Sea-level and ocean-current control on carbonate-platform growth, Maldives, Indian Ocean

Sea-levelandocean-current controlon carbonate-platformgrowth,Maldives, Indian OceanChristian Betzler,* Jorn Furstenau,*,1 Thomas Ludmann,* Christian Hubscher,* SebastianLindhorst,* Andreas Paul,† John J. G. Reijmer† and Andre W. Droxler‡

*Department of Geosciences, University of Hamburg, Hamburg, Germany†Department of Sedimentology and Marine Geology, VU University Amsterdam, Amsterdam, TheNetherlands‡Department of Earth Science MS-126, Rice University, Houston, TX, USA

ABSTRACT

Multichannel high-resolution seismic and multibeam data were acquired from the Maldives-isolated

carbonate platform in the Indian Ocean for a detailed characterization of the Neogene bank architec-

ture of this edifice. The goal of the research is to decipher the controlling factors of platform evolu-

tion, with a special emphasis on sea-level changes and changes of the oceanic currents. The stacking

pattern of Lower to Middle Miocene depositional sequences, with an evolution of a ramp geometry

to a flat-topped platform, reflects variations of accommodation, which here are proposed to be pri-

marily governed by fluctuations of relative sea level. Easterly currents during this stage of bank

growth controlled an asymmetric east-directed progradation of the bank edge. During the late mid-

dle Miocene, this system was replaced by a twofold configuration of bank development. Bank growth

continued synchronously with partial bank demise and associated sediment-drift deposition. This

turnover is attributed to the onset and/or intensification of the Indian monsoon and related upwell-

ing and occurrence of currents, locally changing environmental conditions and impinging upon the

carbonate system. Mega spill over lobes, shaped by reversing currents, formed as large-scale pro-

grading complexes, which have previously been interpreted as deposits formed during a forced

regression. On a regional scale, a complex carbonate-platform growth can occur, with a coexistence

of bank-margin progradation and aggradation, as well as partial drowning. It is further shown that a

downward shift of clinoforms and offlapping geometries in carbonate platforms are not necessarily

indicative for a sea-level driven forced regression. Findings are expected to be applicable to other

examples of Cenozoic platforms in the Indo-Pacific region.

INTRODUCTION

Carbonate-platform development is governed by factors

such as changes of physical accommodation space, nutri-

ent content of the water, antecedent topography, water

temperature and salinity which together dictate the rela-

tion of accommodation to supply. Classification of plat-

forms and internal platform geometries providing

temporal snapshots can become equivocal because they do

not necessarily distinguish the relative importance of

these factors (Lukasik & Simo, 2008).

The Neogene was the time of the evolution of the ice-

house world (Shackleton et al., 1975; Zachos et al., 2001).During this time, fluctuations in polar ice volume appar-

ently controlled eustatic sea-level variations (Miller et al.,1991, 2005, 2011). Carbonate platforms are reliable

recorders of such sea-level fluctuations (Schlager, 1992).

Betzler et al. (2000) proposed that the major Cenozoic

carbonate banks of the Bahamas Bank (Atlantic Ocean)

and the Queensland Plateau (NE Australia) not only

recorded synchronously sea-level changes but also ocean-

ographic and atmospheric circulation events through sig-

nificant compositional and architectural changes. The

middle Miocene geometries of the Maldives tropical iso-

lated platform were introduced as an example to demon-

strate sea-level control on carbonate bank-margin

development (Tcherepanov et al., 2008; Catuneanu et al.,2009). Previous work, which was based on seismic studies,

ubiquitously related the evolution of the platform to rela-

tive sea level changes (Aubert & Droxler, 1992; Purdy &

Bertram, 1993; Aubert & Droxler, 1996; Belopolsky &

Droxler, 2003, 2004a,b) – though sequence-stratigraphic

interpretations were not straight forward. Purdy & Ber-

tram (1993) inferred a diachronous pattern of atoll devel-

opment from variations in onlap depth at the time of

carbonate bank initiation, and Aubert & Droxler (1996)

noted that banks simultaneously drowned and continued

Correspondence: Prof. Dr. Christian Betzler, Geologisch-Palaeontologisches Institut, Bundesstr. 55, 20146 Hamburg,Germany. E-mail: [email protected] Present address: PGO Oslo Norway

© 2012 The AuthorsBasin Research © 2012 Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists172

Basin Research (2013) 25, 172–196, doi: 10.1111/j.1365-2117.2012.00554.x

EAGE

growth and, therefore, concluded this situation to be

incompatible with a sole sea-level control.

Such a turnover in carbonate bank development can be

triggered by environmental changes (e.g. Erlich et al.,1993; Sattler et al., 2009), of which the Early–Middle

Miocene period provides numerous examples of global

paleoceanographic, -climatic and -geographic signifi-

cance. This global relatively warm phase, which peaked in

the middle Miocene climatic optimum (Zachos et al.,2001), is followed by the late Middle Miocene cooling

accompanied by a re-organization of ocean circulation

(Woodruff & Savin, 1989; Flower & Kennett, 1994),

deep-water cooling (Lear et al., 2000) and substantial

growth of the East Antarctic ice sheet (Lewis et al., 2007).Cooling and re-organization of ocean circulation is known

to have impacted on carbonate-platform geometries else-

where. At the Australian Miocene platforms on the Mar-

ion Plateau, such changes were triggered by the

increasing strength of westerly winds and provoked

drowning and subsequently re-deposition of sediments by

currents (Isern et al., 2004; Eberli et al., 2010). In the

Straits of Florida and at the flanks of the Bahamas, the

beginning of drift deposition is proposed to reflect the

onset of the modern conveyor-type circulation in the

Atlantic (Anselmetti et al., 2000).In the Indian Ocean, where the herein studied Mal-

dives carbonate platform is located, enhanced uplift of the

Himalayan and Tibetan region (Clift et al., 2008) concurswith the installation of the seasonally reversing Indian

monsoon, which provides for upwelling and terrigenous

influx (Kroon et al., 1991; Rea, 1992; Zheng et al., 2004).Betzler et al. (2009) discussed that partial drowning epi-

sodes in this carbonate edifice were triggered by the onset

of this oceanographic regime, by current-controlled injec-

tion of nutrients into the shallow-water realm. This

hypothesis relies on the discovery in multibeam data of

isolated drowned current-shaped banks overlying an

extensive bank with barrier reefs. Current control on this

drowning and on the subsequent carbonate-platform evo-

lution is corroborated by the presence of sediment-drift

bodies in the Maldives (Betzler et al., 2009).The aim of this study is to characterize the distinct seis-

mic facies and depositional geometries which occur in the

Maldives carbonate platform and to demonstrate how

these distinct architectural elements reflect sea-level and

current control. It will be shown that the partial drowning

reflects a turning point from a platform growth with a

dominant sea-level and minor current control to a more

dual control by ocean currents and sea level: Since the late

Miocene, the Maldives are strongly impinged by the

regime of monsoonal currents, which are as important for

the physical stratigraphy as sea-level changes. The style

of platform growth, by its complexity triggered by the

seasonally reversing currents, differs from other carbonate

current-controlled carbonate platforms, where windward-

leeward gradients occur under unidirectional wind and

current regimes (Hine et al., 1981; Eberli & Ginsburg,

1987, 1989; Schlager et al., 1994; Eberli et al., 2010). The

herein developed stratigraphic model for the Maldives

therefore may serve as an analogue for other carbonate

edifices in the monsoonal-controlled Indopacific realm,

and for carbonate platforms which grew under compara-

ble regimes at other times of the earth history, such for

example in the Tethys.

GEOLOGICAL SETTING

Physiogeographic situation

The Maldives archipelago in the central equatorial

Indian Ocean is an isolated tropical carbonate platform

constituting the central and largest part of the Chagos-

Laccadives Ridge, which is located southwest of India

(Fig. 1). A north–south oriented double row of atolls

enclose the Inner Sea of the Maldives. North-south-

wards the atolls are separated from each other by inter-

atoll channels, which deepen towards the Indian Ocean

(Purdy & Bertram, 1993). The Inner Sea is a bank-

internal basin with water depths of up to 550 m. The

Maldives total an almost 3-km-thick carbonate sedi-

mentary succession accumulated since the Eocene, away

from any terrigenous input (Aubert & Droxler, 1992;

Purdy & Bertram, 1993).

The archipelago comprises about 1200 smaller atolls,

lying near or slightly above sea surface. Discontinuous

marginal rims formed by such small atolls surround

lagoons with water depths of up to 50–60 m. These

rims are interrupted by deep passages, allowing for

strong currents within the atoll lagoons and subsequent

sediment reworking and re-deposition, as well as for the

growth of patch reefs (Ciarapica & Passeri, 1993). Mod-

ern marginal reefs are composed of robust-branching

corals and coralline algae, whereas the lagoonal reefs

show domal corals, and detrital sand and rubble facies

(Gischler et al., 2008). Muddy sediments are present

only in the smaller atolls’ lagoons protected by a contin-

uous marginal reef rim (Ciarapica & Passeri, 1993; Gis-

chler, 2006).

The oceanward margins of the Maldives archipelago

are generally steeply inclined, with dips of 20–30° downto 2000 m of water depth. On the Inner Sea side, stepped

atoll slopes have the same dip angles, but reach down to

water depths of only 150 m, where the gradient rapidly

declines (Furstenau et al., 2010). The Inner Sea is charac-terized by periplatform ooze deposition (Droxler et al.,1990; Malone et al., 1990), locally accumulated into sedi-

ment-drift bodies (Betzler et al., 2009).Climate and oceanographic setting of the Maldives is

dictated by the seasonally reversing Indian monsoon sys-

tem (Tomczak & Godfrey, 2003). Southwestern winds

prevail during northern hemisphere summer (April–November), whereas northeastern winds prevail during

winter (December–March). Winds generate oceanic cur-

rents, which are directed westwards in the winter and

eastwards in the summer. Interseasonally, a band of

Indian Ocean Equatorial Westerlies establish, enforcing

© 2012 The AuthorsBasin Research © 2012 Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 173

Hybrid control on carbonate-platform growth

strong, eastward-flowing surface currents showing veloci-

ties of up to 1.3 m s�1. Currents reach to water depths

of 200 m and more with only slightly reduced velocities

(Tomczak & Godfrey, 2003). Within the modern atolls’

passages, currents can exhibit velocities of up to

2 m s�1 at the surface (Preu & Engelbrecht, 1991)

accounting for winnowing in the passages and in the

lagoons, where hard bottoms form (Ciarapica & Passeri,

1993; Gischler, 2006).

Structuraland stratigraphic framework

The Maldives formed on an lower Paleogene (60–50 Ma)

volcanic basement (Duncan & Hargraves, 1990). The

(a) (c)

(b)

Fig. 1. Location map of the Maldives. (a) The Maldives are situated in the central equatorial Indian Ocean. (b) The box shows the

outline of the study area. (c) Closer view of the study area. The position of seismic lines of different vintages and wells used within this

study is indicated. Please note the different seismic data sets. The thin dotted line traces the complete R/VMeteor leg M74/4 vessel

track. Isopachs trace thickness of upper middle to upper Miocene prograding complexes (re-drawn after Belopolsky & Droxler,

2004a).


C. Betzler et al.

long-term subsidence rate of the Maldives is roughly in

the range of 0.03–0.04 mm yr�1 based on deep core data

from the well ARI-1 (Fig. 2) (Aubert & Droxler, 1996;

Belopolsky & Droxler, 2004a). In contrast, sedimentologi-

cal data from Rasdhoo atoll indicate a maximum subsi-

dence rate of 0.15 mm yr�1 during the past 135 000

years (Gischler et al., 2008). Faulting of the Maldives

archipelago is reported to be restricted to pre-Miocene

times (Purdy & Bertram, 1993).

The Maldives comprise an approximately 3-km-thick

shallow-water carbonate succession (Belopolsky & Drox-

ler, 2004a). Carbonate production established during the

Early Eocene when flat-topped carbonate banks began to

form on topographic highs, which were created by the

volcanic basement during Eocene to early Oligocene

times. During the late Oligocene, bank margins were

characterized by elevated rims which separated bank-inte-

rior areas from the open ocean. During the early Miocene,

these banks partially drowned and carbonate production

became restricted to narrow bands at the respective most

oceanward exposed areas. The Miocene is characterized

by progradation of bank margins towards the Inner Sea as

recognized in different previous interpretations of reflec-

tion seismic data, irrespective of seismic resolution,

though details of the interpretation differ (Purdy & Ber-

tram, 1993; Aubert & Droxler, 1996; Belopolsky & Drox-

ler, 2004a). Aubert & Droxler (1996) differentiated the

prograding margins into four Neogene units N2–N5, with

N2 comprising the main phase of bank-margin prograda-

tion (Fig. 3). N3–N5 were mutually seen as its waning

stage, with N3 sediments interpreted as having the ten-

dency to be preferentially accumulated in front of the pro-

grading bank margins. N3 isochrones reveal that this unit

is locally linked to areas of bank-margin dismemberment,

which in turn are associated with partial drowning and

channel erosion (Aubert & Droxler, 1996). The study of

intermediate-resolution seismics of Shell (Belopolsky &

Droxler, 2004a) corroborates this general organization of

prograding units throughout the Maldives, but relabels

the N3 unit as middle Miocene prograding complexes in an

expanded sequence-stratigraphic interpretation. In the

upper Miocene and Pliocene, the Inner Sea basin was

filled, while bank margins dominantly aggraded (Belopol-

sky & Droxler, 2004a), but also showed further partial

drowning after deposition of N5 (Aubert & Droxler,

1996).

METHODSAND DATA

The study of depositional geometries is based on the

interpretation of a set of multichannel reflection seismic

lines acquired during R/V Meteor cruise leg M74/4 in

December 2007, and the integration of the published low-

to medium-resolution industrial reflection seismic lines

1973/74 shot for Elf (Purdy & Bertram, 1993; Aubert &

Droxler, 1996) and 1989/90 for Shell (Belopolsky &

Droxler, 2004a), respectively. The Shell seismic data set

covers the Inner Sea and the inter-atoll passages. The Elf

seismic grid also transects most of the Maldivian atolls

and offers good penetration depth across the atolls and

their drowned parts. Stratigraphic interpretation of seis-

mics is made via correlation to published data of explora-

tion wells NMA-1, ARI-1 and ODP Site 716. This study

is complemented by multibeam data, which was continu-

ously recorded during the cruise of R/VMeteor.The newly acquired high-resolution seismic data set

consists of approximately 1400 km of reflection seismic

profiles. Seismic signals were generated by two clustered

GI-guns, each with a volume of 45 in³ for a 105-in³ gen-erated injector volume. A digital 144-channel streamer

array with an active length of 600 m and an asymmetric

group interval was used. The data were digitized with

seven SeaMUX 24 channel 24-bit digitizing modules

(HTI, Mineral Wells, Texas, USA), configured in six

multiple arrays totalling 144 channels. The shot point

distance during the entire cruise was 12.5 m. The domi-

nant frequencies centre around 100–120 Hz. Processing

of reflection seismic data was done using the software

package ProMAX 2D (Halliburton-Landmark, Houston,

Texas, USA). The data are processed to zero phase, fil-

tered in time and f-k domain, and corrected for dip

moveout. In basinal areas, a suppression of multiple

reflections was achieved by predictive deconvolution of

prestacked data. Amplitude losses were compensated by

a power function.

The Shell and Elf seismics was digitized using the ko-

geo seismic toolkit (Philipp Konerding, Hamburg, Ger-

many). Interpretation and visualization were done using

the software package Petrel (Schlumberger, London,

UK). Depending on depth, the vertical resolution of the

newly acquired data is approximately 4–6 m compared

with only 10–25 m of the Shell seismic data (Belopolsky &

Droxler, 2004a). The vertical resolution of the Elf seismic

data is lower. Seismic interpretation was performed on

time-migrated data in time domain. As the continuity of

the reflections in part is weak, the instantaneous phase was

also used for tracing. In the following, depths are approxi-

mated using an average sonic velocity of 2500 m s�1 for

the carbonates (Anselmetti & Eberli, 2001; Belopolsky &

Droxler, 2004a), if not stated otherwise.

Exploration well ARI-1 is situated in the Inner Sea

east of Ari atoll in 348 m of water depth. A 3315-m

thick, upper Eocene to Pleistocene sedimentary column

was penetrated. Total depth was reached at 3365 m in

a 50-m thick basal unit composed of weathered basaltic

flows (Fig. 2) (Aubert & Droxler, 1996). ODP Site

716, drilled in 1987 between Male and Maalhosmadulu

atolls in 533 m of water depth documents 264 m of

upper Miocene to Pleistocene hemipelagic and periplat-

form sediments in the deepest part of Kardiva channel

in the centre of the Inner Sea (Backman et al., 1988).Exploration well NMA-1 situated at Male atoll bottom-

ned in 2222 m of depth and recovered a layer of basalts

being overlain by an early Eocene to Plio-Pleistocene

succession of dolomite, pelagic and vuggy limestone



with corals, algae, bryozoans and larger foraminifers

(Purdy & Bertram, 1993).

Multibeam imaging was performed with a hull-

mounted EM120 multibeam echosounder (Kongsberg

Maritime, Kongsberg, Norway). The EM120 is a high-

resolution sea-floor-mapping system with 256 simulta-

neous beams operating in the 12 kHz range and covering a

swath width of up to 5.5 times the water depth. The beams

are stabilized for roll, pitch and yaw. Data obtained were

post-processed using the software package Neptune (Kon-

gsberg Maritime). Visualization including gridding and

refining of surfaces was done using the software package

Fledermaus (IVS 3D, Banbury, UK). The data were not

corrected for tides, which have a range of 0.4–1.0 m in this

region (Gischler et al., 2008).

Well-to-seismic tie – calibrationandchronostratigraphic framework

Stratigraphic data for wells NMA-1, ARI-1 and ODP Site

716 were taken from Purdy & Bertram (1993), Aubert &

Droxler (1996), Belopolsky & Droxler (2004a) and Rio

et al. (1990). They provide the chronostratigraphic frame-

work for the seismic interpretation. Carbonate lithofacies,

paleobathymetric evaluations and biostratigraphic age

determinations based on cuttings and side wall core analy-

ses for Shell exploration well ARI-1 are first published by

Aubert & Droxler (1996). A vertical seismic profile is used

for time-depth conversion and to tie well data to seismic

data (Fig. 2). For ODP Site 716, which is covered by two

high-resolution seismic lines (Figs 1 and 3b, c), a simple

Fig. 2. (Upper panel) Well-to-seismic

tie for establishment of chronostrati-

graphic framework. Age, biostratigraphy,

gamma-ray log and lithology columns are

taken from industry well ARI-1 (modi-

fied after Belopolsky & Droxler, 2004a).

Depth conversion of seismic line NE-

OMA-P7 was done using an irregular

spaced velocity log from ARI-1 well.

High-resolution seismics is displayed in

red to yellow and blue to turquoise col-

ours corresponding to changes in peak

and trough amplitudes, respectively

(SEG normal polarity). Shell seismic line

after Belopolsky & Droxler (2004b).

(Lower panel) West–east crossing seismic

line NEOMA-P7 (vertical exaggeration–25x) with well ARI-1 (cf. Fig. 1).

1–Eocene, 2–Early Oligocene, 3–LateOligocene, 4–Early Miocene, 5–Middle

Miocene, 6–Late Miocene, 7–Plio-Pleis-tocene.


C. Betzler et al.

time-depth estimation is made based on existing whole-

core P-wave–velocity measurements.

Table 1 provides characteristics of horizons tied into

the seismic data available. The stratigraphic framework

was established for the succession above the Oligocene-

Miocene boundary, since the imaging quality of older

strata is regarded to be too poor for interpretation. It is

based on published horizons O/M, EM1, E/MM, MM3,

Fig. 3. (a) Published seismic units in the Maldives carbonate edifice.N1–N5, PP from Aubert & Droxler (1996) and E-Mio 1/2, M1–M5, L-Mio 1–LP-P according to Belopolsky & Droxler (2004b). (b) Seismic line NEOMA-P65 (vertical exaggeration–20x) runswest-east along the SouthMaalhosmadulu and Goidhoo inter-atoll channel and cross cuts the Kardiva Channel (cf. Fig. 1). (c) General

interpretation from high-resolution seismic data of this study. I–Upper Miocene, II–Lower Pliocene, III–Middle Pliocene, IV–Upper

Pliocene and V–Pleistocene.



MM5 (Belopolsky & Droxler, 2004a) and PB2 (Purdy &

Bertram, 1993), which set a reliable stratigraphic frame-

work throughout the Shell and Elf seismic grid, and is

tied into the newer high-resolution seismic data (Fig. 2).

In addition to the established horizons, the high resolu-

tion of the new seismic lines allow to trace a better

approximation of the base of the Middle Miocene

(bMMio), as defined by Belopolsky & Droxler (2004a) in

ARI-1, and the base of the Early Pliocene (bEPlio),

defined in ODP Site 716 (Rio et al., 1990), throughoutthe seismic grid.

RESULTS

Seismic facies

The sequence-stratigraphic interpretation of the newly

acquired high-resolution seismics is based on analysis of

seismic-reflection pattern in terms of reflector termina-

tion, geometrical relationship, reflection shape, reflection

continuity, as well as of amplitude polarity and strength

(Fontaine et al., 1987). Different seismic facies of the new

high-resolution seismic data cover a complete platform-

to-basin transect. A characteristics summary is given in

Table 2, whereas Fig. 4 provides example images.

The platform areas are mostly characterized by a weak-

amplitude and chaotic seismic-reflection pattern with

single horizontal or basinwards dipping reflections. Inter-

nal geometries generally appear masked (Fig. 4a) by a

strong top-bounding reflection. Where internal platform

geometries are imaged, the inner-platform reflections are

mostly parallel, more or less horizontal, and of high to

moderate amplitude (Fig. 4b). In the Platform zones,

reflections develop laterally into a mounded or concave-

up shape (Fig. 4c). The mound-shaped seismic features

have generally a strong top reflection and internally show

a weak chaotic or transparent reflection pattern. In parts,

these bodies also occur at the basinward margin of the

platform, i.e. towards the Inner Sea, and mark the transi-

tion from the inner-platform area to the slope. The plat-

form slope is characterized by different seismic facies.

Strata either display high to moderate amplitude, sigmoi-

dal-shaped reflections traceable over distances of up to

500 m (Fig. 4d), or form wedge-shaped reflection bun-

dles with moderate- to low-amplitude, oblique-tangential

reflections with a lateral extension of 100–200 m which

downlap the underlying strata (Fig. 4e).

Basinal periplatform to hemipelagic sediments are

shown by parallel to sub-parallel, high to moderately con-

tinuous seismic reflections (Fig. 4f). Reflection ampli-

tudes range from high to low. Parallel to sub-parallel

moderate- to low-amplitude reflections traceable for sev-

eral kilometres and of sigmoid shape are bundled into

wedge-shaped bodies, which internally show offlap, as

well as onlapping and downlapping geometries (Fig. 4g).

In some areas, up to 1 km wide and 20–30 m high ups-

lope-climbing offset wave-shaped reflections of high to

low amplitude occur in the sigmoidal sediment packages

(Fig. 4h). Single wavy seismic reflections show high to

low amplitudes and high to moderate continuity. The co-

occurrence of both depositional geometries attests that

these sedimentary bodies are current shaped, following

the seismic facies overview of contourite deposits pre-

sented by Faugeres et al. (1999). The last seismic facies

corresponds to the submarine dunes (Fig. 4i) which cover

the sea floor of the Inner Sea.

Seismic sequencesand facies distribution

In the following paragraphs, key seismic lines will be used

to illustrate the evolving platform architecture from the

Miocene to the Pleistocene. One data set illustrates the

bank evolution in an area, where an initial middle Mio-

cene drowning episode occurs, another data set allows on

discussion of younger drowning steps (Upper Miocene,

Pliocene). A third data set will be used to discuss the evo-

lution of areas with actively growing carbonate banks.

Drowned lower and middle Miocene bank

Line NEOMA-P65 runs from west to east in the south

Maalhosmadulu and Goidhoo inter-atoll channel and in

the Kardiva Channel (Fig. 1). This line has an excep-

tional quality and is the only one of the entire data set

which allows insight into the platform architecture below

the drowning unconformity which occurs in this part of

the carbonate bank. The line follows the track of a line

shown by Aubert & Droxler (1996; Fig. 13) (Fig. 3a). It

cross cuts a drowned bank towards the West, the Inner

Sea and a drowned banks towards the East. The western

bank, before drowning, prograded, the eastern bank

mostly aggraded (Fig. 3). Following the drowning uncon-

formity, the Inner Sea basin begins to fill up, mostly from

West to East (Fig. 3). Bank growth in the East terminated

later than in the West, above the upper limit of the N3

unit of Aubert & Droxler (1996) (Fig. 3a). After this late

Miocene drowning of the eastern bank, filling of the Inner

Sea is achieved by sediment bodies wedging out at their

western and eastern terminations (Fig. 3b, c).

Table 1. Seismic horizons overview and characteristics. aBelo-

polsky & Droxler (2004a); bPurdy & Bertram (1993)

Horizon

Impedance

contrast Polarity Age

Seabed Strong Positive Recent

bEPlio Weak Negative Late Miocene/Pliocene

MM5a Strong Positive Middle/Late Miocene

MM3a Unstable Positive Middle Miocene

bSN13 Unstable Negative Middle Miocene

E/MMa Strong Positive Middle Miocene

bMMio Unstable Negative Early/Middle Miocene

EM1a Unstable Positive Early Miocene

PB2bO Unstable Positive Early Miocene

O/Ma Strong Positive Oligocene/Miocene


C. Betzler et al.

Table

2.Seism

icfacies

characteristicsandinterpretation

summary

Seism

icfacies

Sedim

entological

interpretation

Externalform

Reflection

characteristics

ofupper

boundary

Internalconfiguration

Impedance

contrast

Continuity

Maskedplatform

Diagenetic

Strongupper

reflection.

Subparallelor

dow

nlapping

overlayingstrata.

Chaoticreflection

pattern,single

horizontaltosubhorizontalor

basinwarddipping

reflectionsappear

Weakor

none

Low

ornone

Inner-platform

Platform

interior

Sheetor

wedge

Strongupper

reflection.

Subparallel

ordow

nlapping

overlayingstrata.

Paralleltosubparallelor

slightly

divergingreflections

Highto

moderate

Highto

low

Reefrim/mound

Interlocking

reeffram

ework

Convex-upshaped

moundor

wedge

Strongupper

reflection

Discontinuoustochaotic

reflections

Weak

Low

Sigmoidslope

Platform

slope

Sheetor

wedge

Dow

nlapsurfacewhen

overlain

bylowstanddeposits

Sigmoidreflections

Highto

moderate,

decreasing

dow

nward

High

Tangentialslope

Platform

slope

Wedge

Strongtop-bounding

reflections

Oblique-tangentialreflections

Moderatetolow

Low

Basinalhem

ipelagics

Periplatform

to

hem

ipelagic

foraminiferaand/or

nannofossilbearingooze

Sheet

Paralleltosub-parallel

reflections

Highto

low

Highto

moderate

Drift

Current-induced

deposited

sedim

ents

Wedge

Paralleltosub-parallelsigm

oid

reflections,show

ingofflap,

aswellasonlapanddow

nlap

Moderatetolow

High

Migratingsandwaves

Bottom

current-induced

re-deposited

sedim

ents

Upslopeclim

bing,offset

wave-shaped

reflections

Highto

low

Moderate

Submarinedunes

Bottom

current-induced

re-deposited

sedim

ents

Sheet

Singlewavyreflections

Highto

low

Highto

moderate



Figure 3b, c shows the upper 1.5 s TWT of this suc-

cession in the newly acquired Line NEOMA-P65. The

asymmetric bank growth is well imaged in the line,

although the area of the Inner Sea contains some sediment

deformation structures which are related to gas migration

which were discussed in detail by Betzler et al. (2011). Inaddition, the sedimentary succession of the western bank

appears to be bended over fault blocks, which according

to Aubert & Droxler (1996) affect the acoustic basement.

For the description and discussion of the predrowning

bank evolution, more detail is shown in Fig. 5 for the

lower to middle Miocene bank-margin progradation

towards the Inner Sea between 0.75 and 1.25 s TWT (ca.935–1550 mbsl). Ten seismic sequences can be differenti-

ated in this succession. Sequences (S) and sequence

boundaries (SB) are numbered 1–10, from the bottom to

the top, and the aggradation/progradation ratio is given

in Table 3. All seismic sequences S1–S10 show a slight

basinward dip of 0.6–1.0°.

Seismic sequence S1. Sequence 1 (S1) (Figs 5 and 6) is the

oldest seismic sequence above the Oligocene-Miocene

boundary. The lower delimiting sequence boundary 1

(SB1) is poorly defined in the high-resolution data set,

but correlates with sequence boundary PB2 defined by

Purdy & Bertram (1993) (cf. Table 1). The platform-inte-

rior area of S1 deposits spans about 5 km from the open

Indian Ocean to the Inner Sea side. S1 is wedge shaped

showing an overall ramp-like morphology with a maxi-

mum thickness of 0.09 s TWT (ca. 110 m) facing the

Indian Ocean and thinning out towards the Inner Sea

with a gentle relief of approximately 1.5° (Fig. 5). The

seismic facies of S1 is entirely the Inner-platform seismic

facies (Fig. 4b), but reflection strength appears to increase

basinwards. The platform interior shows an approxi-

mately 1.5-km wide, progressively filled depression

(Fig. 6) which is rimmed by mound-shaped banks to the

W and E. Reflections dip slightly into this structure from

the West and East, some are characterized by downlaps

(Fig. 6). The platform margin facing the open Indian

Ocean generally aggrades (Fig. 5). The upper sequence

boundary of S1 has an irregular relief.

Seismic sequence S2. Deposits overlying SB 2 onlap this

boundary slightly basinward of the SB 2 bank edge

(Fig. 6). The seismic sequence 2 (S2) is characterized by

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

Fig. 4. Examples of seismic facies defined in the high-resolution seismic data set. Details of the different facies are listed in Table 2.


C. Betzler et al.

Fig.5.(a)Closerview

ofthewestern

partof

seismiclineNEOMA-P65

(verticalexaggeration–7.5x)runningwest-eastthrough

northernKardivaChanneland(b)interpretation.S1–S

10andSB1

–SB10

refertosequencesandsequence

boundaries,respectively,discussed

inthetext.Theblack

lineshow

sthebankedge

grow

thpath.DH–D

iffraction

hyperbola.



distinct basinward-directed lateral growth and steepening

of the platform margin (Fig. 5), which develops from a

ramp-like configuration into a prograding platform with

a steep high-relief margin wedging out basinwards. The

maximum vertical relief is about 0.18 s TWT (ca.225 m) at the end of the sequence. Progradation during

S2 growth led to a platform width of 10 km. The thick-

ness of the sequence ranges from 0.10 s TWT (ca.125 m) in a mostly aggrading platform-interior part, rep-

resented by Inner-platform seismic facies, to a maximum

of 0.20 s TWT (ca. 250 m) at the basinward margin.

Oblique-tangentially arranged reflection bundles with a

dip of up to 11°, interpreted as Tangential slope seismic

facies, downlap onto SB1 towards the Inner Sea. The

Fig. 7. Multibeam image of the eastern

end of Kardiva channel showing the

drowned banks in this area of the Mal-

dives carbonate edifice. Note the occur-

rence of a terrace at 750 m of water

depth, attesting a first backstepping of

the bank margins. The shape of the

drowned isolated carbonate banks attest

that currents where a major controlling

factor of platform shape. This is best

demonstrated by the shape comparison

with the current-controlled active atoll

Gaafaru Falhu (Worldwind satellite

image in the inlay), located 5-km south of

the area shown by the multibeam map.

The white line shows the trace of line

P65 (Fig. 3).

Fig. 6. (a) Closer view of seismic line

NEOMA-P65 (vertical exaggeration–5x;see frame in Fig. 5 for exact location) and

(b) line drawing. During seismic

sequence S1 a depression between 1.05

and 1.15 s TWT (ca. 1310–1435 mbsl) is

laterally filled. This structure is inter-

preted to represent a lagoon. Seismic data

are displayed with adjusted colour scale.

S1–S2 and SB2–SB3 refer to sequencesand sequence boundaries, respectively,

discussed in the text. Numbers along

horizontal axis refer to trace numbers.

Table 3. Aggradation versus progradation of seismic

sequences. Parentheses indicate minimum values, as erosion

could not be excluded

Sequence Aggradation (m) Progradation (m) A/P

S1 110 (0) –S2 125 5000 0.025

S3 60 0 –S4 0 8700 0

S5 85 1300 0.07

S6 110 2000 0.06

S7 (75) 1800 (0.04)

S8 (0) 1500 (0)

S9 (0) 1000 (0)

S10 (0) 1500 (0)


C. Betzler et al.

uppermost part within the platform has high-amplitude

reflections and a strong, irregular reflection on top of S2

which has a relief of 0.2 s TWT (ca. 150 m).

Seismic sequence S3. Although SB 3 is partly masked at

the western flank of the carbonate bank by diffraction

hyperbola (Fig. 5), the level to where the boundary

can be traced corresponds to an approximately 200 m

wide terrace in a water depth of 750 m. A comparable

step, 250 m wide in a water depth of 750 m, occurs at

the eastern flank of the Maldives carbonate edifice

(Fig. 7).

In the lower part of S3, internal reflections onlap and

downlap the slope and toe of slope of the top of the S2

carbonate bank and are gently inclined basinwards

(Fig. 5). The Inner-platform seismic facies of S3 overlies

S2 strata and shows a laterally constant thickness of 0.05 s

TWT (ca. 60 m). Slope deposits reach a thickness of

0.06 s TWT (ca. 75 m). The package of the toe-of-slope

deposits thickens to up to 0.12 s TWT (ca. 150 m). The

vertical relief at the end of S3 is 0.15 s TWT (ca. 180 m),

albeit the slope deposits are less inclined than the S2

deposits (Fig. 5). Reflection amplitude within S3 is mod-

erate to weak. Reflection continuity is moderate to low

within the platform interior, and moderate at and below

the toe of slope. During formation of S3, the platform

margins facing the Inner Sea and the Indian Ocean show

aggradation.

Seismic sequence S4. Deposits in the lower part of S4 on-

lap and downlap SB 4 (Fig. 5; traces 4300–5100), whichis imaged as a very subtle, continuous reflection with an

irregular relief. The sequence is 0.04 s TWT (ca. 50 m)

thick in its western part which faces the Indian Ocean.

Towards the East, it thickens to 0.15 s TWT (ca. 190 m),

and towards the Inner Sea thins basinwards into a Basinalhemipelagics seismic facies (Fig. 5). Internally, Inner-plat-form seismic facies reflections onlap the S3 seismic

sequence at the toe of slope and show a basinwards dip of

1.5°. Reflection amplitude is generally moderate to low,

reflection continuity increases towards the toe of slope.

Within the proximal part of the sequence, parallel to sub-

parallel reflection bundles are laterally interrupted by

convex-up or mound-shaped reflection areas of Reef rim/mound seismic facies (Fig. 8). These zones are each top-

bound by a strong reflection and show a weak-amplitude

to transparent discontinuous or chaotic internal reflection

pattern. Mounds measure around 0.01 s TWT (ca. 12 m)

in height and 200–250 m in width. During the late stage

evolution of S4, a distinct bank edge develops, overlying

(a)

(b)


NEOMA-P65 (vertical exaggeration–5x)and (b) line drawing. Sequences S4–S6are characterized by mound-shaped

reflection bundles being manifested at

the offlap break. Seismic data are dis-

played with adjusted colour scale. v–ver-tical stacking, a–back-reef apron. S4–S7and SB5–SB8 refer to sequences andsequence boundaries, respectively, dis-

cussed in the text. Numbers along hori-

zontal axis refer to trace numbers.



basin-dipping reflections (Fig. 8). The reef rim progrades

around 10 km towards the Inner Sea.

Seismic sequence S5. SB5 corresponds to a reflection

which truncates S4 deposits in the inner platform (Fig. 5;

interval between traces 4600 and 4750). Towards the

Inner Sea, the sequence boundary is imaged as a high-

amplitude reflection, which decreases basinwards. Fur-

ther basinwards this reflection has moderate amplitudes.

During sequence 5 (S5), the bank has a steep-flanked

rimmed platform geometry, prograding towards the Inner

Sea (Figs 5 and 8). Reflections below the offlap break

show the concave-up profile and an oblique-tangential

arrangement to each other, which are attributes combined

in the Tangential slope seismic facies. The vertical relief of

the margin develops from 0.05 s TWT (ca. 60 m) to a

maximum of 0.12 s TWT (ca. 150 m) at the end of S5.

The thickness of the S5 package ranges from 0.07 s TWT

(ca. 85 m) within the inner platform to about 0.14 s

TWT (ca. 170 m) at the platform margin. The sequence

thins basinwards and passes into the Basinal hemipelagicsseismic facies. Reflection amplitude is generally moderate

or high, as is the reflection continuity, with reflections

traceable over distances of 4 km or more (Fig. 8). In the

zone of the slope, reflection amplitude is reduced, individ-

ual reflections can be traced over distances of up to

1.5 km. During formation of seismic sequence S5, the

Inner Sea side bank margin progrades about 1.3 km,

which results in a total platform width of more than

20 km. The bank margin facing the open Indian Ocean

continuously aggraded 0.05 s TWT (ca. 75 m) (Fig. 5).

Seismic sequence S6. Sequence S6 represents a thick ag-

grading platform forms, which is accompanied by a pro-

nounced steepening of the platform flank (Figs 5 and 8).

The thickness of the total package increases towards the

margin facing the Inner Sea, from 0.09 s TWT (ca.110 m) at the Indian Ocean to a maximum of 0.23 s

TWT (ca. 285 m). S6 thins basinwards and passes into

Basinal hemipelagics seismic facies (Fig. 5). Above SB6,

the position of the Reef rim/mound seismic facies shifts

basinwards. This seismic facies generally marks the posi-

tion of the offlap break through sequence S6 and bundles

are stacked vertically to sub-vertically. The vertical relief

of the platform increases during S6 from 0.12 s TWT

(ca. 150 m) at the base to more than 0.22 s TWT (ca.275 m), and dip angles develop from 14° at the beginningto 20° at the end of the sequence. Inner-platform reflec-

tions appear to be slightly bent downwards, i.e. show a

concave-up morphology (Figs 5 and 8). Below this inner-

platform area, the almost vertically stacked Reef rim/mound seismic facies pinches out towards the bank interior

forming a back-reef apron (Fig. 8). Internal reflections

dip towards the platform interior. Both amplitudes and

continuity of reflections in the aggrading part of the plat-

form are high to moderate. In contrast, the slope seismic

facies shows concave moderate- to low-amplitude reflec-

tions of high to moderate continuity arranged in an

oblique-tangential pattern. The platform edge advances

1–2 km towards the Inner Sea.

In contrast to the Inner Sea bank margin, the margin

facing the open Indian Ocean shows a backstepping geom-

etry (Fig. 5). Overlying SB 6, a 600 m wide and 0.017 s

TWT (ca. 20 m) high body occurs, which is mound-

shaped in cross section (Fig. 5). Multibeam data show that

this body forms an irregular step fringing the Maldives

bank margin. Sequence 6 internal reflections of relatively

high-amplitude bend down and away from the margin and

(b)

(a)


NEOMA-P65 (vertical exaggeration–5x)and (b) line drawing. Sequences S7–S10are characterized by an alternation of

oblique-tangentially and sigmoidally pro-

grading geometries. Seismic data are

displayed with adjusted colour scale.

e–erosional truncation, o–oblique-tan-gential clinoforms, s–sigmoidal clino-

forms, m–current moat. S6–S10 and SB7–SB10 refer to sequences and sequenceboundaries, respectively, discussed in the

text. Numbers along horizontal axis refer

to trace numbers.


C. Betzler et al.

towards the platform interior. These reflections are on-

lapped by sub-horizontal, nearly parallel reflections of the

Inner-platform seismic facies (Fig. 5). Sequence S6 is top-

bound by a very strong reflection and in parts covered by

sediments. The sequence top is overlain by the following

sequence S7 in the platform-interior area. Basinwards, S6

is downlapped by lowermost S7 sediments.

Seismic sequence S7. SB7 correlates approximately with

the bMMio (Fig. 5, cf. Table 1). S7 comprises two dis-

tinct reflection packages (Fig. 9). One package is a 4-km

wide wedge Inner Sea-ward of the S6 bank edge. This

package consists of Tangential slope seismic facies, with

prograding growth geometry. Oblique-tangential reflec-

tions downstep and downlap onto the slope and toe of

slope of the underlying sequence S6, respectively. The

dip angle of individual reflections is up to 18°. Reflectionstrength and reflection continuity are low in the steepest

clinoform portion but increase towards the bottomset.

This sub-package is bound by a strong reflection at the

top. The overlying slope package consists of parallel or

sub-parallel sigmoidal reflections of the Sigmoid slope seis-mic facies. The sigmoids dip with up to 30°, have high to

moderate amplitudes, and the reflections are highly con-

tinuous. Single reflections can easily be traced from the

proximal part across the slope and beyond into the basin.

This sigmoidal reflection bundle aggrades 0.14 s TWT

(ca. 175 m) and progrades for more than 0.5 km. In the

bank interior, the Inner-platform seismic facies of S7 con-

formably overlies the S6 seismic sequence. The thickness

of S7 ranges from less than 0.06 s TWT (ca. 75 m) within

the aggrading portion of the platform to a maximum of

almost 0.25 s TWT (ca. 310 m) at the basinward platform

margin from where it very slowly thins distally. The max-

imum vertical relief inferable is about 0.20 s TWT (ca.250 m). Erosional truncation occurs in the upper part of

the sequence at the platform margin facing the open

Indian Ocean (Fig. 5; interval between traces 5200 and

5300). The sequence is has a strong reflection at the top,

where it is unconformably overlain by sediments of Plio-

cene or younger age.

Seismic sequences S8, S9 and S10. Sequences 8–10 (S8–S10) have a similar internal architecture with a repeated

alternation between packages of oblique-tangential and of

sigmoidal reflections. The lower part of the sequences

consist of the wedge-shaped reflection packages accroached

basinwards of the offlap break of the underlying sequence

boundaries (Fig. 9). The wedges are overlain by the bun-

dles of sigmoidal reflections. Aggradation of bank

interiors occurred mostly during phases of sigmoid pro-

gradation (Fig. 9) with maximum thicknesses for S8 of

0.17 s TWT (ca. 210 m), S9 of 0.14 s TWT (ca. 175 m)

and S10 of 0.17 s TWT (ca. 210 m). The upper 0.05 s

TWT (ca. 60 m) of the outermost part of sequence S10

has the chaotic, weak-amplitude reflection pattern of the

Masked platform seismic facies. Sequences S8–S10 show

erosional truncation removing S8–S10 platform-interior

deposits (Figs 5 and 9). Truncation of all four post-early

Miocene sequences S7–S10 starts at a depth of 0.75 s

Fig. 10. Closer view of seismic line NE-

OMA-P65 (vertical exaggeration–2x)with current-related features indicated. t

–low-angle erosional truncation, w–sedi-ment waves, o–offlapping geometries.

Fig. 11. Multibeam map of the inter-atoll passage floor between South Maalhosmadulu and Goidhoo atolls with submarine features

indicated. Water depth ranges from 750 m (pale yellowish green) to 400 m (red-orange). d–submarine dunes, u–uncovered platform.

Seismic line NEOMA-P65 runs in the centre of the stripe.



TWT (ca. 935 mbsl) and reaches down to 0.79 s TWT

(ca. 985 mbsl). This upper boundary is unconformably

overlain by younger sediments, which onlap the slope

(Fig. 5; position around trace 4050, Fig. 9). During depo-

sition of sequences S7–S10, the platform width between

the open Indian Ocean in the West and the youngest pat-

tern of progradation on the Inner Sea side increases by

6 km from approximately 25 km to as much as 31 km.

Post-S10 deposits. The seismic-reflection pattern changes

significantly above the E/MM sequence boundary

(Fig. 5). An eastward prograding sediment body, built of

large-scale sigmoid-shaped clinoform bundles forming a

convex-up shaped wedge, is overlying the entire slope of

S10. The body consists of the Drift seismic facies. In the

western part of the wedge, reflections are arranged in a

parallel to sub-parallel or oblique-divergent way.

Towards the Inner Sea, reflections pass into an oblique-

tangential or sigmoidal pattern, and finally thin into a

Basinal hemipelagics seismic facies (cf. Fig. 3). Internally,

reflections show off- and onlapping geometries, and

reflections or reflection bundles are erosionally truncated

at low angles (Fig. 10). Partially, these erosion surfaces

dip bankwards, i.e. away from the Inner Sea. In the

wedge, an area of laterally offset, stacked wavy reflections,

which can have an erosional base, occur. The whole sedi-

ment body is bound at the top by an erosional unconfor-

mity, which reaches a depth of 0.72 s TWT

(ca. 900 mbsl). This horizon is overlain by horizontal to

sub-horizontal, parallel reflections of high to moderate

continuity and moderate to low amplitude. Towards the

Indian Ocean reflections are wavy, interpreted as large-

scale submarine dunes based on the occurrence of such

dunes on the modern seafloor (Fig. 11).

Thickness of the prograding body decreases towards

the Inner Sea. At the eastern flank of the Inner Sea, the

Basinal hemipelagics changes into a sediment body show-

ing the characteristics of the Drift facies. The strati-

graphic relationship between this body and the eastern

bank is not resolvable, as geometries are masked by the

strong reflection at the bank top.

Upper Miocene and Pliocene drowning

The stratigraphy developed for the high-resolution set

can be correlated with the published lower-resolution

industrial lines. Table 1 provides the correlation of

sequence boundaries defined in the high-resolution seis-

mic data to horizons in the low-resolution data set. In

addition, the developed sequence-stratigraphic subdivi-

sion can be carried to the platform areas that are poorly

imaged in the new data set.

Figure 12 shows the west–east-directed seismic line

MLD-73-12 acquired by Elf (Aubert & Droxler, 1996),

which runs north of Ari atoll across the southern part of

Fuad Bank around 60-km south of line NEOMA-P65

(Fig. 1). Line MLD-73-12 images a more southerly part

of the bank and also shows progradation and steepening

of the bank margin during formation of sequences S1–S10 between 1.2 and 0.7 s TWT (ca. 1500–875 mbsl).

In contrast to the geometries imaged on line NEOMA-

P65, which show drowning after deposition of S10, the

area of the bank shown in line MLD-73-12 continued to

prograde and slightly aggrade beyond seismic horizon

E/MM, but ceased shortly after sequence boundary

MM5. In front of this bank edge, there is a package

with highly continuous, sigmoid-shaped, basinwards

dipping reflections (Fig. 12). Although resolution of the

seismic data is much lower than in the new high-resolu-

tion data, the geometrical characteristics of this package

allow assigning the Drifts seismic facies (Fig. 4g) to this

reflection pattern.

Seismic line NEOMA-P43 (Fig. 13) images the situa-

tion at the bank edge of this platform just 5 km further

Fig. 12. Published (Aubert & Droxler,

1996) (a) uninterpreted seismic line

MLD-73-12 (vertical exaggeration–5x)from Elf running north of Ari atoll west–east through Fuad Bank and (b)

interpretation. S1–S10 refer to sequencesdiscussed in the text.


C. Betzler et al.

northwards (Fig. 1). The higher resolution seismic data

reveal that the high-continuity sigmoids (seismic line

MLD-73-12, Fig. 12) are separated by a moat from the

aggrading bank margin and that individual reflections

become wavy in shape towards the Inner Sea. The sigm-

oids have medium to low amplitudes. The bank margin is

imaged above the first seabed multiple, where it is charac-

terized by aggradation (Fig. 13) and is delineated by

high-amplitude reflections towards basinal strata.

A 1.2 km backstepping of this margin occurs approxi-

mately 0.05 s TWT (ca. 30 m) below the reflection mark-

ing the base of the Lower Pliocene (bEPlio). Above the

step formed by the drowned bank, a high- to medium-

amplitude wedge-shaped bundle of oblique reflections

occurs, which forms a wedge basinwards downlapping

onto the underlying bank top. Discontinuous, high-

amplitude-reflections appear to delineate this bundle to

the west, i.e. towards the bank interior; however, some

Fig. 13. (a) Uninterpreted seismic line

NEOMA-P43 (vertical exaggeration–5x)running north of Ari atoll southwest–northeast through the eastern edge

of Fuad Bank and (b) interpretation.

m–current moat, thick, hatched lines tracereflection multiples.

Fig. 14. Published (Purdy & Bertram,

1993) (a) uninterpreted seismic line MA-

74-53 (vertical exaggeration–5x) from Elf

running west–east through South Male

atoll and (b) interpretation. S1–S10 referto sequences discussed in the text.

Amplitudes and continuity of sub-hori-

zontal platform-interior reflections later-

ally increase seawards. In the respective

clinoform, amplitudes and continuity

abruptly decrease. The transition is

marked by the clinoform edge, which

trace the growth history.



reflections are traceable bankwards across this boundary.

The bank margin is backstepping, until bank growth in

this area of the Maldives terminates, which is indicated by

the occurrence of a continuous flat high-amplitude reflec-

tion draped by medium- to low-amplitude discontinuous

reflections with hummocky shapes. The bank top lies at a

depth of 0.32 s TWT (ca. 400 mbsl). Above SB bEPlio

(Fig. 2), the wedge-shaped bundle, which occurs in front

of the margin of the latest growth stage of the bank, is

overlain by a medium- to low-amplitude reflection pack-

age (Fig. 13). The package consists of high-continuity

divergent reflections, successively developing in shape

from sigmoidal to oblique-tangential. This package

wedges out at the bank edge and, in the upper part of the

succession, is separated from the bank margin by a series

of moats up to 400 m wide and some 10 s of metre deep

(Fig. 13).

Active atolls

Bathymetric maps of the sea floor indicate that the width

of the periplatform apron of the atolls in the study area

varies with respect to the proximity of the channels con-

necting the Inner Sea to the Indian Ocean (Fig. 1). The

aprons are tear-drop shaped (Betzler et al., 2009), andwidest away from the channels; flanking the channels the

apron disappear, and the slopes in front of the reef margin

plunge to the channel floor. For description of the growth

geometries of the active atolls, seismic lines from the

industrial data were chosen which are located where peri-

platform aprons are present.

Internal depositional geometries of active banks indicate

a continuous bank growth since the Early Miocene. This is

best illustrated by Elf seismic line MA-74-53 (Purdy &

Bertram, 1993), which runs west–east through South Male

atoll (Figs 1 and 14). During sequences S1–S6 the plat-

form progrades and aggrades, but only progrades during

S7–S10. Between SB E/MM and SB bEPlio, the growth

mode of the platform successively develops into an aggrad-

ing pattern. During the Miocene, the bank aggrades 0.75 s

TWT (ca. 935 m), and the bank margin progrades approx-

imately 4 km towards the Inner Sea. During Pliocene

times, the margin strongly progrades with a minor down-

stepping component. The late phase of bank growth shows

an aggrading to backstepping geometry.

The stratigraphy of seismic line MA-74-53 described

above was established by jump correlating horizons from

Shell seismic line E310NMA (Belopolsky & Droxler,

2004b) (Fig. 15). The eastern part of Elf seismic line

MA-74-2 (Purdy & Bertram, 1993) is the eastward con-

tinuation of line E310NMA and images the eastern most

part of Male atoll. Eastwards of well NMA-1, the bank

margin’s internal architecture is poorly imaged in the

medium- and in the low-resolution seismic data sets.

Nonetheless, the continuous prograding and aggrading

pattern of the Late Miocene, apparent at South Male

atoll, can also be recognized. The concave-up profile of

reflections near the position of well NMA-1, and the

upwards laterally offset stacking of high-amplitude reflec-

tions between 0.25 and 0.45 s TWT (ca. 310–560 mbsl)

east of NMA-1are interpreted to represent the platform

edge trajectory.

INTERPRETATION AND PRESENTATIONOFA NEWMODEL

Based on low- and intermediate-resolution industry seis-

mic data calibrated to two industry exploration wells

and one ODP hole, a general model for the Eocene to

recent development of the Maldives carbonate platform

has previously been delineated by Purdy & Bertram

(1993), Aubert & Droxler (1996) and Belopolsky &

Droxler (2004a). It shows the large-scale geometrical

Fig. 15. Published (Purdy & Bertram,

1993; Belopolsky & Droxler, 2004b) (a)

uninterpreted seismic line E310NMA

from Shell and eastern part of uninter-

preted seismic line MA-74-2 from Elf,

both running west–east through Male

atoll (vertical exaggeration–5x) and (b)interpretation. Well NMA-1 from Elf is

projected to Shell line. S1–S10 referto sequences discussed in the text.

A–Eocene, B–Oligocene, C–Late Oligo-cene? to Early Miocene, D–Middle Mio-

cene, E–Plio-Pleistocene?, F–Recent(ages after Purdy & Bertram, 1993).


C. Betzler et al.

Fig. 16. Set of schematic cross sections

illustrating the facies architecture devel-

opment through the Miocene to recent

time interval of the Maldives carbonate

platform.

Fig. 17. Chronostratigraphic diagram illustrating the depositional evolution of the Maldives carbonate platform. Figure is not to

scale. For colour coding please refer to Fig. 15. Platform development until earliest Miocene is adopted from Aubert & Droxler (1996).

Grey areas designate the age range of the drowning steps in the area studied. (1) Monsoon intensity is represented by the Arabian Sea

upwelling system reflected by the occurrence of the planktic foraminifer Globigerinoides bulloides (Kroon et al., 1991); (2) Sea-levelcurve fromMiller et al. (2011); (3) Sea-level curve from Kominz et al. (2008); (4) Sea-level curve from Haq et al. (1987).



development through time such as the seismically well

imaged dual bank-margin morphology of the platform

providing a bank-internal Inner Sea basin (Fig. 3). The

transition from Oligocene to Miocene is marked by ret-

rograding and then aggrading margins along the Inner

Sea side. These authors further proposed that the Mid-

dle Miocene and lower Upper Miocene are character-

ized by large-scale, basinwards prograding geometries.

The following middle Miocene to recent bank configu-

ration was described as partially drowned areas and

areas which resisted drowning and kept aggrading to

present sea level. This entire sequence was interpreted

in terms of accommodation variations proposed to result

from fluctuations in relative sea level.

Here, a different interpretation is presented, based

on the combined analysis of newly acquired high-reso-

lution seismic data and published lower-resolution seis-

mic lines (Figs 16 and 17). Based on the studied new

data, which for the first time provide clear insights into

the lower and middle Miocene bank architecture of the

Maldives (Fig. 5), currents together with sea-level

changes are interpreted to have contributed to the Neo-

gene platform evolution. The middle Miocene is the

key interval for the younger evolution of the Maldives,

because at this time reversing currents intensified and

triggered a turnover of platform configuration and

architecture.

A first indication showing the contribution of currents

in the evolution of the carbonate edifice is given by the

asymmetric growth of the western and eastern bank

(Fig. 3). Eastward progradation of the western bank of

around 20 km during the early and early middle Miocene

coincides with a mostly aggradational geometry of the

eastern bank. This is best explained by contribution of a

regular off-bank sediment transport to the leeward side of

the bank, similar to examples of carbonate platforms else-

where (Hine et al., 1981; Eberli & Ginsburg, 1987, 1989;

Schlager et al., 1994; Eberli et al., 2010). In this part of

the succession, variations of accommodation space can be

reconstructed from the bank-margin architecture (Figs 3

and 5).

This pattern is abandoned during the late middle

Miocene, when two different styles of platform devel-

opment existed contemporaneously. In some areas bank

margins drown, while other parts of the platform con-

tinue growing (Fig. 16). This synchroneity of different

stratigraphic stacking patterns is proposed to reflect a

change in the oceanographic and/or climatic factors

governing platform development which triggered a

change and also an intensification in the current

regime, so that currents from then on played a domi-

nant role for shaping platform geometries. The

modern current setup of seasonal balanced monsoonal

impact and strong, inter-monsoonal occurring

Indian Ocean Equatorial Westerlies, enforcing east-

wards directed Wyrtki Jets, provides a template for the

asymmetric filling of the Inner Sea basin since the late

Miocene.

Local tectonic control on relative sea level, so that dif-

ferent parts of the carbonate bank experience different

histories of relative sea level as a controlling factor, is not

supported by the available data. Aubert & Droxler (1996)

and Betzler et al. (2009) showed that individual drowning

unconformities can be traced into different parts of the

carbonate bank at the same depth, which argues against

differential subsidence.

LowerMioceneand lowerMiddleMiocenedevelopment of bankmargins

Reef-rimmed margins

At the Oligocene-Miocene boundary, the Maldives car-

bonate platform partially drowned and bank growth

became restricted to a narrow band at the platform mar-

gins facing the open Indian Ocean (Aubert & Droxler,

1996). Sequence S1 incorporates the upper part of unit

N1 into the first sequence of unit N2 as defined by Aubert

& Droxler (1996) (Fig. 3a). Sequence S1 thus constitutes

the latest stage of a large carbonate bank enclosing the

paleo-Inner Sea, which then was an enclosed basin. The

depression described in the new data is interpreted to rep-

resent a successively filled lagoon of an atoll-like config-

ured bank margin (Fig. 6). During a first progradational

phase of overlying sequence S2, the margin doubled in

width, which was accompanied by the burial of the ante-

cedent topography (SB2), formation of a flat-topped car-

bonate bank, aggradation of the bank interior, and a

severe steepening of the platform slope at the Inner Sea

side (Fig. 5). This is interpreted to be the result of the

establishment of a carbonate bank with a major area of

shallow water, i.e. euphotic, carbonate factory.

A minor backstepping of the bank margins facing the

Indian Ocean affected the Maldives above SB 3 (Figs 5

and 7). This backstepping is shown by the 200–250 m

wide terraces developed at the western and eastern flanks

of the edifice, at a distance of more than 60 km and at a

water depth of 750 m (Fig. 7). Therefore, a regional trig-

ger mechanism for this backstepping event is proposed,

although the available data do not allow for further inter-

pretation. The curve of tectonic subsidence for the North

Male Atoll presented by Purdy & Bertram (1993) do not

indicate any subsidence pulse for the lower Miocene.

During deposition of sequence S3, accommodation-

space creation is proposed to have decreased, provoking

the development of a thick basinal wedge (Fig. 5). This

geometrical arrangement with a wedge-shaped body in

front of the bank top and edge of the underlying sequence

also applies for sequence S4 (Figs 5 and 8), which is inter-

preted to reflect a response to continuing long-term

accommodation-space restriction and possibly downward

shifted carbonate production. This relies on the occur-

rence of an open shelf-like geometry developed in the

lower part of S4, where scattered patch reefs occur

(Fig. 5). Above SB5, the vertical stacking of bank edge

reefs followed by basinward progradation within both,


C. Betzler et al.

sequences S5 and S6, indicate two repeating phases of an

increased accommodation followed by a decrease (Figs 5

and 8).

Depositional geometries of sequence S6 reflect an

increase in accommodation. At the Inner Sea bank mar-

gin, the reef margin is characterized by aggradation, and

westward dipping strata within the wedge-shaped sedi-

ment package indicate that a backreef apron (Figs 5 and

8), suggesting high sediment production (Schlager, 2005).

At the Indian Ocean margin, the S6 margin steps back.

Thus, platform growth is asymmetric.

Margins with hanging shoulders

At SB7, which correlates approximately to the transition

from Lower to Middle Miocene, a fundamental change in

clinoform shape, as well as in the platform growth mode

occurs (Figs 5 and 9). At this boundary, the banks switch

from dominantly aggrading (S4–S6) to dominantly pro-

grading (S8–S10). The clinoform packages of sequences

S7–S10 internally show oblique-tangential geometries

and are proposed to have formed in response to a sudden

decrease in the rate of creation of accommodation. With

respect to the platform to which these bodies are attached

and by their toplapping and offlapping stacking patterns

as well as locally downlapping terminations these pack-

ages each resemble forced regressive deposits (Hunt &

Tucker, 1992; Posamentier et al., 1992) with basinwards

isolated sediment bodies. The sigmoid clinoform pack-

ages of each sequence evolved during creation of accom-

modation space while bank margins prograded, but also

aggraded during bank growth. Thus, deposition of each

package of alternating oblique-tangentially- and sigmoi-

dally-shaped clinoforms is interpreted as the response of

the platform margin to a complete cycle of reduction of

accommodation space followed by creation of accommo-

dation space. Such alternating clinoform geometry has

been reported from carbonate platforms elsewhere, e.g.

Bahamas (Eberli & Ginsburg, 1987, 1989), Mallorca (Po-

mar & Ward, 1994) and Turkey (Janson et al., 2010). Ageneral decrease in stratal thickness, as well as in height of

the aggrading portion, is interpreted to reflect a longer

term slowdown of accommodation creation. Alternatively,

this may result from the reduction in sediment production

due to changing environmental conditions.

The outermost part of the sigmoids in S7–S10 is

gently inclined towards the Inner Sea with an angle of

approximately 2–3° before bending down into the

slope. The lack of a reef-rim-fixed offlap break is

attributed to reworking and redistribution of loose sedi-

ment material by currents and waves, with a significant

portion of bank-top production transported offbank

towards the Inner Sea. In analogy to modern platforms,

preservation of these geometries results from rapid sub-

marine cementation due to good flushing of sediments

(Erlich et al., 1993). The configuration of an asymmet-

ric flat-topped platform resembles the Bahamian model

of an isolated platform being subject to the trade

winds, and exhibiting a wind- and a leeward margin

(Eberli & Ginsburg, 1987, 1989). Consequently, inhibi-

tion of reef growth is proposed to result from the nat-

ure of the substrate, as well as from high sediment

supply from the entire bank. Inadequate substrate and

rapid burial might be accompanied by downcurrent

deterioration of the water quality, comparable to what

is proposed for the Miocene Ermenek platform (Janson

et al., 2010): The margin facing the open ocean would

have much better water quality than the downcurrent

margin, as re-suspension of particulate matter by cur-

rents and wave action would promote reduced water

clarity (Wilson & Vecsei, 2005), which collaterally

impairs conditions for coral growth. Also, off-bank cur-

rents may just sweep coral larvae away.

It can be speculated whether environmental changes

affecting the carbonate factory occurred during formation

of these sequences deposited during the Mid-Miocene

Climatic Optimum (Zachos et al., 2001) (Fig. 17). Halfar

& Mutti (2005) discussed how a community replacement

in neritic environment occurred during this time, with a

bloom of red-algal carbonates in the tropics (Bourrouilh-

Le Jan & Hottinger, 1988). This was interpreted as been

triggered by a global enhancement of trophic resources.

The flat-topped carbonate-platform geometry, espe-

cially in sequences S2–S3, is best explained by growth of

a carbonate bank filling up accommodation up to sea level.

In such a scenario, relative sea-level changes appear as

most probable major controlling factor of bank develop-

ment. The overall basinward inclination of the platform

margin and bending of basinal strata in sequences S1–S6(Figs 3 and 5) is attributed to differential subsidence.

Overall long-term subsidence of 400 m for this time

interval of approximately 10 Myr is estimated.

Although the area of the Maldives carbonate platform

has certainly been affected by factors such as changes of

the dynamic topography (Moucha et al., 2008) or varia-tions of the intraplate stress (Cloething et al., 1985), thelong-term aggradational to progradational growth pattern

of the bank to a certain degree correlates with eustatic

Miocene sea-level changes as reconstructed by Kominz

et al. (2008) and Miller et al. (2011) (Fig. 17). This is

reflected by the clinoform edge trajectory (Fig. 5) of the

bank margin. Sequences S1–S3 are characterized by a

progradation followed by a basinward migration of the

bank edge above SB 4. Sequences S4–S6 show prograda-

tion with increasing aggradation in S6, which turns into

progradation to minor downstepping in S8. Progradation

with aggradation characterize S9 and S10.

Partialplatformdrowning

Synchroneity of drowning and continued bank-margingrowth

Above SB 10, in most areas of the Maldives bank margins

persisted and growth continued through progradation

and slight aggradation (Fig. 12). Other parts of the



platform drowned (Fig. 5). Left behind as persisting

atolls rose, drowned parts began to form passages, sepa-

rating carbonate banks to the north and south, and con-

necting the open Indian Ocean to the Inner Sea (Fig. 7)

(Aubert & Droxler, 1996). This turnover occurred

approximately during the late Middle Miocene (SB E/

MM) and correlates to a positive kick in gamma-ray

intensity in the well ARI-1, emphasizing a change in

lithology from limestone to marly limestone (Fig. 2).

This event of partial platform drowning can be traced

within the entire Maldives archipelago. The penultimate

southern line E970 of the Shell seismic grid (Belopolsky

& Droxler, 2004b), which transects the Maldives archi-

pelago at the southern end of the Inner Sea (ca. 2°40′N;

Fig. 1b), mirrors the situation described from the bank

margin between South Maalhosmadulu and Goidhoo

atolls (Fig. 5). It displays the drowned bank margin at the

eastern side of the Inner Sea and the SB E/MM being on-

lapped by basinwards restricted sediments. With ca.0.72 s TWT (ca. 900 mbsl), the depth of the drowning

unconformity at the bank top coincides fairly well with

the drowning depth observed throughout the high-resolu-

tion seismic grid. The synchroneity of drowning and con-

tinued bank-margin growth as well as the constant

position of the drowning unconformities contradict a sea-

level mechanism for this phenomenon (Aubert & Droxler,

1996).

The shape of a series of drowned banks in the eastern

Kardiva Channel (Kaashidhoo Kuda Kandu; Fig. 1),

probably late middle to late Miocene in age, indicates that

partial drowning of the Maldives carbonate platform coin-

cides with the onset or intensification of a current system

(Fig. 7). This is interpreted based on the perfect similar-

ity of shape elongation of this drowned bank with still

active atolls of the Maldives located just 5 km away of the

drowned edifice. Elongation of the drowned atoll indi-

cates occurrence of an westerly current, which would

complement the previously existing easterly system. The

combined effects of short-term sea-level rise as well as

current-induced upwelling and subsequent injection of

nutrients into the surface waters are proposed to have

provoked environmental stress and to have reduced or

even prevented further reef growth (Betzler et al., 2009).A similar control on drowning by current-driven nutrient

input into shallow waters was presented by Mutti et al.(2005) as a cause for Paleogene-Neogene carbonate bank

drowning at the northern Nicaraguan Rise. The differ-

ence between this established example and the herein pre-

sented case study is that the Maldives partial drowning

was triggered by a system of seasonally reversing cur-

rents.

A precursor of current-induced upwelling at the flanks

of the Maldives is proposed to be reflected by the asym-

metry of sequence S6, with its backstepping margin facing

the Indian Ocean, probably records a first charge of this

monsoonal impact. Such a margin backstepping, in other

cases, such as the Neogene Liuhua carbonate platform in

the China Sea (Erlich et al., 1990) has been related to ele-

vated nutrient input, which there is corroborated by anal-

ysis of carbonate facies and geochemistry (Sattler et al.,2009).

The interpretation of post-S10, i.e. post-drowning,

deposits rely on several observations (Figs 5 and 7). First,

the geometry of this prograding packages is characterized

by a convex-up form, as opposed to the clinoform geome-

tries of the underlying carbonate bank (Fig. 5). This and

the regular bedding is taken as indication that the pro-

grading bodies do not bear any reef bodies capable to

build a steep bank edge. Second, as shown by Belopolsky

Fig. 18. Block diagram sketch showing the middle Miocene to

Pliocene evolution of the western part of the Maldives carbonate

edifice. Partial drowning initiated in passages separating active

banks. Sediment depocentres developed at the down-current

flanks of the drowned bank and were reworked by contourite

currents. Numbers refer to location of key seismic lines dis-

cussed in the text.


C. Betzler et al.

& Droxler (2004a) in isopach maps, lobe-shaped accumu-

lation maxima are only attached to the margin of the

drowned bank adjacent to present day inter-atoll passages

and hence exclusively to areas which drowned during the

late middle Miocene (Aubert & Droxler, 1996). This is

also displayed in the line P6 (Fig. 3) which shows the

southward thinning of this sedimentary interval, away

from the area covered by line P65. This is taken as an

indication that the sediment deposited in the lobes was

delivered by a point source. Third, the prograding bodies

contain sediment waves (Fig. 5), which indicate sediment

reworking through bottom currents. According to Fau-

geres et al. (1999) such waves form through the action of

contourite currents. Fourth, the sediment bodies at their

end facing the drowned bank are separated from the bank

by a channel-like depression (Figs 5 and 8) interpreted as

a current moat as shown for contourites by Faugeres et al.(1999). Therefore, the lobe-shaped body is interpreted as

a current-reworked ‘mega spill over’ deposit, formed on

the down-current side of the drowned carbonate bank

under the co-influence of a contour-parallel current.

Figure 18 shows the main steps which are proposed for

this middle to upper Miocene evolution. The lower

middle Miocene is characterized by a carbonate bank

shedding sediment into the Inner Sea along a line source.

During the late middle Miocene, an additional point

source delivery of sediment started, feeding the lobes

(Fig. 18). Grey arrows show the different current direc-

tions affecting lobe development and are in line with the

regime of seasonally reversing monsoon-triggered flows.

The current-affected ‘mega spill over’ lobes are thought

to consist of an admixture of pelagic and neritic compo-

nents; it is proposed that neritic components are mostly

derived from the shallow carbonate banks fringing the

Inner Sea and the passages between the banks. It remains

open for speculation if the channels were also loci of a

thriving carbonate factory which may have additionally

contributed sediment into the drift bodies. Such a factory

may have been subjected to elevated nutrient contents,

triggered by the forcing of the oceanic currents through the

channels. The Kalukalukuang Bank (Roberts et al., 1987),which is situated in the southern Makassar Street between

Borneo and Sulawesi (Indonesia), may serve as a modern

analogue for environmental conditions which prevailed

within the passages. Scattered coral reefs occur on this

bank, with no corals occurring below 15 m of water depth.

On the up to 100 m deep bank, seasonally reversing mon-

soon currents are responsible for upwelling of nutrient-rich

waters, favouring growth of Halimeda bioherms in water

depths of 20–60 m. Mounds are not consolidated bodies,

and reworking of the mound crest is indicated by bedforms

at the sediment surface (Roberts et al., 1988). Therefore,such a carbonate factory can be seen as a potential source

shedding particles into the lobes.

Passages between the banks eventually widened during

the late Miocene through drowning of further bank areas

(Figs 7, 12, 13 and 18. This widening triggered a broad-

ening of the current stream, sedimentologically reflected

by a more regular sediment input into the Inner Sea and

the compensation of the lobe depositional relief during

the late Miocene and early Pliocene (Fig. 1).

In addition to the onset of bank demise in the upper

Middle Miocene (Fig. 17), two further phases of partial

platform drowning are recognized in the area studied.

Bank-interior geometries prior to drowning, however,

cannot be reconstructed with the same detail, as the

drowning unconformities mask the bank-interior succes-

sion. The drowning episodes are reflected by the upper

Miocene backstepping margin of the bank and finally the

complete lower Pliocene drowning of the bank which are

shown in Fig. 13. These two steps may be correlated with

phases of enhanced Indian monsoon intensity (Kroon

et al., 1991), based on planktic foraminiferal associations

in the Arabian Sea. These phases are found to be linked to

coeval uplift episodes of the Asian continent (Zheng

et al., 2004) and subsequent enhanced rates of erosion.

The record of enhanced erosion is provided by rates of

Himalayan and Tibetan sediment influx to the deep

Indian Ocean (Rea, 1992).

Activeatolls

The evolution of the active atolls since the middle Miocene

cannot be interpreted in great detail, as only low-resolution

industrial lines provide insights into these complexes

(Figs 14 and 15). The clinoform edge trajectory through

sequence S10 is comparable to the trajectory identified in

the drowned middleMiocene bank (Fig. 5), although over-

all progradation does not surpass 10 km. This low value

compared with the 20 km of the drowned bank corrobo-

rates the evidence for a system of easterly currents during

the early and early middle Miocene bank growth. During

the late Miocene, both bank margins of the active atolls

prograded and aggraded, the Pliocene is characterized in

both cases by pure aggradation with slight downward shift

of the bank edge (Figs 14 and 15), although the bank edge

of North Male Atoll migrated around 10 km, compared to

1–1.5 km in SouthMale Atoll.

Both bank margins shifted into an aggradational mode

approximately during the late Pliocene to early Pleisto-

cene. This trend of bank growth is in line with the long-

term eustatic sea-level lowering shown by Miller et al.(2011). As expressed by Aubert & Droxler (1996), the

change from progradation to aggradation is best explained

by initiation of the regime of high-amplitude and high-

frequency sea-level changes. It is proposed that in such a

system and under a persistent current winnowing of the

slopes, banks are forced into an aggradational mode,

because they do not develop slope aprons which are nec-

essary to allow progradation of the bank edge.

CONCLUSIONS

A detailed reconstruction of the Miocene to recent seismic

facies and geometries of the Maldives carbonate platform



allows to show that this edifice is subjected to two major

controlling factors: sea-level changes and currents. The

platform geometries can be explained as a result of varia-

tions of the relative impact of these factors on sediment

accumulation, but probably also on the carbonate factory.

By tracing bank-internal geometries and facies, it can be

shown that the Maldives carbonate platform during the

late Middle Miocene was subjected to a strengthening of

the current regime, which in time correlates with the

onset and/or intensification of the Indian monsoon.

Lower and middle Miocene strata are grouped into 10

sequences that formed in response to variations of accom-

modation space primarily driven by relative sea level.

Sequences 1–6 (S1–S6) show the development from an

isolated shallow ramp with downslope buildups to a

steep-flanked reef-rimmed carbonate platform. Bank edge

reefs protect the lagoon, where backreef aprons occur. At

the transition from sequence 6–7, a general switch from

dominantly aggrading to dominantly prograding bank

margins occur, which is accompanied by a fundamental

change in clinoform shape. Each of the sequences 7–10(S7–S10) is composed of deposits, which are proposed to

have formed in response to a forced regression, and is

overlain by deposits formed during re-flooding of the

bank margins. These sequences show no bank edge reefs,

similar to other carbonate platforms of the Indopacific

realm, where a community replacement in neritic envi-

ronment occurred.

The upper Middle Miocene is characterized by the

appearance of a large-scale clinoform bodies, lobate in

shape, attesting the onset of current amplification. These

bodies are attached to passages where bank segment

drowned, while in other parts bank grew on. Lobes are

interpreted as ‘mega spill overs’ fed by easterly currents,

and reworked by a current system flowing obliquely or

normally to this main stream. Such currents flowing in

different directions are in line with the seasonal reversals

linked to the monsoon. To further drowning steps

affected the Maldives, during the late Miocene and the

early Pliocene.

Hence, since late middle Miocene times the Maldives

show a twofold configuration of bank development of per-

sisting growth, which is accompanied by partial platform

drowning and associated localized deposition of contou-

rites and drifts in areas just some kilometre apart. This

interpretation implies that a downward shift of clinoform

tops and offlapping geometries in carbonate platforms are

not necessarily indicative for a sea-level driven forced

regression or aggrading bank margins. The Maldives pro-

vide an example, where stratal patterns are an equivocal

base for the interpretation of sea-level fluctuations. It

shows that sediment accumulation rate is a consequence

of rate of creation of accommodation, production rate,

and transport rate. In the case presented, the interpreta-

tion of the seismic geometries suggests that all three var-

ied both spatially and temporally. Such depositional

systems are probably more frequent in the geological

record than previously thought in sequence-stratigraphic

models that tend to ignore basic observations. Findings

are expected to be applicable to other examples of Ter-

tiary platforms in the Indo-Pacific region.

ACKNOWLEDGEMENTS

We thank the efficient technical assistance of CaptainWal-

ter Baschek, the officers and the crew of the R/V Meteor,who all together contributed significantly to the success of

the cruise, and the Shipboard Scientific Party of R/V

Meteor cruise leg M74/4 for substantial onboard support.

The Bundesministerium fur Bildung und Forschung and

the Deutsche Forschungsgemeinschaft is gratefully

acknowledged for project funding (NEOMA, 03G0667A;

Be1272/20). We wish to thank Paradigm for providing

ProMax software for seismic data processing and Schlum-

berger for providing Petrel for seismic data analysis. The

in-depth and constructive reviews of Gregor Eberli, Klaas

Verwer, Xavier Janson, Peter Burgess, KenMiller and the

journal editors significantly helped us and are greatly

acknowledged. Philipp Konerding is thanked for digitiz-

ing the Elf seismics paper prints. Torge Schumann is

thanked for helping to polish the manuscript.

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Manuscript received 19 March 2011; In revised form 05April 2012; Manuscript accepted 30 April 2012.


C. Betzler et al.

Sea-level and ocean-current control on carbonate-platform growth, Maldives, Indian Ocean

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