1992, Fujioka, K., Nishimura, A., Matsuo, Y., and Rodolfo, K. S., Correlation of Quaternary tephras...

Taylor, B., Fujioka, K., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 126

2. CORRELATION OF QUATERNARY TEPHRAS THROUGHOUT THE IZU-BONIN AREAS1

Kantaro Fujioka,2 Akira Nishimura,3 Yoshiko Matsuo,4 and Kelvin S. Rodolfo5

ABSTRACT

Quaternary marine tephras in the Izu-Bonin Arc offer significant information about explosive volcanic activities of the arc.Visual core descriptions, petrographic examinations, and chemical and grain-size analyses were conducted on tephras of backarc,arc, and forearc origin. Tephras are black and white and occur in simple and multiple modes with mixed and nonmixed ashes ofblack and white glass shards. The grain size distributions of the tephras are classified into three categories: coarse, whitepumiceous, and fine white and black well-sorted types. The frequency of occurrence of the white and black tephras differs withinthe tectonic settings of the arc. Chemically, the Quaternary tephras in this region belong to low-alkali tholeiitic series with lowerK2O and TiO2 than normal ordinary arc volcanic materials. Several tephras from different sites along the forearc correlate witheach other and with tephras in the Shikoku Basin site and with Aogashima volcanics. These volcanic ashes resemble those inother backarc rifting areas, such as in the Fiji, Okinawa (Ryukyu), and Mariana regions.

INTRODUCTION

The distribution of Quaternary marine tephras around the JapaneseIslands has been well documented by studies of piston core samplestaken in the region (Machida and Arai, 1983, 1988; Furuta et al.,1986). However, tephras along the Izu-Bonin Arc are not as wellstudied, except for those of Oshima Island, south of Tokyo (Naka-mura, 1964). Drilling along the Izu-Bonin Arc provides an excellentopportunity to document the chemistry and distribution of theseopean-ocean tephras and to correlate them, if possible, to those aroundthe Japanese Islands. Moreover, obtaining information on the vol-canic ash layers along the Izu-Bonin Arc is of primary importance forunderstanding the tectonic evolution of the arc.

During Ocean Drilling Program (ODP) Leg 126, we recovered alarge number of tephra layers from the Aogashima and Sumisujimaarea in the Izu-Bonin Arc south of Hachijojima Island, several hun-dred kilometers from Tokyo. Seven sites were drilled around theactive volcanic islands of Aogashima and Sumisujima: three sites inthe forearc basin (Sites 787, 792, and 793), two sites on the volcanicfront (Sites 788 and 789), and two sites in the active backarc SumisuRift (Sites 790 and 791), resulting in a cross-sectional traverse of thearc through the forearc to the backarc (Figs. 1 and 2).

Site 787 in the Aogashima canyon, Site 792 at the fork of theAogashima canyon, and Site 793 in the middle part of the upper slopebasin were drilled in the forearc of the Izu-Bonin Arc (Fig. 1). Thethree sites have similar Oligocene to Holocene lithologies (Fig. 3).Three major lithologic units are evident: gravity flow, hemipelagicsediments, and volcaniclastic materials (Taylor, Fujioka, et al., 1990).The sedimentation rates of the three units at each of these three sitesare also identical to each other: high in the gravity flow facies, verylow in the hemipelagic facies, and moderately high in the volcaniclas-tic facies (Taylor, Fujioka, et al., 1990). More than 300 tephra layerswere successfully recovered from these three sites.

Site 788 was drilled on the footwall of the Sumisu backarc rift, awell-studied backarc rift of the Izu-Bonin Arc (Fujioka, 1983a, 1983b,

Taylor, B., Fujioka, K., et al., 1992. Proc. ODP, Sci. Results, 126: College Station,TX (Ocean Drilling Program).

2 Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo164, Japan (present address: Japan Marine Science and Technology Center, 2-14 Nat-sushima, Yokosuka, Kanagowa 238, Japan).

3 Marine Geology Department, Geological Survey of Japan, 1-1-3 Yatabe, Tsukuba305, Japan.

4 Institute of Geosciences, Shizuoka University, 836 Oya, Shizuoka 422, Japan,department of Geological Sciences, University of Illinois at Chicago, P.O. Box 4348,

Chicago, IL 60680, U.S.A.

1988; Brown and Taylor, 1988; Murakami, 1988; Nishimura andMurakami, 1988; Yamazaki, 1988; Ikeda and Yuasa, 1989; Fujioka etal., 1990) (Fig. 4). At this site, thick pumiceous layers were recoveredwith Pliocene altered pumice beds at the base.

Sites 790 and 791 were drilled 2.5 km apart in the eastern half-graben of the eastern Sumisu Rift (Fig. 4). Three major lithologies,including basement basalts, were identified in these Sumisu Rift sites.The upper half of the section consists of coarse pumiceous beds withfour or five cycles of coarse pumice units underlying thin nannofos-sil-rich hemipelagites. The basement of the rift basin was charac-terized by a frothy vesicular basalt similar in chemical compositionto the backarc basin basalt (Gill et al., 1990; Taylor, Fujioka, et al.,1990; Leg 126 Shipboard Scientific Party, 1989; Leg 126 ScientificDrilling Party, 1989).

We recovered more than 500 volcanic ash layers and ashy intervalsin the Quaternary sediments along the Izu-Bonin Arc during Leg 126.This report documents the chemical nature of these volcanic ash layersand the accumulation rate of the volcaniclastic materials in the region.We also summarize the features and timing of the explosive volcanismthat took place along the Izu-Bonin Arc during the Quaternary.

METHODS

Smear Slide Observations

To determine the mineral assemblage, mode, and morphology ofthe glass shards and the nature of the lithic fragments of the volcanicash layers, we made smear slides of each tephra unit following theprocedure outlined in the explanatory notes of the Leg 126 InitialReports volume (see Taylor, Fujioka, et al., 1990).

Grain-size Analysis

The grain-size distribution of the tephras were determined with alaser light grain-size analyzer for particles smaller than 500 µm; forthose larger than 500 µm, the samples were first sieved, and then theirweights were measured.

Materials finer than 44 µm were eliminated by sieving. The 500µm fraction was analyzed with an automatic laser-beam particleanalyzer (Model SALD 1100 of Shimadzu Co Ltd., Japan). Sedimentswere put into a glass vessel with distilled water. After being mixed inan ultrasonic cleaner for 10 s, the samples are put into the quartz cellof the laser unit. In this unit, laser light refracts suspended materials,and the ratio of the reflected light from particles inside the quartz cellare measured by a photodetector. The accuracy of the grain-sizedistribution for each tephra layer is more than 95%.

23

K. FUJIOKA, A. NISHIMURA, Y. MATSUO, K. RODULFO

34°N

32C

139° E 141' 143C 30c

Figure 1. Bathymetric map of the Izu-Bonin Arc-trench system between 30°N and 34°N and locations of the drilling sites (solid circles) of Leg 126.Contour interval = 500 m (numbers on contour lines indicate depth in km).

Arc

788/9V

3 0 0 0 790/1Backarc Rift

-6000

- 9000 m °

V.E.= 14

forearc High

r* —.

Upperslope

Outerarc High

793 -707 vac;

b a s ' n \784 783

\ T r e n c / j f \ Y

\ -d100 km λ / V

Figure 2. Schematic cross section of the Izu-Bonin Arc with drilling sites. V.E. = vertical exaggeration.

Refractive Indices of Glass dure, the refraction index value follows the temperature change of theimmersion liquid, which is automatically controlled by the instru-ments thermomodule. When the glass flakes in the immersion liquiddisappear under the microscope, the operator sends a signal to the

The refractive indices of glass shards from the volcanic ash layerswere determined using the automatic refractive index measurementsapparatus of Kyoto Fission Track Co. Ltd., Model RIMS-86 follow- microcomputer. After the procedural steps are repeated two times (foring the procedures outlined in Yokoyama et al. (1986). In this proce- ascending and descending temperatures) on a glass flake, the micro-

24

Site 787 Site 792 Site 793Graphic lithology/grain size

c s fs csI ' '

Graphic lithology/grain sizec s fs cs g

' ' ' '

Graphic lithology/grain sizec s fs cs g

I I

100 —

200 —

300 —

400

200

*j-:-:>??>^?^ Nannofossils

>?;j>;̂ ;|;->;-̂ Silt/siltstone and clay/claystone

Vitric and pumiceous sand/sandstone

Pumiceous gravel

Conglomerate

Volcanic-clast breccia

Lapilli tuff

Aπdesitic pyroclastics

Andesitic lavas

Figure 3. Lithostratigraphic diagram of Izu-Bonin forearc Sites 787, 792, and 793. Solid line indicates the bottom of the Quaternary sediments cored during Leg 126.

Key to graphic lithology

IS * S *1 Bioturbation

I Q ° j Sedimentary intraclasts

|*V ^ ÷S | Volcanic pebbles and cobbles

| » •* ^a| Andesite fragments

I Massive flow

δoo

I


Rift marginSite 788

Sumisu Rift BasinSite 790 Site 791

Unit I

387 TD

Unit IA

Unit IIA

SUnit HB

M~£UJéri

m230249

279

374 TD

•^28

— 500

/

Nannofossils

Silt/siltstone andclay/claystoneVitric and pumiceoussand/sandstone

Pumiceous gravel

Conglomerate

Mafic volcanic rocks

•834

Igneousbasement /

/

/

/

/

1145TD

Site 790 Site 7912000

Site 789 Site 788

Vertical exaggeration 2.1

Figure 4. Lithostratigraphic diagram of backarc Sites 790/791 and arc Site 788 (after correlation paper in Taylor, Fujioka, et al, 1990).Upper sketch is the stratigraphic correlation of the backarc sites; lower sketch is the seismic profile of the MCS Line Bon II-4 acrossthe eastern footwall uplift and eastern half-graben of the Sumisu Rift.

26

CORRELATION OF QUATERNARY TEPHRAS

computer calculates the mean and standard deviation of the refractiveindex. The accuracy of the refractive index value of a single glassflake is > 99%.

Chemical Analyseswith the Electron Probe Microanalyzer

An electron probe microanalyzer was used to determine the percent-age of 12 major elements of the glass shards as well as the minerals inthe tephra layers. The tephras were cleaned with an ultrasonic cleaner indistilled water to eliminate clay fractions. Dried samples were impreg-nated on acrylic plates with 5-mm punches of epoxy resin, and polishedthin sections of the volcanic ash layers were prepared. The experimentalconditions for analyzing the glass shards were as follows: 15 kV, 1.2 ×I0""7 Å, beam diameter of 10 µm, 10-s count for every oxide. Oxides anda single mineral were used as a standard for each oxide. The accuracy ofeach oxide ranges from 90% to 98%, depending on the element. SeeFujioka et al. (1980) and Furuta and Arai (1980a, 1980b) for details ofthe microprobe procedures.

VISUAL OBSERVATIONSOF VOLCANIC ASH LAYERS

Visual core descriptions and smear slide and binocular obser-vations of the volcanic ash layers were conducted on board theJOIDES Resolution and during shore-based studies on the charac-terization of each ash layer. A list of all the tephras analyzed is givenin Table 1. The colors of the tephras are white, gray, green, and black,depending on the chemistry of the glass shards, lithic fragments, andminerals. The glassy tephras are silt-sized, and the scoria and pumicerange from sand to granule in size. The thickness of the volcanic ashlayers ranges from several tens of centimeters (maximum =146 cm)to a few millimeters, depending on their distance from the sourceareas and the degree of bioturbation caused by postdepositionalprocesses. Almost all the tephras are less than 10 cm thick, buttephras several tens of centimeters (60 cm) often occur in turbiditeshaving scoriae and pumices. These thicker layers may be resedi-mented mixed layers. Sedimentary structures such as laminae, bio-turbation, and grading were also observed.

The volcanic glasses recovered during Leg 126 occur as discretelayers and small pods or pockets or are dispersed in the muddymatrices. Some tephra layers have distinct grading structures, someare mixed with rhyolitic and basaltic material, and some are resedi-mented. Strong burrow structures caused by bottom-dwelling organ-isms, such as Zoophycos, Chondrites, and Planolites (Taylor, Fujioka,et al., 1990) also occur in the tephra layers, but these are rare.

Visually, the tephras are classified into two types: simple layer andmultiple layer. They also can be classified into mixed and unmixed.The simple layer has a definite boundary in a mud matrix and consistsof black and white tephras mixed to form a discrete layer. The multiplelayer consists of sets of tephras that have not been diluted by the mudmatrices and that have sharp contacts from layer to layer. The exist-ence of the sharp boundaries in the multiple-layer volcanic ash layerssuggests that explosive volcanism may take place regularly, with shortvolcanic quiescent periods intervening in which little or no mudmatrix accumulates.

Two types of dark-colored volcanic ash layers were frequentlyencountered: (1) black, coarse (scoriaceous) and (2) black, fine, wellsorted. Two types of light-colored volcanic ash layers were alsofrequently encountered during visual core observation: (1) white,coarse (pumiceous) and (2) fine, white. These four visually distinctvolcanic ashes suggest that their origins are different from each other.

Smear Slide and Binocular Observations

Smear slide descriptions of all the volcanic ash layers sampled arepresented in Table 2. Characteristic modes of occurrences of the volcanic

glass shards are classified into three major types: translucent, black,and mixed. The translucent type contains more than 85% glass shardswith no lithic fragments. The black type has a few lithic fragmentsand 15%^0% crystals. The mixed type contains both black andtranslucent volcanic glass shards. The shapes of the volcanic glassshards are bubble wall (bw), tabular (tub), and pumice (pm) for thetranslucent type and brown-colored, bubble-wall (brbw) and crystal-rich type for the black ashes. Common mineral assemblages for allthe volcanic ash layers identified under the petrographic and binocu-lar microscopes are Plagioclase, clinopyroxene, and opaque minerals.Orthopyroxene and biotite are rarely encountered. Hornblendes arepresent but rare in the white ash and crystal-rich black ashes.

ACCUMULATION RATESOF THE VOLCANIC ASH LAYERS

To calculate the accumulation rates of the volcaniclastic materials,we assumed that the input of volcaniclastic material was instantane-ous as compared with the hemipelagic muds. Figures 5, 6, and 7illustrate the accumulation rate of the tephras obtained this way. Theaccumulation rates of the ash at backarc Sites 790 and 791 show adistinct maximum since 0.2 Ma. The accumulation rate of the vol-caniclastic materials of the forearc sites (Figs. 5 and 6) shows twomaxima, one around 1.0-0.6 Ma and another at 0.2-0.4 Ma. The olderpeaks include the rapid accumulation of the dark-colored ashes (i.e.,the basaltic ashes). However, the younger peak consisting mostly ofrhyolitic ash layers has a much higher accumulation rate. The accu-mulation rate of the forearc region is quite similar in its pattern tothose obtained in the ashes distributed in the south Boso Peninsula(Kotake, 1988).

GRAIN-SIZE DISTRIBUTIONOF THE VOLCANIC ASH LAYERS

The grain size of the volcanic ash layers contains informationabout (1) the origin of the ash, (2) the source region of the tephra, and(3) the depositional mechanism of tephras. The grain-size distributionof the volcanic ash layers of the forearc sites are classified into threetypes of distribution patterns (Figs. 8Aand 8B and Table 3). The firsttype is characterized by white, coarse ash layers with a mediandiameter around 3(f> (Inmann's Φ scale; Inmann, 1952) and a well-sorted cumulative curve; skewness is small. Examples of this typeof grain-size distribution include Samples 126-792A-4H-4,96-98cm, and 126-792B-19H-2, 36-38 cm. The second characteristicdistribution is a black ash that has a well-sorted cumulative curvewith a 3-4(j) median diameter; skewness is small, but kurtosis ishigh. Examples of this type of grain-size distribution includeSamples 126-792A-3H-5, 78-80 cm, and 126-793A-3H-2, 50-52cm. The third characteristic grain-size distribution is typified by afine white ash layer, either well or poorly sorted, with a mediandiameter of 40. The poorly sorted tephras are skewed toward thefiner portions, whereas skewness is small in the well-sorted por-tion. Examples of the third grain-size distribution type includeSamples 126-788C-11H-4, 108-110 cm, 126-792A-1H-5, 81-83cm, and 126-793A-3H-2, 69-71 cm. Tephras with well-sorted,fine median diameters may be the result of fallout deposits, where-as those skewed toward a finer grain size may be the result ofsubaqueous volcanism (Sparks et al., 1981; Walker, 1971).

The tephra grain-size distribution suggests that distribution types1 and 2 represent input from a nearby source (i.e., the Izu-Bonin Arc),but the black tephras with low kurtosis may come from secondaryprocesses. The third type of tephra grain-size distribution may repre-sent input from the western part of the Philippine Sea.

The backarc tephras are classified into two types of grain-sizedistribution curves: coarse pumiceous and fine glass. Characteristicfeatures of the coarse tephras include a broad size distribution rangingfrom clay to granule size (Nishimura et al., this volume); medium

27


Table 1. Volcanic ash layers at Site 790.

Layer Core, section,interval (cm)

Thickness(cm) Description and remarks

Hole 790B1 1H-1, 14(top)-140(base)

2H-22H-2, 145-150*2H-3,*O-52H-3, 7-112H-3, 17-202H-3, 39-462H-3, 148-150*2H^, *0-182rW, 61-742H-4, 127-150*2H-5, *O~1382H-6,42-773H-1,27-563H-1,70-150*3H-2, *0-293H-2, 58-150*

3H-3,*O-1373H-4, 29(top)-7(base)7H-37H-3, 27-317H-3, 63-667H-3, 85-937H-3, 115-150*

7H-4, *0-150**7H-5, **0-150***7H-6, ***0-1307H-6, 143-1478H-1,+0-150*+

8H2, *0-908H-2, 106-150*

8H-3, *O-858H-3, 143-1488H-4, 0-388H4, 48-62SHA, 115-150*

8H-5, *0-98H-5, 38-131

8H-6, 14-178H-6, 33-358H-6, 54-638H-6,74-92

8H-7,43(top)-15(base)10H-5,10H-5, 22-2410H-5, 32-150*

726

5/3/2

4

720

376/85

352944/65

152/77

3628

433/3/250/55/

230/45/35/50

490/25/11

/1/23/7O/17/339/1/40

/9/32/8535/310/2/226/18

11/41/41

93/2/7/2/2

1622

Thick pumice layer

Very dark gray(5Y3/l)/brownish black(5YR2/l/grayish black(N2); silt/silt/v.f.sand

Brownish black(5YR2/l); siltBrownish black(5YR2/l); siltGrayish black(N2)/brownish black(5YR2/l); siltGrayish black(N2); silt (laminated at lowest 5 cm)

Light olive gray(5Y6/l); siltLight olive gray(5Y4/l); silt/v.f.s-m.s(grading)

Brownish black(5YR2/l); siltGrayish black(N2); siltGrayish black(N2); silt/f.-m.sand(laminated)

Olive gray(5Y5/2)/olive gray(5Y4/2)/dark olive gray(5Y3/2);silt/silt-m.sand(grading)

Thick pumice layer 11

Brownish black(5YR2/l); siltOlive black(5Y2/l); siltOlive gray(5Y4/l)/olive gray(5Y4/l); silt/silt/siltGray(5Y5/l)/olive gray(5Y4/2)olive gray(5Y4/l )/(5Y4/l )/olive

gray(5Y4/2)/(5Y4/2)/(5Y4/2);silt/silt/v.f.sand/v.f.sand/f.sand

Reddish gray(10R6/l); siltLight gray(5Y7/1 )/gray(5Y5/l )/(5Y5/1 )/olive

gray(5Y4/l)/(5Y5/l)/(5Y4/l)dark olive gray(5Y3/2)/olive black(5Y2/l):pumice granule (maximum diameter=3.5 cm)/mixture of pumice pebbleandsilt/silt/silt/silt/silt/v.f.sand/f.sand

Brownish black(5YR2/l)/grayish black(N2)/oliveblack(5Y2/l)/(N2)/(N2)/dark olive gray(5Y3/2); silt/v.f.sand/silt withgrayish black v.f.sand thin layers/v.f.sand/f.sand/v.f.sand

Dark olive gray(5Y3/2);v.f.sandOlive black(5Y2/l); silt/f.sand(laminated)Olive black(5Y2/l)/grayish black(N2)/olive black(5Y2/l); silt/v.f.sand/siltOlive black(5Y2/l j/grayish black(N2); silt with v.f.sand

laminated/v.f.-f.sand(grading)

Brownish black(5YR2/l)/olive black(5Y2/l)/grayish black(N2); f.sand withpumice m.sand laminated/silt with grayish black(N2) v.f.sand thinlayers/v.f.-m.sand(grading)

Dark olive gray(5Y3/2); siltDark olive gray(5Y3/2); siltGray(5Y5/l);siltOlive black(5Y2/l)/grayish black(N2)/olive black(5Y2/l Vgrayish

black(N2)/olive black(5Y2/l); silt/v.f.sand/silt/v.f.sand/siltThick pumice layer III

Brownish black(5YR2/l); siltOlive gray(5Y4/l)/dark greenish gray(5GY4/l)/(5Y4/l )/very dark

gray(5Y3/l)/(5Y4/l)/(5Y4/l)/olivegray(5Y4/2)/(5Y4/l)/(5Y4/2)/(5Y4/l)/(5Y4/2)/dark olivegray(5Y3/2)/(5Y3/2)/gray(5Y5/l )/olive black(5Y2/l )/(5Y3/2); siltyclay/silty clay/silt/silt/silt/v.f.sand/silty v.f.sand/silt/v.f.sand/silt/c.silt/v.f.sand/f.sand/f.sand/m.sand

Hole 790C-3334

3536373839

4041424344454647

49

505152

53

10H-6,*0-146

1HA 70-72\HA, 115(topM3(base)SH 1Jπ-j

5H-3, 55-595H-3, 67-705H-3, 875H-3,91-1115H-3, 111-150*5H-4, *0-605H^, 96-1365H-5,6-155H-5, 20-355H-5, 43-955H-6, 6-215H-6, 62-725H-6, 825H-6, 855H-6, 120-1506H-1,+0-150*

6H-2, *0-316H-2, 47-656H-2, 80-1446H-3, 34-135

6H-4,5-51

5/5/83/2/23/8/1/4/5/6/44/34/14/2/8/20

23628

43<l2039/60

26/1491549/?1510<l<l++2/28+++150/31

1857/729/73

5/25/16

Olive gray(5Y4/l); v.f.sandThick pumice layer IV

Olive(5Y5/4); f.sandOlive(5Y5/4); f.sandDusky green(5G3/2); siltGrayish black(N2); clayey silt-f.s(grading)Grayish black(N2)/dark gray(N3); silt/c.sand(grading)

Dark gray(N3)/black(Nl); clayey silt/m.sandDark gray(N3); siltDark gray(N3);clayey silt/v.f.sandDark gray(N3)/grayish black(N2); clayey silt/m.sandMedium dark gray(N4); silt-v.f.sand(grading)Medium dark gray(N4); silt-v.f.sand(grading)Dusky green(5G3/2); siltDusky green(5G2/l); siltMedium light gray(N6)/olive gray(5Y4/l); silt/siltOlive gray(5Y4/l)/olive gray(5Y4/l); f.-m.sand/pumiceous

m.-c.sand(grading)(laminated)

Light olive gray(5Y6/l); f.sand(grading)Light olive gray(5Y6/l )dark gray(N3); silty clay/m.sandLight olive gray(5Y6/l)/medium gray(N5); silt/m.sand(grading) (pumice

maximum diameter=l mm)Light olive gray(5Y6/l )/light olive gray(5Y6/l )/dark gray(N3); f.s/silty

clay/m.sand

28


Table 1 (continued).

Layer Core, section,interval (cm)

Thickness(cm) Description and remarks

54

55

565758

5960

61

62636465

67(,8h<;

70

717273747576

7778

79SO81

*586

89

9091

9293

94959697

99100101

[03[ 04105106

109110111112113114115116117118

119120121

1232-

6 H 4 , 60-81

6 H 4 , 99-140

6H-5, 7-776H-5.96-I186H-5, 141-150*6H-6, *0-376H-6,41^176H-6, 73-142

7H-l,+0(top)-18(base)8H-CC11X-1,251IX-2, 10-1611X-2, 45-5011X-3, 17-30I1X-3, 109-11011X-4, 32-4711X-4, 70-7211X-4, 110-12011X-4, 139-150*

12X1,5-1412X-1,20-2312X-1,26-2812X-1,35-37I2X-1, 40-4212X-1,45-69

12X-1, 71-7612X-1, 130-140*12X-CC, *O-3313X-1, 18-44I3X-1,50-5213X-1,66-681 3 X 1 , 71-7213X1,7913X1. 8113X-2, 15-5513X-3, 71-7514X-1.+O-714X-1,87-8814X1, 136-147

15X-1,7-815X-1, 12-17*15X-CC, *0-5I5X-CC, 10-22+16X1, +0-28

16X-1, 99-10416X-1, 10616X-2, 8-1216X-2, 35-3816X-4, 63-6716X-5, 52-6616X-5, 118-12016X-6, 17-2016X-CC, 0-416X-CC, 18-2017X-1, 130-14517X-7, 27-32I7X-CC, 34-3518X-1, 14518X-3 135-13618X-4, 58-6018X-4, 10018X-4, 123-12419X-1,46-5319X-1,65-6819X-1,94-9619X-2, 98-10619X-3, 52-6019X-3, 81-8519X-4, 92-150*

19X-5, *0-319X-5, 57-5919X-6, 13-1419X-6, 2019X-6, 39^1520X-2, 63-150*20X-CC, 28-50+

11/10

2/23/16

52/1814/829/17

611/53/5

++125O+

<l6512/1110/521015/4/3/3

9

2225/4/6/4/5

543+

7/5/4221< l<l31/94++716/5

1I0

12+++7/3/16/2

5<l4λ1/3142341/1152/2/11<l12<l17328

436/14/11

2114/22176/16+

ve gray(5Y6/l )/dark gray(N3); clayey silt/f .sand (laminated)ve gray(5Y6/l)/dark gray(N3); clayey silt/silty v.f.sand

Light olive gray(5Y6/l )/dark gray(N3); silty clay with scattered pumice/f .sandwith an erosive base

Olive gray(5Y4/l)olive gray(5Y4/l)/dark gray(N3); silty v.f.sand/siltyclay/m.sand

Light olive gray(5Y6/l )/dark gray(N3); clayey silt/f.sand (laminated)

Light ol

Light olive gray(5Y6/l): v.f.sandLight olive gray(5 Y6/1 )/light olive gray(5Y6/1 )/dark giay(N3); silty clay/silty

f.sand/f.sandThick pumice layer V

SiltVery light gray(N8); clayey silt-v.f.sand(grading)(10Y6/2); silt(grading)Olive gray(5Y5/2)/very light gray(N8); clayey silt-v.f.sand(grading)/clayey siltVery light gray(N8); siltOlive gray(5Y5/2)/dark gray(N3); clayey silt/c.sandMedium light gray(N6); v.f.sandDark gray(N3); silt(grading)Olive gray(5Y5/2)/olive gray (5Y5/2#/light gray(N7)/medium dark gray(N4);

silt/m.sand/silt/v.f.sand with and erosive baseOlive gray(5Y4/l); siltOlive gray(5Y4/l); siltOlive gray(5Y4/l); siltOlive gray(5Y4/l); siltOlive gray(5Y4/l); siltOlive gTay(5Y4/2)/olive gray(5Y4/l)/olive gray(5Y4/2)/olive

black(5Y2/l)/olive black(5Y2/l); silt/silt/m.sand with pumicec.sand/f.sand/silt

Olive black(5Y2/l); f.sandLight olive gray(5Y4/l); silt

Gray(5Y5/l)/olive gray(5Y4/l)/olive black(5Y2/l); clayey silt/silt/f.sandOlive gTay(5Y4/2); siltOlive black(5Y2/l); siltOlive black(5Y2/l); siltOlive black(5Y2/l); siltOlive black(5Y2/l); siltGray(5Y5/l)/gray(5Y5/l); clayey silt/siltGray(5Y5/l);siltGray(5Y5/l);siltLight brownish gray(5YR6/l); clayey siltVery dark gray(N2)/olive gray(5Y4/l); f.sand/scoria granule (maximum

diameter= lcm)Olive black(5Y2/l); silty v.f.sandOlive black(5Y2/l); m.sand

Olive black(5Y2/l);m.sandGray(5Y5/l)/very dark gTay(5Y3/l)/gray(5Y5/l )/very dark gray(5Y3/l);

silt/silt/silt/v.f.sandOlive gray(5Y4/l); siltOlive gray(5Y4/l); silt(5 mm thick)Olive gray(5Y5/2); siltGray(5Y5/l);siltDark olive gray(5Y3/2)/olive gray(5Y4/l); siltVery dark gray(5Y3/l); m.sandLight olive gray(5Y6/l); siltOlive gray(5Y4/2); siltDark olive gray(5Y3/2); siltGray(5Y5/l)/dark olive gray(5Y3/2); siltOlive gray(5Y4/l); silt-sandy silt(grading)Very dark gray(5Y3/l)/olive gray(5Y4/2)/olive black(5Y2/l); silt/silt/m.sandLight olive gray(5Y6/l); siltSiltSiltVery light gray(N8); siltVery light gray(N8); siltVery light gray(N8); siltOlive gray(5Y4/2) siltOlive gray(5Y4/2) siltGray(5Y5/l);siltGray(5Y5/l);siltOlive gray(5Y4/2); clayey siltOlive gray(5Y4/2); clayey siltDark gray(N3)/grayish black(N2)/olive black(5Y2/l); pebble-granule gravel

with sandy silt

Olive gray(5Y4/2); siltMedium light gray(N6); siltMedium light gray(N6); silt pocketOlive gray(5Y5/2)/light olive gray(5Y6/l); siltPumice gravel scattered in sandy mud (maximum diameter=4 cm)Light olive gray(5Y6/l )/grayish black(N2); sand/scoria (maximum

diameter=2 cm)

Note: Sample 126-790B-10H-6, 140 cm, is correlative to 126-790C-1H4,57 cm. v.f. = very fine, v.f.s. = very fine sand, m.s. = medium sand, f. = fine, m.= medium, and c = coarse. The list for forearc Site 792 is given in Fujioka et al. (this volume).

*Continue to the next * below.+Continue to the next + or ++ below.

29

Table 2. Petrographic characteristics of backarc tephras at Sites 790/791.

Core, section,Interval

126-790A-

1H-1,1H-3,1H-6,2H-1,2H-2,2H-4,2H-5,2H-7,3H-1,3H-3,4H-3,4H-5,2H-3,2H-3,2H-4,2H-6,4H-2,4H-6,5H-1,5H-4,6H-4,7H-2,7H-6,8H-3,8H-4,8H-4,8H-5,8H-5,8H-6,

10H-4,10H-6,11H-6,15X-1,

126-790C-

1H-2,2H-6,3H-1,3H-5,3H-6,4H-1,4H-2,4H-3,4H-4.

(cm)

139-14152-5449-5196-9824-26

139-141148-150

30-3266-6840-4283-8530-32

9-1126-2863-6568-7024-26

139-141142-143133-135

31-33129-131146-147

8-1034-3660-647-9

46-4884-86

130-132123-125100-102

58-60

133-135128-130110-112

10-1244-46

113-11559-6121-2365-67

Lithology Thickness Grain size(cm)

Max gl(mm) (mm)

Constituent mineralspI cpx opx op

Glass shape Refractive Index(mean) (min) (max)

Comment

1.50851.5087

1.50671.5063

1.5125 1.5172

1.5131

v.l.sand- I.sand 0.3 92/ 8/0 pum.br br mixed

v.t.sand-

v.f.sand-v.f.sand-

m.sand

v.f.sandf.sandf.sand

0.3

0.120.20.25

80/10/10

93/ 7/088/12/090/10/0

pum.bw

bwbwbw

(.sand - c.sand 0.2 0.25 crystal<Max 0.5mm)



4H-5,4H-6,5H-1,5H-3,5H-5,5H-5,5H-6,6H-2,6H-2,6H-3,6H-4,6H-4,6H-5,6H-6,6H-6,7C-4,8C-4,8H-6,

10X-1,11X-2,11X-3,11X-4,11X-4,12X-1,12X-1,13X-1,13X-2,14X-2.16X-1,16X-5,17X-7,18X-4,19X-1,19X-4,19X-6,20X-3,20X-5,

126-791A-

2H-5,3H-1,4H-4,4H-5,5H-2,5H-3,5H-4,

(cm)

97-9933-3579-8172-744-6

104-106116-118

51-53139-141

80-8227-2978-8021-2310-1292-9444-46

140-142140-142

50-5210-12

1 10-1 1250-5276-7821-2365-6741-4364-65

135-1379-11

63-6529-31

121-12394-96

137-13977-79

109-11120-22

74-7692-94

104-10699-101

142-14478-8016-18

Lithology Thickness(cm)

Grain size

v.f.sand- m.sand

v.f.sandv.f.sand

.sand - m.sand/.f.sand* f.sand

v.f.sand- m.sandv.f.sand- m.sandv.f.sand- f.sand

v.f.sandf.sand

v.f.sand.sand - m.sand

v.f.sand- f.sand

v.f.sand- f.sandv.f.sand- m.sand

Max gl(mm)

0.4

0.150.15

0.40.15

0.450.350.25

0.20.2

0.10.4

0.2

0.250.4

Max li(mm)

0.2

0.05

0.10.150.3

0.15

0.050.15

0.10.15

Mode(gl%/cry%/.i%)

90/ 7/3

90/10/090/10/0

95/ 5/088/1 0/2

80/12/889/10/193/ 6/1

95/ 5/0

65/30/585/10/5

95/ 5/0

87/10/390/ 8/2

Constituent minerals

pi

++

+ +

++++++

++++

++++

++

+ +

++

cpx

++

++

opx

+

hb

++

++++

++++

+++

+

+ +

++

bi op

+ +

++

++

+

++

+ +

++

Glass shape

bw

bwbw

bwbw>cry

bw>brbw>pum

bw

bwpum.tub

bwpum.tub

pum

bwbw

Refractive Index(mean)

1.508

1.5087

(min)

1.5051

1.5075

(max)

1.5105

1.5097

Comment

br glass mixed

op rich

very fine

br.pum.li mixedbr glass mixed

muddy

muddy

muddy

muddy

muddy

muddy



6H-2,7H-2,8H-7,9H-6,

10H-5,12H-3.14H-4,15H-1,15H-4,16H-1,17H-2,17H-6,18H-1,18H-1,19H-1,19H-6,20H-1,20H-2,21H-1,22H-6,22H-6,23H-2,25X-2,28X-1,30X-CC33X-1,39X-1.42X-1,

(cm)

60-6280-8229-31

134-136117-119102-104

35-37130-131121-122

48-51110-111

20-215-7

53-5545-47

132-1342-4

91-9322-2478-80

110-11298-10020-2225-27(18-20)50-5290-9231-33

Lithology Thickness(cm)

Grain size

v.f.sand- f.sand

v.f.sandv.f.sand

v.f.sand- m.sandv.f.sand- f.sand

v.f.sand- m.sandv.f.sand- f.sand

Max gl(mm)

0.2

0.150.10.3

0.25

0.30.3

Max li(mm)

0.050.05

0.15

0.250.2

Mode(gl%/cry%/li%)

96/ 4/0

80 /15 /580/20/598/ 2/0

94/ 5/195/ 5/0

Constituent minerals

pi

++

++++++++

++++

cpx

++

opx hb bi op

++

++++

++++

Glass shape

bw

bwbwbwbw

bw>pumbw

Refractive Index(mean)

1.50541.507

1.51261.5072

(min)

1.50231.5053

1.5091.5057

(max)

1.50911.5097

1.51331.5101

Comment

Max crystal 0.35mm

muddy

muddy

0 T

Ash frequency (no./m.y.)

100 200 300 400 500

B

0 . 5 -

CD<

1.0-

0 T


Ash accumulation rate (m/m.y.)

20 40 60 80

Light-colored ash

Intermediate and undetermined ash

Dark-colored ash

Possible ash in unrecovered part 'óT

1.0 H

Figure 5. Accumulation rate of the Quaternary volcaniclastic materials of forearc Site 792. A. Ash frequency indicated by ash number per million years, calculatedby the constant sedimentation rates of muddy hemipelagites. B. Ash sedimentation rate obtained by the same method as in A.

;

10 8 6 4 2_ L I ' '

B

0.275 Ma

0.830 Ma

ß 1P 100 80 60 40 20

410

705

335

2Q 4,0 6,0 8p 100

Black White

Granule size Sand size

(number)

Silt size

Black White (cm)

Figure 6. Frequency (A) and thickness (B) diagram of the Quaternary tephras at forearc Site 793.

33


0 T

Ash frequency (no./m.y.)100 200 300 400 500

B

0 .5 -

cc

CDen<

1.0-

0 T

Ash accumulation rate (m/m.y.)100 200 300

0 . 5 -

Possible ash in unrecovered partCDen<

1.0-

Figure 7. Frequency (A) and accumulation (B) diagram of the tephras at backarc Sites 790/791.

diameters of the pumice fragments range from 200 to 600 µm.Examples of this type include Samples 126-791A-22H-6, 28-32 cm,and 126-791A-4H-3, 6-10 cm (Fig. 9). This type of grain-size distri-bution represents resedimented deposits (Sparks et al., 1981; Taylor,Fujioka, et al., 1990). In contrast, the fine glassy tephras have a well-sorted distribution curve, with median diameters ranging from 60 to100 µm. Examples of this type include Samples 126-790B-7H-6,146-147 cm, and 126-790C-7H-4, 44-46 cm (Fig. 9). Fine glassyashes are often associated with coarse thick pumiceous tephras, sug-gesting they originate from a subaqueous eruption (Fisher, 1984;Kokelaar et al., 1985; Fiske, 1986; Fiske and Cashman, 1989).

CHEMISTRY OF THE VOLCANIC GLASS SHARDS

Tephras from Leg 126 sites belong to the low-alkali tholeiiticseries on Kuno's silica-alkali diagram (Figs. 10-12; Kuno, 1966).Generally, tephras at Site 788 are slightly different from those at thebackarc and forearc sites in several respects. First, the tephras at Site788 have very low SiO2, Na2O, and K2O concentrations (Table 4).Second, at the forearc and backarc sites, tephras extremely enrichedin K2O were recognized in some intervals. Sometimes K2O contentsexceeded 4 wt%. These kinds of tephras may possibly come from thewestern regions, as discussed below.

The total alkali content of Site 788 tephras is quite constant, thatis, Na2O and K2O change sympathetically with time. The total alkalicontents of Site 790 tephras vary only slightly, but the youngest onesare quite low in K2O, Na2O, and total alkali. Tephras at Sites 792 and793, however, vary greatly over time. See Appendix (on microfichein back pocket) for the chemical composition data of the volcanicglasses at arc Site 788 and backarc Sites 790/791.

CORRELATION OF THE ASH LAYERS

Establishing a correlation of the volcanic ash layers is necessaryto determine the volcanic history in the Izu-Bonin areas as well as inother regions around the Japanese Islands. Although most of the 300tephras identified in this study have different but similar chemical andmineralogical compositions, several have quite distinct characteris-

tics that are comparable to tephras identified in other studies. Here weshow four examples of correlations of tephras from the Izu-Boninforearc to the other sites and volcanic islands. The distinctive proper-ties of these volcanic ash layers are listed in Table 5.

Samples 126-792A-3H-1,80-94 cm, and126-793A-4H-2, 96-98 cm

These tephras have similar refractive indices and chemical com-positions as well as glass shard types and mineral assemblages. Theirage is estimated to be equivalent to Zone CN14b.

Samples 126-792A-5H-3,131-133 cm, and442A-5-3,30-34 cm

These two ash layers have similar refractive indices and chemicalcompositions as well as similar glass shard types and mineral assem-blages. Therefore, these two ashes may be correlative with each otheralthough they are several hundred kilometers apart. The origin of thistephra may be further west of the Shikoku Basin, for example, fromthe Ryukyu Arc.

Black Tephras at Site 792 and Aogashima Volcanics

The chemical compositions of the Kurosaki volcanics, in thelowermost sequence of Aogashima Island, have distinctly differentchemical characteristics as compared with younger sequences, that is,low TiO2 and K2O contents and high FeO and CaO contents (Takadaet al., in press). The chemical compositions of some black volcanicash layers at Site 792 are quite similar to those of the Kurosakivolcanics. Examples include Samples 126-792A-4H-1, 117-119 cm,and 126-792A-4H-3, 20-22 cm.

Samples 126-792A-1H-5,81-83 cm, and Aso-4

The white coarse volcanic ash layer at Hole 792 A has quite similarmineral assemblages, refractive indices, and chemical compositionsas well as ages and glass shard shapes to that of Aso-4 (Furuta et al.,1986) of the Aso Volcano, Kyushu. The type of glass shard in the

34


White fine (poor sorting)Coarse WhiteBlack whiteFine (well sorting)

3 4 5

Grain size(0)

2 3 4 5 6Grain size(0)

Figure 8. Grain-size characteristics of four types of tephras. A. Cumulative frequency grain-size curve for all thetephras at Site 792, Izu-Bonin forearc. B. Frequency grain-size distribution curve for volcanic ash layers at Site 792.Scale of the grain size is represented by Φ (phi).

Aso-4 is bubble wall with a pale brown color. The age of this tephrais estimated to be 70-80 ka according to stratigraphic data obtainedon land (Machida and Arai, 1983) and around 80 ka according to U-Thage determinations (Omura, 1984).

DISCUSSION

The chemical characteristics of the tephras from the Pliocene-Pleistocene ages have low K2O contents similar to the keratophyre ofthe Paleozoic rift as well as low-K rhyolite in the rift area (Gill, 1981;Gill et al., 1984; Reagan and Meijer, 1984; Hawkins and Melchior,1985; Brouxel et al., 1987; Hochstaedter et al., 1990a, 1990b).

Volcanism in this stage shows the characteristic basaltic andrhyolitic bimodal distribution in both chemical and visual composi-tions. This kind of bimodal volcanism was noted in the basin and

range province of the Cascade region of the North America, whereextensional stress fields predominated from Eocene to Holocenetimes (Christiansen and Lipman, 1972; Eaton, 1982, 1984). Kennettet al. (1979) noted the characteristic volcanic pulse of the Quaternaryperiod in the circum-Pacific regions and named this pulse "Cas-cadian." The Izu-Bonin explosive volcanism is also similar in char-acter to the Cascadian event.

The numbers of Pliocene-Pleistocene volcanic ash layers increasedin this area, and thick tephra piles accumulated (Fujioka et al, thisvolume). From the grain-size distribution and chemical compositions, wedetermined that the tephras came mostly from along the present Izu-Bonin Arc, although some tephras possibly came from a more westernpart of the Philippine Sea (i.e., from the Ryukyu Arc).

Along the Ryukyu Arc, the late Miocene to Pliocene ShimajiriGroup yielded many coarse tephras including pumices (Konishi,

35

Table 3. Grain-size analyses data for Sites 788,790, and 791.

SiteCore, sectionInterval (cm)

Cumulativevolume percentages



Class interval

(Φ)1.75-2.00-1.752.25-2.002.50-2.252.75-2.503.00-2.753.25-3.003.50-3.253.75-3.504.00-3.754.25-4.004.50-4.254.75-4.505.00-4.755.00-6.007.00-6.00

Class interval

(Φ)1.75-2.00-1.752.25-2.002.50-2.252.75-2.503.00-2.753.25-3.003.50-3.253.75-3.504.00-3.754.25-4.004.50-4.254.75-4.505.00-4.755.00-6.007.00-6.00

788C11H-338-40

0

0

0.1

3.3

11.125.541.858.571.781.387.6

92

94.795.697.399.3

790B2H-4

63-65

0

0

0

0

0.4

1.5

7.4

15.932

47

61.173.883.387.594.699.3

788C11H-4

108-110

0

1.2

3.1

14

26.942.857.270.779.786

89.792.393.994.6

97

99.1

790B2H-6

68-70

0

0

0

1.4

5.7

14.229

45.962.675.384.390.694.395.497.399.6

790B4H-2

24-26

0

0

0

2

7.5

18.132.848.963.875.383.789.993.995.297.399.4

790A1H-1

139-141

0

0

0

0

0.5

1.8

8.6

18.335.250.664.676.885.789.795.699.4

790B4H-6

139-141

0

1.1

2.8

12.122.936.149.362.372.980.986.691

93.994.997.299.3

790A1H-3

52-54

0

0

0

1.3

5.3

13.327

42.758.771.280.587.592.193.896.699.4

790B7H-2

129-131

0

0

0

1.5

5.9

14.528.444.259.972.381.588.492.894.296.6

99

790A2H-1

96-98

0

0

0

0.5

3.1

8.8

24.142.862.677.587.894.397.798.299.199.8

790B7H-6

146-148

0

0

0

1.9

7.4

18.234.552.467.778.986.191.1

94

94.996.7

99

790A2H-2

24-26

0

0

0

1.8

6.9

16.830.946.360.872.180.486.891.192.896.299.3

790B8H-3

10-Aug

0

0

0

1.3

5.1

12.725.941.358.271.782.189.5

94

95.297.299.6

790A2H-4

139-141

0

0.7

1.8

9.4

20.536.752.868.4

79

86.490.9

94

95.896.4

98

99.6

790B8H-4

34-36

0

0

0

1.9

7.2

17.532.348.764.2

76

84.490.393.894.996.999.2

790A2H-5

148-150

0

0

0

0.2

1.8

5.4

17.232.149.263.173.982.287.789.693.698.2

790B8H-4

60-62

0

0

0

0.5

3

8.4

23.241.460.875.485.692.295.896.697.999.7

790A3H-1

66-68

2

8.4

17.317.331.745.157.366.674.980.985.689.191.992.7

95

97.2

790B8H-57- 9

0.7

5

11.332.352.270.681.7

90

93.295.396.397.197.697.898.599.1

790B8H-5

46-48

0

2.1

5.321.838.756.470.18 2

88.893.3

. 95.797.398.198.398.999.5

790B2H-3

11-Sep

0

0

0

0

0.9

3.1

12.926.145.161.374.985.392.194.397.799.8

790B8H-6

84-86

0

0

0

0

0.3

1

6.1

14.333.9

52

68.681.189.191.394.799.3

790B2H-3

26-28

0

0

0

0

0.8

2.8

11.523.139.2

53

64.774.381.1

8 4

91.297.7

UJIO

I

>>

X

>

G

p

ö

o






Class interval

(Φ)1.75-2.00-1.752.25-2.002.50-2.252.75-2.503.00-2.753.25-3.003.50-3.253.75-3.504.00-3.754.25-4.004.50-4.254.75-4.505.00-4.755.00-6.007.00-6.00

Class interval

(Φ)1.75-2.00-1.752.25-2.002.50-2.252.75-2.503.00-2.753.25-3.003.50-3.253.75-3.504.00-3.754.25-4.004.50-4.254.75-4.505.00-4.755.00-6.007.00-6.00

790C3H-5

12-Oct

0000

0.62.19.6

20.137.853.467

78.286

89.395.199.3

790C11X-212-Oct

0000

1.34.517.534.254.470.382.190.294.996.197.899.7

790C3H-6

44-46

0000

0.62.19.6

20.238

53.667.278.386

89.295

99.4

790C11X-3

110-112

0001

4.511.726.543.961.975.585.191.595.196.197.999.9

790C5H-3

72-74

000

0.93.910

22.537.352.864.973.980.885.4

87.392

97.3

790C12X-121-23

00.51.37.717.632.848.864.776.585

90.393.996

96.697.999.4

790C5H-56-Apr

00.30.85.914.829.445.261.173.482.38892

94.395.197.199.1

790C12X-165-67

00001

3.514.428.949

65.979.989.895.797.198.699.7

790C5H-6

116-118

0000

0.72.410.822.642

58.572.382.689.291.595.599.2

790C13X-141-43

01.43.515.329

45.460.274.183.590

93.896.297.597.998.899.7

790C6H-2

51-53

0000

0.82.711.723.942.658.471.581.688.390.795.199.1

790C13X-264-65

000

1.25

12.626.542.759.973.383

89.994

95.297.299.3

790C6H-2

139-141

01.64.117.431.647.360.672.880.986.790.392.994.595.297.199.1

790C14X-2

135-138

2.810.621.332.843.854.164.374.583.289.693.896.698.198.699.399.9

790C6H-3

80-82

000

0.11.54.916.932.250.264.876.284.990.692.696

99.2

790C16X-19- 11

0000000000000000

790C6H-4

27-29

0000

1.13.815.330.449.965.677.886.692

93.696.299.6

790C17X-729-31

000

1.14.611.826

42.760.373.883.590.294.195.297.399.8

790C6H-4

78-80

01.53.917

31.748.963.376.484.489.993

95.196.396.797.899.4

790C18X-4

121-123

0000000000000000

790C6H-6

12-Oct

0000

0.93.112.926.145.261.474.884.891.19396

99.3

790C19X-194-96

0000

1.44.818.636.256.572.383.891.595.896.898.399.8

790C6H-6

92-94

000

0.52.77.420

35.452.565.975.883.287.989.693.898.4

790C20X-520-22

01.74.418.232.547.861

73.382.188.492.395

96.597.198.499.6

790C7H-2

44-46

0000

0.72.410.421.438.353.166

77.185.288.995.499.6

O

SP

HO

o

TO

X

?






Class interval

(Φ)1.75-2.00-1.752.25-2.002.50-2.252.75-2.503.00-2.753.25-3.003.50-3.253.75-3.504.00-3.754.25-4.004.50-4.254.75-4.505.00-4.755.00-6.007.00-6.00

Class interval

(Φ)1.75-2.00-1.752.25-2.002.50-2.252.75-2.503.00-2.753.25-3.003.50-3.253.75-3.504.00-3.754.25-4.004.50-4.254.75-4.505.00-4.755.00-6.007.00-6.00

791A2H-5

74-76

0

0

0

0.6

3

8

20.335.553.4

68

79.487.993.294.897.399.8

791A39X-190-92

0

0

0

1.4

5.6

13.827.643.459.371.8

81

87.992.393.896.699.4

791A4H-5

99-101

0

0

0

0

0.6

2

9.6

20.540.357.873.184.792.194.397

99.6

791A42X-131-33

0

0

0

1

4.2

10.723.2

38

54.467.677.885.690.692.495.999.1

791A5H-2

142-144

0

0.7

1.8

8.9

18.632.1

47

62.274.884.190.394.496.797.398.699.8

791A5H-3

78-80

0

0

0

2.5

9.2

22.138.756.371.382.289.293.896.397.198.499.8

791A5H-4

16-18

0

0

0

0

1

3.5

14.328.648.364.778

87.793.695.397.799.7

791A6H-2

60-62

0

0

0

0

1

3.4

13.727.244.859.972.783.190.292.796.499.3

791A7H-2

80-82

0

0

0

0

0.6

2.2

9.8

20.437.753.467.679.487.790.995.799.3

791A10H-5

117-119

0

0

0.1

2.8

9.2

21.135.250

62.972.980.385.989.6

91

95

98.7

791A14H-435-37

0

0

0

1.7

6.6

16.130.245.961.473.682.689.393.594.897

99.3

791A18H-1

5- 7

0

0

0

0

0.7

2.4

10.521.840.656.9

71

81.688.490.594.198.7

791A18H-153-55

0

0

0

0

1

3.4

13.727.144.358.670.379.485.487.592.497.6

791A30X-CC18-20

1.6

7.1

14.829.343.256.467

76.683.688.792.194.495.896.397.799.3

791A33X-150-52

0

0

0.1

3.2

10.724.740.757.270.780.887.692.495.296

97.799.5

c

C>>

GO

X

ICPHCΛ

Cp

σT I

o


100

* 50

2j7h

/5 10 50 100 500 1000

Grain size (µm)

B

100

g 504)

11

5 \ /

1

u

f i /J/

//1

10 50 100 500 1000

Grain size

Figure 9. Cumulative grain-size distribution of the Quaternary volcanic ash layers. Grain sizes areindicated by microns (µm) in this diagram. A. Forearc Site 792. Broken lines indicate black ash layersand solid lines indicate white ash layers. (1) Sample 126-792B-8X-3, 60-62 cm; (2) Sample126-792A-1H-1, 38^0 cm; (3) Sample 126-792A-8H-3, 40-42 cm; (4) Sample 126-792A-5H-2,141-143 cm; (5) Sample 126-792A-4H-5,19-21 cm; (6) Sample 126-792A-9H-4,6-8 cm. B. BackarcSite 790. Broken lines indicate black ash layers and solid lines indicate white ash layers. (1) Sample126-790B-5H-5, 43-47 cm; (2) Sample 126-790C-14X-2,135-138 cm; (3) Sample 126-790C-5H-5,4-6 cm; (4) Sample 126-790B-7H-6, 146-148 cm; and (5) Sample 126-790B-8H-6, 84-86 cm.

1965; Letouzey and Kimura, 1985). These tephras are distributedpartly onshore and, for the most part, in submarine areas around thePhilippine Sea.

ACKNOWLEDGMENTS

During Leg 126, Captain Ribbon and the crew of the JOIDES Reso-lution helped us, sometimes beyond the call of duty. These are warmlyacknowledged. B. Taylor, J. Gill, R. Hiscott, and K. Marsaglia gave usvaluable discussion and a warm understanding of the ashy sediments.

Thanks also go to Drs. M. Yuasa, S. Nagaoka, N. Kotake, Y. Hirata,and H. Matsuoka for their critical discussions on the tephras. Mrs.Kanehara and Miss K. Ikari and A. Koizumi helped us during thepreparation of the manuscript.

REFERENCES

Brouxel, M., Lapierre, H., Michard, A., and Albarede, F., 1987. The deep layersof a Paleozoic arc: geochemistry of the Copley-Balaklala Series, northernCalifornia. Earth Planet. Sci. Lett., 85:386-400.

Brown, G., and Taylor, B., 1988. Sea-floor mapping of the Sumisu Rift,Izu-Ogasawara (Bonin) Island Arc. Bull. Geol. Surv. Jpn., 39:23-38.

Christiansen, R. L., and Lipman, P. W., 1972. Cenozoic volcanism and plate-tectonic evolution of the western United States. II. Late Cenozoic. Philos.Trans. R. Soc. London, Ser. A, 271:249-284.

Eaton, G. P., 1982. The Basin and Range Province: origin and tectonicsignificance. Annu. Rev. Earth Planet. Sci., 10:409-^140.

, 1984. The Miocene Great Basin of western North America as anextending back-arc region. Tectonophysics, 102:275-295.

Fisher, R. V., 1984. Submarine volcaniclastic rocks. In Kokelaar, B. P., andHowells, M. F. (Eds.), Marginal Basin Geology: Volcanic and Associated

39


o Site 7924- Site 793

F

2

1.5

1

0.5

70 80 %0

o Site 792+ Site 793

10

^ 8

O 6

6 4CMco 2

Arc high

o Site 788

50 60 70 80

O

2

1.5

1

0.5

Q

Arc high

o

Site 788

50 60 70 ^ 0

5O

o o Hote790A&B• Hole790C+ Hole 791A

wt.%

Figure 10. SiO2-alkali diagram of the Quaternary volcanic ash layers at forearcSites 792 and 793, arc Site 788, and backarc Sites 790/791.

o Hole790A&B• Hole790C+ Hole 791A

wt.%

Figure 11. SiO2-TiO2 diagram of the Quaternary tephras at forearc Sites 792and 793, arc Site 788, and backarc Sites 790/791.

Sedimentary Processes in Modern and Ancient Basins. Spec. Pub!., Geol.Soc. London, 16:5-28.

Fiske, R. S., 1986. Shallow marine volcanic field in the Mio-Pliocene Shira-hama Group, Izu Peninsula, Japan. Geol. Soc. Am. Abstr. Programs, 1986Annual Meeting, 602.

Fiske, R. S., and Cashman, K. V., 1989. Submarine fallout deposit in theShirahama Group (Mio-Pliocene), Japan. Abstr. Vol., Int. Assoc. Volcanol.Chem. Earth's Interior, Gen. Assem., Santa Fe, New Mexico, No. 92.

Fujioka, K., 1983a. History of the explosive volcanism of the Tohoku Arc fromthe core sediment samples of the Japan Trench. J. Volcanol. Soc. Jpn.,28:41-58.

, 1983b. Where were the "Kuroko deposits" formed, looking for thepresent day analogy. Min. Geol. (Tokyo), Spec. Vol., 11:55-68.

-, 1988. A possible nascent rift in northern Izu-Ogasawara arc. InTectonics of Eastern Asia and Western Pacific Continental Margin.1988 DELP Tokyo Int. Symp, and Sixth Japan-U.S.S.R. GeotectonicSymp., 25. (Abstract)

Fujioka, K., Furuta, T., and Arai, F, 1980. Petrography and geochemistry ofvolcanic glass: Leg 57, Deep Sea Drilling Project. In von Huene, R., Nasu,N., et al., Init. Repts. DSDP, 56,57, Pt. 2: Washington (U.S. Govt. PrintingOffice), 1049-1066.

Fujioka, K., Nishimura, A., Koyama, M., and Kotake, N., 1990. Bimodal arcvolcanism and backarc rifting along Izu-Bonin Arc. Eos, 11:956.

Furuta, T, and Arai, F., 1980a. Petrographic and geochemical properties oftephras in Deep Sea Drilling Project cores from the north Philippine Sea.In Klein, G. de V., Kobayashi, K., et al., Init. Repts. DSDP, 58: Washington(U.S. Govt. Printing Office), 617-627.

, 1980b. Petrographic properties of tephras, Leg 56, Deep SeaDrilling Project. In von Huene, R., Nasu, N., et al., Init. Repts. DSDP, 56,57, Pt. 2: Washington (U.S. Govt. Printing Office), 1043-1048.

Furuta, T, Fujioka, K., and Arai, F, 1986. Widespread submarine tephra aroundJapan, petrographic and chemical property. Mar. Geol., 72:125-146.

Gill, J. B., 1981. Orogenic Andesites and Plate Tectonics: New York (Sprin-ger-Verlag).

Gill, J. B., Stork, A. L., and Whelen, P. M., 1984. Volcanism accompanyingbackarc basin development in the Southwest Pacific. Tectonophysics,102:207-224.

Gill, J., Torssander, P., Lapierre, H., Taylor, R., Kaiho, K., Koyama, M.,Kusakabe, M., Aitchison, J., Cisowsky, S., Dadey, K., Fujioka, K., Klaus,A., Lovell, M., Marsaglia, K., Pezard, P., Taylor, B., and Tazaki, K., 1990.Explosive deep water basalt in the Sumisu backarc rift. Science,248:1214-1217.

Hawkins, J. W, and Melchior, J. T, 1985. Petrology of Mariana Trough andLau Basin basalts. J. Geophys. Res., 90:11431-11468.

Hochstaedter, A. G., Gill, J. B., Kusakabe, M., Newman, S., Pringle, M.,Taylor, B., and Fryer, P., 1990a. Volcanism in the Sumisu Rift. I. Major

40


Fore arc

4

* 3tO 2

CM

* 1

%

5

4

"5 3

1

Ho öö 7rJ

Arc high

o Site 792+ Site 793

80

Site 788

50 60 70 80

o Hole790A&B• Hole790C+ Hole 791A

SiO2 wt.%

Figure 12. SiO2-K2O diagram of the Quaternary tephras at forearc Sites 792and 793, arc Site 788, and backarc Sites 790/791.

element, volatile, and stable isotope geochemistry. Earth Planet. Sci. Lett.,100:179-194.

Hochstaedter, A. G., Gill, J. B., and Morris, J. D., 1990b. Volcanism in theSumisu Rift. II. Subduction and non-subduction related components. EarthPlanet. Sci. Lßtt., 100:195-209.

Ikeda, Y., and Yuasa,M., 1989. Volcanism in nascent back-arc basins behindthe Shichito Ridge and adjacent areas in the Izu-Ogasawara arc, north-west Pacific: evidence for mixing between E-type MORB and island arcmagmas at the initiation of back-arc rifting. Contrib. Mineral. Petrol,101:377-393.

Inmann, D. L., 1952. Measures of describing the size distribution of sediments.J. Sediment. Petrol, 22:125-145.

Kennett, J. P., MacBirney, A. R., and Thunell, R. C, 1979. Episodes ofCenozoic volcanism in the circum-Pacific region. J. Volcanol. Geotherm.Res., 2:145-163.

Table 4. Chemistry of volcanic glass shards, Sites 788,790/791,

Site

790/791788792/793

SiO2

65%-80%70%-80%65%-80%

TiO2 K2O NajO

0.1%-1.0% 0.5%^.0% 1.0%-6.0%0.3%-1.0% 0.2%-0.7% 1.3%-2.7%0.1%-1.0% 0.2%-4.5% 1.6%-4.7%

and 792/793.

Totalalkali

2.5%-8.4%

Kokelaar, B. P., Bevins, R. E., and Roach, R. A., 1985. Submarine silicicvolcanism and associated sedimentary and tectonic processes, RamseyIslands, SW Wales. Geol. Soc. London, 142:591-613.

Konishi, K., 1965. Geotectonic framework of the Ryukyu Islands (Nansei-shoto). J. Geol. Soc. Jpn., 71:437-457.

Kotake, N., 1988. Upper Cenozoic marine sediments in southern part of theBoso Peninsula, central Japan. J. Geol. Soc. Jpn., 94:187-206.

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Sparks, R.S.J., Wilson, L., and Sigurdsson, H., 1981. The pyroclastic depositsof the 1875 eruption of Askja, Iceland. Philos. Trans. R. Soc. London, Ser.A, 299:241-273.

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Taylor, B., Fujioka, K., et al., 1990. Proc. ODP, Init. Repts., 126: CollegeStation, TX (Ocean Drilling Program).

Walker, G.P.L., 1971. Grain size characteristics of pyroclastic deposits. / .Geol, 79:696-1'14.

Yamazaki, T., 1988. Heat flow in the Sumisu Rift, Izu-Ogasawara (Bonin) Arc.Bull. Geol Surv. Jpn., 39:63-70.

Yokoyama, T., Danhara, T., and Yamashita, T., 1986. Anew refractometer forvolcanic glass. /. Volcanol Soc. Jpn., 25:21-30.

Date of initial receipt: 2 January 1991Date of acceptance: 22 October 1991Ms 126B-116

41


Table 5. Correlation of marine tephras by mineral assemblage, chemistry, glass type, refractive indices, and age of the volcanic ashlayers.

Core,section, interval (cm)

AgeR.I.(Mean)R.I.(Range)

Mineral assemblageGlass shape

Thickness (cm)Glass grain size(Max)

Mode

792A-3H-1, 86-88

CN14b1.5067

1.5058-1.5080pl++,hb+

bw14 (ML)

0.2595/5/0

793A-4H-2, 96-98

CN14b-a1.5067

1.504-1.5047pl++,hb+

bwDispersed in mud

0.1594/5/1

2) 792A-5H-3,131-133

CN14a1.5013

1.4998-1.5024pl++,cpx+,hb+,op+

bw90 (ML)

0.295/5/0

442A-5-3,30-34*

p.lacunosa1.501-1.5041.500-1.505

pl+,hb

0.2

Core,section, interval (cm)

AgeR.I.(mean)R.I.(range)

Mineral assemblageGlass shape

Thickness (cm)Glass grain size(Max)

Mode

3) 792B-1H-1,50-52

CN151.5065

1.5056-1.5073pl++bw7

0.05

Kurasaki volcano 4) 792A-1H-5.81-83

CN151.5096

1.509-1.5108pl++,cpx+,hb+,op+

bw3

0.5593/5/2

Aso-4**80.000B.P.

1.506-1.514pl,hb,opx,cpx

bw>>pum

42


cm

135

140

145

cm20

30

cm80

90

cm

110

115

120

Plate 1. Core photographs. 1. Sample 126-790B-7H-6,145-148 cm, simple white volcanic ash layer. 2. Section 126-792A-3H-1,80-94 cm, multiple ash layers with crystal-rich, coarse black ash at the bottom and 10 cm of thick, gray, mixed-ash layer above theblack ash. 3. Section 126-790C-5H-5,20-35 cm. multiple black ash layers with fine, sandy ash in the bottom of the tripartite (Taylor,Fujioka, et al., 1990). 4. Section 126-793A-7H-2, 109-120 cm, sharp-based, parallel-laminated, mixed volcanic-ash layer withnotable grading structure inside (Taylor, Fujioka, et al., 1990).

43


1 mm

Plate 2. Stereographic photographs of the volcanic ash layers. 1. Sample 126-791 A- 10H-5,117-119cm, example of poorly sorted coarse white pumice. 2. Sample 126-793A-1H-2, 118-120 cm, exampleof mixed volcanic glass shards, consisting of white and dark gray pumices and scoria.

44


100 µm

100 µm

100 µm

100 µm

Plate 3. Photomicrographs of smear slides of the volcanic ash layer. 1. Sample 126-793A-7H-3, 35-37 cm, fine, white, bubble-wall-type glassshards. 2. Sample 126-792A-1H-6, 119-121 cm, tabular volcanic glass shards in the coarse, white tephra. 3. Sample 126-793A-8H-5, 28-30cm, brown glass in the fine, black tephra. 4. Sample 126-792A-6H-3, 130-132 cm, crystallized brown volcanic glass in the coarse black tephra.

45

1992, Fujioka, K., Nishimura, A., Matsuo, Y., and Rodolfo, K. S., Correlation of Quaternary tephras...

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