An extremely large magnitude eruption close to the Plio-Pleistocene boundary: reconstruction of eruptive style and history of the Ebisutoge-Fukuda tephra, central Japan

Journal of Volcanology and Geothermal Research 107 (2001) 47±69

www.elsevier.nl/locate/jvolgeores

An extremely large magnitude eruption close to the Plio-Pleistocene boundary: reconstruction of eruptive style and history of the Ebisutoge-Fukuda tephra, central Japan K. Kataoka a,*, Y. Nagahashi b, S. Yoshikawa a a

Department of Geosciences, Osaka City University, Sugimoto 3-3-138, Sumiyoshi-ku, Osaka 558-8585, Japan b Faculty of Education, Fukushima University, Kanayagawa 1, Fukushima 960-1296, Japan Received 17 November 1999; revised 9 August 2000; accepted 22 November 2000

Abstract An extremely large magnitude eruption of the Ebisutoge-Fukuda tephra, close to the Plio-Pleistocene boundary, central Japan, spread volcanic materials widely more than 290,000 km 2 reaching more than 300 km from the probable source. Characteristics of the distal air-fall ash (.150 km away from the vent) and proximal pyroclastic deposits are clari®ed to constrain the eruptive style, history, and magnitude of the Ebisutoge-Fukuda eruption. Eruptive history had ®ve phases. Phase 1 is phreatoplinian eruption producing .105 km 3 of volcanic materials. Phases 2 and 3 are plinian eruption and transition to pyroclastic ¯ow. Plinian activity also occurred in phase 4, which ejected conspicuous obsidian fragments to the distal locations. In phase 5, collapse of eruption column triggered by phase 4, generated large pyroclastic ¯ow in all directions and resulted in more than 250±350 km 3 of deposits. Thus, the total volume of this tephra amounts over 380±490 km 3. This indicates that the Volcanic Explosivity Index (VEI) of the Ebisutoge-Fukuda tephra is greater than 7. The huge thickness of reworked volcaniclastic deposits overlying the fall units also attests to the tremendous volume of eruptive materials of this tephra. Numerous ancient tephra layers with large volume have been reported worldwide, but sources and eruptive history are often unknown and dif®cult to determine. Comparison of distal air-fall ashes with proximal pyroclastic deposits revealed eruption style, history and magnitude of the Ebisutoge-Fukuda tephra. Hence, recognition of the Ebisutoge-Fukuda tephra, is useful for understanding the volcanic activity during the Pliocene to Pleistocene, is important as a boundary marker bed, and can be used to interpret the global environmental and climatic impact of large magnitude eruptions in the past. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Explosive eruption; Distal tephra; Plio-Pleistocene boundary; Ebisutoge-Fukuda tephra

1. Introduction Recognition of widespread tephras and understanding the related large explosive volcanic activity have been addressed in late Pleistocene to Holocene * Corresponding author. Fax: 181-6-6605-3176. E-mail address: [email protected] (K. Kataoka).

tephrostratigraphy in Japan (e.g. Kikai-Akahoya tephra: 6.3 ka, Machida and Arai, 1978; Aira-Tn tephra: 22±25 ka, Machida and Arai, 1976; Aso-4 tephra: 84±89 ka, Machida et al., 1985). The ShishimutaAzuki tephra (0.85 Ma: Kamata et al., 1997) is one of the oldest for which details of volcanic activity have been clari®ed in Japan. Recently, widespread tephra layers have also been correlated across several basins

0377-0273/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0377-027 3(00)00300-0

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K. Kataoka et al. / Journal of Volcanology and Geothermal Research 107 (2001) 47±69

and far back into the Pliocene to lower Pleistocene (e.g. Sauri tephra: ,3.4 Ma; Ohta tephra: ,4.4 Ma, Satoguchi et al., 1999). However, many questions about these numerous old tephras and large explosive eruptions are not solved yet; where was the eruptive source? what were the eruption style and eruptive history? how much volume was ejected? what was the impact on environments? and so on. The dif®culty of interpretation and reconstruction of eruption history in ancient volcanoes is due to many reasons. (1) The preservation potential of older deposits is less than that of recent one. Pyroclastic deposits near the source are the best candidates carrying information regarding explosive eruptions, but being unconsolidated, i.e. non-welded pyroclastic ¯ows, these are often easily degraded especially in humid regions. (2) In an active arc setting, sediments are distributed sporadically, so correlation of those sediments is very time-intensive. (3) If it is preserved at a distal location, discrimination between plinian fallout and co-ignimbrite ash is dif®cult, since these deposits are vitric ®ne grained and may have been derived from a complex eruption (Lajoie and Stix, 1992; Cas and Wright, 1987). (4) Old eruptions lack eyewitnesses who watched and monitored the eruption directly. There is high probability that distal ashes contain much information lost by weathering and/or scouring in proximal pyroclastic fall and ¯ow deposits. Distal ashes mainly result from large magnitude silicic eruptions which winds may transport the plinian fallout ash and/or co-ignimbrite ash to the distal area far from the vent. Discrimination and characterization of distal ashes offer information about explosive volcanic activity (e.g. eruptive style and history) in ancient time that is signi®cant because of the catastrophic impact that could affect local and global environments. The eruption of the Ebisutoge-Fukuda tephra (1.75 Ma; Yoshikawa et al., 1996), in the Plio-Pleistocene boundary, is considered as one of the extremely large volcanic events of ancient times. This tephra is stratigraphically signi®cant, being in the boundary layer, and for its extensive regional occurrence. In this contribution, we ®rst describe the characteristics of distal fall units and proximal pyroclastic deposits and rede®ne them. The goal is to describe the eruption history and magnitude of the Ebisutoge-

Fukuda eruption by integrating lithofacies variations within distal ashes, and the relationship between proximal pyroclastic deposits and distal air-fall ashes. We also construct the stratigraphy and isopach map in order to estimate its volume and discuss explosivity of this tephra. 2. Outline of the Ebisutoge-Fukuda tephra The Fukuda Volcanic Ash (Itihara et al., 1975; Fukuda V.A.) and its correlatives are very important key beds in the Japanese Pliocene to Pleistocene succession. This ash and correlatives are exposed at more than 60 localities. They are found as far as 300 km from their probable eruption center near Mount Hotakadake in the Japan Northern Alps, central Honshu Island (Fig. 1). The Takidani Granodiorite and Hotaka Andesite, exposed in the Northern Alps, form a volcano-tectonic graben believed to be the eruption source (Harayama, 1992, 1998; Nagahashi, 1996). However, the real eruption center already disappeared due to erosion and uplifting of the mountain. The Fukuda V.A. and its correlatives are intercalated and distributed in the Plio-Pleistocene sediments of Honshu Island (Fig. 1, Table 1), namely the Osaka Group, Kobiwako Group, Tokai Group, Uomuma Group, and Kazusa Group (Yoshikawa et al., 1994, 1996; Tomita and Kurokawa, 1997). The Osaka and Uonuma Groups mainly consist of ¯uvio-marine deposits, and the Kobiwako and Tokai Groups are ¯uvio-lacustrine. The Kazusa Group comprises shallow to deep marine sediments. This tephra is stratigraphically above the Olduvai subchron and the age is inferred to be approximately 1.75 Ma (Yoshikawa et al., 1996). Stratigraphy and characteristics of the Pliocene to lower Pleistocene pyroclastic ¯ow deposits in the Takayama and Omine Areas, central Japan, have been described in detail by Nagahashi (1995) and Nagahashi et al. (1996). These pyroclastic ¯ow deposits are dacitic to rhyolitic in composition, and are intercalated in alluvial fan to ¯uvial sediments. The Chayano Tuff I and II, and Ebisutoge Pyroclastic Deposits can be correlated with the Fukuda V.A. and correlative ashes. Nagahashi et al. (2000) suggested grouping these volcaniclastic deposits together as a single unit: `the Ebisutoge-Fukuda

K. Kataoka et al. / Journal of Volcanology and Geothermal Research 107 (2001) 47±69

49

Fig. 1. (a) Distribution of the Plio-Pleistocene sediments and localities where the Ebisutoge-Fukuda tephra are exposed. This tephra reached over 300 km far from eruption center, the Mount Hotakadake (open triangle). Alphabets indicate the sites of representative columnar sections in Fig. 5. (b) Index map of Takayama and Omine Areas, near to the probable eruption center.

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K. Kataoka et al. / Journal of Volcanology and Geothermal Research 107 (2001) 47±69

Table 1 Summary of nomenclature of the Ebisutoge-Fukuda tephra Local name

Place

Distance from probable source

Occurrence of fall units

Reference

In proximal area Ebisutoge Pyroclastic Deposits, Chayano Tuff I and II Takagariyama Tuff II

Takayama Area

, 28 km, WSW

Nagahashi (1995)

Omine Area

28±34 km, NE

Nagahashi et al. (1996)

In distal area Fukuda Volcanic Ash Gokenjaya Volcanic Ash

Osaka Group Kobiwako Group

240±320 km, SW 170±200 km, SW

A1, B A1, A2U, B

Tokai Group Uonuma Group Kazusa Group

, 160 km, SSW , 140 km, NE 240±260 km, ESE

A1, A2U, B A1, A2L, A2U, B A1, A2L, A2U, B

Karegawa Volcanic Ash Tsujimatagawa Volcanic Ash Kd 38 Volcanic Ash

tephra'. Therefore, Ebisutoge-Fukuda tephra here means, volcaniclastic material produced by the series of eruptions, i.e. Fukuda V.A. and its correlative ashes, the Chayano Tuff I and II, and the Ebisutoge Pyroclastic Deposits. 3. Description of distal fall deposits Distal fall units of the Ebisutoge-Fukuda tephra (i.e. the Fukuda Volcanic Ash and correlatives) were originally divided into mainly three units, namely unit A1, unit A2 and unit B, by Yoshikawa et al. (1996).

Itihara et al. (1975) Kobiwako Research Group (1981) Mori (1971) Kazaoka et al. (1986) Mitsunashi et al. (1959)

However, units A2 and B have been further subdivided on the basis of new data in this study. Unit A2 is subdivided into subunit A2 Lower and subunit A2 Upper (obsidian layer). The lower part of unit B de®ned by Yoshikawa et al. (1996) in the Kobiwako and Tokai basins belongs to a part of unit A2 in this paper. Therefore, here we rede®ne these fall units on the basis of stratigraphical position, lithofacies and petrographical features. Petrographic data on the mineralogy (including heavy minerals) and the shape and refractive index of glass shards, were collected for 1/4±1/16 mm fraction of ash. Classi®cation of glass shard shape is based

Table 2 Summary of diagnostic (bold) and characteristic features used to de®ne a distal air-fall unit of the Ebisutoge±Fukuda tephra Fall unit

Grain size

Color

Characterisic features

Unit B

Very ®ne to silt

Reddish-brown to reddish-violet

Well-sorted vitric ash, ®ner-grained than A1, A2, normally graded bed, heavy mineral concentration in the base, containing brown colored andesitic glass shards

Unit A2U

Medium to ®ne

Gray to black (in the east); gray to light-gray (in the west)

Coarsest among all fall units, richest in free crystal amounts, reverse grading conspicuous, including obsidian fragments

Unit A2L

Medium-®ne to very ®ne

Light-gray to gray

Normally graded bed, occurrence of visible free crystals

Unit A1

Fine to very ®ne

White to light-gray

Well-sorted vitric ash, ®ner grained than A2, plural normal grading, containing accretionary lapilli

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51

on Yoshikawa (1976), in which volcanic glass shards were divided into four types (viz.; H-, C-, T- and Otype). H-type has bubble wall shard or platy shard, Ttype for ®ber and pumice type. C-type is intermediate between H- and T-types, and O-type is for others.

(hypersthene to eulite) are predominant heavy minerals, with subordinate amounts of amphibole, biotite, zircon and clinopyroxene.

3.1. Stratigraphy and distribution of fall units

The unit A2 is coarse-grained and crystal-rich ash. It can be subdivided into two portions, lower and upper parts, based on lithofacies and petrographical characteristics (Figs. 4 and 5, Tables 2 and 3). The lower part of unit A2 (subunit A2L) is found only in the Uonuma and Kazusa basins (east of the source). The upper part (subunit A2U) occurs in the Kobiwako, Tokai, Uonuma and Kazusa basins.

Variation in the lateral distribution pattern of each fall unit results in a different stratigraphical appearance at every basin (Figs. 2, 3, 4 and 5). In the Uonuma (northeast from the source) and Kazusa (southeastward) basins, all fall units can be observed, unit A1, subunit A2L, A2U, and unit B in stratigraphic order. In the Tokai (westward) and Kobiwako (westward) basins, subunit A2L is absent. Unit B directly overlies unit A1 in the Osaka (westward, most distal) basin. The occurrence, lithofacies character, petrographic features, and chemical composition of glass shards are described for each unit separately and are shown in Tables 2, 3 and 4. 3.2. Unit A1 Lithofacies: The unit A1, white to light-gray in color, occurs widely in all ®ve basins we investigated (Figs. 1, 5, and 9). It consists of well-sorted vitric ash. Thickness is 13±40 cm (or more), and increases towards eruption center (Fig. 9). Grain size also increases towards source, and is medium-®ne sand to silt sized in Osaka and Kobiwako basins (approximately 200±350 km away from the vent), medium to very ®ne sand sized in Tokai basin (,160 km away), medium-®ne to very ®ne sand sized in Uonuma basin (,140 km away), ®ne to very ®ne sand sized in Kazusa basin (,250 km away). The base of the unit is non-erosive, but sharp. The bed shows mantle bedding and ®ning upward. Accretionary lapilli (0.5±5 mm in diameter) are found and are diagnostic of this unit. Occasionally this unit shows plural normal grading with accretionary lapilli concentrations in the middle of the graded part. Petrographic aspects: Unit A1 is largely composed of colorless glass shards (.95%) with minor amounts of feldspar (,3%), trace amounts of quartz and heavy minerals (1% or less). Glass shards are mainly H-type and C-type shape with refractive index in range from 1.498 to 1.503. Chemically glass shards are rhyolitic in composition. Opaque minerals and orthopyroxene

3.3. Unit A2

3.3.1. Subunit A2 Lower Lithofacies: Subunit A2L overlies the unit A1 and is distributed in basins east of the eruption source. It is light-gray to gray and 10±15 cm thick in the Uonuma Group, and 4±5 cm in the Kazusa Group. It consists of well-sorted medium-®ne to very ®ne sand sized vitric crystal ash. This unit is the lowest stratigraphic unit with visible crystals in the ®eld. The bed has a nonerosive basal surface and is normally graded. Petrographic aspects: Colorless glass shards dominate (more than 95%). Subordinate minerals are feldspar (1±3%), heavy minerals (,2%), and quartz (,1%). Glass shape is mainly H- and C-type. The refractive index of glass ranges from 1.501 to 1.504 (1.5025 in mode), rarely it goes up to 1.512 in some localities. Similar to unit A1, glass is rhyolitic in chemical composition. Orthopyroxene (hypersthene to eulite) and opaque minerals are the predominant heavy minerals with subordinate amounts of amphibole and clinopyroxene, minor amounts of zircon. 3.3.2. Subunit A2 Upper Lithofacies: Subunit A2U is gray to black in east of the vent. It is 13±18 cm thick in the Uonuma Group and 6±7 cm in the Kazusa Group. It consists of wellsorted medium to ®ne crystal-rich ash, and is more crystal-rich than other units. Its contact with unit B is sharp but gradational with subunit A2L. In the Uonuma basin, ,140 km northeast of the eruptive source, the lower two-thirds of this subunit is reversely graded with increasing amounts of phenocrysts (Figs. 4 and 5). This reverse grading is terminated by crystal-richest black part, which we call

e-2 e-1

Kazusa Group e-4 e-3

d2-3 d2-2 d2-1

Uonuma Group d2-5 d2-4

c1-1

Tokai Group c1-3 c1-2

b1-1

Kobiwako Group b1-3 b1-2

Osaka Group a3-2 a3-1

A2L A1

B A2U (obsidian layer)

B A2U (obsidian layer) A2U A2L A1

B A2U (obsidian layer) A1

B A2U (obsidian layer) A1

B A1

Unit name

95 95

91 82

96 99 96

93 89

98

94 94

98

97 95

94 97

3 3

6 11

3 1 3

5 7

2

5 4

2

2 3

5 3

0 *

0 1

1 0 1

1 2

0

* 1

0

0 1

0 0

2 2

3 6

1 * *

1 2

*

1 1

*

1 2

1 *

H.M.

42 42

30 41

35 56 71

19 7

41

13 27

53

12 32

34 62

45 37

59 53

52 38 26

61 43

55

46 49

46

46 44

43 34

C

11 20

11 5

11 6 2

21 49

4

38 22

1

41 24

22 4

T

H

Qz

Gl

Fl

Glass shard (%)

Mineral composition (%)

2 1

0 1

1 0 *

0 1

0

3 1

0

2 1

1 0

Oths.

0 0

0 1

0 0 0

0 6

0

1 3

0

0 2

1 0

Obs

0 0

4 0

0 0 0

37 0

0

10 1

0

10 1

7 0

Br

Fraction to total glass particles (%)

1.502±1.553 (1.5025) 1.501±1.5025 (1.5015), 1.509±1.516, 1.551; Obs: 1.502 1.501±1.512 (1.501) 1.501±1.503 (1.501)

1.502±1.556 (1.5025±1.503) 1.5015±1.504 (1.502); Obs: 1.5015±1.503 (1.5025) 1.501±1.507 (1.5025) 1.501±1.503 (1.5025) 1.501±1.503 (1.502)

1.501±1.5565 (1.502) 1.502±1.503 (1.502); Obs: 1.502±1.503 (1.502) 1.498±1.502 (1.500±1.502)

1.500±1.5565 (1.502) 1.500±1.503 (1.5020); Obs: 1.502±1.5025 1.499±1.503 (1.501±1.502)

1.501±1.556 (1.501±1.5025) 1.498±1.503

Glass index (mode)

0 1

0 0

0 ± 0

0 0

1

0 0

±

± ±

± 1

Bi

1 1

111 1

11 11 1

111 11

11

111 111

±

111 111

111 11

Am

111 111

1 111

11 111 111

11 111

11

11 1

111

11 11

± 1

Opx

Heavy mineral composition

0 ± ±

± ±

±

± 0

±

0 0

11 ± 1 ±

0 0

0 0

0 0 0

± 0

0

0 0

0

± 0

111 111

111 11

111 111 111

111 111

111

11 1

111

1 11

11 111

Ap Opq ± 0 11 0

± 0 11 0

± 1 ±

± 0

±

± ±

1

± ±

0 0

Cpx Zr

Table 3 Representative petrographic features of distal fall units. Sampling site and horizon are shown in Figs. 1 and 5. Abbreviations are Gl, glass; Fl, feldspar; Qz, quartz; H.M., Heavy minerals; H, H-type glass; C, C-type glass; T, T-type glass; Obs, obsidian fragment; Br, brown colored glass; *, less than 1% ; Bi, biotite; Am, amphibole; Opx, orthopyroxene; Cpx, clinopyroxene; Zr, zircon; Ap, apatite; Opq, opaque. 111: .30%; 1 1 : 30±10%; 1: 10±5%; ±: ,5%

52 K. Kataoka et al. / Journal of Volcanology and Geothermal Research 107 (2001) 47±69

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53

Table 4 Representative microprobe analyses of glass shards from distal fall deposits of the Ebisutoge-Fukuda tephra. Note: average values are in weight percent; values in parenthesis are standard division for number of analyses n; total Fe as FeO; total recalculated on the basis of nine major elements

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2 O n

Unit A1

Unit A2 Lower

Unit A2 Upper

Unit A2 Upper (obsidian layer)

Unit B (rhyolitic glass)

Unit B (andesitic to dacitic glass)

76.39 (0.67) 0.11 (0.03) 13.38 (0.36) 1.41 (0.13) 0.06 (0.07) 0.05 (0.02) 0.89 (0.10) 3.02 (0.38) 4.69 (0.14) 10 WDS

75.90 (0.35) 0.09 (0.02) 13.49 (0.14) 1.41 (0.13) 0.05 (0.05) 0.17 (0.34) 0.92 (0.10) 3.08 (0.22) 4.89 (0.14) 10 WDS

76.12 (0.30) 0.10 (0.02) 13.51 (0.18) 1.38 (0.06) 0.04 (0.04) 0.06 (0.03) 0.90 (0.11) 3.05 (0.12) 4.85 (0.08) 10 WDS

76.09 (0.28) 0.11 (0.03) 13.45 (0.13) 1.44 (0.01) 0.07 (0.05) 0.08 (0.03) 0.97 (0.09) 2.97 (0.21) 4.83 (0.04) 10 WDS

75.99 (0.54) 0.12 (0.05) 13.47 (0.14) 1.54 (0.18) 0.07 (0.02) 0.07 (0.06) 1.09 (0.30) 3.02 (0.45) 4.62 (0.23) 7 WDS

61.77±68.07 0.40±0.65 15.04±18.64 6.30±11.24 0.05±0.11 0.21±0.45 3.89±6.32 2.70±4.94 1.20±2.43 10 EDS

`obsidian layer'. This obsidian layer is characterized by abundance of crystals and obsidian fragments (Figs. 4 and 6). The same dark-black band is found in the Kazusa basin although subunit A2U lacks the part which is correlative to reversely graded part in the Uonuma basin. Subunit A2U in Tokai and Kobiwako basins, distal from the source (.160 km west of the vent), is gray to light-gray. The reversely graded part seen elsewhere is absent. Although crystal concentration is less, the main characteristics of the obsidian layer match those east of the source. Thickness decreases from 6 to 2 cm with increasing distance from source. It consists of well-sorted medium-®ne to very ®ne sand sized vitric ash. This subunit shows mantle bedding with a nonerosive base. The contacts with unit A1 and unit B are sharp. Petrographic aspects: The lower two-thirds of subunit A2U of the Uonuma basin, is mainly composed of glass shards (.90%) with minor amounts of feldspar (,3%), trace amounts of quartz and heavy minerals (1% or less). Glass shards consist of C-type with subordinate H- and T-type. The refractive index of rhyolitic glass ranges from 1.501 to 1.507 (1.5025 in mode). Orthopyroxene (hypersthene to eulite) and opaque minerals are the predominant heavy minerals with subordinate amounts of amphibole, and minor amounts of clinopyroxene and zircon. In the obsidian layer, obsidian fragments make up 1±6% of total glass shards. Glass shards dominate in

mineral composition (more than 80%). Subordinate are feldspar (3±11%), heavy minerals (1±6%), and quartz (1±2%). Feldspar is more conspicuous in this unit than in all others. The glass shards are mainly Cto H-type. In colorless glasses refractive indices range from 1.500 to 1.503, and in obsidian fragments from 1.5015 to 1.503. High index glass slightly mingled in the sample from Kazusa Group. Heavy minerals

Fig. 2. Distal fall deposits of the Ebisutoge-Fukuda tephra in the Osaka basin, ,300 km SW from source, Osaka (Fig. 1). Fall units (unit A1 and B) are overlain by reworked volcaniclastic deposit. Reddish-brown color of unit B is diagnostic of this unit.

SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2 O n

Chayano Tuff I

77.51 (0.40) 0.09 (0.01) 12.28 (0.29) 1.44 (0.01) 0.07 (0.01) 0.04 (0.00) 0.87 (0.03) 2.62 (0.54) 5.07 (0.43) 3 WDS

Chayano Tuff I

76.79 (0.36) 0.17 (0.15) 13.07 (0.10) 1.47 (0.18) 0.11 (0.08) 0.03 (0.04) 0.97 (0.06) 2.35 (0.33) 5.05 (0.36) 6 EDS

77.12 (0.59) 0.08 (0.01) 12.39 (0.52) 1.45 (0.03) 0.09 (0.02) 0.04 (0.00) 0.79 (0.06) 3.44 (0.24) 4.60 (0.27) 7 WDS

Chayano Tuff II 76.94 (0.24) 0.15 (0.08) 12.99 (0.15) 1.52 (0.08) 0.05 (0.04) 0.01 (0.02) 0.92 (0.10) 2.93 (0.22) 4.48 (0.20) 10 EDS

Chayano Tuff II (pumice) 76.86 (0.20) 0.18 (0.05) 13.01 (0.11) 1.49 (0.15) 0.08 (0.08) 0.00 (0.01) 0.93 (0.09) 3.14 (0.10) 4.30 (0.10) 11 EDS

Ebisutoge P.D. unit-A (pumice) 76.84 (0.20) 0.14 (0.08) 13.06 (0.14) 1.55 (0.11) 0.06 (0.07) 0.03 (0.05) 0.89 (0.04) 3.19 (0.23) 4.25 (0.35) 11 EDS

unit-B (pumice)

77.13 (0.28) 0.08 (0.01) 12.79 (0.19) 1.43 (0.04) 0.06 (0.04) 0.04 (0.01) 0.88 (0.03) 3.36 (0.29) 4.24 (0.09) 7 WDS

unit-B (pumice)

77.06 (0.20) 0.14 (0.06) 13.05 (0.12) 1.50 (0.18) 0.05 (0.07) 0.02 (0.04) 0.95 (0.06) 2.91 (0.23) 4.34 (0.26) 12 EDS

unit-C (pumice)

76.96 (0.19) 0.13 (0.08) 13.11 (0.14) 1.52 (0.22) 0.07 (0.08) 0.01 (0.02) 0.93 (0.08) 2.88 (0.26) 4.39 (0.35) 10 EDS

unit-D (pumice)

Table 5 Representative microprobe analyses of glass shards and pumice from proximal pyroclastic deposits of the Ebisutoge±Fukuda tephra. Note: average values are in weight percent; values in parenthesis are standard division for number of analyses n; total Fe as FeO; total recalculated on the basis of nine major elements

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include mainly opaque minerals, orthopyroxene (hypersthene to eulite), and amphibole with subordinate clinopyroxene, zircon and biotite. 3.4. Unit B Lithofacies: The ®nest reddish-brown to reddishviolet ash, which we designate as unit B, is found in all basins. This unit can be identi®ed very easily by its diagnostic color (Figs. 2, 3 and 4), a typical feature of Fukuda V.A. and correlatives, which distinguishes it from other volcanic ashes. Thickness is 7±22 cm, increasing towards the source, especially in the west (Figs. 5 and 9). It consists of well-sorted ®ne to very ®ne sand to silt sized vitric ash. The base of the unit is non-erosive, but contact with unit A1 or unit A2 is sharp. Generally, the bed is normally graded and shows mantle bedding. At the bottom of unit, the concentration of heavy minerals is common. Petrographic aspects: Glass shards are predominant (.90%) with minor amounts of feldspar (.5± 10%), traces of heavy minerals and quartz (,2%). Glass shards are mainly C- to H-type. This unit is characterized by the presence of the brown colored T- to C-type glass. The refractive index of colorless rhyolitic glass normally ranges from 1.499 to 1.503, although exceptionally brown colored andesitic glass may have refractive index of up to 1.557. Heavy minerals are mainly amphibole with subordinate amounts of opaque minerals and orthopyroxene (hypersthene to eulite), minor amounts of biotite, zircon and clinopyroxene. 4. Proximal pyroclastic deposits From the above description and isopach maps of the distal fall deposits, the source of this tephra is inferred to be in central Honshu Area, Japan. The Pliocene to lower Pleistocene pyroclastic ¯ow deposits in central Honshu Area have been discussed in detail by Nagahashi (1995) and Nagahashi et al. (1996). The proximal pyroclastic ¯ow deposits, rhyolitic in composition, corresponding to distal fall deposits of Ebisutoge-Fukuda tephra are named the Chayano Tuff I, Chayano Tuff II, and Ebisutoge Pyroclastic Deposits, in ascending order, in Takayama Area (Figs. 1 and 7; west of the source), and the Takagariyama Tuff II in Omine Area (east of the source).

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4.1. Chayano Tuff I and II Lithofacies: The Chayano Tuff I consists of pyroclastic fall and non-welded pyroclastic ¯ow deposits related with phreatic eruption. Non-welded pyroclastic ¯ow deposit has a thickness of 0.2±2.7 m in northeast of Takayama Area. It contains mainly very ®ne sand sized vitric ash, and centimeter sized pumice and accretionary lapilli. South of the Takayama Area pyroclastic fall deposit, 1.4 m thick, comprises of vitric ash with accretionary lapilli (,3 cm in diameter). The Chayano Tuff II is non-welded pyroclastic ¯ow deposits, less than 1.4 m thick, consists of chie¯y very ®ne sand sized vitric ash. It contains pumice (0.4±2 cm in diameter) and accretionary lapilli (,1 cm in diameter). Petrography: Ash of the Chayano Tuff I and II chie¯y contains volcanic glass with trace amounts of plagioclase, quartz, and heavy minerals. The rhyolitic glass is colorless with C- and H-types, and ranges from 1.498 to 1.502 in refractive index. Heavy minerals consist of mainly opaques and amphibole. Heavy minerals in pumice of the Chayano Tuff II contain mainly opaques and orthopyroxene (eulite) and minor amounts of amphibole and zircon. 4.2. Ebisutoge Pyroclastic Deposits Lithofacies: The Ebisutoge Pyroclastic Deposits (Ebisutoge P.D.) has four units, namely -A, -B, -C, and -D in ascending order. The preserved total thickness of Ebisutoge P.D. is more than 40 m. The unit-A, less than 0.2 m thick, consists of pumice (0.5±1.5 cm in diameter) and lithic fragments, and is interpreted to be a pyroclastic fall deposit. The unit-B is 10 m thick, and is composed of ®ne sand sized vitric ash including pumice (0.5±1.5 cm in diameter) and lithic fragments (,1 cm in diameter). It is interpreted to be a pyroclastic ¯ow deposit. The unit-C, 0.1±0.2 m thick, is a pyroclastic fall deposit consisting of pumice (diameter of 4 cm) and lithic fragments. The unit-D is mostly composed of welded pyroclastic ¯ow deposit with more than 30 m thick. According to volume estimation (will be discussed later), it is thought that this unit was thicker originally and most of non-welded part of unit-D was eroded. Eutaxitic texture, parallel alignment of lenticular juvenile clasts (pumice and obsidian) is seen in welded zone. Lithic fragments (1±1.5 cm in diameter) are abundantly included. Pumice are concentrated in the

Fig. 3. (a) and (b) Panorama view and schematic illustration of distal fall deposits in the Kobiwako basin, ,200 km SW from the eruptive source, Shiga. Three fall units (unit A1, obsidian layer, and unit B) shows mantle bedding, whereas reworked part is abutting these fall units. Ruler is 30 cm long.

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Fig. 4. (a)±(c) Distal fall deposit in the Uonuma basin, ,140 km NE of the source, Niigata (Fig. 1). (a). Fall units A1 (eruptive phase 1), A2 (phases 2±4), and B (phase 5) are marked. Especially unit B is scoured by overlying reworked part. Scale is approximately 1 m long. (b) Unit A2 is characterized by the ®rst occurrence of visible crystals and subdivided into subunit A2L and A2U. Reversely graded with an upward increase in crystal amounts is typical in unit A2. (c) Crystal and obsidian fragment concentration (obsidian layer) is conspicuous in subunit A2U.

Fig. 5. Representative columnar sections of distal Ebisutoge-Fukuda tephra. Refer to Fig. 1 for locations. Unit A1 and B are extensively dispersed. All fall units are observed in the Uonuma basin, being near and downwind side from the source. Between unit A1 and B, the obsidian concentrated layer interbedded in the Kobiwako and Tokai Groups. This part is much richer in crystal than the other parts. In the Uonuma and Kazusa Groups, crystal rich layer unit A2 can be subdivided into upper part and lower part. In the upper part of unit A2 (subunit A2U), gray to black colored obsidian layer is observed with reverse grading. Phenocrysts component increases along with coarsening upward structure in the subunit A2U.

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Fig. 6. Photomicrograph of obsidian glass (obs) found at obsidian layer in Uonuma Group (a) and (b) and Tokai Group (c) and (d). Flow foliation is observed in reddish-brown to brown colored obsidian fragments.

bottom of the unit with 3±4 cm in average, 15 cm in maximum diameter. Diameter of lens (obsidian) increases (1 to 8 cm) upward of the unit. Petrography: Pumice included in the Ebisutoge P.D. (unit -A to -D), mostly consists of volcanic glass with trace amounts of plagioclase, quartz, and heavy minerals. The glass shard constitutes of H-and C-types, ranges 1.500 to 1.503 (1.501 in mode) in refractive index. Chemical composition of pumice is rhyolitic and matches to those in distal fall deposits (Table 5). Opaque minerals and orthopyroxene (eulite) are predominant heavy minerals with trace amounts of biotite, amphibole, clinopyroxene, and zircon. The matrix of pyroclastic ¯ow deposits in units-B and -D, is vitric ash with amphibole exceeding orthopyroxene.

parts. Lower part, 0.5±1.7 m thick, and consisting of resedimented volcaniclastics, is correlated with the Chayano Tuff I and II through unit-C of Ebisutoge P.D. Although petrographic features of this resedimented part do not match the pyroclastics precisely, obsidian fragments included in this lower part is notable. The upper part, 2.7±15 m thick, is pyroclastic ¯ow deposits whose characteristics correspond to unit-D of Ebisutoge P.D. (Nagahashi et al., 1996).

4.3. Takagariyama Tuff II

Inter-basinal correlations of Ebisutoge-Fukuda tephra have been discussed in previous literature (Yoshikawa et al., 1994, 1995, 1996; Tomita and

Takagariyama Tuff II can be divided into two

5. Discussion 5.1. Similarities of proximal pyroclastic deposits and distal fall deposits

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Fig. 7. Representative sections of proximal pyroclastic deposits of the Ebisutoge-Fukuda tephra after Nagahashi et al. (1996).

Kurokawa, 1997; Nagahashi et al., 2000). Integrating these with the newly obtained data show that proximal and distal Ebisutoge-Fukuda tephra are similar in: (1) petrography: mineral assemblage (glass concentration, heavy mineral composition), refractive index of glass, including orthopyroxene of eulite composition; (2) chemical composition: glass shards and pumice are rhyolitic (Tables 4 and 5) although unit B of the Fukuda V.A. (and correlatives) contains traces of andesitic glass (Table 4); and (3) stratigraphic data and geologic age: magnetostratigraphical and biostratigraphical data of the Plio-Pleistocene succession, intercalated with distal fall of the Ebisutoge-Fukuda tephra, shows that this tephra is above the Olduvai subchron in the Matuyama chron (Yoshikawa et al., 1996). Proximal pyroclastic ¯ow deposit of this tephra is also inferred to be in Matuyama chron (Nagahashi et al., 2000). Geological age of distal Ebisutoge-

Fukuda tephra has been estimated as 1.75 Ma based on ®ssion track age, biostratigraphy, and magnetostratigraphy (Yoshikawa et al., 1994, 1995, 1996). The ®ssion track age of the Nyukawa Pyroclastic Flow Deposit, immediately below the Ebisutoge Pyroclastic Deposits, is 1.76 ^ 0.17 Ma (Harayama, 1998). From its stratigraphic position, diagnostic lithofacies, petrographic features, and lateral dispersal of tephra unit, plausible correlation between units of the distal fall deposit and proximal pyroclastic deposit is inferred as follows (Fig. 8). Unit A1 of the Fukuda V.A. is correlated with the Chayano Tuff I and II, unit A2L with unit-A to -B of the Ebisutoge P.D., unit A2U (obsidian layer) with -C of the Ebisutoge P.D., and unit B with unit-D of the Ebisutoge P.D., respectively. Details of each correlation are discussed along with eruption style and history in next section.

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5.2. Eruption styles and history of the EbisutogeFukuda tephra Correlation of proximal pyroclastics with distal volcanic ash indicates that the Ebisutoge-Fukuda tephra was ejected in ®ve eruption phases and a posteruptive resedimentation (Fig. 8). The distribution pattern of distal fall deposits of each eruption phase was different (Fig. 9). Pyroclastic materials do not contain reworked volcaniclastics between eruption phases. Distal fall deposits in the Osaka, Kobiwako, Tokai, Uonuma and Kazusa basins, also suggest that fall units accumulated continuously. These facts show the ®ve eruptions were part of a continuous event without any signi®cant volcanic quiescence. 5.2.1. Eruption phase 1 The Chayano Tuff I and II, ®ne-grained and with conspicuous accretionary lapilli, formed by a phreatoplinian eruption, is correlated with unit A1 of Fukuda V.A. The unit A1 shows plural normal grading with one to two layers of accretionary lapilli concentration. This shows that evidence of phreatic eruption are preserved in distal areas. Fine-grained deposits in both proximal and distal areas and extremely wide dispersal of unit A1 suggest that these eruption phases were related with explosive phreatoplinian eruption. A concentric isopach map also suggests that these deposits formed by large magnitude eruption (Carey and Sparks, 1986; Koyaguchi, 1996). 5.2.2. Eruption phases 2 and 3 Eruption phases 2 and 3 are a plinian eruption and column collapse which generated the pyroclastic ¯ow. Firstly, plinian eruption caused unit-A of the Ebisutoge P.D. Then eruption column collapsed to generate the ignimbrite as unit-B of the Ebisutoge P.D. The subunit A2L of the Fukuda V.A., relatively coarsegrained, and with visible crystals, resulted from this plinian and ignimbrite forming eruption, and deposited as distal plinian ash and/or co-ignimbrite ash. Subunit A2L is found only east of the eruption center (in the Uonuma and Kazusa basins), and this suggests that the eruption cloud followed westerly prevailing winds. Lower intensity of eruption is re¯ected by the small thickness and distribution of unit-A and -B of the Ebisutoge P.D.

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5.2.3. Eruption phase 4 A large plinian eruption occurred in phase 4. The plinian fall deposits correspond to unit-C of the Ebisutoge P.D. in the proximal area and the subunit A2U (obsidian layer) of the Fukuda V.A. in the distal area. Signi®cantly, the obsidian-bearing unit is found both in distal (the Uonuma, Kazusa Kobiwako and Tokai basins), and in proximal areas (the Omine Area; lower part of the Takagariyama Tuff II). From this we infer that the plinian eruption cloud could be extended towards east and west. Reverse grading in the unit A2 suggests that intensity of ejection increased to cause the vent enlargement similar to the model of Wilson et al. (1980). The obsidian layer could re¯ect the vent wall erosion that triggered column collapse with generation of large scale pyroclastic ¯ow. Kamata et al. (1997) described obsidian fragments in the distal plinian fall deposits and interpreted it as a precursor to the climactic plinian fall and pyroclastic ¯ow. Hildreth and Mahood (1986), Suzuki-Kamata et al. (1993), and Kamata et al. (1994) indicated that the plinian and/or the distal plinian fall deposit sometimes contains obsidian with the same chemical composition as other glass shards. The obsidian concentration in subunit A2U of the Fukuda V.A. occurs at the top of the inversely graded part. The obsidian glass shards have the same refractive index as the colorless glass ones. These facts suggest that subunit A2U consists of distal plinian ash re¯ecting the transition from plinian column collapse to pyroclastic ¯ow. Reverse grading is obvious in subunit A2U, especially in the Uonuma basin, being nearer to the source than other basins. Particles from the plinian event which reached the other distal basins also underwent sorting by wind, resulting in crude reverse grading. In the deep sea deposit from the Kazusa basin, primary information from the eruption was not well preserved, because of the multiple effects of wind sorting, water sorting and penecontemporaneous reworking during the settling. The wide range of glass refractive index in subunit A2U suggests mingling of glass shards caused by these effects. Even within the same strata and in the same basin, fall deposits sometime show differences in lithofacies and petrography when settled into the deep sea (Satoguchi et al., 2000). 5.2.4. Eruption phase 5 After the large plinian eruption of phase 4, the

Fig. 8. Schematic illustration of the eruption history and distal ejecta distribution in each basin. Phase 1 phreatoplinian eruption formed the Chayano Tuff I and II in proximal area, and unit A1 of Fukuda V.A. in distal place. Phases 2 and 3 are characterized by plinian eruption (unit-A of Ebisutoge P.D.) to pyroclastic ¯ow (unit-B of Ebisutoge P.D.). Subunit A2L of Fukuda V.A. is the distal ash of these two phases. Plinian eruption occurred in phase 4, ejected unit-C of Ebisutoge P.D. near the vent and formed subunit A2U of Fukuda V.A. as distal plinian deposit which contains obsidian fragments conspicuously. In the phase 5, eruption column collapse triggered by phase 4, generated large pyroclastic ¯ow (unitD of Ebisutoge P.D.) in all directions. Co-ignimbrite ash reached to more than 300 km away from the eruption center forming unit B of Fukuda V.A.

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eruption column collapsed with vent enlargement. This phase generated huge pyroclastic ¯ows in all directions and much co-ignimbrite ash, resulting in unit-D of the Ebisutoge P.D. and unit B of the Fukuda V.A., respectively. Pyroclastic ¯ow deposits distributed westward (as unit-D of Ebisutoge P.D.) and eastward (as upper part of Takagariyama Tuff II) from the eruption center (Figs. 1 and 7). The unit B of the Fukuda V.A., is ®ner-grained and more widely dispersed, and is interpreted as a co-ignimbrite ash of this pyroclastic ¯ow. The isopach map of unit B of the Fukuda V.A. is almost concentric in pattern. This suggests deposition of co-ignimbrite ash accompanied by large ignimbrite, similar to the other tephras (e.g. Aira-Tn tephra, Machida and Arai, 1976; Toya tephra, Machida et al., 1987). This eruption was so drastic that the co-ignimbrite ash spread more than 300 km away from eruption center. Although the magnitude of this eruption suggests that this phase may relate with caldera collapse, none of the topographical remnants indicating caldera morphology could be identi®ed due to erosion after the eruption. Andesitic glass included in unit B of Fukuda V.A. can not be found in unit-D of Ebisutoge P.D. From the volume ejected during phase 5, most of non-welded part of unit-D inferred to be degraded. The andesitic glass shards, ejected in the ®nal stage of this phase, probably were included in the eroded part of proximal pyroclastic ¯ow deposits. 5.2.5. Posteruptive resedimentation The reworked volcaniclastic part (with 1±10 m thick, Fig. 3) of the Fukuda V.A. is found above fall units several hundred kilometers away from the eruption center (Yoshikawa et al., 1995, 1996; Kataoka et al., 1998). In the Tokai basin, 160 km from the eruption source, the thickness of reworked part is ,10 m. We can easily imagine that ejecta during eruptive phase 1±5 would be eroded quickly after eruption. The supplied volume of sediment throughout the catchment area of sedimentary basins had increased drastically resulting in huge thickness of the reworked part. 5.3. Volume estimation and magnitude of the Ebisutoge-Fukuda tephra Several extrapolation methods for fallout tephra volume estimation have been proposed from isopach

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maps (e.g. Rose et al., 1973; Pyle, 1989, 1995; Fierstein and Nathenson, 1992). Pyle (1989) proposed logarithm of thickness vs square root of area plot as a better way to calculate fallout tephra volume based on exponential thinning law which was supported by many works empirically (e.g. Porter, 1973; Kittleman, 1973). Fierstein and Nathenson (1992) and Pyle (1995) supported the method of Pyle (1989) with some modi®cations. Whereas Walker (1980) proposed another volume calculation method by eolian concentration of crystals. Since estimated volume by Pyle (1989) and Fierstein and Nathenson (1992) are generally several times smaller than that from crystal concentration method by Walker (Pyle, 1989; Fierstein and Nathenson, 1992; Rose, 1993), volume calculation methods have been problematical and controversial. Although we cannot assess which method is appropriate, Pyle's method is useful since it might give us minimum volume estimation of the Ebisutoge-Fukuda tephra. Then we can know `at least' how much volume of ejecta produced by eruptions and can reveal explosiveness of volcanic activity. Compared with younger and better exposed volcanic products, the extrapolated volume of the Ebisutoge-Fukuda tephra lacks accuracy, re¯ecting fewer exposures. We also note that these tephra might have undergone compaction, and the original tephra bed was undoubtedly thicker than what we measured. Eruption of the Ebisutoge-Fukuda tephra dispersed volcanic material over 290,000 km 2 and reaching more than 300 km from the probable eruption center. The isopach map corresponding to each eruption phase is shown in Fig. 9. Exponential decay of thickness is observed even in the compact tephra (Fig. 10). The isopach map of phase 1 (unit A1 of the Fukuda V.A.) is concentric, and hence volume can be properly estimated. About 105 km 3 of volcanic materials was ejected during eruption phase 1. Volcanic products from eruption phases 2 and 3 are indistinguishable in distal locations. Combined volume from both phases is approximately 13 km 3. Volume of the obsidian layer erupted in phase 4 is 17 km 3, though it excludes the reverse graded part due to less observation points. Volume ejected by eruption phase 5 is 123 km 3, and this volume is only for co-ignimbrite ash and excludes the volume of ignimbrite. Sparks

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Fig. 9. Distribution pattern of each eruptive phase. (a) Isopach map showing concentric pattern in the unit A1 of Fukuda V.A. (phase 1) due to large phreatoplinian eruption. (b) Subunit A2L (phases 2 and 3) is only distributed in the east of the vent. (c) Obsidian concentrated layer of subunit A2U has wide dispersal during phase 4. (d) The unit B (phase 5) also has concentric distribution due to ignimbrite ¯ow in all directions caused by large eruption column collapse. All thickness in cm.

and Walker (1977) estimated amount of vitric ®ne ash elutriated from ignimbrite. In their case, the mean minimum loss of vitric ®ne from ignimbrite is estimated as 35%, and at most 50% of the original magma. Thus, volume of ignimbrite and accompanying co-ignimbrite ash of phase 5 is roughly more than 250±350 km 3, although only a few cubic kilometers of volume of ignimbrite are preserved in present outcrops.

Total integrated volume throughout these eruption phases is more than 380±490 km 3 (Table 6), and this volume is still an underestimate. From the volume of products, the Volcanic Explosivity Index (VEI: Newhall and Self, 1982) of the Ebisutoge-Fukuda tephra is considered to be greater than 7. The total bulk volume of the Ebisutoge-Fukuda tephra here estimated is almost equal to other Pleistocene representative widespread tephras, for

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Fig. 10. (a) Log thickness vs distance plot of unit A1. (b) Log thickness vs square root of area plot for unit A1. Measurement along SW direction from the source. The straight line suggests exponential decaying of the thickness.

instance, Aira-Tn tephra (.450 km 3, 22±25 ka; Machida and Arai, 1976; Machida, 1999), Aso-4 tephra (.600 km 3, 84±89 ka; Machida et al., 1985) and Ata tephra in Japan (.300 km 3, 95± 110 ka; Machida and Arai, 1992; Machida, 1999), Los Chocoyos tephra, Guatemala (,420 km 3, 85 ka; Drexler et al., 1980; Rose et al., 1987; Chesner et al., 1991), and Morphi tephra, Mediterranean Sea (.200 km 3 of distal fallout tephra, ,374 ka;

Pyle et al., 1998) and surpasses many other widespread tephras. Representative widespread tephra, has been associated with ignimbrite, and often deposited as coignimbrite ash (Sparks and Walker, 1977; Machida and Arai, 1976). The Kikai-Akahoya tephra (,6.3 ka; Machida and Arai, 1978), Aira-Tn tephra, Aso-4 tephra, Youngest Toba tephra (,75 ka; Rose and Chesner, 1987) are good examples of widely

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Table 6 Estimates of volume of Ebisutoge-Fukuda tephra. Note: equation V ˆ 13.08Tmaxbt 2 is from Pyle (1989). A, Area enclosed within isopach; Tmax, extrapolated maximum thickness; bt, thickness half-distance Unit name

Eruption style

Isopach thickness (cm)

Area A (km 2)

A 1/2 (km)

Tmax (m)

bt (km)

Extrapolated volume (km 3) V ˆ 13.08Tmaxbt 2

Unit B

Ignimbrite

10 15

2.61E 1 05 1.10E 1 05

5.10E 1 02 3.31E 1 02

0.32

172.6

123.8 (excluding ignimbrite body)

Subunit A2U (obsidian layer)

Plinian

3 5

1.41E 1 05 9.20E 1 04

3.76E 1 02 3.03E 1 02

0.43

55.3

17.1 (excluding reversely graded part)

Subunit A2L

Plinian to ignimbrite

5 10

7.02E 1 04 2.62E 1 04

2.65E 1 02 1.62E 1 02

0.30

58.1

13.1

Unit A1

Phreatoplinian

10 20 30 40

2.89E 1 05 1.18E 1 05 5.88E 1 04 2.97E 1 04

5.37E 1 02 3.43E 1 02 2.42E 1 02 1.72E 1 02

0.74

104.9

107.1

Ebisutoge P.D. Unit-D

Ignimbrite

Total volume

dispersed co-ignimbrite ash. Therefore, the extensive dispersal pattern, thickness, and volume of the widespread tephra re¯ect intrinsic characteristics of ignimbrite volcanism. However, unlike other widespread tephras, the Ebisutoge-Fukuda tephra also shows phreatoplinian eruption during phase 1 (Chayano Tuff I and II; unit A1 of Fukuda V.A.). The recognition of fall units with varying physical character is also unusual when compared with other widespread tephras. There is dif®culty distinguishing distal plinian ash from co-ignimbrite ash in a large eruption (Kamata et al., 1997; Lajoie and Stix, 1992; Cas and Wright, 1987). However, in the case of early Pleistocene Shishimuta-Azuki Tephra, Kamata et al. (1997) showed that it is possible to identify a paired occurrence of distal plinian ash and co-ignimbrite ash from a single sequence of eruptions based on physical and chemical changes in the deposits. The EbisutogeFukuda tephra also suggests that one can distinguish several fall units in the distal tephra deposits, each with a corresponding eruptive style and magnitude. Thus, the Ebisutoge-Fukuda tephra represents a widespread tephra which reveals the relation between distal and proximal characteristics, and aids our

. 124±230 (km 3) (excluding coignimbrite ash) . 380±490 (km 3)

understanding of large scale eruptions and depositional processes of the distal ashes. The large mass of ejecta suggest that eruption of the Ebisutoge-Fukuda tephra could have affected environmental and climatic conditions globally at the Pliocene to Pleistocene boundary. Successional paleontological analysis in the Tokai Group revealed paleoenvironmental change around the EbisutogeFukuda tephra horizon as reported by Tado Collaborate Research Group (1998). They found assemblages of pollen, diatom and beetle fossils change above the Ebisutoge-Fukuda tephra layer, and concluded that the shift to a period of colder climate occurred after the deposition of this tephra. This cooling could be triggered by the eruption of Ebisutoge-Fukuda, like as other large eruptions resulting in worldwide cooling (i.e. volcanic winter). Further comprehensive studies are necessary for deducing the nature and magnitude of environmental impact of the Ebisutoge-Fukuda tephra. Wide dispersal of the Ebisutoge-Fukuda tephra offers signi®cant possibility to establish the stratigraphy and chronology in extensive terrestrial sediments and in deep sea drilling core sediments around Japan. Being a boundary bed of the Pliocene to Pleistocene, this tephra can be a marker in sediment records.

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6. Conclusions

References

In this study we reconstructed the ancient EbisutogeFukuda eruption by comparing the facies variation, distribution pattern, and petrographic features of proximal and distal volcaniclastic deposits. The Ebisutoge-Fukuda tephra re¯ects ®ve eruption phases and posteruptive resedimentation. Phase 1 is phreatoplinian eruption producing more than 105 km 3 of volcanic materials as unit A1 of the Fukuda V.A. and Chayano Tuff I and II. Phases 2 and 3 are a plinian eruption and transition to pyroclastic ¯ow resulting in formation of subunit A2L of Fukuda V.A. and unit-A, -B of the Ebisutoge P.D. Plinian activity also occurred in phase 4, which ejected conspicuous obsidian fragments to the distal locations (subunit A2L of Fukuda V.A. and unit-C of Ebisutoge P.D). In phase 5, collapse of eruption column triggered by phase 4, generated large pyroclastic ¯ow (unit-D of Ebisutoge P.D.) in all directions with much of co-ignimbrite ash (unit B of Fukuda V.A.), resulted in more than 250± 350 km 3 of deposits. The total volume of this tephra amounts over 380±490 km 3 indicating the VEI of greater than 7. The extensive dispersal, huge thickness and total bulk volume of Ebisutoge-Fukuda tephra resulted not only from an ignimbrite forming eruption (with co-ignimbrite ash) but also from a phreatoplinian eruption (with distal plinian). This tephra, with wide dispersion and explosiveness, is very important as a marker bed in the Plio-Pleistocene boundary, and may have caused global environmental and climatic changes. Thus, the Ebisutoge-Fukuda tephra can be good example for elucidating numerous older tephras whose eruption style, history and magnitude are still unknown.

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Acknowledgements We thank Dr Yasufumi Satoguchi and Mr Shogo Konishi for their help in the ®eldwork. Our gratitude to Dr Takamoto Okudaira for his assistance with microprobe analysis. Dr Takeshi Nakajo is acknowledged for providing comment on this manuscript. We thank Dr M. Satish-Kumar for improving content and English. Professor W.I. Rose and an anonymous reviewer are acknowledged for their constructive comments on this manuscript.

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