The leaking bucket of a Maldives atoll: Implications for the understanding of carbonate platform drowning

Marine Geology 366 (2015) 16–33

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The leaking bucket of a Maldives atoll: Implications for the understanding of carbonate platform drowning Christian Betzler a,⁎, Sebastian Lindhorst a, Thomas Lüdmann a, Benedikt Weiss b, Marco Wunsch a, Juan Carlos Braga c a b c

Institut für Geologie, CEN, Universität Hamburg, Hamburg, Germany Institut für Geophysik, CEN, Universität Hamburg, Hamburg, Germany Departamento de Estratigrafía y Paleontología, Universidad de Granada, Granada, Spain

a r t i c l e

i n f o

Article history: Received 24 December 2014 Received in revised form 17 April 2015 Accepted 26 April 2015 Available online 28 April 2015 Keywords: Lagoon Relict platform Indian Ocean

a b s t r a c t Seismic and multibeam data, as well as sediment samples were acquired in the South Malé Atoll in the Maldives archipelago in 2011 to unravel the stratigraphy and facies of the lagoonal deposits. Multichannel seismic lines show that the sedimentary succession locally reaches a maximum thickness of 15–20 m above an unconformity interpreted as the emersion surface which developed during the last glacial sea-level lowstand. Such depocenters are located in current-protected areas flanking the reef rim of the atoll or in infillings of karst dolinas. Much of the 50 m deep sea floor in the lagoon interior is current swept, and has no or very minor sediment cover. Erosive current moats line drowned patch reefs, whereas other areas are characterized by nondeposition. Karst sink holes, blue holes and karst valleys occur throughout the lagoon, from its rim to its center. Lagoonal sediments are mostly carbonate rubble and coarse-grained carbonate sands with frequent large benthic foraminifers, Halimeda flakes, red algal nodules, mollusks, bioclasts, and intraclasts, some of them glauconitic, as well as very minor ooids. Finer-grained deposits locally are deposited in current-protected areas behind elongated faros, i.e., small atolls which are part of the rim of South Malé Atoll. The South Malé Atoll is a current-flushed atoll, where water and sediment export with the open sea is facilitated by the multiple passes dissecting the atoll rim. With an elevated reef rim and tower-like reefs in the atoll interior it is an example of a leaky bucket atoll which shares characteristics of incipiently drowned carbonate banks or drowning sequences as known from the geological record. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Carbonate platform drowning occurs when the sediment accumulation rate is lower than the rate of increase in accommodation space, and the platform top is therefore submerged below the euphotic zone terminating shallow-water carbonate production (Schlager, 1981). Carbonate platforms may drown entirely or partially, when shallow-water carbonate factories punctually continue to accumulate neritic carbonates, forming relic banks growing to sea level. Sediments juxtaposed or overlying the drowned platforms often attest for the occurrence of strong bottom currents. This has been interpreted as a consequence of the acceleration of relatively sluggish ocean tides and currents by the sharp topography of the drowned banks (Schlager, 1998, 1999). The Maldives carbonate platform provides an example where past bank drowning and onset of currents are probably not two independent processes (Betzler et al., 2009, 2013; Lüdmann et al., 2013). Since the late Miocene, vigorous monsoonal-driven currents and nutrient upwelling force the Maldives atolls into an aggradational to backstepping ⁎ Corresponding author. E-mail address: [email protected] (C. Betzler).

http://dx.doi.org/10.1016/j.margeo.2015.04.009 0025-3227/© 2015 Elsevier B.V. All rights reserved.

mode (Betzler et al., 2009). With their 50 to 80 m deep lagoons and the elevated atoll rims, the individual atolls of the Maldives can be classified as empty buckets (Schlager, 1981) or incipiently drowned carbonate banks (Read, 1985) which provide a natural laboratory to test relationships between empty bucket morphology, drowning, and corresponding controlling factors. This study documents the facies and stratigraphy of such a carbonate bank. The goal of this investigation is to elucidate whether the physical impact of currents in the lagoon of such a carbonate body is a major, previously underestimated or even disregarded controlling factor of tropical carbonate platform evolution. Using seismic, multibeam and sedimentological data acquired in the lagoon of the South Malé Atoll (SMA) it will be shown that bottom currents pervasively control sedimentation in the 50 m deep water body. It is discussed how such currents are a factor that can contribute to carbonate platform drowning. 2. Geological setting The Maldives archipelago consists of 22 atolls with 1300 islands and faros, which are small ring-shaped reef complexes (Purdy and Bertram, 1993). Water depth of the lagoons ranges between 31 and 82 m. The

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atolls of the Maldives are relicts of a formerly larger platform (Aubert and Droxler, 1992, 1996; Purdy and Bertram, 1993; Belopolsky and Droxler, 2004; Betzler et al., 2009). During the late Miocene and early Pliocene, stepwise disarticulation of a N–S striking megabank occurred, which led to a complex geometry consisting of a western and an eastern strip of neritic carbonates separated by the hemipelagic basin of the Inner Sea. Starting of the partial platform drowning of the Maldives coincides with the inception of drift sedimentation in the Inner Sea (Betzler et al., 2009). The drowning initiated as passages separating the atolls, and later affected larger bank areas (Aubert and Droxler, 1996). Postdrowning relict banks, which were submerged during these later stages, have elongated outlines indicating current shaping; active atolls of the Maldives have complex growth patterns, with a co-existence of bank margin progradation, aggradation, and backstepping (Betzler et al., 2009, 2013). Progradation is restricted to current sheltered areas, where periplatform drifts (Betzler et al., 2014) accumulate at the atolls flanks. Current-exposed flanks aggrade or step back. The Maldives are affected by a monsoonal triggered seasonal reversing current regime (Schott and McCreary, 2001). The Southwest Monsoon Current (SMC) flows from June to October, and the Northeast Monsoon Current (NMC) from December to April. During the transitions, eastward directed surface jets with velocities of up to 1.3 m/s develop in the open ocean. Monsoonal currents form upwelling cells, producing elevated nutrient values around the reefs (Preu and Engelbrecht, 1991). Location of the upwelling cells is at the downstream sides of the archipelago, and thus seasonally alternates depending on the monsoonal-driven current pattern (Anderson et al., 2011). Betzler et al. (2009) expressed the hypothesis that these monsoon-driven currents and the resulting nutrient injection into the shallow water are a major controlling factor of the Neogene platform evolution. Whereas there is a reasonable understanding of the sedimentology, the stratigraphy and the sequence stratigraphic stacking pattern in the Inner Sea, little is known about the sedimentary dynamics acting in the Maldivian atolls. Observation on sedimentation processes in the lagoons dates back to Darwin (1842) who described how the currents affect the lagoons: “The currents of the sea flow across these atolls, …, with considerable force and drift the sediment from side to side during the monsoons, transporting much of it seaward; yet the currents sweep with greater force round their flanks” (p. 108). In the passages of the atoll rims of the Maldives, monsoon and tidal currents have velocities of up to 1.5–2 m/s (Preu and Engelbrecht, 1991; Owen et al., 2011), triggering winnowing in the passages and in the lagoons where hard bottoms occur (Ciarapica and Passeri, 1993; Gischler, 2006). Bianchi et al. (1997) reported azooxanthellate corals at the slope of the atolls in water depths as shallow as 15 m and in the passes of the atoll rims. In such passes, the depth boundary between zooxanthellate and azooxanthellate corals depends on the current intensity, being shallower in the current-swept passes, which is attributed to strong currents favoring growth of rheophilic forms (Bianchi et al., 1997). Strong currents also erode the outer atoll flanks (Ciarapica and Passeri, 1993). Gischler (2006), Parker and Gischler (2011), Klostermann and Gischler (2014) and Klostermann et al. (2014) analyzed the sedimentary succession in Rasdhoo Atoll (Fig. 1), which is a small atoll enclosed by a reef rim with two passages. The marine postglacial deposits in this atoll which date back to 8 ka BP are little more than 4 m thick in the current protected areas, away from the passages. No data exist for the larger atolls with multiple passages. Naseer (2003), in a comparison of Indian and Pacific Ocean atoll water depths, however, forwarded that Maldivian lagoons are infilling slower than other Indo-Pacific atolls located in the trade wind zone. The time of main wave action in the Maldives is during the 8 months of summer monsoon with E-directed waves, producing a swell in the lagoons (Kench et al., 2006). The winter monsoon induces a shorter episode of minor, W-directed wave action. Kench and Brandner (2006) and Kench et al. (2009) showed that the islands undergo extreme

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rates of gross shoreline change between monsoonal seasons and that the beach width varied by up to 53 m. On an annual basis, there is a minimal net shoreline change, indicating a spatially balanced shoreline pattern. For an island with a diameter of less than 200 m, it was calculated that between 9 and 23 × 103 m3 of sediment volume is moved seasonally. Assuming that the sediment input of the reefs onto the islands is an ongoing process, this implies that the sediment dispersal system is open and that a certain amount of material is remobilized and transported into the lagoon. For an island located in North Malé atoll in the eastern atoll string of the Maldives, Morgan and Kench (2014) showed that seasonal varying wave incidence affects sediment transport pathways with high bank and off-reef export to the reef slope and the lagoon. 3. Methods A research cruise was performed in late November 2011 with the dive safari vessel HOPE CRUISER. Forty five seismic profiles were recorded with a total length of 331.5 km. The seismic equipment used for the survey consisted of a boomer-plate AA301 (Applied Acoustic, U.K.) as acoustic source, which was operated with a power of 300 J/shot, a 24channel MicroEel analog streamer (Geometrics, U.S.A.), and a global positioning system (Hemisphere, Canada). To maximize the resolution and quality of the seismic data, the source used different shot intervals from 0.533 s to 0.7 s for the lagoon areas and up to 1.2 s for the Inner Sea. The applied recording length was 0.2 s TWT. During the survey the vessel moved at a speed of 3 to 3.5 knots. The processing of the seismic data was performed with the software package SU-SEISMIC UNIX (Colorado School of Mines, U.S.A.). The software Petrel (Schlumberger, U.S.A.) was used for the interpretation and correlation of various seismic profiles. Sea floor mapping was performed with the mobile multibeam system SwathPlus (S.E.A. Beckington Castle, U.K.) at a sonar frequency of 117 kHz. The swath was stabilized for roll, pitch and yaw. Vertical sound profiles through the water column were recorded at a regular term as a patch test for geometry correction. The data was processed using the software S.E.A. (S.E.A. Beckington Castle, U.K.) and CARIS (CARIS, Fredericton, Canada). For the interpretation of the multibeam data the software Fledermaus (IVS 3D) was used. Multibeam tracks and maps were merged with pansharpened LANDSAT 8 images in ARCGIS (ESRI, Redlands, USA). Surface sediment samples were taken with a Van-Veen grabsampler which was handled with a portable electric BuLiteK crane (Hamburg University). The geographical position and the water depth were recorded by GPS and a Humminbird Fishfinder 587ci HD echosounder. A total of 65 samples were taken and texture was characterized with the Dunham classification (Dunham, 1962). A first description of the color, the macroscopic components, and the texture of the samples was performed aboard by using a stereoscopic microscope. In the laboratory, split-samples were freeze-dried and infused with resin to produce thin sections. For grain-size analysis, further split samples were freeze-dried and weighed. Samples were wet sieved (N2 mm, 2 mm–500 μm, 500 μm–250 μm, 250 μm–63 μm, b63 μm). The distinct fractions were dried and weighted to determine the percentage of each grain-size fraction. Sediment composition was analyzed with a binocular for the grain sizes of 0.5–2 mm and larger 2 mm. The nature of the grain size smaller 63 μm was analyzed with a scanning electron microscope. Element mapping in thin section was performed with a LEO 1455 SEM with EDX. 4. Results 4.1. Seismic and hydroacoustic facies Seismic facies (Figs. 2, 3) are defined by the criteria of the seismic interpretation and stratigraphy introduced by Mitchum et al. (1977).

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Fig. 1. A, B: Location of the Maldives in the Indian Ocean. The rectangle in B indicates position of South Malé Atoll (SMA). NMA: North Malé Atoll. C: Pansharpened Landsat 8 image of SMA (pixel resolution: 30 m) with the multibeam tracks acquired during the CURRDROW cruise in November 2011.

4.1.1. Facies 1: fringing reef deposits Facies 1 is characterized by medium-amplitude reflections with a flat horizontal to subhorizontal part laterally passing into an inclined and curved shape (Fig. 2). The change from the flat to the inclined part forms a well-defined edge. Reflections delimit wedge-shaped bodies, which in the deeper parts interfinger with reflection of other seismic facies. Wedge-shaped bodies are interpreted as fringing reefs, with the flat top representing the reef flat.

4.1.2. Facies 2: mound ridges Facies 2 forms up to 50 m wide and 50–60 m long, and around 1 m high ridges at the sea floor (Fig. 2). In cross-section, the ridges have convex tops, imaged by a strong reflection. The reflection delimiting the ridge base in the subsurface appears concordant with a high amplitude. Partially, a basal velocity pull up occurs at the ridges' lower boundary. This points towards a higher interval velocity in the ridge compared to the underlying facies. The internal stratification of the sediment bodies shows continuous convex reflections of medium amplitudes. The ridges are interpreted as carbonate banks and faint bedding indicates that the ridge-shaped mounds formed rather through sediment deposition than through bioconstruction. A mechanism responsible for such features

with a positive relief for example is sediment trapping in seagrass banks. 4.1.3. Facies 3: embryonic patch reefs This facies forms up to 50 m wide and up to 3 m high subsurface bodies with an elongate to subcircular lens-shaped top (Fig. 2). The upper and lower boundaries are concordant high-amplitude reflections. The internal reflection pattern is discontinuous to chaotic with high amplitudes. Based on the shape in cross section and the reflection pattern, this facies is interpreted as representing buried biogenic carbonate buildups. The reflections in these patch reefs laterally interfinger with the surrounding sediments, indicating a concurrent deposition of the distinct facies. 4.1.4. Facies 4: patch reefs Facies 4 is recognized as a flat-top or mound-shaped circular elevated structure at the present-day seabed (Fig. 2). The diameter of the bodies is up to 250 m, the top of the mounds is up to 10 m higher than the surrounding sea floor. The internal reflection pattern consists of discontinuous convex to chaotic, medium-amplitude reflections. At the toe of slope of the mounds, diverging layers laterally pass into the surrounding

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Fig. 2. Examples of seismic facies with line drawing showing the corresponding interpretation. Blue areas depict the relevant part of the seismic section.

deposits. Facies 4 is interpreted as patch reef deposits, with reef talus sediments at the toe of slope. 4.1.5. Facies 5: pinnacle reefs Facies 5 is recognized as peak-shaped structures which are 100– 200 m wide at their base and up to 20 m high (Fig. 2). The internal reflection pattern consists of chaotic low-amplitude reflections. Due to masking by the steep flanks, the lower boundary cannot be recognized. The reflections below the structure are attenuated and show strong positive velocity anomalies (pull-up). Layers at the toe of slope of the pinnacles are divergent and interfinger with surrounding deposits. Facies 5 is interpreted as biogenic carbonate buildup, i.e., pinnacle reefs. The divergent reflection patterns are interpreted as talus deposits. 4.1.6. Facies 6: back reef debris apron This facies is restricted to the lagoonward side of passages connecting the atoll lagoon and the open sea, where it forms 500–600 wide and up to 3 m high sediment wedges (Fig. 3). Internally, the facies is characterized by discontinuous sigmoidal reflections of low to medium amplitudes. Reflections partly downlap onto the lower boundary of the wedges; the upper boundary is characterized by toplaps. Medium amplitude reflections form the base and the top of the wedge. This facies

is interpreted as back-reef apron, which formed a sort of spillover sediment body. 4.1.7. Facies 7: restricted lagoonal facies This facies is restricted to depressions, up to 500 m wide, imaged in the subsurface of the lagoon (Fig. 3). It consists of continuous subparallel to parallel reflections of medium to low amplitudes and fills the depressions with an onlap configuration. The continuous and subparallel, nearly horizontal configuration of these reflections implies a deposition under hydrodynamically protected conditions. Faint reflections indicate little contrasts in acoustic impedance within this facies. 4.1.8. Facies 8: open lagoonal facies This facies is restricted to depressions. The facies forms the top of the infill of the subsurface depressions or the base of present-day depressions (Fig. 3). Facies 8 consists of discontinuous inclined to wavy reflections of medium amplitudes. The inclined to wavy reflection configurations imply a deposition under shoal-water conditions above the fair weather wave base. 4.1.9. Facies 9: sediment waves This facies appears as convex, up to 5 m wide and 1 m high elongated structures on the recent sea floor and consists of high-amplitude

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Fig. 3. Examples of seismic facies with line drawing showing the corresponding interpretation. Blue and green areas depict the relevant part of the seismic section.

reflections (Fig. 9). An internal reflection pattern is not visible. This facies is interpreted to represent sediment waves. 4.1.10. Facies 10: drift sediments Facies 10 consists of continuous subparallel to parallel reflections of low amplitudes (Fig. 3). From the outer shape, the structures can be subdivided into two types. The first type forms 500 wide and up to 8 m high elongated mound-shaped structures between topographic highs. It consists of continuous parallel or convex reflections of low amplitudes. The structures are laterally delimited from highs by current moats and downwards bordered by a high amplitude reflection. The second type forms large-scale elongate structures with a laterally extension of 100 m to over 1 km and a thickness of up to 10 m. The bathymetric framework controls the overall geometry and the internal reflection pattern consists of continuous parallel reflections of low amplitudes. This type is restricted to low relief depressions and slopes. The hydroacoustic coverage of the study area is not complete, but the data show that distribution and thickness of these drift sediments within the lagoon interior is linked to the present-day bathymetry of the atoll and the presence of the passages in the atoll margin. Following the classification after Faugères et al. (1999) the first type is a channel-related drift, which is related to passageways between bathymetrical highs. These drift bodies are delimited by marginal

moats, which are formed by bottom currents. After Faugères et al. (1999), the second type shows separated/plastered drift geometry. This type is characterized by alongslope migration and large lateral extension. 4.1.11. Karst (K) The most prominent karst features in South Malé Atoll are represented by circular depressions at the lagoon floor, which are interpreted as blue holes which formed during lowstand exposure of the carbonate bank. Karst facies also is recorded in the seismic data, where it is characterized by reflection patterns with high variations (Fig. 3). In general, karst facies shows discontinuous medium-amplitude reflections with subparallel to chaotic configurations (Kindinger et al., 2010). In many cases, the reflections trace depressions, which are interpreted as sediment filled collapse structures. Detailed views of the karst facies are shown in the seismic lines which are presented below. 4.2. Seismic lines In the following, a series of seismic lines depicting the stratigraphy in key areas of SMA are presented. Line 23 is located atollward of a 2.5 km wide faro delimiting the SMA towards the NW (Figs. 1, 4). It runs northwest to southeast, away from the atoll rim over a distance of 3700 m in a

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Fig. 4. Seismic line 23, line drawing, and multibeam image covering the lagoonward side near a passage connecting the lagoon with the Inner Sea (see Fig. 1 for location). The northwestern part of the profile is characterized by moderately continuous reflections. In the middle part of the profile, two depressions are imaged in the subsurface, which are downwards bounded by Horizon A. The infill of the depressions in the upper part consists of moderately continuous subparallel reflections. The lower infill part is characterized by continuous parallel reflections, which onlap on Horizon A. Note the erosive depression behind the passage displayed in the multibeam map. Numbers refer to seismic facies described in the text.

maximum water depth of 50 m (Fig. 4). Adjacent to the faro, up to profile distance 1000 m, there is a submarine terrace at a water depth of 30 to 35 m which is characterized by the karst facies (K) overlain by small patch reefs (3) and patches of the mound ridge facies (2) at its edge. East of the terrace, the sea floor drops to a water depth of 50 m, where two depressions are imaged in the subsurface, bound by a high-amplitude single reflection at the base. This horizon shows a certain lateral continuity in this and other lines, and is therefore taken as a reference horizon termed Horizon A.

The two depressions are separated by an elevated area with karst facies. The northwestern flank of this elevation is characterized by lowangle inclined reflections, which in the depressions are truncated by Horizon A. The infill of the subsurface-depressions from the sea floor down to 74 ms TWT is characterized by discontinuous subparallel reflections of the open lagoonal facies (8). Below, there is a package of restricted lagoonal facies (7) which onlap onto Horizon A. Line 6 runs from the mouth of a passage between two faros into the atoll over a total length of 6300 m and a maximum water depth of 54 m

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Fig. 5. Seismic line 6, line drawing and multibeam map. This line shows the stratigraphy in the inner part of SMA (see Fig. 1 for location). The western part of the line is characterized by inclined reflections, which downlap onto Horizon A. In the subsurface three depressions are imaged, which are separated by three mound-shaped structures with chaotic internal reflections.

(Fig. 5). At the passage mouth, below small mound ridges (2), there is package of back reef debris apron facies (6) bound at its upper side by toplaps, and by downlaps onto Horizon A at the base. The sea floor in the remaining section has an irregular relief. From profile length 750 to 3400 m, there are three depressions imaged in the subsurface, each traced by Horizon A. The depressions are separated by two embryonic patch reefs (3) and are infilled by a succession of restricted lagoonal deposits (7) overlain by open lagoonal sediments (8). These open lagoonal deposits laterally interfinger with the embryonic patch reef facies. Below Horizon A, the section is characterized by discontinuous to moderately continuous subparallel reflections of medium to high amplitudes, which are interpreted as karstification (K). A view of the Inner Sea facing flank of SMA and of a passage connecting the SMA lagoon with the Inner Sea is provided in Seismic line 11 (Fig. 6). The seismic line, which runs in a passage between two faros has a minor and patchy penetration. The slope of SMA down to a water depth of 100–105 m dips with an angle of approximately 10° into the Inner Sea. Two terraces occur, with continuous convex highamplitude reflections (1) at a water depth of around 56 and 80 m. In the passage, the sea floor appears barren of sediment cover, and has an irregular relief with a karst blue hole at the lagoonward entrance of the passage. The passage north of the elongated faro is deeper than the southern one. It plunges from a water depth of 55 m at the entrance to the Inner Sea to 90 m in a depression towards the atoll interior. The eastern limit of this submarine trough is linear, and the prolongation of the lineament cross cuts the blue hole. A representative view of the stratigraphy in the inner part of SMA is provided by Line 36 (Fig. 7). The segment shown has a length of 2800 m and was recorded in a maximum water depth of 54 m. The sea floor of the lagoon is characterized by a 2100 m wide depression. Multibeam data coverage does not allow defining its overall shape, but it is

characterized by an almost linear SW–NE trending western border, which also delimits the orientation of the edges of nearby faros. Mound ridges (2) and a pinnacle reef (5) arranged in a step-like and retreating pattern cover the flanks of the depression. The low has a thin infill with discontinuous, medium-amplitude reflections attributed to the open lagoonal facies (8), bound by Horizon A the base. A patch reef (4) occurs at profile length 1300 m, in the center of the depression. The convex internal reflections of the patch reef facies laterally interfinger with the open lagoonal deposits. Below Horizon A, the line is characterized by the karstified facies (K). A further stratigraphic record of the interior of SMA is given in Fig. 8. Line 13 has a total length of 2400 m and was recorded in a maximum water depth of 56 m. The sea floor is covered by 150 m wide and 8 m high patch reefs (4) and up to 50 m wide elevations of the mound ridge facies (2). In the lower part of the patch reefs, reflections laterally grade into subparallel reflections of the surrounding open lagoon facies (8). In the northern part of the profile, there are drift sediments (10) separated from the patch reefs by moats. As in the other seismic lines, Horizon A separates the welldefinable facies packages from underlying discontinuous subparallel reflections of the karst facies (K). A tower-shaped, 500 m wide and 10 m high karst feature in the inner SMA lagoon is depicted in Fig. 9. Its internal medium-amplitude reflections (K) are discontinuous and chaotic. The reflections grade downdip into divergent to subparallel reflections of medium amplitudes. The signals underneath the mound are not interpretable. The areas around the mound are characterized by drift sediments (10), which are delimited from the elevation by current moats. The lower boundary of the drift sediment package is marked by Horizon A. The top of the mound has a thin cover consisting of a reef rim (2) and an embryonic patch reef (3) in the center.

Fig. 6. Seismic line 11 through a passage connecting the lagoon with the Inner Sea (see Fig. 1 for location). The multibeam map shows the irregular sea floor in this and the northern seaway. Karstification is reflected by the blue hole (BH) and the through north of it, which reaches a depth of 90 m.

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Fig. 7. Seismic line 36 in the lagoon interior with multibeam map (see Fig. 1 for location). The line cross cuts a depression characterized by karstification, which is rimmed by a series of backstepping reef terraces (mound ridge seismic facies). Note that submarine terrace limits and active shoals are aligned along a SW–NE trending lineament.

A stratigraphic record of the SMA lagoon in an area adjacent to an 11 km long faro separating the lagoon from the Indian Ocean towards the east is given in Fig. 10. The upper interval of the profile, in a water depth between 40 and 50 m, is dominated by the drift facies (10), which show continuous parallel medium-amplitude reflections. This package downlaps onto a series which consists of underlying embryonic patch reef facies (3). The drift (10) and the embryonic patch reef facies (3) are separated by a medium-amplitude reflection. The embryonic patch reefs overlie Horizon A at a depth of around 75 ms TWT.

4.3. Sediment composition Sediment composition and grain size characteristics for a west to east sample transect through SMA is shown in Fig. 11. Sediment in the SMA is carbonate gravel to sand (Fig. 12A, B) with carbonate contents higher than 95%. Main components are red algae, Halimeda, bivalves, gastropods, and bryozoa. Large benthic foraminifers occur in some of the samples and are represented by Alveolinella, Amphistegina, Heterostegina, Operculina and soritids. Red algae and Halimeda are

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Fig. 8. Seismic line in the lagoon interior with multibeam map (see Fig. 1 for location) showing the widespread occurrence of drowned patch reefs. Note that that the northern flank of the drowned reef in the center part of the seismic line is lined by a moat delimiting a sediment drift body.

most abundant in the areas near the atoll rim, whereas gastropods, Operculina, and Alveolinella are slightly more abundant in the atoll center. In many cases, gastropods, bryozoa, and some of the large benthic foraminifers are infilled with a wackestone to fine-grained packstone. In the eastern part of the transect, rhodoliths are the dominant element (Figs. 11, 12A). Algal nodules are a few centimeters in maximum dimension (up to 5.5 cm), mostly ellipsoidal and discoidal in cases in which the shape is conditioned by a bivalve shell or a laminar coral nucleus. Rare specimens are spheroidal. The surfaces of the rhodoliths are smooth to slightly warty and lumpy in a few cases. From a compositional point of view the nodules are in fact foralgaliths (Prager and Ginsburg, 1989) or foralgal macroids. Acervulinids are 40% or more of the biogenic

structure of the nodules. Minor serpulids and bryozoans can also occur. The nuclei are all biogenic, from mollusk shell to coral fragments. In many cases the original nucleus is dissolved. The nodules are in general pervasively bored by sponges and secondarily by bivalves. The resulting galleries are sometimes lined by new growths of acervulinids and rarely by new algal growths. The galleries can be empty, or partially to totally filled by a grainstone. In a few cases there are several generations of Entobia boring; the first generation is filled with a wackestone/packstone; these fillings are bored by new empty or grainstone filled Entobia galleries. Halimeda fragments and large benthic foraminifers can occur as bioclasts within the borings. The dominant corallines are thin laminar thalli of Lithothamnion, intergrown with acervulinids. Lithothamnion muelleri can be identified

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Fig. 9. Seismic line 30 cross-cutting a drowned reef mound (see Fig. 1 for location). The multibeam map shows that the drowned reef is part of a complex with several similar drowned circular to sub-circular reef complexes. Note that the reef is lined by moats delimiting drift deposits.

in well preserved examples. Sporolithon plants (Sporolithon molle and Sporolithon sp.) are also common components together with Lithoporella sp. and minor Spongites sp. and Lithophyllum acrocamptum. Peyssonnelaceae occur in many samples always as secondary components. Thin section analysis shows that sand and silt sized components are bioclasts, although in one sample there are very minor amounts of ooids (Fig. 12C). In addition to the biogenic components, the samples contain wackestone and packstone intraclasts, some of them with a brown or green stain (Fig. 12B, D, E). EDX analyses of the green particles in a thin section of Sample 30 show that the particles are enriched in Si, Fe, and Al, and therefore probably contain glauconite. Deposits are coarse grained with a packstone to rudstone texture (Fig. 12A, B), and carbonate mud content is below 10%. Along the transect, the finest deposits occur in the interior of the lagoon.

Sediments of SMA are fine grained (Fig. 12F) in areas where the lagoon is separated from the open Indian Ocean by elongated faros (Figs. 1, 10, 13). Such deposits, which correspond to the plastered drift bodies, are peloidal packstones with mollusk debris and some bioclasts. Miliolid foraminifers, Operculina and Halimeda also occur as larger components. In such areas, carbonate mud content is higher than 90%, and component diversity is reduced compared to the other deposits in the lagoon. 5. Discussion 5.1. Seismic stratigraphy of the atoll A stratigraphic scheme showing the distribution of the distinct facies has been developed for the sedimentary succession in SMA from tracing

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Fig. 10. Seismic line covering the area protected by an elongated faro separating the lagoon from the Indian Ocean (see Fig. 1 for location). This area is current protected and thus characterized by laterally continuous depositional packages. The multibeam map shows that the sea floor is a flat, lagoonward dipping surface.

depth position and extension of seismically defined facies (Fig. 14). The prominent high-amplitude single reflection marked as Horizon A in Figs. 4 and 5, as well as 7–10 occurs throughout the data set. This horizon characterized by a high contrast in acoustic impedance truncates underlying reflections; the sedimentary sequence above onlaps this surface. It is therefore interpreted as an erosional truncation and exposure surface, probably characterized by a certain degree of lithification. It is probable that this surface formed during the last glacial sea-level lowstand, when the atoll floor was subaerially exposed. Based on their seismic facies characterization (Figs. 4, 6–8, 10), depressions which are traced by this surface are interpreted as karst sinkholes. The sediments in the interior of SMA above Horizon A locally reach a thickness of 15–20 m in the filling of the subsurface depressions. The oldest sediments of the SMA lagoon are represented by a sediment package of restricted lagoonal facies (Facies 7) which is limited to subsurface depressions (e.g., Fig. 4). This package onlaps onto Horizon A. Thin horizontal and undisturbed seismic reflections as well as the onlap configuration are interpreted to reflect deposition under a rising water level in a hydrodynamically protected environment. Stratigraphically younger deposits consist of the back-reef debris aprons (Facies 6) at the atollward side of the passages separating the faros near the atoll margin, in areas close to these atoll passages (Fig. 5). The internal reflections partly downlap onto the lower boundary of the facies package; the upper boundary is characterized by toplaps. In the lagoonal interior, continuing sea-level rise and thus changes in the hydrodynamic conditions caused the transition from a restricted lagoon (Facies 7) to an open lagoon environment (Facies 8). The

restricted lagoonal facies change into the open lagoonal facies (Facies 8) without a conspicuous marker or surface at a constant depth of 74 ms TWT. Such sediment packages crop out at the present-day sea floor in certain areas, such as the current moats (Figs. 8, 9). It is proposed that the wavy reflections (e.g., Line 6, Fig. 5) within this sediment package indicate the occurrence of bedforms such as submarine sediment dunes. Drift sediments (Facies 10) and associated deposits characterized by continuous parallel reflections of medium amplitude overlie the open lagoonal facies. In the central parts of the lagoon, moats separate the drift bodies from elevated areas (Fig. 9), attesting for the erosive flow regime around such highs. In other lagoonal areas, sediments were accumulated in depressions or in current protected areas behind larger faros (Fig. 10). 5.2. Late Pleistocene to Holocene atoll evolution A reconstruction of the relative chronostratigraphy of the lagoonal succession is proposed by correlating the depth position of the distinct facies with the post-glacial sea-level evolution (Bard et al., 1990, 1996, 2010; Camoin et al., 2004; Edwards et al., 1993; Fairbanks, 1989; Fleming et al., 1998; Montaggioni, 2005; Nunn and Peltier, 2001; Siddall et al., 2003; Yokoyama et al., 2001; Zinke et al., 2003). Isostatic correction is estimated as unnecessary, because the Maldives represent a tectonically stable region with a linear subsidence rate. The long-term subsidence rate of the Maldives is in the range of 0.03–0.04 mm year−1 (Belopolsky and Droxler, 2004); for the last 135,000 years, a maximum

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Fig. 11. Composition and granulometry of sediment samples recovered along a west to east sample transect through the SMA lagoon (see Fig. 1 for location). Sample composition is separately shown for the fraction from 500 μm to 2 mm and for the fraction N 2 mm.

subsidence rate of 0.15 mm year−1 has been proposed by Gischler et al. (2008). With respect to the ice load of continental margins, the Maldives archipelago can be considered as a far-field site (Peltier and Fairbanks, 2006). Therefore, the total isostatic correction required is negligible compared to the values of the eustatic sea level rise following the last glacial maximum. The curve for Pleistocene sea-level fluctuations after the last interglacial implies that the atoll lagoon of South Malé was exposed for around 55 ky. With the elevation difference between sea level and the top of the exposed atoll during the last glacial maximum, the platform would have contributed as orographic enhancement to the local rainfall patterns (e.g., Sobel et al., 2011). The exposed atoll plains acted as a large catchment area for precipitation, which induced karstification. This episode is reflected in the depressions, the sinkholes, and blue holes. Seismic and multibeam data indicate that karst features such as sinkholes and valleys occur throughout the lagoon, from the passages separating the faros to the lagoon interior. The deepest karst valley imaged in the data set (90 m) is located near the border of the atoll (Figs. 1, 6). Such a distribution therefore seems not to entirely follow the concept of Purdy (1974) and Purdy and Bertram (1993) that karst solution is stronger in the center of atoll lagoons. After the LGM termination around 19 ka BP, sea level rose with a moderate rate of b5 mm/year (Fleming et al., 1998) until 15 ka BP and

reached a level around 97 m below the present position. The time period between 14.6 and 14.3 ka is characterized by an accelerated sea-level rise of about 40 mm/year (Deschamps et al., 2012) during Meltwater Pulse 1A (MWP-1A). According to Montaggioni (2005), MWP 1A caused a widespread reef-drowning event in the Indo-Pacific region, which however is questioned by Woodroffe and Webster (2014). In the Maldives, this episode of slower rise followed by an accelerated sealevel rise may be reflected by terraces at the slope of SMA and at the flanks from the atolls elsewhere (Fürstenau et al., 2010). The effect of meltwater discharge decreased after 14.3 ka BP, and sea level rose with an estimated rate of 7.5 mm/year in the central Pacific (Bard et al., 1996). A phase of reef growth during this time interval is probably recorded in the Maldives, where a terrace is located at a water depth of 56 m (Fig. 6, Fürstenau et al., 2010). Subsurface depth position of the restricted lagoonal facies packages, which appear in every paleo-depression mapped out in the subsurface data set, indicate that these formed in the time period between Meltwater Pulse 1A and 1B. This first creation of accommodation in the lagoon was not necessarily marine accommodation, but could also represent establishment of a fresh-water lens in the atoll interior, lying above the surrounding sea-level (Vacher, 1988). The thickness of this lens would depend on the abundance of rainfall and the permeability of the rocks above, and just below, sea level (Schlanger et al., 1963).

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Fig. 12. (A) Coarse-grained deposits from the eastern part of the transect in SMA (sample 13). Main components are red algal nodules (living and dead) as well as Halimeda flakes. (B) Carbonate sand from the interior of SMA (sample 30). Components are bioclasts and stained relict grains. (C) Thin section photograph of sample 14 with an ooid, Halimeda flake and Heterostegina. (D) Thin section photograph of sample 30 with bioclasts and stained intraclasts. (E) Thin section photograph of sample 31 with gastropod and red-algal nodule debris, other bioclasts, Alveolinella, and stained bioclasts. (D) Thin section photograph of sample 26 from a current-protected area.

It is proposed that the partial marine flooding of the lagoon areas started around 11.9 to 11.7 ka BP. This proposed time span is comparable to published ages of the flooding of the Mayotte lagoon of 11.9 to 11.3 ka BP (Zinke et al., 2003). During the subsequent phase of moderate sea-level rise, the sea ingressed into the passages between the areas now occupied by the faros and flooded the small banks within the breaches. This provided full marine conditions in the areas near the passages and probably in large parts of the atoll, inducing an initial reef growth. This reef growth and the continuous erosion of the preHolocene rim by wave action formed the reef debris aprons which accumulated at the lagoonward side of the buildups. With continuing sea-level rise and flooding of the lagoon, water exchange between the lagoon and the open sea was eventually enhanced

and patch reef growth started. The reefs nucleated on topographic highs, as documented elsewhere by Kendall and Schlager (1981). The low-energy water conditions of the restricted lagoon areas were replaced by shallow water conditions of an open lagoon with sediments deposited around wave base. This is indicated by the change from the seismic facies with laterally continuous layering to deposits with discontinuous and low-angle inclined bedding (Figs. 8, 9). The depth position of this transition correlates with the sea-level position during Meltwater Pulse 1B. Precise timing and amplitude of MWP-1B are still a matter of debate (Woodroffe and Webster, 2014), because this event was originally detected as hiatus between individual drill cores collected at different depths off Barbados (Bard et al., 2010). During MWP-1B, the sea level presumably rose with an approximated rate of 19 mm/year

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Fig. 13. Composition and granulometry of sediment samples recovered along a west to east sample transect in a current-sheltered area at the lagoonward side of a faro (see Fig. 1 for location).

(Zinke et al., 2003). Drowning of the embryonic patch reefs in the subsurface of the lagoon is tentatively attributed to this event. This reef drowning also affected the fringing reefs at the atoll slopes of North Ari and South Malé, which were established during the time interval between MWP-1A and -1B (Fürstenau et al., 2010). This also correlates with the reef-drowning event and the prominent gap in reef growth at the end of Meltwater Pulse 1B in the entire Indo-Pacific region (Montaggioni, 2005), although no drowning event was observed in the reefs of the Huon Peninsula or Tahiti (Woodroffe and Webster, 2014). After the termination of MWP-1B, sea level rose with an estimated rate of 9 mm/year (Zinke et al., 2003). In the Indo-Pacific realm, coral communities colonized the newly submerged topographic highs in the time window between 10 and 7 ka BP (Montaggioni, 2005), which is also proposed as the time window for patch reef establishment in the SMA lagoon. The post-glacial sea-level rise ended at around 5 to 4 ka BP (Nunn and Peltier, 2001). At this time, the entire atoll interior and atoll perimeter of SMA were flooded. The base of the drift deposits in the lagoon can be taken as a pinning point for the establishment of the present-day current system. The time of initiation of these drifts can be estimated to some degree, because the drift sediments buried the patch reefs which established earlier. In the sediments of the Inner Sea, this onset of lagoonal flushing seems to be recorded by an increase in the content of bank-derived calcareous mud and aragonite (Paul et al., 2012).

Samples recovered from the recent sedimentary surface attest for the impact of currents on the sea floor throughout the lagoon (Figs. 12, 13). Muddy sediments with a wackestone to fine-grained packstone texture were only recovered in current sheltered areas, away from the passages. This is similar to the sediment distribution of Ari Atoll, located west of SMA (Fig. 1) described by Gischler (2006). In the rest of the lagoon, even in the center of the 20 km wide atoll water body, the grain size fraction b 63 μm barely reaches 10% of the bulk sediment. Inner lagoonal sediments are either current-winnowed carbonate sands and gravels rich in red algal nodules (Fig. 12A) or carbonate sands rich in stained relict grains (Fig. 12B). The red-algal nodules have a morphology and composition that can be expected for a deep-water setting: small, smooth, with a high proportion of encrusting foraminifers and dominance of melobesioids corallines (Lithothamnion in this case) and Sporolithon, with minor Lithoporella, as well as Peyssonnelia (Lund et al., 2000; Webster et al., 2009; Braga, 2011). The intense, pervasive, multistory boring, sometimes lined by new encruster growth indicates a very low rate of sedimentation around the nodules (Bassi et al., 2012). This image of a pervasive current impact in the lagoon is in line with the distribution of elongated faros and islands throughout this water body. As discussed by Purdy and Bertram (1993), Kench et al. (2009), and Schlager and Purkis (2013) such elongation is a consequence of current and wave impact.

Fig. 14. Stratigraphic model for the post LGM succession in the SMA with schematic distribution of seismic facies (F1–F10).

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5.3. The leaking bucket In atolls, the rim reefs commonly shed sediment into the lagoon in a form of a debris apron that progrades away from the rim (Montaggioni, 2005) and progressively occupies the lagoon (O'Leary and Perry, 2010). These debris aprons through progradation are the major driver for the atoll lagoon infilling (Purdy and Gischler, 2005; Barrett and Webster, 2012; Rankey and Garza-Pérez, 2012). The filling process of the investigated atoll lagoons in the Maldives archipelago seems not to follow this model. From the base of the sediment drift packages onwards, sedimentary depocenters in the SMA lagoon appear to be current controlled with wide areas characterized by non-deposition and even erosion in current moats throughout the lagoon (Figs. 3, 8, 9). This current regime in the atoll can be established because the discontinuous reef rim allows the inflow of wind- and tide-generated currents that attenuates the deposition of the debris aprons in the proximity of the passages such as in atolls elsewhere (Zinke et al., 2001; Purdy and Gischler, 2005), and rework these reefal sediments. The lagoon water returns to the open sea through the passages and transports potential lagoon infill from the debris aprons into deep water. Purdy and Gischler (2005) proposed that such atolls with discontinuous rims are probably never able to fill the enclosing lagoon completely and introduced the term “leaky bucket” for these rim configurations. The collected data indicate that this circumstance applies for the SMA, where lagoonal depocenters are located in current-protected areas. The leaky bucket effect may even be enhanced in the Maldives lagoons which are subjected to the complex interaction of tidal and seasonally reversing monsoon-driven currents. 5.4. An incipiently drowned carbonate bank? In several respects, the SMA shares characteristics of drowned carbonate banks and carbonate platform drowning sequences described from the geological record elsewhere. The atoll is characterized by tower structures and elevated rims (Read, 1985), and its interior is occupied by facies rich in large benthic foraminifers and rhodoliths similar to Neogene carbonate platforms in the South China Sea (Erlich et al., 1993; Sattler et al., 2009). Glauconitic grains indicate that parts of the lagoon are characterized by low sediment accumulation rates. Finally, the atoll is flanked by drifts (Betzler et al., 2009, 2013; Lüdmann et al., 2013) and a large part of the atoll interior is covered by current-dominated deposits. A juxtaposition and superposition of current-dominated deposits and drowned carbonate banks is a common scenario in carbonate platforms from southeast Asia, east Australia, and the Tethyan Mesozoic (Zempolich, 1993; Bracco-Gartner et al., 2004; Isern et al., 2004; Marino and Santantonio, 2010), which has been attributed to the acceleration of sluggish ocean tides and currents at the sharp topography of the drowned banks (Schlager, 1998). The available data indicate that one of the major players of this scenario is the seasonally reversing currents impinging onto the Maldives and flowing through the atoll. Remote sensing data tracing chlorophyllα concentrations in and around the Maldives show that currents inject high nutrient contents into the shallow water (Sasamal, 2007; Anderson et al., 2011). Temporarily, these nutrient injections induce chlorophyll-α concentrations of 0.34 mg/m3 thus placing the carbonate factory near the coral turn on/turn off zone (Schlager, 2005). Currents also physically affect the sedimentation through reworking and erosion. Storz and Gischler (2011) discussed that even the coral growth rates in the Maldives are affected by current strength, with less yearly extension but thickening of the skeleton during the summer monsoon. Further, currents are also proposed to affect the stratigraphy of lagoonal deposits. In more enclosed lagoons, the stratigraphic succession is preserved during sea-level lowering, and additional sediment may be deposited in the lagoon during sea-level falls (Paterson et al., 2006). In the leaky bucket lagoons, however, during sea-level lowerings the currents can remobilize part of the sediment, ultimately exporting it out of

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the lagoon. Although monsoonal currents affect the Maldives since the middle Miocene (Betzler et al., 2009, 2013; Lüdmann et al., 2013), the sediment winnowing effect in the lagoons is thought to have been amplified with the inception of the Pleistocene high-amplitude sea-level fluctuations, leading to the configuration of reef towers elevated about 30–80 m above the lagoonal bottoms. In a broader geological context, the Maldives have the potential to add new facets to the discussion about unfilled accommodation space on carbonate platforms, which has serious consequences for the understanding of the stratigraphic record, especially with regards to cyclostratigraphic interpretations. Unfilled accommodation space, in this case represented by an empty atoll bucket, is increasingly recognized as a common feature of Holocene or icehouse carbonate platforms (Eberli and Grammer, 1999; Zinke et al., 2001; Gischler et al., 2003; Paterson et al., 2006; Eberli, 2013; Eberli et al., 2008) and is seen as a consequence of the rate of sediment supply being lower than the rate of increase in accommodation space (Purdy and Gischler, 2005; Paterson et al., 2006). Where infilling of accommodation space occurs, it is mainly accomplished through the lateral progradation of one facies (Eberli et al., 2008). Therefore, if accumulation of lagoonal sediments through lateral progradation is inhibited, only reefs aggrade, and lagoons end up unfilled. To our knowledge, this effect was not taken into account yet, and in the models of carbonate-platform development (see Paterson et al., 2006 for review), only unidirectional currents were assumed. 6. Conclusions The South Malé atoll is a leaky bucket atoll which shares characteristics of drowned carbonate banks or drowning sequences known from the geological record. Its lagoon is a sediment-starved depositional system, and position of depocenters is current controlled. Erosional moats at the flanks of highs such as drowned patch reefs are located at several places in the lagoon, even in the center of the atoll. The sediments of the lagoon are mostly grainy bioclastic carbonate sands with less than 10% of mud. Low amounts of carbonate muds are interpreted to reflect current winnowing at the lagoon floor, as is also indicated by the intense borings of the red-algal nodules. Reduced sedimentation rates are also shown by the occurrence of glauconitic grains in the lagoon interior. Fine-grained sediments are located in current sheltered depocenters along the downcurrent current-protected flanks of faros. It is proposed that the atoll serves as an example which shows how strong currents can contribute to the drowning of carbonate banks. Acknowledgments We want to thank the captain and the crew of our temporary research vessel HOPE CRUISER and our friend Moosa who helped us to achieve our goals with their extraordinary support. The Ministry of Fisheries and Agriculture in Malé is thanked for providing the research permit FA-D2/33/2011/11 for this project. The support before and after the cruise by Christian Hübscher is gratefully acknowledged. Dr Shiham Adam, as usual, helped and supported us with different logistic questions. Freimut von Borstel is thanked for sample preparation and support during sample analysis. The Deutsche Forschungsgemeinschaft is thanked for providing the financial support of this project through the grant BE1272/21. The manuscript benefitted from the very constructive and helpful reviews by John Reijmer and an anonymous reviewer. References Anderson, R.C., Adam, M.S., Goes, J.I., 2011. From monsoons to mantas: seasonal distribution of Manta alfredi in the Maldives. Fish. Oceanogr. 20, 104–113. Aubert, O., Droxler, A.W., 1992. General Cenozoic evolution of the Maldives carbonate system (equatorial Indian Ocean). Bull. Cent. Rech. Explor. Prod. Elf-Aquitaine 16, 113–136. Aubert, O., Droxler, A.W., 1996. Seismic stratigraphy and depositional signatures of the Maldive carbonate system (Indian Ocean). Mar. Pet. Geol. 13, 503–536.

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