Geochemical Journal, Vol. 39, pp. 69 to 82, 2005
Pyroclastic deposits of Mio-Pliocene age in the Arakan Yoma-Andaman-Java
subduction complex, Andaman Islands, bay of Bengal, India
TAPAN PAL,1 TANAY DUTTA GUPTA,1 PARTHA PRATIM CHAKRABORTY2* and SUBHAS CHANDRA DAS GUPTA1
1
Project: Andaman, Op: WSA, ER, Geological Survey of India, Kolkata, India
2
Department of Applied Geology, Indian School of Mines, Dhanbad, India
(Received June 19, 2003; Accepted May 19, 2004)
A thick sequence of bedded tuff alternated with non-volcanogenic turbidites is present in the Archipelago Group of
rocks (Mio-Pliocene age) of the Andaman Islands. The tuff occurs in three different facies: a) Facies A, white massive tuff
with ill defined bedding contacts; b) Facies B, dominantly green and white tuff with ill defined turbidite Tabc, Tab Bouma
sequence; and c) Facies C, well defined ash turbidites of Tab, Tabc, Tacde sequence. Dominance of cuspate shards with no
pervasive alteration and absence of blocky shards are features of subaerial eruption for the pyroclastic rocks. Absence of
glass welding and plastic deformation together with the presence of good sorting and recurrence of ash turbidites indicate
that subaerially-erupted ash landed in water and behaved as a cold subaqueous flow. Field features show change of flow
character from subaqueous debris flow (for Facies A) to recurrent high to low concentration turbidity current (for Facies
B), and ultimately to low particle concentration turbidity current (for Facies C). All the tuff varieties are vitric to crystovitric in character, and contain broken crystals of quartz, plagioclase, mica, and glass shards of delicate shape (crescent,
cuspate, curved) without any welding features. The tuffs do not contain any lithic fragments of volcanic rocks. Petrography
as well as XRD studies show that glass alteration is common in Facies A and is least in green tuff of Facies B. Alteration
of glass to clinoptilolite and analcime has been linked to burial diagenesis. The similarity of chemical composition of
glass shards (mainly dacite) and bulk rocks, however, indicates negligible chemical change during diagenesis. The origin
of Andaman tuff in a convergent margin tectonic setting is assumed as the basis of high Zr/Nb and Zr/Y.
Keywords: pyroclastic, Archipelago Group, Andaman, subaerially-errupted, convergent margin
gested possible diagnostic features in those flow products. For the first time, a detailed account of pyroclastic
deposits of Mio-Pliocene age from the Andaman Islands
is presented.
The Andaman and Nicobar Islands, form the central
part of the 5000 km long Burma-Sunda-Java subduction
complex. Major tectonostratigraphic elements in these
islands strike approximately parallel to the trend of the
Java Trench. The predominance of sedimentary rocks and
local thrust-emplaced ophiolite slices and olistoliths (Ray,
1982) are consistent with an outer arc setting for the
Andaman and Nicobar Islands (Chakraborty and Pal,
2001). The Archipelago Group of rocks, which constitute
an important stratigraphic subdivision within the 3150 m
thick (Roy, 1983) Tertiary sedimentary succession exposed in this island chain, has been studied for the last
four decades (Karunakaran et al., 1967; Ray, 1982). The
Group is described as an interbedded sequence of sandy
limestone, claystone, calcareous sandstone and shale
(Karunakaran et al., 1967). On the basis of the litho-assemblage and fossil content, a marine origin has been
suggested for Archipelago sediments (Ray, 1982;
Srinivasan, 1988). Although several studies have been
carried out on this stratigraphic unit (e.g., Pawde and Ray,
INTRODUCTION
Recognition of subaerial or subaqueous depositional
environment for a pyroclastic flow product is difficult in
the rock record, and often leads to controversy (cf., Cas
and Wright, 1987). A subaqueous flow may either be fed
directly from subaqueous eruption or from a subaerial
eruption that is reworked and deposited under subaqueous
conditions (Fisher and Schmincke, 1984; Cas and Wright,
1987; McPhie et al., 1993). For a number of the
pyroclastic deposits described in the literature, an origin
has been proposed whereby subaerial eruptions, upon
entering the sea behaved as subaqueous flows.
Subaqueous pyroclastic flow products in sequences of
different age have been described (e.g., of Ordovician age
by Wright and Mutti, 1981; of Mio-Pliocene age by
Cousineau, 1994 and others). White (2000) categorized
different eruption-fed subaqueous flow types, and sug*Corresponding author (e-mail: [email protected])
*Present address: Central Petrological Laboratory, Geological Survey
of India, 15 A & B Kyd Street, Kolkata 700016, India.
Copyright © 2005 by The Geochemical Society of Japan.
69
Fig. 1. a) Regional structural elements around the Java-Andaman-Burma trench (after Mitchell, 1985). b) Generalised geological
map of Andaman islands, box marking C is the study area of Havelock island. c) Geological map of the part of the Havelock
Island.
1963; Srinivasan, 1988) no in-depth discussion is available in the literature for any volcanogenic event of Archipelago time from the main islands of Andaman and
Nicobar. Reports of the presence of glass shards within
Archipelago sediments of Ritchies Archipelago
(Srinivasan, 1988) and occurrence of acid tuff from South
70
T. Pal et al.
Andaman (Pal et al., 2002) prompted the present study
of Mio-Pliocene volcanogenic activity along this plate
margin. Extensive volcanic activity has been reported
from the Late Miocene onwards in Sumatra at the southern part of this subduction complex (cf., Hall, 2002). In
the present investigation, based on field observations and
upper mantle-crustal section of oceanic
plate forming a part of anaccretionary prism
metamorphic rocks, tectonites, cumulates, plagiogranite-diorite-andesite suite, basalt and pelagic
sediments
Ophiolite Group
Cretaceous to Palaeocene
trench sediments
conglomerate sandstone and shale
Mithakhari Group
(1400 m thick)
Lower to middle Eocene
forearc basin
interbedded sequence of sandstone, siltstone and shale representing siliciclastic turbidite deposits
Andaman Flysch Group
(300 m thick)
Upper Eocene-Oligocene
forearc region
pyroclastic deposit with siliciclastic sediments in lower part, pyroclastic deposits interbedded with
carbonate in the intermediate part, and dominantly carbonate turbidite in the upper part
Archipelago Group
(400 m thick)
Mio-Pliocene
marine sediments and subaerial
soil, beach sand and shell limestone
Pleistocene-recent
Stratigraphic unit
Age
The Andaman and Nicobar group of islands are in the
southeastern part of the Bay of Bengal and form part of
the 3000 km long chain running from Arakan-Yoma in
the North to Sumatra and Java in the South (Fig. 1(a)).
The Indian Plate is subducted northwards below these
plates and below the Sino-Burma Plate along the
geophysically-traced Arakan-Yoma-Andaman-Java trench
(Curray and Moore, 1974; Karig et al., 1979;
Mukhopadhyay, 1988). The Narcondam and active Barren volcanoes representing a magmatic arc are present to
the east of this main Island chain. Major constituents of
these islands are: (a) oceanic crust (in the form of
ophiolites) and trench sediments, together forming an
outer arc; and (b), subaerially-exposed sediments of the
fore arc. Table 1 summarizes the lithostratigraphic subdivisions, and their inferred tectono-depositional framework (revised after Ray, 1982; Chakraborty et al., 1999;
Chakraborty and Pal, 2001; Pal et al., 2003).
The ophiolite sequence comprising tectonite
(ultramafic)-cumulate (layered mafic and ultramafic)
plagiogranite-diorite suite-basalt-pelagic sediments, is
identified as fossilized oceanic crust (cf., Gass, 1990).
The Cretaceous age of the ophiolite succession is indicated by the foraminiferal assemblage of pelagic
sediments (Roy et al., 1988). The metasediment and
metabasalt units associated with ophiolites (either at their
sole or as caught-up patches) are related to the emplacement history of the thrust slices (Pal et al., 2003). The
ophiolites are present as dismembered slices and interleaved with ophiolite-derived clastic sediments. These
ophiolite-derived clastics (the Mithakhari Group), represented by lensoid conglomerate and sandstone beds in
dominantly shale facies, are interpreted as trench
sediments (Chakraborty et al., 1999). A thick sedimentary pile of sandstone and shale representing classic
turbidites forms the Andaman Flysch Group. The sedimentary sequence of the Archipelago Group comprises a
thick pile of pyroclastic deposits, limestone, sandstone
and shale. A Mio-Pliocene age is inferred for the Archipelago Group of rocks on the basis of foraminiferal and
nannofossil assemblage (Ray, 1982). A conglomeratic
Table 1. Lithostratigraphy and tectonic setting of Andaman and Nicobar Islands
GEOLOGICAL SETTING
Lithologic character
Tectono-depositional setting
petrographic studies, recurrent felsic volcanic events have
been documented in the sediments of the Archipelago
Group exposed on South Andaman and the Havelock Islands of Andaman group of islands. The present study
deals with the pyroclastic deposits in terms of their: (a)
field disposition; (b) geochemical composition; (c) mineralogy and characterization of glass; and d) diagenetic
alteration (of tuff). The nature of eruption, mode of transport and deposition, and possible diagenetic alteration of
the deposits are then examined.
Mio-Pliocene pyroclastic deposits from Andaman Islands, India
71
Table 2. General features of different facies of Andaman tuff
Volcaniclastic facies
Facies A
Facies B
Facies C
a) Dominant colour, contact nature
white to pink, sharp, non erosive
with underlying turbidites.
massive-parallel stratification
towards top.
poor
green to white, gradual with facies
A, erosive to nonerosive with mud.
massive-normal grading-parallel
lamination crude cross lamination.
good with different glass- and
crystal-rich layers.
glass shards, crystals and volcanic
dust.
bicuspate-tricuspate.
quartz, plagioclase zircon.
grey white to white, erosive with
underlying mud.
normal grading-cross laminationparallel lamination towards top.
moderate to poor.
fresh to feeble alteration.
primary accumulation subaqueous
turbidity deposit.
feeble
secondary accumulation
subaqueous distal turbidites.
b) Sedimentary structures
c) Sorting
d) Framework constituents
e) Shape glass shards. Crystals
f) Secondary alteration
g) Nature of deposit
glass Shards crystals, volcanic dust
few crystals.
platy to curved.
quartz, plagioclase, mica zircon
(accessories).
prevalent but not strong.
primary accumulation sub aqueous
debris flow deposit.
shell limestone of Pleistocene to Recent time consists of
shell fragments together with pebbles of various rocks
embedded in calcareous cement. Late Mesozoic-onward
subduction along this plate margin resulted in the uplifting of Cretaceous ophiolites and Eocene sediments by a
series of thrust slices in an accretionary prism setting;
subsequent N-S normal and E-W strike slip faults caused
the development of forearc basins in Oligocene and MioPliocene time (Pal et al., 2003).
FIELD ASPECTS
OF
TUFFS AND ASSOCIATED ROCKS
The Archipelago Group was studied in the HubdeypurM i l e Ti l e k s e c t o r o f S o u t h A n d a m a n a n d t h e
Krishnanagar-Shyamnagar area of Havelock Island (Fig.
1). Exposures of the volcaniclastic sequence, overlying
or sandwiched between nonvolcanic (background)
turbidite successions, can be laterally traced for hundreds
of meters in either of the sections studied. Between different sections however, the volcaniclastic units vary in
bedding character, grain size, depositional structure, and
associated non-volcanic sediment character. Classification of the volcaniclastic units is in three different facies
types based on color, bedding character, primary sedimentary structure, composition and sorting of framework
grains, and commonly observed alteration patterns (Table 2). The sections of South Andaman and Havelock Island vary between themselves both in terms of
volcaniclastic facies types present, and their associated
non-volcanogenic sediment character.
South Andaman
In the western part of the South Andaman Island Archipelago, younger sediments conformably overlie the
sediments of the Andaman Flysch Group while in the eastern part of the island, Archipelago rocks are juxtaposed
72
T. Pal et al.
fine glass shards, crystals and
volcanic dust.
platy, angular curved. quartz,
plagioclase, mica.
with rocks of the Mithakhari Group along a faulted contact. In both the Hubdeypur and Mile Tilek sections, the
tuff beds occur between non-volcanogenic sequences
comprising sandstone-shale alternations (Figs. 2(a) and
(b)). In the Hubdeypur section, the tabular nonvolcanogenic sandstone beds, occasionally amalgamated,
have an invariably sharp base (erosional) and planar, gradational top. Internally, these parallel-sided greywacke
sandstone beds with coarse-tail normal grading represent
well-defined Bouma cycles (Tacde, Tacd). In the Mile Tilek
section, the sandstone units are finer-grained (fine sandstone/siltstone), without amalgamation and with dominance of bottom-truncated Bouma cycles (Tcde, T ce). In
South Andaman sections the pyroclastic sequence interleaving with these non-volcanogenic successions represent two facies variants of the tuff (facies A and B). In
the Hubdeypur area, both facies A and B are present
whereas in Mile Tilek area only facies B is present (Fig.
2). Maximum thicknesses of the tuff layer recorded in
Hubdeypur and Mile Tilek sections are 14.46 m and 3.87
m respectively. White colored tuff of facies A occupies
the basal part of the Hubdeypur section with a uniform
physical character. Beds of this facies are tabular in geometry, meter scale in thickness and found amalgamated
with poorly defined bedding contacts. Tuff units of this
facies are dominantly massive with rare indistinct stratification and lack any basal scour. Fractures, joints and
faults are frequently found transgressing the tuff beds of
facies A. Tuff facies B, occurring towards the top part of
the Hubdeypur section, show colour variation from green
to white to pinkish white and lateral variation in
depositional structures. In a down current transect, the
tuff beds of this facies vary from a coarse-grained massive unit with a scoured base (Bouma Ta) to units with a
massive and overlying plane laminated subdivision having low angle truncations (Bouma Tab; Fig. 3(a)) to units
Fig. 2. Measured lithologs of Archipelago sections at a) Hubdeypur, South Andaman, and b) Krishnagar-Shyamnagar area,
Havelock island.
with planar, nonerosional base and internal ripple drift
lamination followed by thin parallel lamination (T cd).
Locally, beds of green tuff are normally graded (grain
size varying from fine sand to clay) with internal stratification varying upward from massive to plane lamination
and finally to ripple cross-lamination. The sequence is
repeated with a scoured base and shows a poorly developed T abcd succession. Distal equivalents of facies B is
observed in the Mile Tilek area where individual tuff beds
with internal stratification are thin and resemble Tc–e, Tde
succession.
The dominant coarse-grained tuff beds of facies A with
a poor grading, poor stratification and lacking of basal
scour, are products of high grain concentration in flow.
The rare occurrence of indistinct, undulatory laminae towards the top part of some of these flows, however, reflects dilution of these flows towards their top. Rapid
emplacement from one depositional event may cause such
coarse, massive or poorly developed normal graded beds,
lacking internal structure (Cousineau, 1994). The locally
formed, poorly defined stratification possibly formed in
response to a high suspended load fall out rate (Lowe,
1988) of frictional freezing as high-density sub flows
developed at the top of the flow. Absence of fines in this
facies is typical of volcanic debris flow (Fischer, 1984).
Facies B tuff beds with the undoubted presence of Bouma
cycles, however, were possibly deposited from turbulent
flows with superheated steam as the medium. Definite
indications of down current reduction in grain concentration within the flow and increase in flow dilution are
analogous to down current flow transformation in normal gravity flows (Fischer, 1983) resulting from wet
pyroclastic surges (Sparks and Walker, 1977).
Mio-Pliocene pyroclastic deposits from Andaman Islands, India
73
(a)
(b)
(c)
Fig. 3. Photograph showing turbidity features in Andaman tuff
(hand specimen) a) shows change from normal graded layer,
low angle scour, cross lamination, parallel lamination, high
angle cross lamination of facies B, b) normal grading of facies
and high angle cross bedding of facies C, c) sygmoidal cross
laminations of facies C.
Havelock Island
On Havelock island, tuff beds alternate with non-calcareous siltstone/mudstone in the Krishnanagar area, and
are interbedded with a fossil-bearing clastic limestoneshale in a section 3 km SE of the Shyamnagar area. The
thin siltstone/mudstone interbeds in the Krishnanagar
74
T. Pal et al.
section are laterally extensive between two successive tuff
unit and internally massive without any wave or current
features. The non-volcanogenic sediment associated with
tuff sediment in Shyamnagar area alternates between parallel-sided clastic, fossiliferous carbonate beds and fissile shale, which can be traced laterally for tens of meters. Both lower and upper contacts of the carbonate bed
are planar; while the lower contact varies between sharp,
erosive to nonerosive, the upper contact is invariably gradational. Internally, individual carbonate beds reveal a
structural sequence from graded to cross-laminated to
parallel laminae towards the top (Bouma Tacd, Tc–e). Two
facies types of tuff (facies B and C) are present in the
Havelock area. Facies C in the Krishnanagar area contains tuff beds with medium sand to silt-sized grains and
well defined normal grading (Fig. 3(b)). From the base
upward, individual tuff units change in structural pattern
from graded bedding with scoured base to parallel lamination to cross lamination and then again to parallel lamination; these are typical structural features of a Tabce, Tab,
T abc , T acde , Bouma sequence. Normal grading and
sygmoidal geometry of the cross lamination of this facies
are also clearly seen in hand specimen (Fig. 3(c)). Soft
sediment deformation features such as convolute lamination, pseudo-nodule structures and small clastic dykes,
are abundant in many tuff layers. In the down-current direction, tuff beds of facies C changes sedimentary character similar to that of facies B of South Andaman, but
here the interbedded non-volcanogenic sediments are clastic carbonates with a change from graded beds to parallel
laminae to cross laminae to parallel laminae towards the
top. The entire sequence is overlain by carbonate turbidite.
The siltstone/mudstone interbeds without any wave/
current features in the Krishnanagar Section are consistent with deposition below storm wave base where supply
of siltstone presumably took place through buoyant sediment plumes during episodes of climatic/tectonic perturbation. The normally-graded clastic carbonate beds with
well-documented Bouma cyclicity in the Shyamnagar
section were deposited from low-density turbulent flows
(cf., Walker, 1978). Observation of similar features in tuff
beds of facies C also suggests their deposition from lowdensity ash turbidite. The abundance of convolute soft
sediment deformation features deposition and dewatering
of the ash turbidite beds before consolidation (Lowe and
LoPiccolo, 1974). Vibrations generated by volcanic tremors might have initiated and enhanced liquefaction and
dewatering (Cousineau, 1994).
ANALYTICAL TECHNIQUES
Geochemical and petrographic studies of the
pyroclastic rocks were undertaken using the following
methods:
(a)
(a)
(b)
Fig. 6. Back scattered image of zeolite crystals by Scanning
Electron Microscope showing platy shape of analcime (marked
as A) and needle shape of clinoptilolite (marked as B).
Fig. 4. Microphotographs of tuff showing typical shapes of
glass shards a) under plane polarised light. b) Typical cuspate
shaped glass shard.
and monochromator.
X-ray fluorescence (XRF): Apparently homogeneous
individual, bulk layers of the tuff were analysed by XRF
spectrometer (Philips PW 1400) at 45 kV and 55 mA for
major oxides and 70 kV and 40 mA for trace elements.
The samples were fused into glass discs. Natural standards supplied by United States Geological Survey and
CRPG (France) were used.
Induced Coupled Plasma Optical Emission
Spectrometry (ICP OES): Rare Earth Element (REE)
analysis was done by JUBIN YVON JY 38 with a natural
standard (J-1A from Japan) and an aspiration rate of 1.5
ml/min. and pneumatic nebulizer.
Electron Probe Micro Analysis (EPMA): For mineral
and glass composition polished sections were analyzed
with a CAMECA Sx51 at 15 kV, 12 nA. The natural standards were supplied by BRGM, France.
PETROGRAPHY
Fig. 5. Scanning Electron Microphotograph show different
shapes of glass shards in the left part and right part is the enlarged portion of the box marked in left part showing typical
glass alteration features (A-glass, B-clinoptilolite, C-analcime).
X-ray diffraction (XRD): This study was undertaken
to identify the mineral assemblage of different tuff facies
types. Powdered samples were studied by Automated
Powder Diffractometer APD-15 using Cu-K a radiation
Thin sections of fifteen samples collected from different facies types of Archipelago tuff were studied. The
massive tuff of facies A is vitric to crysto-vitric; predominantly comprises delicately shaped glass shards, with
variable amounts of broken phenocrysts of euhedral
quartz, plagioclase, muscovite, orthoclase, zircon and
ilmenite. Glass in some cases is altered to zeolite. In white
tuff (Facies A), glass is altered commonly to form zeolite
whereas glass alteration is a minimum in green tuff (dominant member in Facies B). Due to the presence of different glass shards e.g., platy, sickle, bicuspate, tricuspate,
crescent and horn shape (Figs. 4(a) and (b)) the tuff can
be grouped into type 5 of Wohletz (1983). High magnification photographs (Fig. 5) show typical shapes of glass
Mio-Pliocene pyroclastic deposits from Andaman Islands, India
75
Fig. 7. XRD chart of Andaman tuff. In fresh green tuff (sample No. 117D) no zeolite has been formed, in the lighter green tuff
(sample No. 117B1) clinoptilolite has been formed from glass and white tuff shows presence of both clinoptilolite and analcime.
shards. A cuspate shard with gas bubble is shown in Fig.
4(b). Quartz crystals are acicular to irregular to bi-pyramidal shapes and extinguish sharply under crossed
nicols. Plagioclase is lath-shaped, orthoclase is equant in
shape, and glass rinds around crystals are also seen. In
green tuff of facies B, sorting has produced alternate glassrich and crystal-rich layers. High magnification photographs by scanning electron microscope show two stages
of alteration in glass shards of white tuff (Fig. 5) with
glass altered to one zeolite that is again replaced by another zeolite mineral. The formation of crystals of one
variety of zeolite (analcime) from another variety of zeolite (clinoptilolite) is common in white tuff (Fig. 6).
The non-volcanogenic sandstone associated with tuff
comprises microcline, quartz, biotite, muscovite,
plagioclases and tuff fragments as framework grains, and
76
T. Pal et al.
chloritic matrix. The presence of fresh microcline,
plagioclase and tuff fragments suggest a neovolcaniclastic
origin (Critelli and Ingersoll, 1995). In the sandstone unit
immediately underlying Facies A tuff in Hubdeypur section, rock fragments, plagioclase and quartz are framework grains where rock fragments are represented by
basalt, tuff, ultramafic, sandstone and siltstone. On the
other hand, limestone interbedded with facies B in
Havelock islands is fossiliferous calcarenite to calcilutite.
X- RAY DIFFRACTION STUDY
In order to examine the mineral assemblages of the
different types of tuffs, representative samples from white
tuff of Facies A (Sample 117C), dark green tuff of facies
B (Sample 117D) and lighter green tuff of facies B (sam-
Table 3A. Major oxide analysis (in %) of Andaman tuff by XRF method
Sample No.
H1a
H1b
H1d
H1f
117a
117B1
117B2
117C
117d
479
SiO2
Al2 O3
*Fe 2 O3
FeO
MnO
MgO
CaO
Na 2 O
K2 O
TiO2
P2 O5
L.O.I-(H2 O)
63.32
15.00
4.61
.29
.27
1.86
2.28
.83
2.30
.66
.06
8.49
59.50
17.68
4.57
.19
.25
.87
2.79
1.19
2.64
1.23
.18
8.86
61.38
15.48
5.87
.19
.20
1.92
2.11
.50
2.31
.83
.05
9.11
65.66
14.22
1.29
.19
.01
.67
4.13
1.61
.73
.21
.04
11.20
69.69
15.25
1.61
0.38
0.03
1.59
1.46
0.67
3.13
0.25
0.06
5.84
69.77
12.85
.50
.19
.01
.91
2.87
.25
2.04
.18
.03
10.37
69.55
14.58
.75
.19
.04
.72
2.43
.51
2.57
.22
.07
8.27
66.34
15.53
1.84
.19
.04
1.80
2.25
.07
2.04
.22
.03
9.59
68.81
12.87
0.46
0.18
0.02
1.08
2.70
.27
1.80
.20
.02
9.25
75.40
14.09
0.94
0.36
0.06
0.63
0.07
5.56
0.81
0.18
0.04
1.81
Table 3B. Major oxide analysis (in %) of one tuff sample by
EPMA method
Sample No.
117B1
Points
1
2
3
4
5
SiO2
Al2 O3
FeO
MnO
MgO
CaO
Na 2 O
K2 O
TiO2
Cr 2 O3
NiO
BaO
69.44
12.90
0.10
0.00
1.20
2.49
0.99
1.66
0.01
0.00
0.03
0.41
73.80
13.14
0.07
0.00
1.05
2.42
0.92
1.46
0.00
0.03
0.00
0.40
66.77
19.17
1.06
0.02
1.24
1.76
0.84
0.96
0.04
0.00
0.00
0.24
75.37
14.07
0.26
0.04
1.62
2.38
1.60
1.61
0.03
0.03
0.04
0.44
70.01
13.35
0.75
0.00
1.18
1.92
0.74
1.15
0.06
0.90
0.01
0.34
*FeO and Fe 2O 3 were determined from wet chemistry from total Fe2O3 measured in XRF method.
ple 117B1) were investigated by X-ray diffraction. The
XRD charts (Fig. 7) show primary minerals mainly as
quartz, crystobalite, alkali feldspar (albitic) and
clinoptilolite to analcime as secondary (zeolite) minerals. Correlation of the XRD charts clearly shows that in
the fresh dark green variety of tuff no zeolite has formed.
In lighter green tuff, clinoptilolite has formed whereas in
white tuff, peaks for clinoptilolite-analcime can be observed. The XRD study therefore shows glass alteration
is gradual from white to green tuff with maximum in white
tuff of Facies A and minimum in green tuff of Facies B.
Petrographic observations also show similar alteration
pattern.
GEOCHEMISTRY
The analysed major oxides (Table 3A) are normalised
to 100% anhydrous and are given in wt%. The silica value
ranges from 59.50% to 69.77% except one sample show-
Fig. 8. The total alkali-silica diagram showing composition of
Andaman tuff (after Le Bas et al., 1986).
Mio-Pliocene pyroclastic deposits from Andaman Islands, India
77
Table 4. Trace element analysis (in ppm) of Andaman tuff
Sample No.
H1a
H1b
H1d
H1f
117a
117B1
117B2
117C
17d
479
Rb
Sr
Zr
Y
Nb
Ni
Cr
Co
Ba
92
513
180
22
16
75
71
40
540
57
359
220
22
14
45
8
36
407
108
541
207
19
14
65
50
39
593
22
1712
302
17
12
14
<5
16
1515
84
822
182
23
7
18
<5
20
4838
33
2331
319
16
11
19
<5
15
1855
88
2657
360
23
12
15
214
18
5060
29
198
206
16
10
20
48
17
1566
44
2020
190
16
11
14
<5
16
1630
44
76
109
16
13
23
<5
22
95
Fig. 9. A chemical plot of trace element of Andaman tuff normalized against chondrite (using MINPET 2.0 software).
Fig. 10. Zr/Nb plot of Andaman tuff showing compressional
tectonic set up set up (after Loomis et al., 1994).
ing 75.40% with an average of 65.07%. Total alkali content ranges from 2.11% to 3.83% with an average of
2.80%. The contents of alkali and silica show that
Andaman tuff is, in general dacitic in composition with a
few falling in the andesite and rhyolite fields (Fig. 8).
The tuff has undergone partial diagenetic alteration in
diagenesis as reflected by the growth of zeolite in expense of glass. To know the individual composition of
glass shards, one sample (No. 117B1) was analysed by
EPMA (Table 3B) which show SiO 2 variation from
66.77% to 75.77% with an average 71.7%. The same sample in bulk composition has a SiO2 content of 67.77%.
The total alkali contents of glass shards are also similar
to those determined by bulk XRF analysis. The change in
chemical composition during diagenesis of the glass
shards is thus minimal.
Some trace elements (e.g., Rb, Ba, Sr, and LREE) show
considerable variation whereas others (Zr, Y, Nb) have
limited variation (Table 4). The higher abundances of Ba
and Sr relative to Rb (Fig. 9) coupled with low Rb/Sr and
Rb/Ba are consistent with predominant plagioclase rather
78
T. Pal et al.
than K-rich feldspar fractionation (cf., Anderson et al.,
2000). XRD and petrographic results of the tuff samples
corroborate this contention. High values of Zr/Nb and Zr/
Y and the Zr-Nb discriminatory diagram (Fig. 10) suggest the origin of Andaman tuff in a convergent plate
margin (cf., Loomis et al., 1994); a volcanic arc setting
is also inferred from the Rb abundances (<100 ppm) and
(Y + Nb) values (<100 ppm) (Fig. 11) (Pearce et al., 1984).
The analyses of REE (Table 5) normalised against
chondrite are plotted in Fig. 12. The REE pattern and LaN/
YbN and CeN/YbN (7.06 and 3.84 respectively) show
LREE enrichment relative to HREE.
DISCUSSION
Nature of eruption
Glass shards of different shapes viz. curved, v-shape,
crescent and cuspate are the fragments of elongate, thin
pipe-shaped bubble wall vesicles (cf., Fisher and
Table 5. Analysis of rare earth elements (in ppm) of Andaman tuff
Sample No.
117a
117B2
117C
117D
H1b
H1f
H1a
H1d
117B1
479
La
Ce
Nd
Sm
Eu
Gd
Dy
Er
Yb
Y
46.7
59.6
42.9
6.65
0.20
5.8
5.4
3.30
2.90
30.8
34.8
45.5
32.6
6.8
1.45
5.85
6.5
4.3
4.1
47.1
22.6
33.3
17.8
3.0
0.20
2.8
2.2
1.55
1.75
14.4
24.3
33.6
20.3
3.75
0.90
3.2
2.55
1.40
1.3
14.4
35.5
46.0
32.9
6.9
0.95
6.55
6.2
4.10
4.2
38.4
24.05
32.2
17.4
2.9
0.55
2.65
2.3
1.45
1.6
14.0
20.0
31.5
18.2
5.10
0.20
4.95
4.8
3.0
4.35
34.2
17.8
29.8
14.7
3.05
0.10
2.9
2.8
1.85
2.2
15.6
22.5
31.8
17.5
3.0
0.85
2.7
2.4
1.40
1.5
14.4
20.3
35.05
13.9
2.60
0.25
2.3
2.2
1.4
1.95
15.6
Fig. 11. Rb-Y + Nb plots of Andaman tuff indicating the volcanic arc origin (after Pearce et al., 1984) VAG-volcanic arc
granite, ORG-ocean ridge granite, WPG-within plate granite,
COLG-syn-collision granite.
Schmincke, 1984). Dominance of juvenile constituents
in the form of such typical shaped glass shards and absence of blocky/angular glass shards may indicate
subaerial eruption rather than phreatomagmatic eruption
(Heiken, 1972; Heiken and Wohletz, 1985). The absence
of pumice and welding texture together with weak alteration of glass also negates the possibility of subaquous
eruption. Maintaining their original shape, glass shards
can be transported long distances in cold debris flows (cf.,
Cas and Wright, 1987).
Reworking and transport of pyroclastic flows in
subaqueous conditions rather than under subaerial conditions is inferred from features such as: (a) different
shapes of glass shards, characteristic of subaerial eruption; b) interbedding with nonvolcanic turbidite sequence;
(c) good sorting producing crystal rich and glass rich layers; (d) normal size grading in beds; e) absence of any
Fig. 12. A chemical plot of rare earth elements element of
Andaman tuff normalized against C1 chondrite (using MINPET
2.0 software).
volcanic vent in the form of dyke or plug nearby; and f),
absence of double grading (Wright and Mutti, 1981;
Cousineau, 1994; White, 2000). Sorting of pyroclasts can
be attained well through water in subaqueous deposition
rather than in air (Cashman and Fiske, 1991). The presence of dominant cuspate shards indicates subaerial eruption. All the features therefore, suggest that ash of
subaerial eruptions landed in the sea and was transported
subaqueously to form subaqueous deposits (cf., Cas and
Wright, 1987; Sigurdsson et al., 1991; Whitham, 1989).
During flow under these conditions, delicate shapes of
glass shards can also be maintained (cf., Cas and Wright,
1987; Cousineau, 1994). The supply of pyroclasts from
slumping of syndepositional subaerial eruption may also
be possible. In that case, recurrent ash falls with progressively finer material were deposited into the sea to form
repeated graded beds.
Transport and deposition
In facies A, massive beds with poorly developed grad-
Mio-Pliocene pyroclastic deposits from Andaman Islands, India
79
ing, poorly defined internal stratification and poor sorting are product of rapid deposition from subaqueous debris flows (Lowe, 1982; Nemec and Steel, 1984), more
common for debris derived from volcanoes (Fisher, 1984).
Crude normal grading, indicative of limited turbulence,
is a common feature of subaqueous debris flow rather than
in an ideal cohesive debris flow (Nemec and Steel, 1984).
High particle concentration inhibited development of traction bed form and caused massive deposition (cf., Wright
and Mutti, 1981). The ill-defined stratification towards
the top of tuff beds is caused by high suspended fall out
rate (Lowe, 1988) or by frictional freezing as high density sub flows developed at the top of the flow (Cousineau,
1994). This process was repeated to produce a stack of
massive beds.
In Facies B, changes up section from ill-defined normally-graded beds to parallel laminae are a product of
high to low concentration turbidity currents (Walker,
1978) which results both from decrease in grain size and
suspended fall out rate (Stix, 1991; Cousineau, 1994).
These features suggest this facies represents a medial
turbidite.
Facies C with features typical of classical turbidites
and dominance of bottom-truncated Bouma sequences
possibly represent a distal turbidite succession. The
graded beds with scoured base and other current structures indicate emplacement by a low concentration current with limited turbulent flow (Middleton and Hampton, 1973; Wright and Mutti, 1981). Recurrences of mud
with volcaniclastics indicate that normal deep-sea sedimentation was progressing almost continuously with repeated interruption by felsic debris. Subsequently ash
turbidites became less ash laden and the current more turbulent. With lowering of the fall out rate felsic debris progressed into distal turbidites and ultimately produced thin
ash beds. Recurrence of tuff beds in the form of Facies B
above facies C (e.g., in Havelock area,) suggest that tuffs
were derived by repeated eruption, and the influx of felsic
tuff interrupted deposition of carbonate turbidites.
Diagenetic alteration
Glass, the major constituent in Andaman tuff, is unstable and can easily be altered during diagenesis by reacting with pore water. In high-SiO2 glass shards, zeolite
in the form of clinoptilolite at first replaced glass and
then changed to analcime with progressive diagenesis (cf.,
Iijima and Utada, 1971; Iijima, 1986; Ogihara, 2000). Due
to microdissolution-precipitation with increasing burial,
clinoptilolite dissolved and analcime precipitated
(Oghihara and Iijima, 1990; Ogihara, 1996). The
diagenetic change of glass can be controlled by two factors: grain size and burial depth. XRD results show that
fresh green tuff is zeolite-free whereas lighter green tuff
c o n t a i n s c l i n o p t i l o l i t e a n d w h i t e t u ff c o n t a i n s
80
T. Pal et al.
clinoptilolite-analcime. Since green tuff and white tuff
more or less have the same grain size, the observed change
could be linked to burial diagenesis only. The formation
of clinoptilolite may indicate temperature range for
diagenesis as 40 to 55∞C (Iijima, 1986).
CONCLUSIONS
The pyroclastic deposit of the Archipelago Group
originated from subaerial eruption and the ash, after landing in the ocean, was transported and deposited by density currents. The deposit represents a subaqueous debris
flow in the lower part, and medial to distal turbidites in
the upper part. The Archipelago Group of rocks consisting of pyroclast deposits is present dominantly in the eastern part of the Andaman Islands. In the Andaman subduction complex, arc volcanoes are present in the eastern
part of the main Andaman Islands. Since present subduction at this Plate margin started during Cretaceous time,
and the predominant occurrence of tuff is restricted to
the eastern part of the main islands only, it may be surmised that inner arc volcanoes were also supplying felsic
magma during Mio-Pliocene time. Felsic volcanism during Mio-Pliocene age has also been reported from the
Southeast Asian Plate. In Sumatra, although short-lived
plutonism has been reported in the early Eocene, major
Cenozoic volcanic activity dates back to the Early
Miocene only. The recent volcanism from Barren Island
of Andaman Sea is basaltic to basaltic andesite in composition. Probably with time the arc volcanism changed
from felsic to basaltic composition.
Acknowledgments—The authors are thankful to Sri N. Das,
Dy. Director General, Op: WSA, Eastern Region, Geological
Survey of India (GSI) for his constant encouragement in the
course of the work. Assistances from Shri S. K. Bhaduri and
Dr. L. G. Mondal, Chemical Division, ER, GSI for respective
XRF and ICPOES studies; from Shri S. K. Shome for SEM
analysis; from Dr. N. C. Pant and Sr. A. Kundu, EPMA Laboratory, Faridabad, GSI for EPMA analyses, and Dr. Mrs. K.
Roychowdhury, Mineral Physics Division, GSI for XRD studies are thankfully acknowledged. Professor R. J. Arculus provided a constructive journal review.
REFERENCES
Anderson, A. T., Davis, A. M. and Lu, F. (2000) Evolution of
Bishop tuff rhyolitic magma based on melt and magnetite
inclusions and zoned phenocrysts. J. Petrol. 44(3), 449–
473.
Cas, R. A. F. and Wright, J. V. (1987) Volcanic Successions.
Allen & Unwin, London, 528 pp.
Cashman, K. V. and Fiske, R. S. (1991) Fallout of pyroclastic
debris from submarine volcanic eruption. Science 253, 275–
280.
Chakraborty, P. P. and Pal, T. (2001) Anatomy of a forearc sub-
marine fan: Upper Eocene-Oligocene Andaman Flysch
Group, Andaman Islands, India. Gond. Res. 4(3), 477–486.
Chakraborty, P. P., Pal, T., Dutta Gupta, T. and Gupta, K. S.
(1999) Facies pattern and depositional motif in an immature trench-slope basin, Eocene Mithakhari Group, Middle
Andaman, India. J. Geol. Soc. India 53, 271–284.
Cousineau, P. A. (1994) Subaqueous pyroclastic deposition in
an Ordovician forearc basin: An example from the SaintVictor formation, Quebec Appalachians, Canada. Jour. Sed.
Res. A64(4), 867–880.
Critelli, S. and Ingersoll, R. V. (1995) Interpretation of volcanic versus palaeovolcanic sand grains: An example from
Miocene deep marine sandstone of the Tonga Group (Southern California). Sedimentology 42, 783–804.
Curray, J. R. and Moore, D. G. (1974) Sedimentary and tectonic processes in the Bengal deep sea fan and geosyncline.
The Geology of Continental Margins (Burke, C. A. and
Drake, C. L., eds.), 617–627, Springer Verlag, New York.
Fischer, R. V. (1983) Flow transformation in sediment gravity
flows. Geology 11, 273–274.
Fischer, R. V. (1984) Submarine volcaniclastic rocks. Marginal
Basin Geology. Volcanic and Associated Sedimentary and
Tectonic Processes in Modern and Ancient Marginal Basin
(Kokelaar, B. P. and Howells, M. F., eds.), Spec. Publ. Geol.
Soc. London 16, 5–27.
Fischer, R. V. and Schmincke, H. U. (1984) Volcaniclastic sediment transport and deposition. Sediment Transport and
Depositional Processes (Pye, K., ed.), 351–388, Blackwell
Scientific Publications, Oxford.
Gass, I. G. (1990) Ophiolites and oceanic lithospheres. Proc.
Symposium, “Troodos1987” (Malpas, J., Moores, E. M.,
Panayiotou, A. and Xenophontos, C., eds.), Publ. Geological
Survey of Cyprus, 733 pp.
Hall, R. (2002) Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-drafted reconstructions, model and animations. J. Asian Earth Sci.
20, 353–431.
Heiken, G. and Wohletz, K. (1985) Volcanic Ash. University of
California Press, Berkeley, 256 pp.
Heiken, R. (1972) Morphology and petrography of volcanic
ashes. Geol. Soc. Am. Bull. 83, 1961–1988.
Iijima, A. (1986) Occurrence of material zeolites. Nendo
Kagaku, 26, 90–103.
Iijima, A. and Utada, M. (1971) Present day zeolite diagenesis
of Neogene geosynclinal deposits in the Niigata Oil Field
Japan. Avances in Chemistry Series 101, Molecular Sieve
Zeolites-1 (Gould, R. F., ed.), 342–349, American Chemical Society, Washington, D.C.
Karig, D. E., Suparka, S., Moore, G. F. and Hehanussa, P. E.
(1979) Structure and Cenozoic evolution of the Sunda arc
in the Central Sumatra Region. Geological and Geophysical Investigations of Continental Margins (Watkins, J. S.,
Montadert, L. and Dickinson, P. W., eds.), Mem. Am. Assoc.
Petrol. Geol. 29, 223–237.
Karunakaran, C., Ray, K. K. and Saha, S. S. (1967) A revision
of the stratigraphy of Andaman and Nicobar Islands, India.
Bull. Nat. Inst. Sci. Ind. 38, 4–36.
Le Bas, M. J., Maitre, R. W., Streckeisen, A. and Zanettin, B.
(1986) A chemical classification of volcanic rocks based
on the total alkali-silica diagram. J. Petrol. 27, 745–750.
Loomis, J., Weaver, B. and Blatt, H. (1994) Geochemistry of
Mississippian tuffs from the Ouachita mountains, and implications for the tectonics of the Ouachita Orogen, Oklhoma
and Arkansas. Geol. Soc. Am. Bull. 106, 1158–1171.
Lowe, D. R. (1982) Sediment gravity flows II. Depositional
models with special reference to the deposits of high-density turbidity current. Jour. Sed. Pet. 42, 775–793.
Lowe, D. R. (1988) Suspended load fallout rate as an independent variable in the analysis of current structure.
Sedimentology 35, 765–776.
Lowe, D. R. and LoPiccolo, R. D. (1974) Characteristics and
origin of dish and pillar structures. J. Sed. Petrol. 44, 484–
501.
McPhie, J., Doyle, M. and Allen, R. (1993) Volcanic textures:
a guide to the interpretation of textures in volcanic rocks.
CODES Key Centre, University of Tasmania, Hobart, 198
pp.
Middleton, G. V. and Hampton, M. A. (1973) Sediment gravity
flows: Mechanics of flow deposition. Soc. Econ. Paleont.
Miner. Short course 1, Anaheim, 1–38.
Mitchell, A. H. G. (1985) Collision-related fore-arc and backarc evolution of the Northern Sunda arc. Tectonophysics 116,
323–334.
Mukhopadhyay, M. (1988) Gravity anomalies and deep structure of the Andaman-arc. Marine Geophy. Res. 9, 197–210.
Nemec, N. and Steel, R. J. (1984) Alluvial and coastal conglomerates: Their significant features and some comments
on gravity mass-flow deposits. Sedimentology of Gravels
and Conglomerates (Koster, E. H. and Steel, R. J., eds.),
Canadian Society of Petrolium Geologists Memoir 10, 1–
31.
Ogihara, S. (1996) Diagenetic transformation of clinoptilolite
to analcime in silicic tuffs of Hokkaido, Japan. Mineralium
Deposita 31, 548–553.
Ogihara, S. (2000) Composition of clinoptilolite formed from
volcanic glass during burial diagenesis. Clays and Clay
Minerals 48(I), 106–110.
Ogihara, S. and Iijima, A. (1990) Exceptionally K-rich
clinoptilolite-heulandite group zeolites from three offshore
boreholes of Northern Japan. Eur. J. Mineral. 2, 819–826.
Pal, T., Dutta Gupta, T. and Dasgupta, S. C. (2002) Vitric tuff
from Archipelago Group of rocks (Mio-Pliocene) of South
Andaman. J. Geol. Soc. India 59, 111–114.
Pal, T., Chakraborty, P. P., Dutta Gupta, T. and Singh, C. D.
(2003) Geodynamic evolution of the outer arc-forearc belt
in the Andaman Islands, the central part of the Burma-java
subduction complex. Geol. Mag. 140(3), 289–307.
Pawde, M. B. and Ray, K. K. (1963) On the age of greywackes
in south Andaman. Science and Culture, 30, 279–280.
Pearce, J. A., Lippard, S. J. and Roberts, S. (1984) Characteristics and tectonic significance of supra-subduction zone
ophiolites. Marginal Basin Geology (Kokelaar, B. P. and
Howells, M. F., eds.), Spec. Publ. Geol. Soc. Lond. 16, 77–
94.
Ray, K. K. (1982) A review of the geology of Andaman and
Nicobar islands. Geol. Surv. Ind. Misc. Pub. 42(2), 110–
125.
Roy, D. K., Acharyya, S. K., Ray, K. K., Lahiri, T. C. and Sen,
Mio-Pliocene pyroclastic deposits from Andaman Islands, India
81
M. K. (1988) Nature of occurrence and depositional environment of the oceanic pelagic sediments associated with
the ophiolite assemblage, South Andaman Island. Indian
Minerals 42, 31–56.
Roy, T. K. (1983) Geology and hydrocarbon prospects of
Andaman and Nicobar. Petroliferous Basins of India
(Bhandari, L., Venkatachala, B. S., Kumar, R., Swamy, S.
N., Garga, P. and Srivastava, D. C., eds.), 37–53, Petrol.
Asia Jour., KDMIPE, ONGC, Dehradun.
Sigurdsson, H., Carey, S., Mandeville, C. and Bronto, S. (1991)
Pyroclastic flows of the 1883 Krakatau eruption. EOS,
American Geophysical Union Transactions 72, 377–381.
Sparks, R. S. J. and Walker, G. P. L (1977) The significance of
vitric enriched air-fall ashes associated with crystal enriched
ignimbrites. J. Volcanol. Geotherm. Res. 2, 329–341.
Srinivasan, M. S. (1988) Late Cenozoic sequences of AndamanNicobar islands; their regional significance and correlation.
Indian J. Geology 60(1), 11–34.
Stix, J (1991) Subaqueous, intermediate to silicic-composition
82
T. Pal et al.
explosive volcanism: a review. Earth Science Reviews 31,
21–53.
Walker, R. G. (1978) Deep-water sandstone facies and ancient
submarine fans: Models for exploration for stratigraphic
traps. Am. Assoc. Petrol. Geol. Bull. 62, 932–966.
White, J. D. L (2000) Subaqueous eruption-fed density currents
and their deposits. Precambrian. Res., 101, 87–109.
Whitham, A. G. (1989) The behaviour of subaerially produced
pyroclastic flows in a subaqueous environment: evidence
from the Roseau eruption, Dominica, West Indes. Marine
Geology 86, 27–40.
Wohletz, K. H. (1983) Mechanisms of hydrovolcanic pyroclast
formation: grains size, scanning electron microscopy, and
experimental studies. J. Volcanol. Geotherm. Res. 17, 31–
63.
Wright, J. V. and Mutti (1981) The Dali ash, islands of Rhodes,
Greece: A problem in interpreting submarine volcanogenic
sediments. Bull. Volcanol. 44, 153–167.