Gondwana Research, L? 4, No. 3, p p . 477-486. 0 2001 International Association for Gondwana Research, Japan. ISSN: 1342-937X

Anatomy of a Forearc Submarine Fan: Upper Eocene - Oligocene Andaman Flysch Group, Andaman Islands, India

Partha Pratim Chakraborty' and Tapan Pal2

Department of Applied Geology, Indian School of Mines, Dhanbad - 826 004, India Geological Survey of India, Eastern Region, Calcutta - 700 016, India

(Manuscript received July 10,2000; accepted Janua y 10,2001)

Abstract

Detailed facies analysis in the upper Eocene-Oligocene Andaman Flysch Group reveals fourteen different facies, grouped into three different associations. These facies associations represent two anatomical divisions of a submarine fan, viz. inner fan and depositional lobe in middle fan. Paleocurrent pattern, high percentage of quartz in the sandstones and outer arc derived clasts in the inner fan channel conglomerates suggest dual sediment supply in this fan. A longitudinal geometry of the basin has been inferred where juxtaposition of sediments of different proximality deterred development of any definite sequence pattern. A tectonically induced active sediment supply in a rising sea level stand is thought to be responsible for development of the fan.

Key words: Andaman flysch, facies, submarine fan, paleocurrent, sequence.

Introduction

Deep sea fans, both modern and ancient, have been the focus of sedimentological study for last three decades (Mutti and Ricci Lucci, 1972; Middleton and Bouma, 1973; Shanmugam and Moiola, 1988). Descriptions of submarine fans in active-margin setting are also available in literature (Ito, 1998). Turbidite package that occur as channel-lobe association is thought to represent a submarine- fan sequence in rock record. Unlike fans where lobes are directly attached to the feeder channels, the record of fans with detached lobes is very meager in geologic record (Shanmugam and Moiola, 1988).

Despite geological investigations in Andaman islands for last four decades the thick (3150m) sedimentary sequence (Roy, 1983) exposed in these islands (viz. Mithakhari Group, Andaman Flysch Group and Archipelago Group) received very little sedimentological attention except for broad lithological description. Only recently, Chakraborty et al. (1999) attempted a detailed facies analysis for Mithakhari sediments, and Bandopadhyay and Ghosh (1998) provided a petrographic account for the sedimentary formations.

The late Eocene-Oligocene (Pawde and Ray, 1963) Andaman Flysch Group (ca. 750m thick) exposed in isolated patches over a strike length of 270 km in

Andaman group of islands extending from north to south, Andaman offers an opportunity to study a submarine fan sequence with development of detached lobes in its mid fan part. Juxtaposition of these sediments in an outer arc (Sengupta et al., 1990) prompted the earlier workers to suggest a forearc environment for these sediments. Required paleoenvironmental analysis, based on depositional facies identification and comprehensive basin evolution are, nonetheless, yet to be done. This paper deals with the analysis of submarine fan facies and classification of fan subenvironments in order to bring out the paleogeography and basinal geometry against the regional tectonic backdrop. Because of overall scarcity of good exposures of Andaman Flysch Group of rocks the work was done in isolated stretches along the coastal tracts of Shibpur-Kalipur section of North Andaman, Corbyn's Cove section of South Andaman and Smith island of North Andaman (Fig. 1). Steep dip of beds (> 80°) allowed study of thick vertical sequences within short width of the belts.

Geological Background

Andaman group of islands represent the subaerial part of subduction zone complex having active subduction of the Indian plate on its west along the Java trench (Curray and Moore, 1974). Dominant sedimentary prisms and

478 P.P. CHAKRABORTY AND ‘r. PAL

STRATIGRAPHIC UNIT (GROUP) AGE

Archipelago Pliocene lo Miocene

- Uncunlormity - Oligocene to Upper Zooene

Andaman Flysch

- - Unconlormable / Tranriiional

Miakhari Middle lo Lower Eocene

- Teclonic I Unconlormable -

Cretaceous Ophiolie

GENERAL STRATIGRAPHY OF THE AREA

(after Ray, 1982)

local thrust emplaced ophiolite slices and olistoliths (Ray, 1982) in the subduction environment indicate that the islands represent parts of an outer arc.

Andaman Flysch Group, an important stratigraphic subdivision of the accumulated sediment pile, overlies and onlaps the Mithakhari Group (Fig. l), a lithopackage consisting of dismembered ophiolite bodies and laterally impersistent, highly immature sediments, belonging to trench inner slope (Chakraborty et al., 1999). Unconformably the flysch sediment is overlain by the Archipelago group of sediments, Miocene-Pliocene in age, comprised of biostromal and biohermal limestones, siltstones and chalks of outer shelf to open marine origin (Roy, 1983).

Presence of Nummulites atacicus and Assilina papiliata indicate middle Eocene age for Mithakhari sediments. Assemblage of small tests of radiolaria, silicoflagelletes, sponge spicules, small planktonic foraminifera and rare diatoms in siltstone and chalks identify lower Miocene age for Archipelago Group. Sandwiched between these two Oligocene age has been tentatively assigned for Andaman Flysch sediments.

Facies Analysis

Laterally persistent (over 100s of metres), numerous Bouma cycles and profusion of fluidisation structures characterize the Andaman Flysch Group. Facies are defined in Andaman Flysch sediments based on

Fig. 1. Location map of different studied sections of Andaman Flysch Group of sediments, Andaman islands. Regional tectonic set-up is shown in the inset. Generalised stratigraphic column is shown on the right.

sedimentary features observed in field viz. rock type (conglomerate, sandstone, shale, limestone), texture (matrix-supported or clast-supported) and sedimentary structures including types of grading (inverse, normal or ungraded). A total of 14 constituent facies types have been recognized and their characteristics and interpretations are summarized in table 1.

Facies Associations and their Paleoenviron- mental Significance

Facies associations are defined on the basis of abundance, geometry and stacking pattern of the constituent facies and their compatibility with actualistic arc-trench depositional model (Macdonald, 1993). The absence of any shallow-water features, storm layers and bioturbation precludes a shallow water deposition for the Andaman Flysch sediments. Mutual relationship between different facies types and motif of facies succession development pattern in three studied sections represent two anatomical subdivisions of a submarine fan (viz. inner or upper fan and middle fan) and basin plain.

a) Facies association I : inner or upper fan

section of North Andaman is characterised by

to fine grained facies,

This facies association present in Shibpur-Kalipur

i) wide spectral lithologic variation from very coarse

ii) abrupt vertical facies transitions, iii) close association of very coarse (bouldery) inferred

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FOREARC SUBMARINE FAN, ANDAMAN ISLAND 479

Table 1. Description and interpretation of facies comprising the Andaman Flysch Group.

Facies class Lithology Facies class Description Interpretation designation subdivision

A Limestone

Shaley mudstone

Laminated sandy/ silty mudstone

B

C

D

E

F

Fine sandstone alternate with silty mudstone

Massive sandstone

Pebbly sandstone

Parallel stratified sandstone

Cross stratified sandstone/siltstone

Matrix - supported conglomerate

Clast supported conglomerate

Boulder conglomerate

A1

A2

A3

B1

C 1

c 2

C3

c4

D1

D2

E l

E2

F1

F2

Grey biomicrite with randomly oriented shell fragments, presence of glauconites Grey homogenous shale, silty mudstone, massive with very rare graded interbeds of sandstone. Presence of foraminifera1 and molluscan fossils Variegated colour, heterogenous silty mudstone, parallel laminated with common plane or ripple laminated sandstone/ siltstone Ungraded to feebly graded grey, fine sandstone alternating with black mudstone containing wispy silt interlaminae. Sandstone with sharp, flat or irregular basal contacts, internally plane or ripple laminated. Thick beds of light grey noncalcareous to slightly calcareous fine to medium grained amalgamated sandstone. Interbedded with subfacies B1. Even and parallel sided beds. Shale clast horizons commonly above and oriented paralie1 to lower bed contacts. Normally graded, channel geometry with scoured sole features flow dispersion, Horizontal ungraded stratifications, amalgamation common Horizontal, inverse graded stratifications

Small scale planar cross .- stratifications (av. set thickness 1.5 cm) of climbing nature Small scale (set thickness 1.2 cm) trough cross strata Normal, coarse tail grading and sharp scoured basal contact Ungraded to inverse coarse tail grading, flat base. Absence of erosional features Graded stratified, with vertical normal grading and lateral gradation from a core of massive clast supported conglomerate to finer stratified sandstone Ungraded to normally graded, matrix infilled beds. Basal contact shallow scoured, commonly with sediment injection features. Normal grading, if present, is observed only in umer bed uortion.

Low energy, below wave base

Rapid suspension fallout with intermittent sandy dispersion from waning turbulent flow, below wave base Fine grained low concentration silty turbidites below wave base

Low energy environment, with intermittent incursion of tractive currents transporting sand across a muddy substrate followed by suspension fall out. Very rapid deposition from sandy dispersions associated with waning turbulent flow in a high energy flood condition

High concentration turbulent flow dispersion or hyperconcentrated flood

Upper flow regime tractive transport of sand grains Frictional freezing of high density traction carpet at the base of a large scale turbidity current Lower flow regime condition, deposited from linear crested dune bedfoms Traction current deposition from sinuous crested dune bedforms Channelised high concentration turbulent flow dispersion Rapid freezing in a debris flow deposition

High concentration deposition from turbulent gravel flows proximal to source

Boulder lags from sandy debris flows proximal to source

high density turbidity current or debris flow channelised deposits (Fig. 2) and fine grained inferred overbank deposits,

iv) overall tendency of upward decrease in maximum depth (from 2.3m to 0.4m) and increase in width : depth ratio (from 0.02 to 0.07) of the channels (Fig. 3),

v) occurrence of poorly sorted and rounded large (max. diam. 89cm) clasts of basalt, chert and limestone in the conglomerates indicating nearby provenance,

vi) occurrence of thick matrix-supported debris flow units with non erosional base,

vii) dominant northeastward paleocurrent pattern (Fig. 4a) and

vi) abundance of syndepositional deformational features such as sand volcano and fluidisation pipe. In undisturbed parts, however, there is excellent preservation of strata and laminations within strata.

A channel - levee complex is envisaged for the

Gondwana Research, V. 4, No. 3, 2001

480 P.P. CHAKKABORTY AND T. PAL

Fig. 2. Coarse grained channel-fill in facies association 1. Note the pebbly/bouldary base and the relatively finer (pebbly sandstone) top (hammer length 28 cm).

Ungraded cionglomerate

Graded granular sandstone Reverse graded conglomerate p 3

Fig. 3 . Detailed lithologs measured along Shibpur - Kalipur stretch, North Andaman. Note upward reduction in depth of channels correlated selectively over the entire stretch.

n = 25 n = 1 3

Fig. 4. Paleocurrent pattern in facies associations 1 (a), I1 (11) and 111 (c). Note distinct northeastern mode in inner fan part, while the major direction is broadly southward.

association. Such a complex typically develops either on inner submarine fan or on basin plains. On mid fans or outer fans sediments are much less differentiated between channel and overbank. Frequent juxtaposition of contrasting lithologies precludes the idea of basin plain paleogeography for this facies association. Upward shallowing and widening of the channels (Fig. 3 ) indicates progressive channel abandonment (cf. Shanmugam and Moiola, 1988). Presence of channel forms, proximal source of conglomerates, wide variability in conglomerates in terms of their bed thickness and flow behaviour point towards an upper fan channel - levee set-up for deposition. High coarse : fine grained facies ratio (1.3 : 1) and clast - supported nature of poorly sorted conglomerates strengthens this contention. While conglomerates and pebbly sandstones dominate the channel fills, sandstone, siltstone and shale (facies A3, B1,C3 and D l ) , volumetrically minor in proportion,

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FOREARC SUBMARINE FAN, ANDAMAN ISLAND 48 1

represent channel margin deposits formed by overbank processes. The finer grained levee products are occasionally breached by ungraded clast supported conglomerates of non-erosive nature (facies E2) or thinly bedded boulder conglomerate of both normal (Fig. 5) and reverse graded nature (facies F1 and F2). These are products of either debris flows or high concentration turbidity current (Dickie and Hein, 1995) generatedduring the periods of high discharge (Chakraborty et al., 1999).

Fig. 6. Laterally persistent parallel sided beds in facies association I1 (exposure length along left of the photograph 120m).

Fig. 5. Thinly bedded normal graded boulder conglomerate in facies association I (hammer length 28cm).

Channel conglomerates, in general, grade normally to pebbly sandstone which in rare cases show imbricated clasts and poorly defined cross laminae (av. set thickness 32cm). Presence of normal grading and basal scouring in the conglomerates suggest deposition from turbulent high concentration flow dispersion (Dickie and Hein, 1995). Multiple channel-fill succession of equally coarse grained material in an exposure of 45m width in the present section suggests channel instability.

b) Facies association II: mid fan lobe

of South Andaman is characterised by

A3,Bl,Cl,C2,C3 and D I ,

in the scale of 100s of meters (Fig. 6),

This facies association I1 in the Corbyn’s Cove section

i) predominance of relatively finer grained facies types

ii) parallel sided sandstone beds of lateral continuity

iii) presence of well defined Bouma cycles (T a r Tbe,

iv) absence of basal channeling and frequent

v) stacked sheet sand deposits with no distinct vertical

vi) grain size range of sandstones varying between

vii) occurrence of laterally persistent slumped layers

T *> Tdr’ Tb,) (Fig. 71,

amalgamation of sandstone beds,

variation in bed thickness (Figs. 8a,b),

medium to fine sand grade,

(Fig. 91,

Fig. 7. Tab divisions of Bouma cycle present in lobe sandstone interbeds in facies association I1 (Ta : massive graded division, Tb: plane laminated division).

viii) presence of flute marks and tabular scours at the sole of sandstone beds and

ix) paleocurrent direction derived from sole features point broadly towards south (Fig. 4b).

Non channelised nature, stacked sheet sand deposits with wide lateral continuity of beds and domination of bottom truncated Bouma cycles in this facies association represents depositional lobe in a distal mid-fan setting (O’Connell et al., 1985). Development of a depositional lobe in mid-fan part is caused by rapid deposition from the current as it leaves the confines of the leveed fan channel of the upper fan. Due to paucity of exposures, documentation of channel - lobe transition could not be made possible in the present case. In an active margin setting, such as the present one, detachment and formation of lobe at the base of slope is triggered by narrow shelf and sediment bypassing on the upper slope. The sheet sandstone units in figure 6 are products of depositional lobe, while the fine grained intervening facies with thin turbidite beds represent lobe fringe environment. Lack of any definite cyclicity pattern and absence of

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482 PI? CHAKRABORTY AND T. PAL

a 48 m

97 m

68 m

1 146.5 m 1955m.

b 12

10

t * E.

$ 6 e 4

2

0 26 51 76 101 l m 151 176 226

Bed Number -

I N D E X

Plane laminated shale

Ripple laminaled m . siltstone

Plane laminaled sandstone

Slump folded sandslone

Massive sandstone

'Exposure gap

Fig. 8. Detailed litholog measured at Corbyn's Cove (a), South Andaman. Plot of bed number vs. sandstone bed thickness up the section; note absence of cyclicity in bed thickness variation (b).

channels or proximal facies up the measured section precludes the idea of progradational development and suggest aggradational nature of this lobe (Ricci Lucci and Valmori, 1980). Such aggradational nature is common in lobes detached from channels (Mutti, 1985). Shanmugam and Moiola (1988), however, argued that both progradation and aggradation are responsible for lobe formation, specially in active margin settings.

C) Facies association 111: basin plain

This facies association, exposed in Smith island section of North Andaman, is characterised by the predominance of hemipelagic mudstone with interbedded thin

Gondwana Research, V. 4, No. 3,2001

FOREARC SUBMARINE FAN, ANDAMAN ISLAND 483

sandstone/siltstone beds exhibiting classical distal turbidites (facies types Al , A2 and B l ) . These fine grained turbidite beds contain regular, irregular and lenticular silt laminae (2 - 10 mm) and thin sand beds ( > l o mm) alternating with mud veneers. Features which characterize this association are

i) lack of channels and large scale scours at the base of sandy/silty interbeds,

ii) absence of any erosional sole feature, iii) presence of load casts and mud injection (flame)

structures, iv) facies distribution similar to those described as ‘mud

turbidite’ by Piper (1978) or ‘partial Bouma sequence’ of Walker (1 967) and

v) southward paleocurrent with wide dispersion (Fig. 4c). A depositional set-up away from the channel axes on the abyssal plain is suggested.

The occasional relatively coarser grained graded sandstone beds in this setting may be the products of either low-density turbidity currents or buoyant plumes (hypopycnal flow suspension fall out). Distinction of these sediments from those of inner fan is based on continuity of bedding, lack of slump features and local presence of thin sandstone beds resulting from unusual flows that partially bypassed the fan. Thinning- and fining - upward hemicycles (Fig. 10) at the basal part of this facies association reflect rise in relative sea level. Intervened by a thick (ca. 9m) shale - limestone interval (facies A1 and A2) these basal fining-upward hemicycles are overlain by coarsening-upward hemicycles defined by thickening of siltstone/sandstone interbeds.

Petrography Microscopic study of Andaman Flysch sandstones reveal

their poor sorting and matrix-supported (> 15%) wacke- type nature (cf. Pettijohn et al., 1973). C1ast:matrix ratio varies from 2.3 in Shibpur-Kalipur section to 0.818 in

Fig. 9. Laterally persistent slump layers in facies association 11 [pen (encircled) length 14cml.

Corbyn’s Cove section. The framework includes dttrital grains of quartz, lithic fragments, feldspar and pyroxene. Modal abundance of detrital grains show quartz content of these sandstones vary from 25.35% to 36.7% and hence classified as quartzwacke (Pettijohn, 1984). Quartz grains, both monocrystalline and polycrystalline, show corroded grain boundaries. While monocrystalline grains with abraded silica overgrowth indicates recycling; those with straight to slightly undulose extinction are, in the present tectonic setting, believed to be derived from plutonic source (Folk, 1974; Abdal - Wahab, 1992). Plutonic origin is also conceived for few polycrystalline quartz grains where two crystals are found with straight to slightly curved intercrystalline boundaries. The other varieties of polycrystalline quartz grains present are i) grains with elongated and crenulated crystals, having sutured crystal - crystal boundaries, ii) grains with straight intracrystalline boundaries and iii) grains with numerous, elongated silt- sized quartz grains, having mutually adjusted boundaries. These grains with ten or more individual crystals are thought to be an excellent indicator of metamorphic source (Scholle, 1979), derived either from metamorphic patches caught up in outer arc ophiolites (Ray, 1982) or from continents in the north.

Lithic fragments include chloritised vitrophyric and porphyritic basalt, chert, fossiliferous carbonate extraclasts, micaceous schist and clasts of sandstone, siltstone and tuff. Minor presence of kaolinised feldspar grains, twinned plagioclase grains of microcline variety and cloritised pyroxene grains are also noticed.

Groundmass of these sandstones include both matrix and cement. Siliciclastic matrix is dominantly constituted of clay minerals, chlorite and fine sized quartz and feldspar. Often the lithic fragments are squashed and formed pseudomatrix. Cements, minor in abundance, are either siliceous (in the form of overgrowth) or carbonate or iron oxide.

Gondwana Resenrch, V. 4, No. 3, 2001

484 P.P. CHAKRABORTY AND T. PAL

Discussion

Neither stratigraphic nor structural evidences provide undoubted clue for distinction of forearc basins from those of accretionary complex (Macdonald, 1993). Often fore arc sedimentation is coeval with uplift and deformation of the seaward basin margin, i.e., with the later stages of trench and trench - slope basin evolution. Forearc basin sediments are, in general, younger and less deformed than those of accretionary basins. In contrast to the underlying intricately folded Mithakhari Group of sediments, deposited in narrow restricted basins of trench inner-slope (Chakraborty et al., 1999), the less deformed Andaman Flysch sediments with regional fold pattern and wide lateral extent of beds suggest wide basinal configuration. Further, a submarine fan succession with its well developed anatomical parts is unlikely to develop in a trench setting, where facies successions are often truncated. A forearc basin setting adjacent to the accretionary arc is thus suggested. The development of this forearc is related to the uplift of the accretionary complex that is evident from the undoubted presence of ophiolite derived clasts within conglomerates and sandstones of the inner fan. Large, bouldery, poorly sorted ophiolite-derived clasts in the channel conglomerates of inner fan identify the accretionary complex as a proximal source for the fan. However, this source alone cannot account for the moderately high percentage of quartz present in these sediments. A distal continental extrabasinal source beyond the northern and northeastern frontiers of Burma is envisaged (Ray, 1982; Karunakaran and Ray, 1968). The dual source of sediments is also apparent in the wide apart paleocurrent directions derived from inner fan and depositional lobe of Andaman Flysch fan (Figs. 4a,b). The basin received sediments longitudinally from north and transversely from the accretionary margin on west.

Considering the dominant paleocurrent direction towards south, the northward location of basin plain facies (Smith Island section) with respect to mid-fan lobe facies is interesting. Transverse (eastward) sediment dispersal was restricted to inner fan channelised part of Andaman Flysch fan. Beyond that, longitudinal dispersion was dominant in non channelised distal mid-fan part which might have followed the axial slope of the basin. Paleocurrent direction in basin plain facies confirms this contention. This broadly orthogonal paleocurrent pattern is suggestive of longitudinal geometry of the basin. Longitudinal geometry is further corroborated by the presence of slumps all along the fan tract, which attests the existence of slope gradient throughout the basin. Detachment of lobe in high slope condition and its progradation along basin axis might have caused the

observed complexity in facies distribution pattern. External (allocyclic) tectonics triggered the large scale sedimentary events in Andaman Flysch fan and resulted the complex sediment distribution pattern, while the depositional pattern of the turbidite facies is explained in terms of autocyclic submarine fan models (cf. Dickinson and Seely, 1979; Ingersoll, 1978). Lack of any systematic sequence development pattern in Andaman Flysch fan may be the result of this longitudinal geometry of the basin where multiple source and juxtaposition of sediments of varying proximalities are expected.

Active supply of coarse grained siliciclastic sediments to a deep water environment is favoured during sea level fall (Mutti, 1985; Posamentier and Vail, 1988). Deposition of slope sand turbidite packets which show lobe and channel-levee features in transgressive and high stand systems tract is also reported ( Sakai and Masuda, 1996) and discussed in terms of possible variations in tectonic setting, climate and physiography of sedimentary basins (Galloway, 1989; Kolla and Perlmutter, 1993). Timings and modes of turbidite deposition in a submarine radial fan environment can be correlated in global sea level chert and constrained in a sequence stratigraphic framework through correlation of chronostratigraphic markers present in submarine fan deposits with those of coeval shallow marine shelf - upper slope products (Ito, 1998). Multiple source, longitudinal nature of Andaman Flysch fan, lack of any chronostratigraphic marker, absence of any correlatable shallow water deposit in this onshore outcrop belt prevents correlation of Andaman Flysch fan with global sea level curve. The problem gets further complicated due to postulation of major eustatic fall (> 140m) in mid - Oligocene time (Haq et al., 1987).

In contrast to most sequence stratigraphic models, the siliciclastic Andaman Flysch fan do not represent either forced regressive lowstand fan or prograded lowstand wedge. The blankets of basin plain pelagic and hemipelagic muds with fluidal massflows (facies association C) with fining upward and coarsening upward hemicycles may be, though not unequivocally, represent an initial transgressive followed by a highstand systems tract. The basal overall fining upward pattern is further constituted by short hemicycles (av. thickness 14m), which are themselves fining upward. The coarsening upward part, at the top, is also punctuated by shorter hemicycles which are themselves coarsening up. The 9 m thick shale - limestone unit (facies A1 and A2) in basin plain facies association rich in authigenic mineral, such as, glauconite that intervene the two stratigraphic segments of distinctive stratal stacking pattern (Fig. 10) is typical product of condensed section related to maximum flooding (Loutit et al., 1988). The overall

Gondwana Research, V. 4, No. 3,2001

FOREARC SUBMARINE FAN, ANDAMAN ISLAND 485

45 m

__ 0

t

L

I-

I

I

I

massive sandrlone

M F S Maxlmum flooding

T s T Transgressive systems bact

H S T Highstand systems traci

accretionary complex. In contrast, the Andaman Flysch forearc fan experienced an active supply of terrigenous clastic sediments to a deep marine environment in transgressive and highstand of sea level (Fig. 11). Such departure can be explained in terms of either unrestricted nature of the Andaman Flysch forearc basin or due to other extrabasinal factors such as strike-slip faulting or renewed extension.

Conclusion

Andaman Flysch fan developed as a longitudinal fan in a forearc setting which received sediments parallel to its basin axis and occasionally transversely from basin margin. Development of a depositional lobe in its distal mid fan part indicates that the fan developed in a system with high sediment discharge, a narrow shelf and a steep slope into the basin. Signatures of rising and high sea level in coeval basin plain succession indicate that Andaman Flysch forearc fan experienced tectonically induced active supply of terrigenous clastic sediments to a deep-water environment. Construction of the accretionary arc and its tectonic uplift in the erosional domain certainly have influenced the fan development in adjacent forearc basin, which later got tectonically juxtaposed to give the present outcrop scenario.

Fig. 10. Detailed litholog measured at Smith Island, North Andaman. Note retrogradational hemicycles a t the base and progradational hemicycles towards top.

tendency of rise in relative sea level is also reflected in the channel-levee association of inner fan by channel abandonment and up section decline in channel depth.

Overall upward shallowing seems to be a characteristic of forearc basins (Macdonald, 1993), where the basin is thought to exist owing to a physical barrier along the

I Parrive Margin (Radial Ian) I Vail Model (Radial Ian)

Val. 1987 M e r elal ,1998

Longiludinal Ian

Fig. 11. Comparative schematic illustration of geometry of depositional sequences of Andaman Flysch Group and other sedimentary basins of different tectonic regimes.

Acknowledgments

The authors are thankful to the Director, Andaman Division, GSI and Dy. Director General, Op. W-S-A and n , E.R, GSI for their constant encouragement both in the field and in writing of this paper. The infrastructural help extended by officers of ALHW is thankfully acknowledged. Reviews from Prof. S.K. Tandon and Prof. P.K. Bose helped in improving the quality of the paper.

References

Abdel - Wahab, A.A. (1992) Provenance of Gebel El - Zeit sandstones, Gulf of Suez, Egypt. Sedim. Geol., v. 75, pp.

Bandopadhyay, P.C. and Ghosh, M. (1998) Facies, petrology and depositional environment of the Tertiary sedimentary rocks, around Port Blair, South Andaman. J. Geol. SOC. India,

Chakraborty, PP., 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, v. 53, pp. 271 - 284.

Curray, J.R and Moore, D.G. (1974) Sedimentary and tectonic processes in the Bengal deep-sea fan and geosyncline. In: Burke, C.A. and Drake, C.L (Eds.), The geology of continental margins. Springer-Verlag, Ny, pp. 617-627.

241-255.

V. 52, pp. 53-66.

Gundztmna Research, K 4, No. 3, 2001

486 PF? CHAKRABORTY AND T. PAL

Dickie, J.R. and Hein, F.J. (1995) Conglomeratic fan deltas and submarine fans of the Jurrasic Labarge Group, Whitehorse Trough, Yukon territory, Canada: forearc sedimentation and unroofing of a volcanic island arc complex. Sedim. Geol.,

Dickinson, W.R. and Seely, D.R. (1979) Structure and stratigraphy of forearc regions. Amer. Assoc. Petrol. Geol.

Folk, R.L. (1974) Petrology of sedimentary rocks. Hemphill, Austin, Texas, p. 182.

Galloway, W.E (1989) Genetic stratigraphic sequences in basin analysis, I. Architecture and genesis of flooding-surface bounded depositional units. Amer. Assoc. Petrol. Geol. Bull.,

Haq, B.U., Hardenbol, J. and Vail, P.R. (1987) Chronology of fluctuating sea levels since the Triassic (250 my to present). Science, v. 235, pp. 1156-1167.

Ingersoll, R.V. (1978) Petrofacies and petrologic evolution of the late Cretaceous forearc basin, northern and central California. J.Geol., v. 86, pp. 335-353.

Ito, M. (1998) Submarine fan sequences of the lower Kazusa Group, a Plio-Pliestocene forearc basin fill in the Boso peninsula, Japan. Sedim. Geol., v. 122, pp. 69-94.

Karunakaran, C. and Ray, K.K. (1968) Tertiary sedimentation in the Andaman-Nicobar geosyncline. J. Geol. SOC. India, v. 9, 32 p.

Kolla,V. and Perlmutter, X. (1993) Timing of turbidite sedimentation on the Mississipi fan. Amer. Assoc. Petrol. Geol. Bull., v. 77, pp. 1129-1141.

Loutit, T.S., Harbendol, J. and Vail, P.R. (1988) Condensed section : the key age determination and correlation of continental margin sequences. In: Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier, H.W., Ross, C.A. and Van Wagoner, J.C. (Eds.), Sea level changes: an integral approach. SEPM Sp. Publ. No. 42, pp. 183-213.

Macdonald, D.I.M. (1993) Controls on sedimentation at convergent plate margins. Sp. Publ. Int. Ass. Sedim., v. 20,

Middleton, G.V. and Bouma, A.H. (Eds.), (1973) mrbidites and deep water sedimentation. Pacific section SEPM Los Angeles, California, pp. 1-38.

Mutti, E. (1985) Turbidite systems and their relations to depositional sequences. In: Zuffa, G.G. (Ed.), Provenance of arenites. D.Reide1, Dordrecht, pp. 65-93.

Mutti, E. and Ricci Lucci, F. (1972) Turbidites of the northern Appenines: introduction to facies analysis. Int. Geol. Rev.,

O’Connell, S . , Stelting, C.E., Bouma, A.H., Coleman, J.M., Cremer, M., Droz, L., Meyer-Wright, A.A., Normark, W.R.,

V. 98, pp. 263-292.

Bull., V. 63, pp. 2-31.

V. 73, pp. 125-142.

pp. 225-257.

V. 20, pp. 125-166.

Pickering, K.T., Stow, D.A.V. and DSDP Leg 96 shipboard scientists (1985) Drilling results on the lower Mississipi fan. In: Bouma, A.H., Normark, W.R. and Barnes, N.E. (Eds.), Submarine fans and related turbidite systems. Springer - Verlag, Ny, pp. 291-298.

Pawde, M.B. and Ray, K.K. (1963) On the age of greywackes in South Andaman. Science and culture, v. 30, pp. 279-280.

Pettijohn, F.J. (1984) Sedimentary r0cks.3‘~ edition. CBS Publishers and Distributors, New Delhi, 620 p.

Pettijohn, F.J., Potter, P.E. and Seiver, R. (1973) Sand and sandstone. Springer - Verlag, Ny 553 p.

Piper, D.J.W. (1978) Turbidite muds and silts in deep-sea fans and abyssal plains. In: Stanley, D.J. and Kelling, G. (Eds.), Sedimentation in submarine fans, canyons and trenches. Hutchinson and Ross, Strondsburg, Penn., pp. 163-176.

Posamentier, H.W. andVail, PR. (1988) Eustatic control on clastic deposition I1 - sequence and systems tract models. In: Wilgus, C.K., Hastings, B.S., Kendall, C.G. St.C., Posamentier, H.W., Ross, C.A. and Van Wagoner J.C. (Eds.), Sea level change - an integrated approach SOC. Econ. Paleont. Mineral., Sp. Publ., v. 42, pp. 125 - 154.

Ray, K.K. (1982) An overview of the geology of Andaman and Nicobar islands. Geol. Surv. India, Misc. Publ. No. 41,

Ricci Lucci, E and Valmori, E. (1980) Basin -wide turbidites in a Miocene over supplied deep sea plain : a geometrical analysis. Sedimentology, v. 27, pp. 241 - 270.

Roy, T.K. (1983) Geology and hydrocarbon prospects of Andaman and Nicobar. In : Bhandari, L., Venkatachala, B.S., Kumar, R., Swamy, S.N., Garga, P. and Srivastava, D.C. (Eds.), Petroliferous basins of India. Petrol. Asia Jour., KDMIPE, ONGC, DehraDun, pp. 37 - 53.

Sakai, T. and Masuda, F. (1996) Slope turbidite packets in a forearc basin fill sequence of the Plio-Pliestocene Kakegawa Group, Japan: their formation and sea level changes. Sedim. Geol., v. 104, pp. 89 - 98.

Scholle, P.A. (1979) A colo-illustrated guide to constituents, textures, cements and porosities. Amer. Assoc. Petrol. Geol. Mem. No. 28, p. 210.

Sengupta, S . , Ray, K.K. and Acharya, S.K. (1990) Nature of ophiolite occurrences along the eastern margin of the Indian plate and their tectonic significance. Geology, v. 18,

Shanmugam, G. and Moiola, R.J. (1988) Submarine fans: characteristics, models, classification and reservoir potential. Earth Sci. Rev., v. 24, pp. 383 - 428.

Walker, R.G. (1967) Turbidite sedimentary structures and their relationship to proximal and distal depositional environments. J. Sedim. Petrol., v. 37, pp. 25-43.

pp. 110-125.

pp. 439 - 442.

Gondwana Research, K 4, No. 3,2001