mass flow deposit south of central andaman trough, andaman...

Indian Journal of Geo Marine Sciences

Vol.46 (08), August 2017, pp. 1519-1527

Mass flow deposit south of Central Andaman Trough, Andaman Sea:

evidence from sub bottom profiler records

Tripathi S. K1*

, Das S.2, Meitei S. I

1, Rajarama K. N

1, Resmi S.

1 , Tewari H.

1, Kashyap M. P

1 & Pradhan R.K.

1

1M&CS, Geological Survey of India, Kolkata-700091, India 2M&CS, Geological Survey of India, Mangalore- 575001, India

*[E-mail: [email protected]]

Received 24 April 2015 ; revised 22 November 2016

A total of 2577 line-km sub bottom profiler data recorded over East Andaman Basin, Sewell and Mergui ridges, south of

Central Andaman Trough, Andaman Sea, revealed distinctive echo types representing different seabed material and seafloor

morphology. Echo Type I, Type-IIIA, Type-IIIB and Type IVA prevail over ridges, slopes of Sewell rise and Mergui ridge. Scarp

head and slides (Types II, Type IVA) occur extensively over the entire slope areas of the ridges. Whereas in East Andaman Basin

area, top sub bottom reflectors are almost flat in all transects and downward it becomes wavy, chaotic and hummocky in nature. This

means that present day depositional environment is relatively calm as compared to bottom unit, which means the bottom most units

is formed by the mass flow deposit. These evidences from echo pattern, shows a successive slope failure and supports frequent

seismic shaking related to the rifting of Central Andaman Trough.

[Keywords: sub bottom reflectors, chaotic, hummocky, mass flow deposit, neo-tectonic.]

Introduction East Andaman Basin (EAB) and Mergui

Basin are NNE-SSW trending elongate sediment

filled sub-basin, are the product of back arc

extensional tectonics1, 2

. EAB comprises a total of

4600m thick sedimentary unit1. This basin was

under shallow to deep water regime since middle

to late Miocene1,3

and post-rift related

sedimentation was up to 2500 m thick4. At

present, main source of enormous quantity of fine

grained sediments in Andaman Sea is from

Irrawaddy-Salween river system, and apart from

this very small amount reaches into deeper parts

via Martaban Canyon 5-9

. Additional contribution

of terrigenous input in southern part of Andaman

Sea is from Malacca Strait10

.

For analysing depositional behaviour of

sediments in EAB, sub bottom profiler has been

used as a tool during SR-003 cruise of Geological

survey of India (R. V. Samudra Ratnakar), which

provides information about three distinguishing

zones viz., surficial, near-surficial and deep

sedimentary layers. These surficial, near surficial

and deep sedimentary structures depend on the

acoustic wave interactions with seabed and

impedance contrast between two layers. Large

impedance contrast between water and rocky

seabed with a considerable smooth surface

indicates that seabed surface behaves as an almost

perfect reflector. On the other hand, in softer

sediments, acoustic impedance difference is much

less which implies that larger energy will be able

to penetrate this boundary. A portion of energy

reflected back whenever the signal encounters a

different material and recorded back by the system

in form of analog signals. These analog signals

used to reconstruct vertical cross section of

sedimentary environment, obtained as an image of

layer boundaries. These reflected echoes mainly

controlled by geomorphology of seafloor,

subsurface geometry and sediment texture11, 12, 13

.

Highly variable and steep surface topography of

the submarine plateau and ridge generally yields

relatively poor acoustic images, hampering

detailed studies on echo types and their regional

distributions. Aim of this paper is to understand

the echo characters and regional distribution of

sediment deposits and further to understand the

INDIAN J. MAR. SCI., VOL. 46, NO. 08, AUGUST 2017

possible mechanism for extensive mass

movements and slope failures.

Materials and Methods Andaman Sea in the northeast Indian ocean

is a unique complex tectonic domain, occurs all

along subduction zone of Indo-Australian plate

beneath the overriding southeast Asian plate

occurs all along the Sunda arc14

, extending from

eastern Himalayan syntaxis to Banda arc, resulting

in oblique convergence in the Andaman-Nicobar

sector14

. Oblique plate convergence include

strike-slip faulting parallel to trench axis,

formation of a sliver plate, spreading related to

pull apart basin formation, active volcanism,

extensive seismicity and mantle upwelling15

.

Present configuration of sea-floor topography of

continental shelf has arrived during termination of

Pleistocene and at the beginning of Holocene16

.

Recent days Back-arc basin is known for well

defined spreading axis and hydrothermal

mineralization. South western part of Sewell rise

is dominated by north-south trending fault system

and western part is dominated by volcanic

features, which is related to arc volcanism and

back arc spreading activity (Fig.1).

Although north western, eastern and south eastern

part of EAB is surrounded by Sewell rise and

Mergui ridge; both these ridges are separated by

inactive spreading centre with relatively smooth

surface topography. Since last few decades

extensive work have been done on tectonics and

geological history of Andaman Sea by various

worker like Rodolfo, 1969a17

; Curray et al.15

, Rao

et al.,18

Kamesh Raju et al.,19

but limited studies

have been carried on sediment types20

. Apart from

fluvial sources EAB also receive material from

pelagic, eolian, weathering of sea-floor rocks and

possible hydrothermal sources18,20,21,22

. The present

spreading rate of central Andaman basin is of

about 38 mm/yr 19, 1

.

Sub bottom profiler (SES-2000, Deep

system manufactured by Innomar Technologie

GmbH, Germany) has been used for survey work.

The equipment works on very low frequency

channel that operates between 2 to 7 kHz.

SESWIN, software was used for data acquisition.

Data processing was performed by ISE software

and processed data converted into different image

format (Tiff).

Fig 1- Location map of the study area along with different

tectonic domain (after Curray, 2005)

Results and Discussion

In survey area, a total of 2577 line km sub

bottom profiler data were recorded, out of that 300

line km data were selected for detailed study,

which cover all structural configuration of the

study area. These representative lines are: 39, 49,

50, 51, 52, 54 and cross line (Fig. 2). In addition

few more lines were also taken into accounts

which have some special structural configuration.

Distinct sea bed morphology with different

gradient settings was noticed: (1) Mergui Ridge in

the south-eastern part (2) EAB in the central part,

and (3) Sewell Rise in the north-western part

(Fig.2). During survey, average sound wave

penetration was around 40m to 80m. Sub bottom

profiler recorded low sediment cover over Mergui

ridge and Sewell rise in comparison to EAB.

Ridge tops and basinal area are dominated by

foraminiferal and siliceous oozes, respectively20

.

Acoustic reflector over ridges reveals that the

sediments are eroding from slopes and get

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TRIPATHI et al.: MASS FLOW DEPOSIT SOUTH OF CENTRAL ANDAMAN TROUGH, ANDAMAN SEA

deposited in the valley portions (Fig. 3). Reflector

characteristics of the youngest units (study from

grab samples) with recent detritus and biogenic

sedimentation, is well matched with the view of

Keller and Richards10

, Rodolfo5 Colin et al.,

6 and

Tripathi 20

. These reflectors in upper parts are

parallel to each other, whereas, sub bottom depth

reflectors below 20 to 40 m b.s.f were merged and

form chaotic, lenticular and hummocky patterns.

These are inferred as mass transport deposits from

the ridges or due to tectonic mixing of the

sediments (Fig.4).

Fig. 2- Detailed bathymetry and physiographic features

along with cruise track, South of Central Andaman

Trough, Andaman Sea. Where SWR= Sewell Rise, EAB=

East Andaman Basin, MR= Mergui Ridge.

The features are similar to echo characters of the

basinal sediments, which are deformed in a bizarre

manner due to presence of numerous fault.

Considering an average sedimentation of 10 cm

per ka in Andaman basin (Rodolfo5), the age of

sediment which recorded in sub bottom reflectors

are approximately between 0.4 to 0.8 Ma.

Channel noticed in the valley portion of

Mergui ridge (Fig-5), where slope varies from 0.50

to 20 and become almost flat in EAB with some

localized stacking and bathymetric depression

(Fig.6). Sediment deposited in EAB is not only

from the Irrawaddy River but also from the slope

of elevated ridges. In younger sediments, acoustic

internal reflectors are almost parallel up to 20-30

m depth from the sea floor.

Strata Types

Based on different acoustic characteristics, the

entire penetration depth is grouped into four major

strata packages to simplify the depositional

environment (Fig.7). First strata package has an

average thickness of 7m to 15m with distinct

acoustic internal reflector, minor growth fault (Fig-

8) and acoustic voids. These acoustic voids in sub

bottom reflector may be due to amalgamation of

fluid or due to the movement of organic gases in

the sediment. Wavy undulation noticed throughout

entire penetration depth, and it might have formed

because of material flow due to non-cohesiveness

(Fig. 9). Second strata package has an average

thickness of 20 to 40 m, with lots of minor

secondary internal reflectors.

Here reflectors are mostly hummocky,

lenticular and chaotic in nature with very few

internal acoustic transparencies (Fig-8).Third

strata package is sandwiched between second and

fourth strata packages. It has an average thickness

of 20 m and mostly chaotic in nature, moreover

somewhat weak in acoustic transparency (Fig.8).

Fourth strata package is recorded around 60m

b.s.f. It seems that the second and third strata

packages might be deposited under turbidity

induced mass transport system, as similar

observation was recorded by Schwab et al.23

and

also affected by tectonics of the nearby area.

Acoustic internal reflection patterns over Sewell

Rise follow the topography and their

sedimentation nature is very similar to Mergui

Ridge.

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Fig. 3- (A) Sub-bottom reflectors along part of line 39,

showing rugged geomorphological terrains formed due to

erosion of older sediments over Mergui ridge. (B) In 3D

Multi beam map dot line represents the track of the Sub

Bottom Profiler.

Fig.4- Sub bottom image along part of line 29 showing (A)

Distinct bottom echo with several continuous internal

reflections of total length 1km (type I-B), Surface echoes are

flat, distinct to indistinct and become wavy with sub bottom

depth 60m b.s.f (TYPE IV-B) (B). Lenticular / hummocky

bedding showing amplitude 1.5m and wavelength 45.94m.

Fig. 5- A 230m wide and 25 m depth channel in valley

portion of Mergui ridge along part of line 49

Fig.6- Bathymetric Profile line [P-P’] taken from Sewell

Rise to Mergui ridge.

Fig. 7- A) Profile section from south to north, B) Sub

bottom profiler section taken along south to north

representing the overall behaviour of the bottom

reflectors.

Fig. 8- Sub-bottom reflectors along part of line 54 showing

Growth fault due to compression or formed by compactness

of the sediment over Sewell Rise.

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Fig. 9- Sub-bottom profiles along part of line 10 showing

hyperbolic or wavy echoes (III). Irregular, wavy surface

echo and distinct to indistinct, continuous internal reflectors

(type III-C). Basal reflectors are mingled and transparent. A

downward increase in wave amplitude (open arrow). NIF=

Normal Internal Fault, CIF= Compressional Internal Fault.

Wavy undulations are 2m in amplitude & 64 m wavelength.

Echo Types

The classification criteria of echo character

emphasized by Damuth24

, Damuth and

Hayes13

, Chough et al.25

and Pratson & Laine26

have been used as base to interpret

sedimentary processes of the study area.

Based on these acoustic properties like clarity,

continuity and geometry, different acoustic / or

echo pattern have been identified from three

different settings, viz- Mergui Ridge, EAB and

Sewell Rise. These echo types are classified

into four major classes: 1) distinct echo (Types

I), (2) indistinct echo (Type II), (3) hyperbolic

or wavy echo (Types IIIA to IIIC), and (4)

combined echo (Types IVA & IVB).

Distinct Echo

Type-I: Echo type-I is characterized by distinct

bottom echo with several continuous, parallel

internal reflectors, have an average thickness of

around 20 to 30 m and conformable one over

other. Surface reflectors of Mergui Ridge are

generally flat on top and moderately sloping

towards the fringe and become flat in EAB.

Acoustic reflectors of the recent sediments are

mostly of distinct type and internal reflections are

very prominent in top few meters (Fig. 10 a, b &

c). This type of echoes may form under relatively

uniform environment, which is supported by the

study of grab sample. The nature of the sample

was pelagic20

. In figure 10c, reflectors are

slightly wavy with diffused internal reflection at

top of ridge. Although of type-I, these reflectors

are faulted at most of the places, might be due to

the effect of neo- tectonic activity.

Fig.10- Sub bottom profiles of distinct echoes and indistinct

type. (a, b & c) Distinct bottom echoes with several

continuous internal reflections (Type I). (d) Indistinct echoes

with diffuse internal reflectors (Type-II)

Indistinct Echoes

Type-II: These echoes are generally wedged

shaped with internally transparent as well as

diffuse echo types (Fig. 10 d & 11h). Sediments

of this type commonly show a very weak internal

reflection to almost transparent behaviour. Most

of the places in EAB, below 40 m from seafloor,

a thick pile of sedimentation illustrate internal

acoustic transparent reflector that probably

indicate uniform massive sediment.

Transparencies reflect internal homogenization,

possibly resulting from the collapse of slope

masses that got mixed with ambient water during

sediment deposition27, 28

. This type of echo

patterns has been reported from the regions of

interbedded pelagic and gravity flow deposits11

.

Hyperbolic or Wavy Echo (Type-III)

Type III-A: This type of echo pattern is

characterised by irregular overlapping hyperbolae

with widely varying vertex elevations above

seafloor (Fig. 11e). Each hyperbola generally

shows very strong surface echo and prolonged

sub bottom echo suggestive of basement highs12,

29, 30.

Type III-B: This echo type has regular,

overlapping hyperbolae with slightly varying

vertex elevation and no internal reflectors. Type

III-B is often bounded by eroded sedimentary

layer (Fig. 12). The hummocky echoes have been

commonly ascribed to bed forms on the surface

of gravity induced mass flow deposits31, 27

.

Type III-C: This echo pattern is characterized by

overlapping hyperbolae with widely varying

vertex tangent to the seafloor. These intense,

irregular hyperboles are recorded in regular bed

1523


form, may form by mass flow/ contour current13

(Fig. 11 g). The wavy topography exhibits a

succession of broad crests and narrow troughs

(Fig. 11g). Waves are 64 m long and 2 m high.

Wave dimensions and thickness of internal

reflective layers vary systematically with change

in depth. In the basal part, this echo type exhibits

mingled reflections that are correlated with

chaotic reflectors overlying the acoustic

transparent homogenized sedimentary layers32

.

Wave amplitude commonly increases with depth

(Fig.4 & Fig. 11g). The irregular, wavy,

continuous reflective layers with internal fault

systems are most likely generated by creep

movements of pelagic materials at much smaller

strains than those formed by slides and slumps32,

33. Wave troughs are commonly associated with

normal or compressional internal faults showing

small scale displacement (Fig.9).

Fig.11- Sub bottom profiles showing, (e) Irregular varying

overlapping hyperbolae; surface echo with distinct to

indistinct vertex elevations (type III-A). (f) Irregular blocky,

lumpy or hyperbolic masses bounded upslope by scarps and

scars (type IV-A). (g) Irregular, intense overlapping

hyperbolae with little or slightly varying sub bottom echo

vertex elevations (type III-C). (h) Wedge shaped transparent

mass (type-II).

Fig. 12- Type-III-B reflector noticed over Mergui ridge

along part of line 23.

Combined Echo

Type IV-A: Scarp heads are underlain by

slightly disturbed sediments or by irregular

drapes or transparent masses. The displaced

masses exhibit various degrees of internal

deformation ranging from intact masses through

folded or faulted internal reflectors to fused

internal echoes (Fig. 11f).

Type IV-B: This type consists of flat, distinct to

indistinct surface echoes and continuous sub

bottom reflections that are initially flat but

become wavy with sub bottom depth (Fig. 4).

Spatial distribution of Reflector Types

East Andaman Basin: It is marked by echoes of

Type-I; Type III-A; Type III-C and Type IV- B.

Large-scale blocky, lumpy or hyperbolic masses

are increasing at sub bottom depths below 20 to

40 m throughout the EAB (Fig. 4). Growth fault,

acoustic chimneys and acoustic voids are noticed

in the sub bottom reflectors. A localized

bathymetric depression is surrounded by

disturbed surface reflectors. However, the basinal

reflectors are dinstict type and have no sign of

distrubance (Fig.13). Sub bottom depth reflectors

are transparent acoustic and chaotic in nature

(Fig. 4). Collapsed reflectors is noticed, which

might have formed due to migration and escaping

of fluid (Fig. 14, A). A drag fault may exhibit a

compression regime and material behaves like a

plastic body (Fig. 15). Upper sequence is

characterized by pelagic sedimentation, which is

well matched with the echoes types reported by

Jha et al.3.

Fig.13- Basinal structure observed at line-52. Transparent

acoustic layer with chaotic reflector pattern.

Mergui Ridge: Generally marked by echoes of

Type-I, II, IIIB and IVA with a localized channel

in valley portion. Echo type IV-B is observed

below the channel, giving information about

mass transportation deposit (Fig-6). As evident at

the edges of Mergui ridge, rugged morphology

may be attributed to erosive processes induced by

bottom currents (Fig. 3). A 50 m wide hump

1524


observed, where strata seems to be up-thrusted

and simultaneously fluid tries to migrate upward

under pressure along the fault plane (Fig. 14B).

Fig. 14- A) Reflectors collapsed may be due to fluid escape

along part of line 45, B) 50-meter wide hump, Strata

thrusted Upward may be due to fluid pressure or by any

upward movement along part of line 23.

Sewell Rise: This part is marked by echoes of

Types-I, II, III and IV-A. Where, sediment

internal acoustic reflectors over the ridges,

conformable over the physiographic slope (Fig.

11f). Although northern slope of the ridge is

marked by type -II echo pattern, here sediment

seems to be moved along a plane (Fig. 16).

Faulted sedimentary layers and scarp drape head

are also noticed (Fig.17). Mass flow deposits in

places may be due to the influence by contour

currents. Similar observations were also observed

over Mergui Ridge.

Fig.15- Sub-bottom reflectors showing drag faulting

between along part of line 49.

Conclusions

Based on acoustic echo types, entire study area is

presumed to be related to post-rift sedimentation.

Sedimentary succession in the upper sequence of

East Basin, having stratified reflectors, may

suggest uniform sedimentation under relatively

tranquil condition. While below 40 m depth b.s.f,

sediment contains chaotic pattern, suggest mass

flow deposits. Chaotic facies in the lower

sequences may be related to rifting of Central

Andaman Trough and subsidence of Basin

sediment, as evident in form internal faults.

Ruggedness in reflector pattern over Mergui and

Sewell ridges suggest low rate of sedimentation,

and contour current eroding the materials from

the edge of ridges and deposited in the basinal

area.

Fig.16- Sub marine landslide noticed along the northern part

of line 19. Inset map of multibeam showing the track of

profile. (Section view shows how material slides along a

plain)

Fig. 17- (A) sub-bottom reflector along part of line 54

showing distinct nature over Sewell rise.(B) In 3D

Multibeam map, a dot line showing the cruise track of

the profile.

Acknowledgement The authors are grateful to late Shri N. R.

Biswas, Director M&CSD, Kolkata for his

motivation in respect to defining the objective of

this work and also a special thanks to Deputy

Director General, Marine Wing, GSI, for

providing all possible facilities to carry out the

study. Heartfelt thanks to Chief scientist and

cruise participants for their dedication and hard

work during data acquisition.

1525


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mass flow deposit south of central andaman trough, andaman...

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