1. IntroductionThe Andaman, Nicobar

1. Introduction
The Andaman, Nicobar, Nias and Mentawai islands
represent the subaerial parts of the forearc ridge in the
Indonesian island-arc system (Fig. 1A). An oceanic trench
(Andaman-Java trench) borders the forearc ridge to the west
of the Andaman Islands, extending southwards into
Indonesia. To the east of the forearc the Quaternary
Narcondum and Barren Island volcanoes rise from a
submarine ridge with a parallel arcuate trend. Barren Island
has exploded very recently (Halder et al., 1992). At the
present time the arc and forearc are separated from
continental mass of the Burma–Thailand – Malaya Peninsula by the Andaman backarc marginal sea. Palaeogeographic reconstructions of the Andaman area indicate that
the Andaman Sea did not exist during the Eocene–
Oligocene period, but began opening only in the Middle
Miocene (Curray et al., 1982; Hall, 1996).
The Andaman arc-trench system has been described as
the result of the active subduction of the Indian oceanic
lithosphere below the Eurasian plate along the AndamanJava trench, from the Cretaceous to the present day (Curray
and Moore, 1974; Karig et al., 1979, 1980; Samuel and
Harbury, 1996). Ophiolite occurrences in Andaman Island
are interpreted as thrust slices of oceanic crust scraped off
the subducting slab at the toe of the accretionary complex
(Moore et al., 1982). Accretion of these thrust sheets,together with thick Palaeogene and Neogene sediments
from the Andaman-Java trench, caused the formation of a
wide accretionary prism, leading to the construction of a
high forearc ridge (Andaman-Nicobar Islands). In the
Andaman Islands, ophiolites of late Cretaceous – Paleocene
age occur as dismembered units in subhorizontal thrust
slices, forming the basement for the Tertiary sediments in
these islands (Sengupta et al., 1990). Middle Eocene to MioPliocene sediments formed in varied tectonic and sedimentary environments were deposited unconformably on the
accreted ophiolite. These sediments are classified in
ascending order into the Mithakhari, Andaman Flysch and
Archipelago Groups (Table 1).
The Mithakhari Group (,1400 m thick), consisting of
melange rocks and interstratified conglomerate, sandstone,
siltstone and shale, is classified into three Formations
(Karunakaran et al., 1968). These are, in ascending order,the Lipa Black Shale, the Hope Town Conglomerate and the
Namunagarh Grit.
The Lipa Black Shale, which includes melange containing rock fragments of diverse lithology and age enclosed in
sheared argillite matrix, has been interpreted as an
olistostromal unit (Acharyya et al., 1989). Melanges from
Nias and eastern Indonesia, similar to those in the Andaman
Islands are inferred to have been formed diapirically (Barber
et al., 1986; Samuel et al., 1997). In Nias, melange blocks
and matrix were derived from all the stratigraphic units,
ranging from Cretaceous to Plio-Pleistocene, exposed on the
island. Mud volcanoes, which extrude mud and rock
fragments at the surface indicate that diapirism still
continues on Nias today. Recent mud volcanoes are also
recorded from Andaman Islands. Samuel et al. (1997)in
their revised stratigraphy of Nias Islands do not show the
melange as a stratigraphic unit. If it was formed in the same
fashion, the melange in the Lipa Black Shale should not be
regarded as a stratigraphic unit.
The Hope Town Conglomerate comprises steeply
dipping, laterally discontinuous, thrusted and faulted strata,
chiefly made up of polymictic conglomerates, subordinate
sandstones, minor shale. Interbedded shale and thin, but
laterally persistent, beds of coal are minor components of
the Mithakhari Group.
The Namunagarh Grit is chiefly made up of subhorizontal to low dipping, more or less continuous and
relatively less deformed strata consisting predominantly of
pebbly and coarse- to fine-grained, green greywacke
sandstones with minor conglomerates and shales.Acharyya
et al. (1989) asserting that the latter two formations are
composed of ophiolite-derived clastics, merged these two
units and described them as the Namunagarh Formation.
Deposition of the Mithakhari sediments in small faultcontrolled trench-slope basins under submarine slope, shelf
and alluvial plain environments are inferred (Ray, 1982;
Chakraborty et al., 1999).
The Oligocene Andaman Flysch, consisting of thick and
extensive occurrences of greywacke sandstones, showing
the profuse development of Bouma sequences, is interpreted
as composed of turbidites formed in forearc submarine fans
(Bandopadhyay and Ghosh, 1998a; Chakraborty and Pal,
2001).
The Pliocene Archipelago Group consists of crossbedded mature sandstones, siltstones, bluish white claystones and carbonate turbidites, formed on shallow marine
shelves (Ray, 1982). Reworked vitric tuffs are reported from
the Archipelago Group (Pal et al., 2002).
Based on records of larger foraminifera and calcareous
algaeRoy et al. (1988)assigned a Middle-Late Eocene age
to the Mithakhari Group. However, clasts of foraminiferal/
algal limestones, common in the conglomerates and
sandstones of the Mithakhari Group, are reworked.
Similarly, on Nias Island off Sumatra, the Oyo Formation,
which resembles the Mithakhari Group, was assigned an
Eocene age from algal/foraminiferal limestone fragments.Later detailed sedimentological and stratigraphic work
(Samuel et al., 1995, 1997; Samuel and Harbury, 1996)
revealed that both reworked Eocene fauna and younger
microfossils are present in the Oyo Formation. The younger
microfossils within the Oyo Formation indicate that it is of
upper Oligocene to Lower Miocene age. It is possible that
the Mithakhari Group is of a similar age.
Pebbly and coarse-grained sandstones, making up the
bulk of the Namunagarh Grit, are best developed near
Namunagarh, South Andaman (Fig. 1A). These rocks are
described as greywacke, composed of rock fragments
(andesite, basalt, quartzite and limestones), plagioclase
and quartz, embedded in green chloritic matrix. The genesis
of these rocks has long been attributed to the weathering and
erosion of an accreted ophiolite (Acharyya et al., 1988,
1989; Acharyya, 1997). Acharyya (op.cit.) and his
colleagues considered that the ophiolites and the Mithakhari
rocks have no genetic relationship to the Andaman-Java
subduction system, but were emplaced over the Andaman
Flysch as thrust packages, from a distal source further to
the east of the Andaman Islands. Curray (1992)did not
accept this suggestion of ophiolite emplacement from the
east. The absence of evidence of arc volcanism, common in
arc-trench systems, in the Palaeogene sediments of the
Andaman Islands, may have led to the development of
this proposal. The history of Tertiary sedimentation in the Andaman-Java accretionary prism still remains a
contentious issue.

ผลลัพธ์ (อังกฤษ) 2:

1. Introduction
The Andaman, Nicobar, Nias and Mentawai Islands
represent the subaerial Parts of the forearc Ridge in the
Indonesian Island-Arc System (Fig. 1A). An Oceanic trench
(trench Andaman-Java) Borders forearc the Ridge to the West
of the Andaman Islands, extending Southwards Into
Indonesia. To the East of the forearc the Quaternary
Narcondum and Barren Island Volcanoes rise from a
Submarine Ridge with a parallel arcuate Trend. Barren Island
has exploded very recently (Halder et al., 1992nd). At the
present time and the Arc forearc are Separated from
the Burma-Thailand of mass Continental - Malaya Peninsula by the Andaman Sea Backarc marginal. Palaeogeographic reconstructions of the Andaman Area indicate that
the Andaman Sea did not exist during the Eocene-
Oligocene period, but only began opening in the Middle
Miocene (Curray et al., 1982; Hall, one thousand nine hundred ninety-six).
The Andaman Arc-trench System has been. described as
the Result of the active Subduction of the Indian Oceanic
lithosphere Below the Eurasian Plate along the AndamanJava trench, from the Cretaceous to the present Day (Curray
and Moore, 1 974; Karig et al., 1 979, 1980; Samuel and
Harbury, one thousand nine hundred and ninety-six. ). Occurrences ophiolite in Andaman Island
are interpreted as thrust slices of Oceanic Crust scraped off
the subducting slab at the Toe of the Accretionary Complex
(Moore et al., 1,982th). Accretion of these thrust Sheets, Palaeogene and Neogene sediments Together with Thick
from the Andaman-Java trench, caused the Formation of a
Wide Accretionary prism, Leading to the Construction of a
High Ridge forearc (Andaman Nicobar Islands-). In the
Andaman Islands, Late Cretaceous ophiolites of - Paleocene
Age occur as dismembered Units in Subhorizontal thrust
slices, Forming the Basement for the Tertiary sediments in
these Islands (Sengupta et al., in 1990). MIDDLE Eocene sediments to MioPliocene formed in sedimentary environments and varied Tectonic were deposited unconformably on the
accreted ophiolite. These sediments are classified in
Ascending Order Into the Mithakhari, Andaman flysch and
Archipelago Groups (Table 1).
The Mithakhari Group (, one thousand four hundred M Thick), consisting of
Melange Rocks and Interstratified conglomerate, Sandstone,
siltstone and shale, is classified Into Three Formations.
(Karunakaran et al., 1968). These are, in Ascending Order, the Lipa Black Shale, the Hope Town Conglomerate and the
Namunagarh Grit.
The Lipa Black Shale, which includes Melange containing Rock fragments of diverse widowogy and Age enclosed in
sheared Argillite Matrix, has been interpreted as an
Olistostromal UNIT. (Acharyya et al., 1989). The Mélanges from
Nias and Eastern Indonesia, similar to those in the Andaman
Islands are inferred to have been formed Diapirically (Barber
et al., 1,986; Samuel et al., the 1997th). In Nias, Melange Blocks
and Matrix were all derived from the stratigraphic Units,
ranging from Cretaceous to Plio-Pleistocene, exposed on the
Island. Mud Volcanoes, which extrude Mud and Rock
fragments at the surface indicate that Diapirism still
Continues on Nias today. Mud Volcanoes are also recent
Recorded from Andaman Islands. Samuel et al. (1 997) in
their revised Stratigraphy of Nias Islands do not Show the
Melange as a stratigraphic UNIT. If it was formed in the Same
Fashion, the Melange in the Lipa Black Shale should not be
regarded as a stratigraphic UNIT.
The Hope Town Conglomerate comprises steeply
dipping, Laterally discontinuous, thrusted and faulted strata,
chiefly Made up of Polymictic conglomerates, subordinate
sandstones. , minor shale. Interbedded shale and thin, but
Laterally Persistent, beds of Coal MINOR are components of
the Mithakhari Group.
The Namunagarh Grit is Made up chiefly of Subhorizontal to low dipping, more or less continuous and
relatively less Deformed strata consisting predominantly of
Pebbly and Coarse- to. Fine-grained, Green greywacke
sandstones and conglomerates with MINOR Shales.Acharyya
et al. (One thousand nine hundred eighty-nine) asserting that the latter Two formations are
composed of ophiolite-derived clastics, merged these Two
Units and described them as the Namunagarh Formation.
Deposition of the Mithakhari sediments in Small Faultcontrolled trench-Slope basins under Submarine Slope, Shelf
and alluvial Plain environments. are inferred (Ray, in 1982;
Chakraborty et al., 1,999th).
The Oligocene Andaman flysch, consisting of Thick and
Extensive Occurrences of greywacke sandstones, showing
the Profuse Development of Bouma sequences, is interpreted
as composed of turbidites formed in forearc Submarine fans
(. Bandopadhyay and Ghosh, 1998a; Chakraborty and Pal,
in 2001).
The Group consists of Crossbedded mature Archipelago Pliocene sandstones, siltstones, bluish White Claystones and carbonate turbidites, formed on shallow Marine
Shelves (Ray, 1982). Vitric reworked tuffs are reported from
the Archipelago Group (Pal et al., 2,002th).
Based on records of larger foraminifera and calcareous
AlgaeRoy et al. (1988) assigned a Middle-Late Eocene Age
to the Mithakhari Group. However, clasts of Foraminiferal /
algal limestones, common in the conglomerates and
sandstones of the Mithakhari Group, are reworked.
Similarly, on Nias Island off Sumatra, the Oyo Formation,
which resembles the Mithakhari Group, was assigned an
Eocene Age from algal / Foraminiferal. detailed sedimentological and stratigraphic limestone Fragments.Later Work
(Samuel et al., 1,995th, 1,997th; Samuel and Harbury, 1,996th)
revealed that both reworked Eocene fauna and younger
microfossils are present in the Oyo Formation. The younger
microfossils Within the Oyo Formation indicate that it is of
Upper Oligocene to Lower Miocene Age. It is possible that
the Group is of a similar Mithakhari Age.
Pebbly and coarse-grained sandstones, Making up the
Bulk of the Namunagarh Grit, are developed near Best
Namunagarh, South Andaman (Fig. 1A). Rocks these are
described as greywacke, composed of Rock fragments
(andesite, basalt, quartzite and limestones), plagioclase
and Quartz, embedded in Chloritic Green Matrix. The Genesis
of these has long been attributed to the Rocks weathering and
erosion of an ophiolite accreted (Acharyya et al., 1,988th,
1989; Acharyya, one thousand nine hundred and ninety-seven). Acharyya (Op.cit.) And his
colleagues considered that the ophiolites and the Mithakhari
Rocks have no Genetic Relationship to the Andaman-Java
Subduction System, but were emplaced over the Andaman
thrust flysch as Packages, from a distal Source further to
the East of. the Andaman Islands. Curray (1,992th) did not
accept this Suggestion of ophiolite Emplacement from the
East. Evidence of the absence of Arc volcanism, common in
Arc-trench Systems, in the Palaeogene sediments of the
Andaman Islands, have LED May to the Development of
this proposal. History of the Tertiary sedimentation in the Andaman-Java Accretionary prism still remains a
contentious Issue.

ผลลัพธ์ (อังกฤษ) 3:

1. Introduction
The, Andaman Nicobar Nias and, Mentawai islands
represent the subaerial parts of the forearc ridge in the
Indonesian. Island - arc system (Fig. 1A). An oceanic trench
(Andaman-Java trench) borders the forearc ridge to the West
of the Andaman. Islands extending southwards, into
Indonesia. To the east of the forearc the Quaternary
Narcondum and Barren Island volcanoes. Rise from a
.Submarine ridge with a parallel arcuate trend. Barren Island
has exploded very recently (Halder et al, 1992). At the
present. Time the arc and forearc are separated from
continental mass of the Burma - Thailand - Malaya Peninsula by the Andaman backarc. Marginal sea. Palaeogeographic reconstructions of the Andaman area indicate that
the Andaman Sea did not exist during the. Eocene -
.Oligocene period but began, opening only in the Middle
Miocene (Curray et al, 1982; Hall 1996).
The, Andaman arc-trench. System has been described as
the result of the active subduction of the Indian oceanic
lithosphere below the Eurasian plate. Along the AndamanJava trench from the, Cretaceous to the present day (Curray
and, Moore 1974; Karig et al. 1979 1980;,,, Samuel, and
Harbury 1996).Ophiolite occurrences in Andaman Island
are interpreted as thrust slices of oceanic crust scraped off
the subducting slab. At the toe of the accretionary complex
(Moore et al, 1982). Accretion of these thrust sheets together with, thick Palaeogene. And Neogene sediments
from the Andaman-Java trench caused the, formation of a
wide accretionary prism leading to, the construction. Of a
.High forearc ridge (Andaman-Nicobar Islands). In the
Andaman Islands ophiolites of, late Cretaceous - Paleocene
age occur. As dismembered units in subhorizontal thrust
slices forming the, basement for the Tertiary sediments in
these islands (Sengupta. Et al, 1990). Middle Eocene to MioPliocene sediments formed in varied tectonic and sedimentary environments were deposited. Unconformably on the
.Accreted ophiolite. These sediments are classified in
ascending order into, the Mithakhari Andaman Flysch and
Archipelago. Groups (Table 1).
The Mithakhari Group (, 1400 m thick), consisting of
melange rocks and interstratified conglomerate sandstone
siltstone,,, And shale is classified, into three Formations
(Karunakaran et al, 1968). These are in order, ascending, Lipa the Black. Shale.