0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.epsl.2004.08.014

* Corresponding author. Tel.: +64 7 374 8211; fax: +64 7 374 8199.

E-mail addresses: b.alloway@gns.cri.nz (B.V. Alloway)8 westgate5489@rogers.com (J.A. Westgate)8 michael.bird@st-andrews.ac.uk

(M. Bird)8 keith.Fifield@anu.edu.au (L.K. Fifield)8 alan.hogg@waikato.ac.nz (A. Hogg)8 ie.smith@auckland.ac.nz (I. Smith).

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l

Correspondence between glass-FT and 14C ages of silicic

pyroclastic flow deposits sourced from Maninjau caldera,

west-central Sumatra

Brent V. Allowaya,*, Agung Pribadib, John A. Westgatec, Michael Birdd,

L. Keith Fifielde, Alan Hoggf, Ian Smithg

aInstitute of Geological and Nuclear Sciences (GNS), Wairakei Research Centre, Private Bag 2000, Taupo, New ZealandbVolcanological Survey of Indonesia (VSI), Jl Diponegoro no. 57 Bandung 40122, IndonesiacDepartment of Geology, University of Toronto, 22 Russell Street, Toronto, Canada M5S 3B1

dSchool of Geography and Geosciences, University of St. Andrews, Fife, Scotland KY16 9AL, United KingdomeDepartment of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University,

Canberra, ACT 0200, AustraliafRadiocarbon Dating Laboratory, Waikato University, Private Bag 3105, Hamilton, New Zealand

gDepartment of Geology, Auckland University, Private Bag 92019, Auckland, New Zealand

Received 19 April 2004; received in revised form 21 July 2004; accepted 17 August 2004

Available online 22 September 2004

Editor: B. Wood

Abstract

Paroxysmal pyroclastic flow deposits sourced from Maninjau caldera in west-central Sumatra are dated at 50F3 ka (n=3)

using the isothermal plateau and diameter corrected fission-track (ITPFT and DCFT, respectively) techniques on glass shard

constituents. In addition, charcoal obtained from tall trees in position of growth within the paroxysmal flow deposit on the upper

flanks for the caldera are also dated at 52.3F2 14C ka (n=2) and 51.1F3.2 14C ka (n=1) using an acid–base, wet oxidation,

stepped combustion (ABOX-SC) and standard acid–base–acid (ABA) 14C techniques, respectively. The close correspondence

in 14C ages of charcoal sample splits analysed at two laboratories (Australian National University, Australia and Waikato

University, New Zealand) verifies the reliability of these 14C techniques up to at least 50 ka.

Based on concordant ages derived from glass-FT and 14C techniques, an age of 52F3 ka is assigned to the latest silicic

eruptive activity at Maninjau caldera. This chronology is further confirmed by the occurrence of a silicic tephra bed that closely

underlies paroxysmal Maninjau deposits at two sections and is correlated with Youngest (75 ka) Toba Tephra (YTT) erupted

from Toba caldera in north-central Sumatra. This study not only provides a much needed regional chronological reference point

Earth and Planetary Science Letters 227 (2004) 121–133

B.V. Alloway et al. / Earth and Planetary Science Letters 227 (2004) 121–133122

for Quaternary deposits in west-central Sumatra but also extends the minimum age range of the glass-FT technique from 75 ka

down to c. 50 ka that is now for the first time within the extended maximum age range of the 14C technique.

D 2004 Elsevier B.V. All rights reserved.

Keywords: pyroclastic flows; Maninjau caldera; Sumatra; glass-FT; 14C techniques; Youngest Toba Tephra

1. Introduction

Indonesia is situated adjacent to the Sunda

subduction system that has produced a 1600-km-long

volcanic arc with as many as 129 historically active

volcanoes, extending from Sumatra in the northwest

to Sulawesi in the southeast of the Indonesian

archipelago. Sumatra, in particular, contains a great

variety and number of volcanoes (Fig. 1). Many of

these volcanoes are dormant but a few have erupted

during historic time (i.e., Marapi and Singgalang-

Tandikat).

To date, much of the volcanological research

conducted in Sumatra has largely focussed on the

eruptive history of Toba caldera (i.e., [4]) and the best

record of Indonesian arc volcanism is from tephra

layers preserved in ODP-sedimentary cores retrieved

from the northeastern Indian Ocean adjacent to

Sumatra (i.e., [5]). Many of these tephra layers have

yet to be correlated to their onshore eruptive source

areas, confirming that there is little known about the

eruptive history of many volcanic centres located

onshore in Sumatra.

Maninjau is a volcanic edifice situated in the

Padang Highlands c. 300 km to the south of Toba

and c. 15 km west of the town of Bukit-Tinggi in

west-central Sumatra, Indonesia (Fig. 2). The central

part of this edifice is occupied by a collapsed

caldera that has a length of 20 km and a width of 8

km that is now occupied by a lake with an

estimated volume of 100 km3. The caldera lake

parallels the Sumatran volcanic front and is located

within the northwest–southeast trending Sumatran

fault zone which runs the entire length of Sumatra

[3,7].

There has been little previous research conducted

in the vicinity of Maninjau. Published material

includes the volcano–tectonic history of Maninjau

[6], 1:250,000 geological quadrangles of Padang [8]

and Solok [9] as well as descriptions of pyroclastic

flow deposits of the Padang Highlands [10] and

regional tectonic controls on volcanism and com-

plex crustal movements [11]. Posavec et al. [11]

suggested that Maninjau caldera was the product of

three major eruptions that blasted out the core of a

large central volcanic edifice and buried the

surrounding region with volcanic ash. However,

only two silicic eruptions were subsequently recog-

nised by Purbo-Hadiwidjoyo et al. [6]. The first

eruption produced an unwelded pumiceous tuff, and

the second, a welded tuff which is presumed to

have erupted from the southernmost part of the

caldera. Unwelded tuff deposits have been mapped

in a radial distribution around Maninjau extending

up to 50 km to the east and 75 km to the southeast

[6]. Pumiceous tuff deposits were also recognised

extending westward to the present-day coastline. In

this vicinity, the westward margin of the tuff

deposits abruptly terminates and forms a continuous

shore-parallel cliff positioned just inland from the

present-day coast. It appears that the tuff extended

further seaward but was subsequently eroded and

cliffed by a high sea level stand. Purbo-Hadiwid-

joyo et al. [6] estimated that the Maninjau tuff

deposits are distributed over 8500 km2 and have a

volume of the 220–250 km3.

2. Stratigraphy

Spectacular exposures of Maninjau-sourced silicic

deposits occur in deeply incised valleys (i.e., Ngarai

Sianok Valley, Section A48) in the vicinity of Bukit-

Tinggi [12], c. 15 km from eruptive source (Figs. 3

and 4A). Here, a number of thin pyroclastic density

current (PDC) deposits (each b1 m thick) are observed

at road level and overlain by a prominent 0.24-m-

thick basal-surge layer and a 60 m+ thick flow unit

with a lithic-rich base (Fig. 4B). The occurrence of

degassing structures confined to within each PDC

deposit and in some instances, erosional contacts,

suggests time lapses between each successively

Fig. 1. Map of western Indonesia showing regional tectonic features (Inset—after [1]) and volcanism. Triangles, volcanoes which have erupted

in historic time; circles, volcanoes in fumarolic state (modified from [2]). The distribution of silicic volcaniclastic deposits in Sumatra [3] are

also shown (dark grey).

B.V. Alloway et al. / Earth and Planetary Science Letters 227 (2004) 121–133 123

emplaced PDC event but not sufficiently long enough

for soil formation and weathering processes to occur.

This sequence is overlain in turn by a c. 11-m-thick

sequence of Andisol material with weathered lapilli

and coarse ash interbeds of basaltic-andesite to

andesite composition.

Fig. 2. Map of west-central Sumatra showing distribution of Maninjau PDC deposits (modified from [6]) and location of descriptive and

sampling sections (this study and [12]).

B.V. Alloway et al. / Earth and Planetary Science Letters 227 (2004) 121–133124

Closer to eruptive source, successive lithic fall

deposits of cobble to boulder size are restricted to

caldera rim sections (i.e., A52/53 and A64; Figs. 2

and 3). In this same vicinity, a thick succession of

crudely stratified and poorly sorted pumiceous

lapilli and ash have inundated and preserved

carbonised trees (10 to 15 m high and c. 0.3 m

diameter) in position of growth (Section A53; Fig.

5). On the eastern lower flanks of Maninjau caldera

and within the Ngarai Sianok Valley (Section A26;

Fig. 2), en-echelon gas pipes are observed within

the paroxysmal PDC deposit that are inclined c. 558in the direction of flow. This suggests low velocity

shearing of propagating gas pipes during and/or

closely following PDC emplacement (Fig. 6).

Further eastward, the thin PDC deposits wedge

out and the overlying voluminous upper unit

persists for a further 50 km. Distally at Mantijung

(c. 38 km from source; Section A45; Fig. 2) c.

0.60-m-thick surge bed can be observed directly

underlying a single c. 5.7-m-thick flow deposit. In

the west towards the Indian Ocean coastline, two

prominent flow units (12 m+) are observed sepa-

rated by a thin (0.2 m) layer of co-ignimbritic ash

(Section A78; Fig. 2). Gas-escape structures from

the lower flow unit extend continuously upward

through the upper unit and suggest near-contempo-

raneous emplacement.

3. Age

The age of Maninjau-sourced silicic deposits is

not well constrained. Purbo-Hadiwidjoyo et al. [6]

x xx x x xxx

x

xxx

xxxx xxxx x

0

5

10

Depth (m)

30

40

50

55

100

Section A64

Sungai Landir(c. 9 km from source)

160

719

277319

917

1117 (Change in scale)

5493Youngest Toba Tephra

(YTT) correlative(A64-2)

54405405 (Change in scale)

x

0

1

2

Depth

(m)

3

4

5

6

100

Section A40

Desa Melintang, Batusangkar(c. 36 km from source)

150

171

210219

251

275

305

331340

373

387

436429

475

500

674

696

xx xxxxx xx

x x xx xxxx

xxxxx

x x xxx x xxxx

x xxx x xx xxx x

xxxxx xxx xx

x xx xxx xx

xxx

x

xx

x xx xx

x xx

x xx x

xxxx

x

xx

xx xx x x

x x x xx

x

xx

x

xxx

x

x xx xxx xxxx xx x xxx x

xx

xx

xx

xxx

x

x

x

xxx xxxxx xxManinjau Tephra

(A40-1)

YTT correlative(A40-2)x xx x x xx

xx

xxx

xxxx xxxx xxx xx x xxx x xx

x xx

x xxx x xxx xX

X

X

X

XX

X XXX

XX X

X X

X XXXX

X XX

X

X

X

X

X

XXX

XX

X X

X X

X

XX X

XX

XX

X XXXX X

XXX XXXX

X

XX X

XX

XXX

X

X XX

X

XX

X

X

XXXXX

XX

X X

X

X

X

X

X

X

X

X

XX

XX XX

XX X

X

X

X

X

X

X

X

X

X

X

XX

X

4405

4205

5200

5002

20

X

X

X

XX X

X

X

X

X

X

X

X

X

X

X18001700

X

XX

X

X

X

X

X

X

X

X

X

X

XX

X

X

X

Height

(m)

3

2

1

Section A48

Bridge Site - Ngarai Sianok(c. 15 km from source)

42

80

151

0

180

54

X

XX

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

XX

X

X

X

X

X

X

X

XX

X

X

X

X

X

X

X

X

60+

KEY

Coarse ash

Lithic lapilli

Fine ash

Andic soil material

Pumiceous lapilli

Silicic tephra bedXX

X

X

XX

X

X

X

XX X

X

X

X

X

X

xxxxxx xxxx xx

xx xx xxxx xxx x x

Siliciclastic PDC

deposit

Lahar deposit

Gas segregation pipes

X

X

X

X

X

X X

Fig. 3. Detailed stratigraphy at Sections A40, A48 and A64 showing Maninjau siliciclastic deposits, YTT correlatives, Andisols and basaltic-

andesite tephra beds erupted from Singgalang, Tandikat and Marapi Volcanoes.

B.V. Alloway et al. / Earth and Planetary Science Letters 227 (2004) 121–133 125

Fig. 4. (A) View of Maninjau PDC deposits spectacularly exposed in the deeply incised Sianok Valley adjacent the town of Bukit-Tinggi. The

location of Section A48 is indicated as well as the location of panel B (inset square). (B) Thin pyroclastic density current (PDC) deposits (each

b1 m thick) are observed (arrowed) beneath a prominent basal-surge layer and a 60 m+ thick flow unit with a lithic-rich base. Note the

occurrence of multigenerational degassing structures confined to, and extending through, successive PDC deposits.

B.V. Alloway et al. / Earth and Planetary Science Letters 227 (2004) 121–133126

reported that five samples from Maninjau were

obtained for fission-track dating by Nishimura et al.

[13]. Although age and locational data were not

given, an age of between 70,000 and 80,000 years

was cited. A single K–Ar age determination of

0.28F0.12 Ma was also determined on plagioclase

from pumice retrieved from rhyolitic ash-flow tuff

located on the upper eastern flanks of the caldera in

the vicinity of Matur [2] (Fig. 2). In this study,

glass-fission-track and radiocarbon techniques were

used in an attempt to cross-check the chronology

and derive an accurate age determination for the

Maninjau siliciclastic succession.

3.1. Glass-FT

The age of the paroxysmal Maninjau PDC deposit

(Section A82) has been determined using the

isothermal plateau corrected fission-track (ITPFT)

and diameter corrected fission-track (DCFT) dating

techniques [14–16]. Both glass-FT techniques have

been clearly shown to be well suited to the dating of

rhyolitic tephra and have permitted a direct and

detailed comparison of stratigraphic and chronologic

data in a variety of proximal eruptive source to distal

sedimentary environments in New Zealand [17–19]

and North America [20–23]. Glass-FT age determi-

Fig. 5. Carbonised trees (10 to 15 m high and c. 0.3 m diameter) in

position of growth at Section A53 on the upper eastern flanks of

Maninjau caldera. Charcoal samples for 14C dating were obtained

from this locality.

Fig. 6. Within the Ngarai Sianok Valley (Section A26), en-echelon gas p

direction of flow and suggest low velocity shearing of propagating gas pi

B.V. Alloway et al. / Earth and Planetary Science Letters 227 (2004) 121–133 127

nations have also been made on the Youngest (75

ka) Toba Tephra (YTT) from sites in northern

Sumatra [4] and distally in northwest India [24].

YTT is the youngest tephra so far dated by the glass-

FT technique and was thought to occur at the

practical lower limit of the technique.

Glass-ITPFT and -DCFT ages for the Maninjau

paroxysmal PDC deposit were derived prior to 14C

dating (Table 1) and a weighted mean age of 50F3

ka (n=3) was determined. Glass-FT dating was

greatly facilitated by coarse grain size of Maninjau

glass providing a large surface area for fission-track

counting under the microscope and high uranium

content within the glass (4.4F0.6 ppm; n=22). This

high U content has produced a higher areal density

of fission tracks than would otherwise be encoun-

tered in equivalent aged silicic samples from else-

where, i.e., Taupo Volcanic Zone, New Zealand

(3.2F0.7 ppm U; n=49) and Aleutian arc and Alaska

Peninsula (3.3F1.3 ppm U; n=12).

3.2. Radiocarbon

Charcoal samples were retrieved from tall trees in

position of growth within paroxysmal deposits on

the upper eastern flanks of the caldera in the vicinity

of Desa Bukit Apit (Section A53; c. 8 km from

eruptive source; Fig. 5). Radiocarbon ages for the

ipes within the paroxysmal PDC deposit are inclined c. 558 in the

pes during and/or closely following PDC emplacement.

Table 1

Glass shard fission-track ages of paroxysmal Maninjau PDC deposit at Section A82, west-central Sumatra

Sample

number

(analyst)

Spontaneous

track density

(102 t/cm2)

Corrected

spontaneous

track density

(102 t/cm2)

Induced track

density

(104 t/cm2)

Track density on

muscovite detector

over dosimeter glass

(105 t/cm2)

Etching conditions,

HF: temp.: time

(%: 8C: s)

Ds

(Am)

Di

(Am)

Ds/Di or

Di/Ds#

Age

(103 yr)

Maninjau glass (A82)

UT1418* (BVA) 0.75F0.07 (101) 0.84F0.08 29.78F0.23 (17039) 5.49F0.05 (13947) 26: 23: 120 6.38F0.14 7.10F0.09 1.11F0.02# 49F5

UT1418* (JAW) 0.75F0.07 (102) 0.83F0.08 28.78F0.21 (18641) 5.49F0.05 (13947) 26: 23: 120 6.38F0.14 7.10F0.09 1.11F0.02# 50F5

UT1418 (BVA) 0.52F0.06 (75) 17.98F0.19 (9135) 5.49F0.05 (13947) 26: 21: 165 6.36F0.12 6.16F0.07 1.03F0.02 51F6

Weighted

Mean

50F3

Japanese glass standard (JAS-G1)

UT1141 (JAW) 23.0F1.36 (286) 40.22F0.46 (7714) 5.39F0.05 (13947) 26: 21.5: 195 0.99F0.06

The population-subtraction method was used. Samples with asterisk corrected for partial track fading by the track-size method [16]; the isothermal plateau (ITPFT) age is given for the

other samples [14]. Ages calculated using the zeta approach and kD=1.551�10�10 year�1. Zeta value is 318F3 based on six irradiations at the McMaster Nuclear Reactor, Hamilton,

Ontario, using the NIST SRM 612 glass dosimeter and the Moldavite tektite glass (Lhenice locality) with an 40Ar/39Ar plateau age of 15.21F0.15 Ma [25]. All samples irradiated in

same can on 27/05/97. Error (F1 S.D.) is calculated by combining the Poisson errors on the spontaneous and induced track counts and on the counts in the muscovite detector

covering the dosimeter glass. Ds=mean spontaneous track diameter and Di=mean induced track diameter; # = D i /Ds ratio determined (c.f. Ds /D i). Glass area for UT1418 estimated

using the point-counting method [15]; an eyepiece graticule was used for UT1141, which is the Japanese glass standard JAS-G1, whose 40Ar/39Ar age is 0.947F0.0005 Ma [26,27].

Number of tracks counted is enclosed in parentheses.

B.V.Allo

wayet

al./Earth

andPlaneta

ryScien

ceLetters

227(2004)121–133

128

B.V. Alloway et al. / Earth and Planetary Science Letters 227 (2004) 121–133 129

paroxysmal Maninjau PDC deposit are presented in

Table 2. Initially, charcoal samples were submitted

to Waikato University and Rafter Laboratory at the

Institute of Geological and Nuclear Sciences (GNS),

for radiocarbon dating and infinite conventional ages

(Wk-5370 and NZA-9396) of N40 and N47 ka were

returned, respectively. Sample splits were then

resubmitted to laboratories at the Australian National

University (ANU) and Waikato University for

redating. ANU uses a relatively new acid–base wet

oxidation, stepped combustion (ABOX-SC) proce-

dure for dating boldQ charcoal samples [28,29]. This

technique has been shown to consistently eliminate

contaminants not removed by conventional pretreat-

ments enabling reliable 14C determinations in the

40–50 ka time range. An ABOX-SC 14C age of

52.3F2 ka (ANU-13404 and -14112) was deter-

mined. Waikato University used the more traditional

acid–base–acid (ABA) extraction process on a blind

sample split and returned a statistically identical age

of 51.1F3.2 ka (Wk-13370).

Calibration of the radiocarbon time scale in terms of

calendar years is problematic in this c. 50 ka age range.

Recent data from the Cariaco Basin [30] appear to

indicate that calendar age is approximately equal to

radiocarbon age at around 50 ka. On the basis of other

new data from amarine core at the Iberianmargin, Bard

et al. [31] alternatively suggest that calibrated ages

Table 2

Radiocarbon ages for charcoal splits (Section A53) associated with Manin

ABA 14C technique

Lab-number d13C x wrt PDB D14C

Wk-5370 �24.9F0.2 �996.6F0.7

NZA-9396 �25.7 �995.6F1.5

Wk-13370 �24.6F0.2 �998.3F0.6

ABOX-SC 14C technique (ANU) Carbon combusted

[mg]

Radi

ANUA-Number Weight

(mg)

340

8C650

8C920

8C a

Raw

activ

(pMC

13404 3.4 0.50 0.01 1.3 0.270

14112 3.7 0.37 0.37 1.4 0.242

Wtd. avg. 0.247

a ABOX-SC 14C ages were obtained on the highest temperature fractb Ages calculated after 13C correction assuming a value of �25x, an

liberated in the lower temperature combustion steps for production of a ta

should be somewhat older than radiocarbon ages at this

time. The magnitude of the offset depends on which

Greenland ice-core chronology the marine core is

synchronised to. With the climate record in the marine

core synchronised to GISP2, the offset is 2000 years,

whereas it is 5400 years if the GRIP chronology is

employed. Note that the Cariaco Basin record was

synchronised to the GISP2 chronology. In this study, a

glass-FT age of 50F3 ka is statistically indistinguish-

able from the two 14C ages of 52.3F2 ka (ANU) and

51.1F3.2 ka (Waikato). Thus, within error limits,

calendar age is approximately equal to radiocarbon age.

3.3. Stratigraphic association

At two sections (A40 and A64) a prominent c.

0.23-m-thick silicic airfall layer closely underlies the

paroxysmal Maninjau flow deposit and is separated by

b1 m of andisol material (Fig. 3). Fresh glass shards

were subsampled from both airfall and overlying

paroxysmal flow deposits for electron microprobe

analysis. The resultant glass major oxide contents

(Table 3) were then used to characterise these deposits

enabling their correlation between onshore sections as

well as to ODP-sites (Fig. 7). On the basis of major

element glass chemistry, the underlying silicic airfall

layer (A64-2 and A40-2) is readily differentiated from

Maninjau-sourced eruptive products but is indistin-

jau prepared using conventional ABA and ABOX-SC procedures

Radiocarbon results

% Modern Age

(yr BP)

Error

(+)

Error

(�)

0.30F0.1 N40,000

0.44F0.15 N47,000

0.2F0.1 51,154 3238 2302

ocarbon results

14C

ity

)

Error

(1r)Bkgd

subt’d 14C

(pMC)

Error

(1r)Age

(yr BP)bError

(+)

Error

(�)

0.057 0.170 0.060 51,110 3,530 2,440

0.027 0.142 0.034 52,560 2,170 1,710

0.024 0.147 0.032 52,280 1,940 1,560

ion (920 8C).d subtraction of a blank of 0.1F0.02 pMC insufficient carbon was

rget.

Table 3

Average major element composition of glass and mineral phases from Maninjau PDC deposit as well as glass compositions of Younger Toba

Tephra (YTT) correlatives

Unit: Maninjau PDC deposit Youngest Toba Tephra (YTT) correlatives

Material: Glass Glass

Sample

locality:

1 (A82)a 2 (A78)a 3 (A64)a 4 (A52)a 5 (A53)a 6 (A40)a 3 (A64-2)a 6 (A40-2)a Average YTT

SiO2 77.10 (0.15) 77.01 (0.17) 76.88 (0.19) 76.92 (0.17) 77.35 (0.15) 77.12 (0.11) 76.96 (0.20) 76.95 (0.18) 77.57 (0.45)

TiO2 0.12 (0.05) 0.11 (0.04) 0.13 (0.05) 0.12 (0.04) 0.12 (0.03) 0.10 (0.05) 0.07 (0.05) 0.09 (0.04) 0.08 (0.05)

Al2O3 12.93 (0.10) 12.94 (0.11) 12.97 (0.07) 13.05 (0.08) 12.90 (0.03) 12.80 (0.09) 12.68 (0.12) 12.61 (0.11) 12.18 (0.23)

FeOt 0.81 (0.07) 0.81 (0.07) 0.75 (0.05) 0.77 (0.06) 0.81 (0.06) 0.73 (0.05) 0.95 (0.07) 0.93 (0.04) 0.80 (0.14)

MnO 0.06 (0.03) 0.07 (0.05) 0.04 (0.05) 0.04 (0.04) 0.12 (0.05) 0.07 (0.04) 0.08 (0.02) 0.07 (0.05) 0.06 (0.03)

MgO 0.06 (0.03) 0.07 (0.04) 0.05 (0.05) 0.05 (0.04) 0.07 (0.05) 0.07 (0.05) 0.05 (0.02) 0.03 (0.03) 0.06 (0.03)

CaO 0.78 (0.03) 0.79 (0.04) 0.75 (0.07) 0.78 (0.02) 0.76 (0.03) 0.79 (0.07) 0.79 (0.07) 0.82 (0.08) 0.76 (0.11)

Na2O 3.49 (0.09) 3.57 (0.12) 3.55 (0.08) 3.52 (0.06) 3.48 (0.06) 3.44 (0.10) 3.29 (0.12) 3.36 (0.16) 3.20 (0.36)

K2O 4.52 (0.09) 4.52 (0.08) 4.64 (0.10) 4.49 (0.08) 4.53 (0.06) 4.82 (0.06) 5.00 (0.11) 4.99 (0.09) 5.09 (0.40)

Cl 0.16 (0.03) 0.16 (0.03) 0.16 (0.01) 0.15 (0.03) 0.16 (0.02) 0.13 (0.03) 0.17 (0.04) 0.16 (0.03) 0.14 (0.04)

Total 95.30 (1.04) 95.80 (1.80) 96.88 (1.14) 95.11 (1.09) 95.65 (1.11) 94.13 (2.02) 91.86 (0.93) 90.54 (0.83) 97.90 (1.21)

H2O 4.70 (1.04) 4.20 (1.03) 4.20 (0.08) 4.89 (1.04) 4.35 (1.18) 5.87 (2.02) 8.14 (0.93) 9.46 (0.83) 2.10 (1.21)

n 17 71 11 10 16 15 10 10 207

Unit: Maninjau PDC deposit

Material: Feldspar b Biotiteb Magnetiteb Ilmeniteb

Sample locality: 7, 5 (A50 and A53) 8 (A45) 5 (A53) 5 (A53)

SiO2 59.88 (0.61) 35.60 (0.50) 0.37 (0.05) 0.32 (0.05)

TiO2 4.42 (0.11) 6.27 (0.24) 42.92 (0.65)

Al2O3 25.02 (0.33) 14.15 (0.28) 1.92 (0.06) 0.25 (0.06)

FeOt 19.43 (0.87) 85.48 (0.37) 52.86 (0.45)

MnO 0.31 (0.04) 0.64 (0.06) 0.94 (0.11)

MgO 10.75 (0.42) 0.81 (0.09) 1.88 (0.16)

CaO 6.45 (038)

Na2O 7.53 (0.24) 0.40 (0.06)

K2O 0.55 (0.05) 8.25 (0.44)

Cl 0.22 (0.02)

Total 99.44 (0.51) 93.53 (1.43) 95.50 (0.49) 99.16 (0.68)

n 16 26 10 10

Analyses made using a JEOL JXA-5A electron microprobe fitted with a LINK EDS detector and ZAF-4/FLS software. Operating conditions: 15

kV accelerating voltage, 0.5 nA beam current, 10 Am beam diameter. Glass analyses recast to 100% on volatile free basis. Totals shown are of

original analyses; difference from 100% considered as H2O content. Average glass composition of YTT taken fromWestgate et al. [22]. Minerals

taken from rhyolitic and rhyodacitic pumice fragments in Maninjau PDC deposit with exception of biotite, which is taken from the matrix. Sample

localities shown in Fig. 1. n=number of analyses; FeOt=total Fe as FeO; standard deviations are enclosed in parentheses.a Analyst—B.V.Alloway.

B.V. Alloway et al. / Earth and Planetary Science Letters 227 (2004) 121–133130

guishable from Younger Toba Tephra (YTT) erupted

from Toba caldera in north-central Sumatra [4,22] and

tephra layer-A identified in ODP Leg-121 Site 758

located c.1000 km to the northwest of Sumatra [5].

The occurrence of a YTT correlative underlying

Maninjau-sourced deposits provides additional evi-

dence supporting the glass-FT and radiocarbon

chronology.

4. Discussion

Detailed stratigraphic evidence suggests that Man-

injau caldera formed during a single eruptive event

that produced a closely spaced siliciclastic succession

comprising thin pyroclastic flow and intervening

surge deposits overlain by one (eastward) and two

closed-spaced (westward) voluminous paroxysmal

Fig. 7. (A) K2O vs. Al2O3, (B) K2O vs. FeOt and (C) K2O vs. CaO (wt.%) composition of volcanic glass shards from Younger Toba Tephra

(YTT; [22,24]) compared with pre-Maninjau YTT correlatives (Sections A64-2 and A40-2) and Maninjau PDC deposits (see Table 3).

B.V. Alloway et al. / Earth and Planetary Science Letters 227 (2004) 121–133 131

flow deposits. The occurrence of degassing structures

confined to within each flow unit suggests time lapse

between each successively emplaced flow event but

not sufficiently long enough for soil formation to

develop. However, for the two closed-spaced (west-

ward) voluminous paroxysmal flow deposits, degass-

ing structures extend upward from the lower through

the upper unit and indicate near-synchronous

emplacement.

The occurrence of successive lithic fall deposits of

cobble to boulder size at sections adjacent to the

caldera rim, as well as carbonised tall trees in position

of growth buried by crudely stratified and poorly

sorted pumiceous lapilli and ash, suggests a low

eruption column and that, close to source, low

velocity PDC emplacement took place in filling the

adjacent valley systems.

The lack of a widespread plinian fall deposit

associated with the Maninjau PDC succession and

conspicuous absence of a Maninjau-sourced tephra

layer from ODP-Site 758 in the Indian Ocean [5] also

favours low-energy and sustained effusive emplace-

ment in preference to high-energy emplacement

resulting from collapse of a high eruptive column.

The paroxysmal PDC deposit does, however, appear

to become more turbulent distally as evidenced by the

occurrence of rip-up clasts, basal surge beds (bc. 0.6

m thick) and flow-oriented carbonised palm trees (cf.

carbonised trees in growth position at Section A53).

The fine grain size (with corresponding high grain

surface area) of the paroxysmal PDC may have been a

factor that promoted heat retention and turbulence

through degassing, and additional incorporation and

vapour flashing of groundwater during flowage.

The entire Maninjau PDC succession is overlain

by Andisol and basaltic-andesite lapilli layers

erupted from adjacent Singgalang–Tandikat and

Marapi Volcanoes. No silicic tephra interbeds have

B.V. Alloway et al. / Earth and Planetary Science Letters 227 (2004) 121–133132

been identified within the postparoxysmal, domi-

nantly andic sequences, suggesting that eruptive

activity at Maninjau caldera was short lived. Based

on concordant ages derived from glass-FT and 14C

techniques, an age of 52F3 ka is assigned to the

latest silicic eruptive activity at Maninjau caldera in

west-central Sumatra.

This study also extends the minimum age range of

the glass-FT technique from 75 to c. 50 ka that is now,

for the first time, within the extended upper range of

the 14C technique. Concordant Glass-FT and 14C ages

differ from previous and disparate FT (c. 70–80 ka)

and K–Ar (c. 0.28 ka) determinations. The c. 52 ka

age for the paroxysmal Maninjau PDC deposit is

supported by the occurrence of an underlying silicic

tephra bed which is geochemically indistinguishable

from the c. 75 ka Youngest Toba Tephra (YTT)

erupted from north-central Sumatra.

Acknowledgments

This work forms part of a postgraduate thesis

conducted by Agung Pribadi (supervised by B.V.

Alloway and I.E.M. Smith) under an NZODA study

award and as part of a Development Cooperation

Program between New Zealand and the Republic of

Indonesia. We acknowledge the financial support of

this research from the Internal Grants Committee,

Auckland University (to BVA) and the Natural

Sciences and Engineering Research Council of

Canada (to JAW). We extend a special note of thanks

to the Volcanological Survey of Indonesia (VSI) who

provided transport and accommodation during field

work. Finally, the manuscript benefited from reviews

by Brad Pillans, Australian National University, and

Chris Turney, Queen’s University, Belfast.

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