0.2 km3), volumes for all these Mt Ruapehu tephras are each < 0.1 km 3. A short period of quiescence at Mt Ruapehu followed the deposition of the Bullot Formation tephras. During this time (ca. 10,000-9800 BP) Pahoka Tephra and Mangamate Tephra were erupted from Mt Tongariro (Topping, 1973). The Holocene tephra record on the volcanic ring plains is dominated by tephras erupted from Mt Tongariro. The lapilli units are particularly distinctive and are more widely dispersed (tephra volumes ranging between 0.2 and 2.0 km 3) than the Holocene Ruapehu eruptives. A less 'intense' period of eruptive activity occurred between ca. 9700 and 2500 BP, during which time Papakai Formation, derived principally from eruptions at Mt Tongariro, was deposited. The Papakai Formation is nonetheless an important unit in that it contains several
15
interbedded distal rhyolitic tephras which are useful time planes. The Tufa Trig Formation tephras, dated between ca. 1850 BP and the present, represent the second main period of activity at Ruapehu volcano. The first eruptions of these tephras began approximately 8000 years after the deposition of Bullot Formation tephras (i.e. ca. 2000 BP), and followed approximately 700 years of activity from Mt Ngauruhoe, during which time incremental units, grouped as the Mangatawai Tephra (Topping, 1973), were deposited. Tufa Trig Formation tephras are very different in character to the older Bullot Formation eruptives and the lapilli units from Tongariro. They are vitric-rich tephras interpreted to be the products of small hydrovolcanic (phreatomagmatic) eruptions through Crater Lake on Mt Ruapehu. Although erupted more frequently (one event approximately every 100 years), Tufa Trig Formation tephras have contributed very little tephra (< 0.1 km 3) to the ring plain. They are, however, useful marker beds for correlating local Onetapu Formation (Donoghue, 1991) debris flow deposits of the Mt Ruapehu ring plain and in establishing a chronology of events where the tephras can be dated. For example, Tufa Trig Formation Member Tf6, dated ca. 650 BP (Donoghue, 1991), is interbedded with laharic sediments deposited along Whangaehu River east of Mt Ruapehu, confirming that at least five lahar events have been channelled down Whangaehu River in recent times.
TEPHRA CORRELATION AT TONGARIRO
VOLCANIC CENTRE Although rhyolitic tephras are found interbedded with local TgVC andesitic tephras in many sections, andesitic tephras are considerably more numerous and are often found as the basal tephra cover beds overlying debris flow and fluvial deposits, and lava flows. Where dated, andesitic tephras are potentially useful chronostratigraphic marker beds, not only locally, but also distally where they are found intercalated with non-volcanic Quaternary sediments, such as gytta and loess. Andesitic tephras derived from both Mts Tongariro and Ruapehu have therefore been fingerprinted using both field and laboratory methods in an attempt to identify diagnostic features by which they may be reliably correlated (Donoghue, 1991). Distinctive ferromagnesian assemblages containing olivine, hornblende (calcic amphiboles), schist xenoliths, accretionary lapilli, and pumiceous clasts showing magma-mingling have all proved helpful in tracing members of Mangamate Tephra, Pahoka Tephra, and members of Bullot Formation (Shawcroft Tephra and Member L3). Olivine, which occurs in two distinct morphologies (non-skeletal and skeletal), is a particularly useful mineral in tephra correlations (Donoghue et al., 1991). The Mt Tongariro-sourced Mangamate Tephra (Poutu Lapilli Member, Waihohonu Lapilli Member, and Te Rato Lapilli Member) and Pahoka Tephra are usually
S.L. Donoghue and V.E. Neall
16
TABLE 1. Diagnosticcharacteristicsof regional marker bed tephras Tephra
Field characteristics
Composition
Ferromagnesian mineral assemblage*
Poutu Lapilli
Angular dark greyish-brown fine lithic lapilli and poorly vesicular, fine, strong brown pumice lapilli; tephra is strongly iron-stained Angular very dark grey medium lithic lapilli and poorly vesicular, fine, yellowish-brown pumice lapilli Angular bluish-grey fine lithic lapilli, and vesicular white, and grey and white colour-banded fine pumice lapilli Grey fine lithic lapilli, and white, and grey and white colour-banded medium pumice lapilli
Scoria, pumice, and lithic lapilli
Cpx + Opx + O1 + Hb (tr) (with forsteritic skeletal olivine)
Scoria. lithic, and pumice lapilli
O1 + Cpx + Opx (with forsteritic skeletal olivine)
Scoria, pumice, and lithic lapilli; lithic fraction contains abundant schist xenoliths Scoria, pumice, and lithic lapilli; lithic fraction contains abundant schist xenolitbs
Hb + Opx + Cpx + Ol (tr)
Waihohonu Lapilli Te Ram Lapilli
Pahoka Tephra
Hb + Cpx + Opx + O1 (tr)
*Cpx = clinopyroxene;Opx = orthopyroxene;Hb = hornblende:OI = olivine; tr = trace. The total mineral assemblageof each tephra also contains plagioclase feldsparand Fe-Tioxides.
sufficiently widespread and distinct to be defined as regional marker beds (Table 1; Figs 2 and 3). They are readily distinguished through a combination of field and mineralogical properties. Isopachs (Fig. 3) indicate that Mangamate Tephra members are unlikely to be preserved as macroscopic tephra layers further south than presently identified, but are very likely preserved in distal environments north of TgVC. Indeed, Mangamate Tephra occurs interbedded with paleosols in the Taupo region, and with medial andesitic and rhyolitic tephra in the Hawke's Bay region; it is also found in fault trenches of the Ruahine and Mohaka faults in Hawke's Bay and is a particularly useful chronostratigraphic marker bed for dating earthquake events (A. Hammond, pers. commun.. 1995). Within TgVC, these tephras are widely used to establish relative ages of local debris flow deposits and of tephras erupted from Mt Ruapehu. The great majority of tephras from Mt Ruapehu are, however, difficult to correlate. Lack of diagnostic field characteristics, restricted distributions, inconsistent exposure, multiplicity of units, similarities in grain size, colour, and composition, and similar ferromagnesian mineral assemblages, make distinction difficult. Detailed examination of the tephras using single-grain techniques, such as electron microprobe analysis of ferromagnesian minerals and volcanic glass, is therefore necessary if individual members are to be correlated proximal to source.
Distinguishing tephra source centres
In stratigraphic studies it is important that the source of tephras be distinguished, especially when working with distal tephras. In working with TgVC tephras there are two important considerations. Firstly, the ability to distinguish distal TgVC tephras from distal andesitic tephras derived from other sources, e.g. Egmont Volcanic Centre (EVC); and, secondly, to distinguish eruptives from the same source, i.e. within TgVC. Studies of tephras at source (Kohn and Neall,
1973; Donoghue et al., 1995), and of distal andesitic tephras (Lowe et al., 1980; Lowe, 1988; Froggatt and Rodgers, 1990), show that it is possible to distinguish TgVC-derived tephras from EVC-derived tephras by (1) comparison of their ferromagnesian mineral assemblages - - TgVC tephras characteristically contain orthopyroxene, often dominant, whilst EVC tephras rarely contain this mineral (Kohn and Neall, 1973; Wallace, 1987); (2) the composition of titanomagnetites (Cr203 and MnO contents) as determined by Kohn and Neall (1973), Lowe (1988), and Donoghue (1991) (Fig. 4); and (3) glass major element composition, specifically the SiO2 and alkali contents, in some instances. Of significance is the ability to distinguish TgVC and EVC tephras by the major element chemistry of phenocryst hornblendes (using magnesium numbers and K20 contents) as determined by electron microprobe (Wallace, 1987; Donoghue, 1991) (Fig. 5; Table 2). The presence of significant amounts of hornblende in TgVC tephras necessarily precludes simple distinction between TgVC and EVC tephras on this basis, since tephras found to contain significant levels of hornblende have previously been attributed an EVC source. The data (Fig. 5), however, further demonstrate that hornblende compositions do not permit distinction between TgVC tephras and those derived from the more northern rhyolitic Taupo Volcanic Centre. Thus, in distal environments where TgVC and Taupo Volcanic Centre tephras are found interbedded, determination of glass composition is necessary to distinguish source. Within TgVC several tephras are readily distinguished on the basis of differences in their field characteristics or ferromagnesian mineral assemblages. Preliminary investigations (Donoghue, 1991) also show that it is possible to distinguish some tephra members, and indeed formations, by their major element glass chemistries. Tufa Trig Formation tephras, for example, have dacitic glass chemistries (Sit2 = 63%; Na20 + K20 = 6.5%), whilst Bullot Formation members L3 and Pourahu have andesitic and rhyolitic glass chemistries, respectively. Thus, in cases where tephras cannot be correlated solely
Tephrostratigraphic Studies at Tongariro Volcanic Centre
A
,75"~o', /
,~. : " - - . _ . - - ~ ,
'
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on field characteristics, ferromagnesian mineral assemblages and mineral proportions, or ferromagnesian mineral chemistry, the investigation of glass shard composition may prove useful.
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17
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Problems with correlation of TgVC andesitic tephras iI
The correlation of units at source is complicated by the multiplicity of units in some formations, the advanced weathering of units in other areas (e.g. to the west of Ruapehu volcano, reflecting lower accretionary rates in soil-forming environments), incomplete stratigraphies, and the absence of interbedded datable material within most formations. Few tephras have been dated directly, and stratigraphers remain reliant upon interbedded distal rhyolitic tephras to build up a chronology, with limited opportunity to date individual tephras and formations. Distally, correlation with either formations or members is also problematic, due to incomplete stratigraphies (especially member representation), winnowing of ferromagnesian minerals, mixing of tephras with sedimentary materials (of similar mineralogy) so that ferromagnesian mineral assemblages are an unreliable means of correlation, and the typically high weathering rates of andesitic volcanic glass and microlite contents which together limit the application of single-grain fingerprinting techniques. What are needed are recognized and reliable means of identifying and correlating andesitic tephras at source and distally, as has been established for central North Island rhyolitic tephras.
/
1
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FIG. 3. Isopachs of regional tephra marker beds: A, Pahoka Tephra; B, Poutu Lapilli; C, Waihohonu Lapiili. Isopachs of Te Rato Lapilli and Okupata Tephra are given in Topping (1973). Contours are in millimetres. Stars indicate measured sections for isopach data.
Historically, tephra studies in New Zealand have focused on central North Island rhyolitic tephras. Following several decades of research we now have clear "modal concepts" of Taupo and Okataina-derived tephras, in terms of their mineralogy and chemistry. At present, the same cannot be said of TgVC tephras, since very little is known about the mineralogy and chemistry of TgVC eruptives, and much of what is already known has come from studies of distal tephras rather than units at source. Consequently, the potential value of TgVC andesitic tephras as stratigraphic marker beds, and thus a tool in Quaternary geomorphological studies, is not fully appreciated. There are many reasons why andesitic tephras may never be as useful as rhyolitic tephras in New Zealand stratigraphic studies. Their typically small volumes and dispersal areas, the high weatherability of andesitic glasses, and the limited variation in mineral assemblages and compositions of tephras from the same source, restrict their application. It seems appropriate, therefore, that future studies at TgVC should focus on correlation techniques, assessing both the suitability of existing methods employed in rhyolitic tephra correlation to andesitic eruptives, and investigating new approaches in correlation. Equally important is the need to establish detailed member chronologies for TgVC eruptives if TgVC tephras
18
S.L. Donoghue and V.E. Nealt MnO wt. % 1.0
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EGMONT I TEPHRA8 I I I ; i
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FIG. 4. Comparison of phenocryst titanomagnetite composition of tephras from Tongariro and Egmont volcanic centres. Compositional fields (dashed boxes) for Tongariro-derived and Egmont-derived tephras are based on data in Lowe (1988). Labelled data show titanomagnetite compositions for tephras of the Bullot Formation (Donoghue, 1991). Bars show standard deviation from the mean.
are to be used as regional markerbeds. TgVC tephras are known to be preserved distally, having been identified in alpine peat bogs and in lake sediments in regions several hundred kilometres from source (Lowe, 1988; Froggatt and Rodgers, 1990; Eden et al., 1993). Preservation in such
1.4~
K20 wt. % .
.
.
.
.
distal environments offers opportunities for the dating of TgVC tephras, and, importantly, providing that the tephras can be correlated to tephra formations at source, an opportunity to refine the proximal member tephrostratigraphy and tephrochronology (Fig. 2).
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aourced:
Tongariro
FIG. 5. Comparison of the major element composition (K20 weight % v s Magnesium Number) of phenocryst amphiboles from andesitic tephras of Tongariro and Egmont volcanic centres, and from rhyolitic tephras of Taupo Volcanic Centre. Taupo and Egmont data are from Howorth (1976) and Wallace (1987).
Tephrostratigraphic Studies at Tongariro Volcanic Centre
19
TABLE 2. Representative mean electron microprobe analyses* of hornblendes in Tongariro Volcanic Centre tephras: Te Rato Lapilli Member of Mangamate Tephra, Pahoka Tephra, and Pourahu Member of Bullot Formation Te Rato Lapilli Mean SiO2 A1203 TiO2
42.87 11.23
FeO t MnO MgO CaO Na20 K20 Total
1.64
42.13 12.02 1.48
14.19
(0.64)
14.56
0.44
(0.12) (0.49) (0. 30) (0. 50)
0.37
12.79 10.92 1.63 0.38 96.06 n=28
1.991 0.186 1.785 0.056 2.868 1.760 0.476 0.073 15.638 61.63
(0. 51 )
Pahoka Tephra Mean
(0.38) (0.15)
Cations on the basis of 23 oxygens Si 6.448 A1 Ti Fe Mn Mg Ca Na K Total Mg No.
Std. Dev.
(0. 04)
(0. 63)
(0. 063) (0. 067) (0.017) (0.083) (0. 016) (0.106) (0. 049) (0.146) (0. 008) (0.102) (1.82)
12.26 11.09 2.07 0.37 96.29 n=17
Std. Dev.
(0. 86) (0.57) (0.34) (0.94) (0.07) (0.68) (0. 35) (0.13) (0. 05) (0. 5 7)
Pourahu Member Mean 42.68 11.88 1.79 12.89 0.20 13.75 11.14
2.16 0.36 96.84 n=3
6.34
(0.104)
6.34
2.13
(0.108) (0.053) (0.122) (0. 009) (0.146) (0. 061) (0.038) (0.010) (0.069) (2.79)
2.07 0.20
0.15 1.83
0.04 2.75 1.79
0.60 0.07 15.73 59.98
1.60 0.02
3.04 1.77 0.62 0.06 15.76 65.50
Std. Dev.
(0. 67) (1.24) (0.26) (1.71) (0. 05) (1.18) (0.11) (0. 08) (0.13) (0. 4 7)
(0.112) (0. 205) (0.031) (0.225) (0. 007) (0. 232) (0. 034) (0.021) (0.025) (0.036) (4.86)
*From Donoghue (1991). Microprobe set up (JEOL-JXA 633): 3 ~tm beam diameter; current 12 na at 15 kV; 3 x 10s (meaned) peak count time; 1 x l0 s background count, n = number of analyses in mean. tTotal Fe as FeO.
ACKNOWLEDGEMENTS The authors especially wish to thank colleagues Drs A.S. Palmer and R.B. Stewart for their involvement and support in the study of the tephrostratigraphy of Tongariro Volcanic Centre. Special thanks are also extended to Drs C.D. Miller and B.V. Alloway for their helpful suggestions and time given in reviewing this manuscript for publication. Several figures were redraughted from originals by Mr H, Kwan, The University of Hong Kong.
REFERENCES Cole, J.W. (1978). Andesites of the Tongariro Volcanic Centre, North Island, New Zealand. Journal of Volcanology and Geothermal Research, 3, 121-153. Cole, J.W. and Nairn, I.A. (1975). Catalogueof the Active Volcanoes of the World Including Solfatara Fields, Part XXII New Zealand. International Association of Volcanology and Chemistry of the Earth's Interior, Rome. Cole, J.W., Graham, LJ., Hackett, W.R. and Houghton, B.F. (1986). Volcanology and petrology of Quaternary composite volcanoes of Tongariro Volcanic Centre, Taupo Volcanic Zone. The Royal Society of New Zealand Bulletin, 23, 224-250. Cronin, S., Neall, V.E. and Palmer, A.S. (1996). Geological history of the north-eastern ring plain of Ruapehu volcano, New Zealand. Quaternary International, 34-36 (this volume). Donoghue, S.L. (1991). Late Quaternary volcanic stratigraphy of the southeastern sector of the Mount Ruapehu Ring Plain, New Zealand. Unpublished Ph.D. thesis, Massey University, Palmerston North.
Donoghue, S.L., Stewart, R.B. and Palmer, A.S. (1991). Morphology and chemistry of olivine phenocrysts of Mangamate Tephra, Tongariro Volcanic Centre, New Zealand. Journal of the Royal Society of New Zealand, 21, 225-236. Donoghue, S.L., Neall, V.E. and Palmer, A.S. (1995). Stratigraphy and chronology of late Quaternary andesitic~ tephra deposits, Tongariro Volcanic Centre, New Zealand. Journal of the Royal Society of New Zealand, 25(2), 115-206. Eden, D.N., Froggatt, P,C., Trustrum, N.A. and Page, M.J. (1993). A multiple-source Holocene tephra sequence from Lake Tutira, Hawke's Bay, New Zealand. New Zealand Journal of Geology and Geophysics, 36, 233-242. Froggatt, P.C. (1983). Toward a comprehensive upper Quaternary tephra and ignimbrite stratigraphy in New Zealand using electron microprobe analysis of glass shards. Quaternary Research, 19, 188-200. Fergusson, G.J. and Rafter, T.A. (1959). New Zealand ~4C age measurements - - 4. New Zealand Journal of Geology and Geophysics, 2, 208-241. Froggatt, P.C. and Lowe, D.J. (1990). A review of late Quaternary silicic and some other tephra formations from New Zealand: their stratigraphy, nomenclature, distribution, volume, and age. New Zealand Journal of Geology and Geophysics, 33, 89-109. Froggatt, P.C. and Rodgers, G.M. (1990). Tephrostratigraphy of highaltitude peat bogs along the axial ranges, North Island, New Zealand. New Zealand Journal of Geology and Geophysics, 33, 111-124. Healy, J. (1964). Dating of the younger volcanic eruptions of the Taupo Region. New Zealand Geological Survey Bulletin, 73, 7-42. Howorth, R. (1976). Late Pleistocene tephras of the Taupo and Bay of Plenty regions. Unpublished Ph.D. thesis, Victoria University of Wellington, Wellington. Kohn, B.P. and Neall, V.E. (1973). Identification of late Quaternary tephras for dating Taranaki lahar deposits. New Zealand Journal of Geology and Geophysics, 16, 781-792.
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S.L. Donoghue and V.E. Neall
Lowe, D.J. (1988). Stratigraphy, age, composition, and correlation of late Quaternary tephras interbedded with organic sediments in Waikato Lakes, North Island, New Zealand. New Zealand Journal of Geology and Geophysics, 31, 125-165. Lowe, D.J. (1990). Tephra studies in New Zealand: an historical review. Journal of the Royal Society of New Zealand, 20, 119-150. Lowe, D.J., Hogg, A.G., Green, J.D. and Boub6e, J.A.T. (1980). Stratigraphy and chronology of late Quaternary tephras in Lake Maratoto, Hamilton, New Zealand. New Zealand Journal of Geology and Geophysics, 23, 481-485. Neall, V.E. (1972). Tephrochronology and tephrostratigraphy of Western Taranaki (N108-109), New Zealand. New Zealand Journal of Geology and Geophysics, 15, 507-557. Topping, W.W. (1973). Tephrostratigraphy and chronology of late Quaternary eruptives from the Tongariro Volcanic Centre. New Zealand Journal of Geology and Geophysics, 16, 397-423. Vucetich, C.G. and Pullar, W.A. (1964). Stratigraphy of Holocene ash in the Rotorua and Gisborne districts. New Zealand Geological Survey Bulletin, 73, 43~i3.
Vucetich, C.G. and Pullar, W.A. (1969). Stratigraphy and chronology of late Pleistocene volcanic ash beds in central North Island, New Zealand. New Zealand Journal of Geology and Geophysics, 12, 784837. Vucetich, C.G. and Pullar, W.A. (1973). Holocene tephra formations erupted in the Taupo area, and interbedded tephras from other volcanic sources. New Zealand Journal of Geology and Geophysics, 16, 745-780. Wallace, R.C. (1987). The mineralogy of Tokomaru silt loam and the occurrence of cristobalite and tridymite in selected North Island soils. Unpublished Ph.D. thesis, Massey University, Palmerston North. Wilson, C.J.N. (1993). Stratigraphy, chronology, styles and dynamics of late Quaternary eruptions from Taupo Volcano, New Zealand. Philosophical Transactions of the Royal Society London, A343, 205-306.