Received: 9 April 2018 | Revised: 20 June 2018 | Accepted: 22 June 2018 DOI: 10.1002/dep2.47

ORIGINAL RESEARCH ARTICLE

Satellite‐ and field‐based facies mapping of isolated carbonate platforms from the Kepulauan Seribu Complex,

Dwi Amanda Utami1,2 | Lars Reuning1 | Sri Yudawati Cahyarini1,2

1Energy and Mineral Resources Group, Geological Institute, RWTH Aachen Abstract University, Aachen, Germany Quantitative facies models from modern carbonate are essential for the interpreta- 2Geotechnology Research Center, tion of their fossil counterparts. The isolated carbonate platforms of the Kepulauan Indonesia Institute of Sciences LIPI, Seribu archipelago has many atoll‐like islands with reef belts exposed to bidirec- Bandung, Indonesia tional monsoon winds. Statistical analysis based on texture and composition reveal Correspondence that there are four sedimentary facies; coral grainstone, coral packstone/grainstone, Dwi Amanda Utami, Energy and Mineral ‐ Resources Group, Geological Institute, coral mollusc packstone and mollusc wackestone. The occurrence of mollusc RWTH Aachen University, Aachen, wackestone in the lagoon is controlled by water depth, while the sand apron and Germany. reef front do not show significant facies separation with water depth. The co‐ Email: [email protected] occurrence of these different facies in the same depth window is contrary to the Funding information common thought that changes in bathymetry should be reflected in facies changes. DIPA 079.01.2.017152/2015, Grant/Award The studied reef systems therefore show aspects of random and ordered facies dis- Number: 3408.001.014.H; Lembaga Pengelola Dana Pendidikan (LPDP), tribution with respect to water depth. A satellite derived environmental facies map Grant/Award Number: RJ-4137/LPDP.3/ generated by an image analysis algorithm indicates that environmental facies dis- 2016 tribution is mainly controlled by water depth, density of seagrass cover and coral abundance. The sand apron can be subdivided into three environmental facies with no, sparse and dense seagrass cover. The deeper water zone can be separated into shallow and deep subtidal parts of lagoons and platform margins. In the lagoon, satellite derived environmental facies directly correlated with sedimentary facies. No direct correlation of environmental facies to sedimentary facies was possible in the sand apron due to the heterogeneity and complexity of the environ- ment. However, the mean sediment grain size is significantly smaller in areas of the sand apron colonized by dense seagrass. This study aims to contribute towards a better understanding of modern equatorial Southeast Asian carbonate systems, delineate modern carbonate facies based on sediment texture and composition with the aid of multivariate statistical analysis combined with statistic based satel- lite mapping, and give insights regarding the correlation between depositional facies and water depth.

KEYWORDS atoll, coral, equatorial carbonate, Holocene, modern carbonate, Monsoon

------This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2018 The Authors The Depositional Record published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists. | Depositional Rec. 2018;4:255–273. wileyonlinelibrary.com/journal/dep2 255 256 | UTAMI ET AL. 1 | INTRODUCTION Toruan, Soedharma, & Dewi, 2013; Umbgrove, 1947), with only a few focusing on carbonate sedimentology (Jor- Cenozoic shallow marine tropical carbonates form major dan et al., 1993; Park, Siemers, & Brown, 1992; Scrutton, hydrocarbon reservoirs in various parts of the world. In 1976a, 1976b). Major carbonate facies zones in Kepulauan Southeast Asia, Cenozoic carbonates are extensive, diverse, Seribu were mapped based on grab and dive samples by and often form hydrocarbon reservoirs in the subsurface Scrutton (1976a, 1976b) and Jordan et al. (1993). However, (Wilson & Evans, 2002). Almost half the cumulative pro- a modern statistically based facies analysis for the Kepu- duction and a sizeable proportion of the remaining oil and lauan Seribu complex is still lacking. gas reserves in Indonesia are found in the Sea, off- Conventional field‐based mapping studies of carbonate shore southeast Sumatra and northwest Java (Figure 1). platforms have long been used to gain dimensional data These fields are hosted by Miocene carbonates of the Batu- about sediment distribution patterns, while the satellite‐ raja and Parigi Formation (Bukhari, Kaldi, Yaman, based studies have the advantage of being able to cover Kakung, & Williams, 1992; Burbury, 1977; Fainstein, larger areas in a more timely and cost‐efficient manner. 1987; Park, Matter, & Tonkin, 1995). Reservoir quality Satellite‐based remote sensing has been established as a and rock properties of these carbonate reservoirs are often beneficial examination tool and has been extensively difficult to predict. Most modern analogue studies for car- applied within the earth sciences, including the mapping of bonate systems come from subtropical latitudes, such as coral reef biota and bottom sediments (Gischler & the Florida Reef Tract (Enos, 1974; Ginsburg, 1956), Lomando, 1999; Kaczmarek, Hicks, Fullmer, Steffen, & Belize atoll and barrier reefs (Purdy & Gischler, 2003), Bachtel, 2010; Lyzenga, 1981; Madden, Wilson, & O'Shea, Great Bahama Bank (Purdy, 1963a, 1963b; Reijmer et al., 2013; Purkis, Rowlands, & Kerr, 2015). Modern carbonate 2009), the Hawaiian chain (Gross, Milliman, Tracey, & platforms are ideal for satellite‐based studies as they gener- Ladd, 1969), the Great Barrier Reef (Orme, Flood, & Sar- ally are only covered by shallow and relatively clear water, gent, 1978) and the South Pacific (Gischler, 2011). How- which facilitates accurate characterization of satellite data ever, Southeast Asian equatorial carbonates are somewhat in marine environments. The combination of statistical different from their subtropical counterparts and therefore satellite image analysis and sediment data was recently pro- should be re‐examined as distinct depositional systems posed as a quantitative technique for producing satellite (Gischler & Lomando, 1999; Park, Crevello, & Hantoro, derived facies maps for carbonate platforms (Kaczmarek et 2010; Wilson, 2002, 2012). Carbonate systems in Southeast al., 2010). Asia form a broad variety of platform types such as land This study follows this approach intending to (i) con- attached shelves, isolated platforms or localized and/or tribute towards a greater understanding of modern equato- ephemeral carbonates (Tomascik, Mah, Nontji, & Moosa, rial Southeast Asia carbonate systems, (ii) compare modern 1997; Wilson, 2002). Carbonate deposition is often domi- carbonate facies with satellite derived environmental facies nated by bioclastic assemblages, few nonskeletal grains and maps and (iii) test to what degree the carbonate facies dis- a notable absence of coated grains (Jordan, Wharton, & tribution on small patch reef systems is related to water Cook, 1993; Lees & Buller, 1972; Scrutton, 1976a, 1976b; depth. Wilson, 2002). The small, isolated carbonate platforms forming the islands of the Kepulauan Seribu Complex therefore provide a useful modern analogue for buried car- 2 | STUDY AREA bonate systems in Indonesia (Park et al., 1995; Wilson, 2002) and other similar Cenozoic carbonates from South- Situated on the outer shelf margin of Sumatra in the Java east Asia and northern Australia (Belde, Back, Bourget, & Sea, Kepulauan Seribu (Thousand Islands) consists of Reuning, 2017; Morgan, George, Harris, Kupecz, & Sarg, numerous coral reef islands ranging in dimension from a 2010; Rosleff‐Soerensen, Reuning, Back, & Kukla, 2012, few metres across to length greater than 7 km. The archipe- 2016). Sediment distribution patterns form the fundamental lago commencing about 25 km northwest of the Java coast- studies on sediment formation, redeposition, sediment line, extends for about 40 km into the Java Sea. Kepulauan dynamics and early diagenesis in modern reefs and carbon- Seribu sits on a NNE‐SSW structural high, the Seribu plat- ate platforms (Gischler, 2011; Gischler, Isaack, & Hudson, form (Figure 1), which is separated from the Sunda Basin 2017). towards the west by the still active N–S‐oriented Seribu Studies of modern carbonate systems in Kepulauan Ser- Fault (Park et al., 2010). The N‐S orientation of the Seribu ibu typically focus on their biota and ecology (Cahyarini, platform likely is also inherited from this structural grain Zinke, Troelstra, Suharsono, & Hoeksema, 2016; Cleary (Park et al., 2010). The E–W‐oriented inter island channels et al., 2014; De Voogd & Cleary, 2008; Farhan & Lim, that can be in places more than 50 m deep are the remains 2011; Hoeksema, 1991; Rachello‐Dolmen & Cleary, 2007; of a late Pleistocene drainage pattern that was later UTAMI ET AL. | 257

FIGURE 1 Study area as indicate in the red box (a) Kepulauan Seribu complex, Java Sea, Indonesia. (b) Location of Semak Daun, Panggang and Pramuka Island in the middle of Kepulauan Seribu complex. (c) Major structural features in the Sunda Basin and the Seribu Platform (after Fainstein, 1987) modified and truncated by river capture. In this respect, the sea‐level fall that led to lateral growth of the patch reef preceding topography has played an important role in systems (Park et al., 1992, 2010). establishing the location and nature of Holocene reefal car- The controlling influence on modern reef growth and bonate build‐ups in Kepulauan Seribu (Park et al., 1992). morphology in Kepulauan Seribu is believed to be the Little is known about the internal architecture of the Seribu seasonal change in wind and current directions (Scrutton, platform. Based on three shallow cores from two islands, 1976a). The Java Sea is a relatively shallow sea with Park et al. (1992, 2010) showed that modern reef growth water depth generally between 40 and 50 m and only initiated in the early Holocene. Vertical reef growth contin- limited connection to the ocean. The Java Sea is charac- ued with a rate of 5–10 mm/year to around 4,500 years terized by the monsoonal wind with a seasonal reversal BP. This stage was followed by a slight late Holocene between the west monsoon (December–February), and the 258 | UTAMI ET AL. east monsoon (April–October). The east monsoon wind is This study focuses on the patch reef systems of Pang- two times more persistent than the west monsoon wind, gang Island, Pramuka Island and Semak Daun Island which but the west monsoon wind shows more extreme varia- are situated in the middle part of the Kepulauan Seribu tions in wind strength (Poerbandono, 2016). The climate chain (Figure 1). Panggang Island is the island in Kepu- in Kepulauan Seribu is tropical with high air humidity of lauan Seribu with the highest population density and is sep- 80 mmHg, a mean temperature of 27°C and a seasonal arated from the less inhabited Pramuka Island by a shallow cycle from 32.3°C to 21.6°C (Sachoemar, 2008). Salinity inter‐reef channel. Panggang is an atoll‐like reef with an measurements in the west Java Sea indicate average val- approximate area of 3.2 km2, and a central lagoon sur- ues of 32‰ total dissolved salts, well below normal rounded by a reefs and its sand apron. From the reef flat open ocean salinities. These relatively low salinities result the water depth increases from about 1 m to about 20 m in from high seasonal rainfall and high runoff associated the deepest part of the lagoon. Some of the reef on the reef with the monsoonal climate. The tidal range in Kepu- crest will be exposed during low tide, showing various lauan Seribu is micro tidal, averaging about 52 cm with kinds of corals dominated by platy Acropora and foliated a range from 15 to 82 cm and a typical diurnal tidal corals (Figure 2). The sand apron on the eastern and west- cycle with one high and one low stand per day (Wyrtki, ern side of the lagoon is roughly two times wider com- 1961). Generally, surface currents in Kepulauan Seribu pared to the sand apron on the northern and southern side show a maximum speed of 0.5 m/s (Farhan & Lim, (Figure 3). Only one small inlet traverses the sand aprons 2011). Wave height in Kepulauan Seribu varies between allowing only very limited connection between the lagoon 0.5 and 1 m during the east monsoon and between 2 and the open sea. There are two small islands on the north- and 3 m during the west monsoon, while mean wave eastern side of Panggang, preventing the reef system from speed is only 1 knot due to the wave absorption by the direct wave exposure from the Java Sea. Pramuka is a island chain (Sachoemar, 2008). fringing reef with an extensive reef flat on the eastern and

FIGURE 2 (a) Platy Acropora and foliaceous Montipora on the reef crest, south of Panggang lagoon, often exposed during low tide. (b) Sand apron with dense seagrass in Pramuka Island, individual corals often occur within the seagrass meadows. Underwater photograph of: (c) Sand apron with sparse short seagrass cover in Pramuka Island. (d) Sand apron with sparse seagrass, Halimeda, and small isolated coral heads in Panggang Island. (e) Sand apron without seagrass in Pramuka Island. (f) Coral framework on the reef front of Semak Daun Island UTAMI ET AL. | 259 northern part, while almost no reef flat exists on the west- 2015, 45 samples), Pramuka (May 2016, 32 samples) and ern part (Figure 3). The island itself lies on the southwest- Semak Daun (May 2016, 25 samples). In shallow water, ern part of the platform, emerging about 2 m above the sea sediments were collected by hand and water depth was surface. Most of the reef flat is covered by less than 1 m measured using a marked stick. Sampling in the inner of water with no distinct reef crest observed. The reef slope lagoon and at the reef slope was conducted by scuba div- on the western side reaches depths of ca 20 m, while on ing and water depth was determined using the diving the eastern side it falls off into much deeper water, reach- gear. Latitude and longitude of sampling locations were ing depths of about 40 m in the Java Sea. Semak Daun is measured with a global positioning system (GPS) and a platform lagoon with an extensive reef flat surrounding a plotted on SPOT‐6/7 satellite imagery. In the laboratory, partially filled shallow lagoon with active coral growth in samples were washed in fresh water and dried using an its centre (Figure 3). Similar to Panggang, the reef flat in oven set to 70°C for 7 hr. Dried samples were first sieved Semak Daun is expanded broadly on the eastern and west- through a 125 μm sieve and a split of the coarse fraction ern side, leaving a narrow reef flat on the northern and was stored in a glass bottle for later point counting analy- southern side. Water exchange between the lagoon and the sis. The remaining portion of the >125 μm fraction was Java Sea is restricted to few inlets on its western side, sieved through 250 μm, 500 μm, 1 mm and 2 mm sieves. while the eastern side of Semak Daun is directly exposed All individual grain‐size fractions were weighed. Mean to the Java Sea. grain size and sorting were calculated using the software Gradistat (Blott & Pye, 2001). Sorting of samples was quantified as graphic standard deviation using the equa- 3 | METHODS tion of Folk and Ward (1957). Values below 2 indicate moderate sorting, while values <4 and >4 indicate poor A total of 102 surface sediment samples were collected and very poor sorting, respectively. Point counting analy- from three islands in Kepulauan Seribu; Panggang (May sis was done on the split of the >125 μm fraction by

FIGURE 3 Location and sorting of sediment samples 260 | UTAMI ET AL. counting 300 grains under a binocular microscope. penetration potential of the infrared bands because of Selected samples of the <125 μm fraction were further increasing absorption of the longer wavelength by water analysed using scanning electron microscopy in the SEM (Kaczmarek et al., 2010). Thus deeper water areas contain laboratory of FMIPA (Institut Teknologi Bandung) and less data on which to classify (Kaczmarek et al., 2010). the SEM/EDS laboratory of UPP Chevron‐Prodi Geologi Because of this limitation, no environmental facies could (Institut Teknologi Bandung). Correlation, cluster and be derived for some sampling points at the deeper part of principle component analysis were conducted using the the reef front. software package PAST (Hammer, Harper, & Ryan, SPOT standard products are delivered application 2001). Sediment samples were classified using a modified ready; therefore, there is no need for pan sharpened and Dunham (1962) classification following the method out- orthorectified imagery in natural colours. Discrimination lined in Gischler et al. (2017) which proposed a threshold of bottom benthic sediments into distinct thematic classes value of 50% matrix to separate mud supported from was conducted using the spatial analysis tool “isocluster grain‐supported textures. Grainstone has 0%–10%, pack- unsupervised classification” in ArcGIS. This method stone 10%–50%, wackestone 50%–90% and mudstone assessed each pixel in the satellite image by conducting a 90%–100% matrix. The <125 μm fraction was defined as calculation based on a compilation of the spectral values matrix. of an individual band. Applying a large number of the- The relationship between water depth and facies diver- matic classes in the unsupervised classification is critical sity was explored using the Shannon evenness index for capturing the wide range of variability in sediment (Shannon, 1948). Rankey (2004) employed the maximum reflectance (Kaczmarek et al., 2010). In this study, 30 entropy concept whereby divergence from a state of dis- spectral classes were utilized to produce an environmental order is statistically assessed using the Shannon evenness facies map, resulting in six distinct environmental facies index to examine facies diversity. Evenness (E) for each groups which were subsequently validated with field water depth is calculated following Purkis et al. (2015) observation. as:

E ¼ 1 ½ð∑pi ln piÞ= lnðnÞ (1) 4 | RESULTS whereby n is the number of facies and p is the proportion 4.1 | Texture of each facies within a depth interval. Evenness (E) can reach values from zero to one, with values near one indi- Most sediment samples are grain‐supported (Table 1) and cating that only one facies is present, meaning that the can be categorized as either grainstone (0%–10% matrix) or facies present can be predicted with confidence for a given packstone (10%–50% matrix) in nearly equal abundance. water depth (Purkis et al., 2015; Rankey, 2004). Because Wackestone (50%–90% matrix) is the only matrix‐sup- the sample distribution is skewed towards shallow water ported texture and forms only a small proportion of all sed- depth, the binning size of the depth intervals with water iment samples. In Panggang Island, the majority of depth were increased following an exponential function sediment samples are grainstone, while packstones domi- (f(x) = 2x), resulting in depth bins from 0–1m,1–3m,3– nate in Pramuka and Semak Daun islands. Sorting is gener- 7m,7–15 m, etc. ally poor, but “very poorly sorted” samples also occur A satellite‐based facies map was produced by statistical frequently (Figure 3). Only one sample classified as “mod- analysis of the colour spectrum of high resolution SPOT‐ erately sorted” was found on the eastern reef flat of Pra- 6/7 satellite images to quantitatively discriminate between muka Island. combined benthic sediment and biota types across the study area. This study applied the simplified workflow of 4.2 | Sediment composition Kaczmarek et al. (2010) with slight modifications to gen- erate SPOT‐6/7 derived environmental facies maps. The Skeletal components, including fragments of corals and SPOT‐6/7 satellite image was used in this study because mollusc shells (Figure 4) constitute most of the sediment in the spatial high resolution (1.5 m) is expected to cover Kepulauan Seribu. Less abundant skeletal components are benthic diversity in the relatively small (±24 km2) study echinoderm spines, Halimeda flakes, crustose coralline area. For this study, only the red, green and blue bands algae, the red algae Amphiroa sp., foraminiferal tests, were used due to low level returns from subsurface fea- sponge needles and tunicate spicule. The benthic foramini- tures within the panchromatic band which was deemed fera Homotrema sp. occurs pervasively, while Calcarina unsuitable for work below the ocean surface (Riegl, sp. is most abundant in sandy environments and Amphyste- Reichert, & Ullrich, 2006), and due to almost no gina sp. is most prolific at the reef slope. Nonskeletal UTAMI ET AL. | 261

grains generally are very rare, limited to only aggregates

– and peloids. Aggregates, usually ca 3 mm in size, consist of bioclasts bound together by marine fibrous or microcrys- talline cement. Peloids were sporadically found in the inner lagoon as opaque and cemented faecal pellets. The statisti- cal analysis shows several statistically significant (p < 0.05) and strong (r > 0.6) correlations (Table 2). The tone. Environmental fine fraction (<125 μm) content increases strongly with water depth (r = 0.87), while the abundance of most bio- clasts, and especially coral fragments (r = −0.77) –– – – 5% 36% 48% 12% 6% 29% 29% 35%

SRM/ SC SAWDSG SAWSSG SAWSG decreases. Other components, which encompass mainly

ef margin/sparse coral, SAWDSG = sand unidentified grains and sponge spicules, show a moderate 18% 27% 27% 27% SSPL/ PM positive correlation (r = 0.36) to the fine fraction. The rea- son for this is likely the small grain size of close to 125 μm for most of the unidentified grains. Foraminifera DSPL/ PM abundance shows a negative correlation (r = −0.51) to the –– –– – –– – fine fraction indicating that most foraminifera seem to be adopted to sandy substrates. 33% 67% 73% 27% 100% – – 4.3 | Facies types DunhamGST PST Environment WST

= number of sample Principle component analysis shows three facies groupings n m)

μ that can be differentiated based on their grain size and the Sorting ( relative importance of coral and mollusc fragments (Fig- m)

μ ure 5). Relatively fine grained sediments containing abun- Mean grain size ( dant molluscs form a broad group on the right side of the

Fines (%) diagram. The central part of the diagram shows a group of samples which are very poorly sorted and contain corals besides molluscs as an important component. A large group Other (%) of relatively coarse grained samples on the right‐hand side of the diagram is dominated by coral fragments. Using cluster analysis (Figure 6) this group can be subdivided Nonskeletal (%) further into a grainstone (upper part) and a packstone/grain- stone dominated facies. In summary, the combination of cluster and principal component analysis allows four facies to be distinguished: coral grainstone, coral packstone/grain- Echinoderm (%) stone, coral‐mollusc packstone and mollusc wackestone (Figure 7, Table 1). Foram (%)

4.4 | Coral grainstone Mollusc (%) The coral grainstone facies is generally very poorly sorted and has the coarsest grain size (mean: 0.6 mm) of all facies

Halimeda (%) in this area. The fine fraction content varies between 0%–15%, with a mean of 7% in packstones (20%) and

Red algae (%) grainstones (80%). Coral fragments are the most abundant components in this facies, on average reaching 60%. Mol-

Coral (%) luscs are quite common and reach an average abundance of 12 12.8 0.5 0.3 17.9 1.5 3.4 0.7 7.1 55.9 99 6.26 18 37.2 3.0 2.5 21.2 4.8 4.6 0.4 3.0 23.3 293 5.89 50 49.0 2.5 5.5 24.5 5.8 3.3 0.2 0.8 8.3 573 3.56 66% 34% 22 59.5 2.9 3.9 16.6 6.8 3.2 0.3 0.2 6.5 620 3.32 77% 23% n Sediment composition, texture and environmental facies for each sedimentary facies. Dunham texture: GST = grainstone, PST = packstone, WST = wackes 17%, followed by foraminifera tests, which on average con- tribute 6%. Fragments of red algae, Halimeda, and echino- derms are also present in a limited amount of only 3%–4% ‐ each. Nonskeletal (0.3%) and other constituents (0.2%) are wackestone mollusc packstone packstone/ grainstone grainstone Mollusc Coral Coral Coral Sedimentary facies facies: DSPL/PM = deeper subtidalapron part with of dense lagoon/platform seagrass, margin, SAWSSG SSPL/PM = = sand shallower apron subtidal with part sparse of seagrass, lagoon/platform SAWSG margin, = SRM/SC sand = apron subtidal without re seagrass. TABLE 1 very rare. This facies occurs from the sand apron to the 262 | UTAMI ET AL.

FIGURE 4 SEM images of fine fraction (<125 μm) from Panggang Island for samples: (a) MPG 4‐1L located in the northwestern sand apron. (b) MPG 3‐3A located in the lagoon. (c) MPG 2‐4A located at the northern reef front. (d) MPG 3‐1C located in the eastern sand apron. Bioclasts are the major sediment components

TABLE 2 Correlation matrix: Significant (p < 0.05) correlations are marked bold. p‐values in upper right part, r‐values in lower left part

Coral Red Algae Halimeda Mollusc Foram Echinoderm Nonskeletal Other Fines Water depth Coral 0.02 0.01 0.74 <0.01 0.25 0.03 <0.01 <0.01 <0.01 Red algae 0.26 0.85 0.02 <0.01 0.80 0.08 0.26 <0.01 0.04 Halimeda 0.30 0.02 0.35 <0.01 0.23 0.20 0.01 <0.01 <0.01 Mollusc 0.04 −0.25 0.10 0.11 0.80 0.76 0.27 0.01 <0.01 Foram 0.31 0.41 0.44 −0.17 0.68 0.45 0.03 <0.01 <0.01 Echinoderm −0.13 0.03 −0.13 −0.03 0.05 0.06 0.98 0.95 0.50 Nonskeletal −0.24 0.19 −0.14 0.03 −0.08 0.20 <0.01 0.88 0.84 Other −0.56 −0.12 −0.28 −0.12 −0.24 <0.01 0.51 <0.01 <0.01 Fines −0.87 −0.34 −0.48 −0.27 −0.51 0.01 0.02 0.36 <0.01 Water depth −0.77 −0.23 −0.45 −0.31 −0.34 0.07 0.02 0.33 0.87 reef front in water depths of 0.3–2 m and sporadically at Panggang, Pramuka and Semak Daun Island in water the deeper reef front from 15 to 20 m. depths between 0.3 m and 21 m (Figure 7).

4.5 | Coral packstone/grainstone 4.6 | Coral–mollusc packstone Nearly 50% of all samples belong to the coral packstone/ All samples in this group have 10%–50% matrix (mean: grainstone facies. It contains 0%–21% fine fraction in 23%) and are classified as packstone. The amount of corals grainstones (66%) and packstones (34%). Coral packstone/ (37%) and molluscs (21%) is moderate, followed by fora- grainstones are characterized by a high amount of corals minifera (5%), echinoderms (4%), red algae (3%) and Hal- (49%) and mollusc fragments (25%), followed by a small imeda (2.5%). Nonskeletal grains are very rare (0.4%). The amount of foraminifera (6%), Halimeda (5%), echinoderms mean grain size of this facies is 0.3 mm and it can be clas- (3%) and red algae (2%). Nonskeletal grains occur only in sified as very poorly sorted. This facies occurs in water trace amounts (0.5%). The mean grain size of this facies is depths between 0.4 and 18 m in the shallow part of 0.5 mm and the samples are dominantly poorly sorted. This lagoons, at parts of the reef front and sporadically on the facies can be found on the sand apron and reef front of sand apron of Panggang Island (Figure 7). UTAMI ET AL. | 263

FIGURE 5 Principle component analysis using texture and composition data

FIGURE 6 Tree diagram of a cluster analysis (Ward's method) based on texture and composition data 264 | UTAMI ET AL.

4.7 | Mollusc wackestone 5 | DISCUSSION This facies shows the highest fine fraction content with an 5.1 | Distribution of sediment, carbonate average of almost 60%. The most common skeletal compo- components and facies nents in this facies are molluscs and corals. Foraminifera tests and echinoderm fragments contribute less than 2% Coral and mollusc are the most abundant sedimentary and 4%, respectively. Halimeda, red algae and nonskeletal grains with minor contributions from Halimeda, red algae, components are rare, contributing on average less than 1%. foraminifera and echinoderms (Table 1). Nonskeletal com- The mean grain size is ca 0.1 mm and samples in this ponents occur only in trace amounts in the form of aggre- facies are generally very poorly sorted. This facies is found gates and faecal pellets (Table 1). In Panggang Island, in water depths between 3 and 15 m at the reef slope and coral fragments occur mainly at the reef front, where corals in the lagoons of Panggang and Semak Daun Island (Fig- are actively growing (Figure 8) and in the sand apron ure 7). where the fragments are washed by waves and tides. In the lagoon, coral fragments are generally sparse (Figure 9a), 4.8 | Satellite‐derived environmental facies except in areas where corals are actively growing (Fig- map ure 8). In Pramuka Island, coral fragments occur mostly in the sand apron near the island and reduce in numbers Six environmental facies (Figure 8) have been derived by towards the outer sand apron and the reef front (Figure 9b). integrating SPOT‐6/7 multispectral data, statistic‐based A possible explanation for this trend indicated by satellite isocluster unsupervised classification, and ground‐truthing data (Figure 8) and field observations (Figure 2b,d) could with field observation. The sand apron can be subdivided be the local contribution from corals growing among the into three facies with no, sparse and dense seagrass seagrass on the sand apron (Figure 2b,d). cover. One facies is formed by areas with active coral Mollusc debris occurs in all sedimentary and environ- growth. The deeper water zone can be separated into a mental facies zones and its distribution shows no clear pat- shallow and a deep subtidal part of lagoons and platform tern (Figure 9c,d). This generally agrees with the notion of margins. Variation in reflectance can be attributed to Scrutton (1976b) that molluscan debris is ubiquitous in the benthic biota and water depth. In general, the environ- Seribu archipelago, a fact that distinguished the modern mental facies map (Figure 8) indicates that the sand systems from their Neogene counterparts. Park et al. (2010) apron with sparse seagrass is the dominant environmental believed that the dominant easterly monsoon has formed a facies and the subtidal reef margin coral facies often large bank consisting of argillaceous foram—mollusc pack- occurs adjacent to the sand apron with dense seagrass stone along the western margin of the platform. However, cover. This might in part be due to the reflectance simi- field observation, sedimentary facies analysis (Figure 7) larity of the chlorophyll‐producing organisms which make and satellite‐derived environmental facies study (Figure 8) coral reefs difficult to distinguish from loose sediments show no evidence to support this argument. Mollusc frag- inhabited by seagrass (Kaczmarek et al., 2010; Luczko- ments are distributed relatively equally along the western vich, Wagner, & Stoffle, 1993; Zainal, Dalby, & Robin- and eastern part of the platform (Figure 9c,d), suggesting son, 1993). Field observations show that the natural the influence of bidirectional monsoon winds rather than complexity of benthic organism associations cannot be an easterly domination. completely resolved by the satellite data. This is most Halimeda is generally a minor constituent (Figure 9e,f) apparent on the sand apron where usually small solitary that occurs mainly in the sand apron and to a lesser degree coral lumps occur among algae and seagrass (Figure 2). on the reef front. It is virtually absent from the lagoon. Cluster analysis has recognized four sedimentary facies, This study agrees with the findings of Jordan et al. (1993) while six environmental facies have been derived from who reported that Halimeda is usually found in the reef the satellite data (Figure 8, Table 1). In general, sedimen- flats and only locally concentrated at the reef front and tary facies do not correlate directly with environmental contrary to the findings of Scrutton (1976b) who observed facies except for the mollusc wackestone facies that only that Halimeda appears to be prolific on the reef flank (ap- occurs in lagoonal environments. In contrast, there is no proximate depth 10 to 40 m). Nevertheless, the observa- close correspondence between the density of seagrass tions reported here cover only the shallow part of the reef cover and sedimentary facies in the sand apron. Gener- flank (down to 22 m) and therefore does not rule out more ally, the texture associated with the subtidal lagoon is abundant Halimeda growth below this depth. wackestone, while packstone and grainstone textures Generally, the sedimentary facies of the studied patch dominate all environmental facies on the sand apron reef systems is comparable to other equatorial carbonate (Table 1). systems from Indonesia. Madden et al. (2013) investigated UTAMI ET AL. | 265

FIGURE 7 Facies distribution based on sediment samples collected in this study as indicated by coloured dots foreshore/backshore and intertidal deposits of the Kale- the fine fraction (<125 μm) and mean grain size. At Pang- dupa‐Hoga in Tukang Besi Archipelago offshore Southeast gang Island, the fine fraction is concentrated in the centre Sulawesi, Indonesia. They found a similar association of of the lagoon (Figure 10a), which also corresponds to the sand‐sized bioclasts dominated by coral and molluscs frag- finest mean grain size (Figure 10c). The sand apron is ments and a general absence of fines and nonskeletal characterized by coarser grained sediments and a low fine grains. They stress the role of waves and storms in the fraction content, whereas the reef front shows a generally fragmentation and accumulation of these skeletal sands and finer mean grain size and an increase in the fine fraction coarse coral debris; a process also important in the mon- content. In Pramuka Island, the coarser mean grain sizes soonal dominated system of Kepulauan Seribu. The shape and low fine fraction contents occur in the west‐southwest of the mud‐sized sediment particles in the Seribu Archipe- sand apron and reef front which is exposed to waves and lago also suggests that maceration and other physical pro- currents from the Java Sea (Figure 10b). In contrast, the cesses play an important role in the breakdown of the fine fraction and the finer mean grain sizes are concen- skeletal components such as coral, mollusc, echinoderm, trated in the east‐northeast sand apron and reef front in red algae and foraminifera (Figure 4). Bioerosion, e.g. by more protected environments (Figure 10d). This asymmetry the sponge Cliona seems to contribute to mud production in grain‐size distribution on Pramuka Island likely reflects (Figure 4) but does not seem to play the dominant role the stronger wind force of the east monsoon compared to suggested by Park et al. (2010). Clear evidence for other the west monsoon in the area. Poerbandono (2016) argued mud sources such as inorganic precipitation or bacterial that this imbalance also results in a north‐westward net induced precipitation as proposed by Scrutton (1976b) is drift of beach sediments. Another expression of the pre- lacking. dominance of the eastern monsoon is likely the broader The dominance of sand‐sized grains is also apparent in sand apron on the eastern side and the location of Pramuka Figure 10 that shows distinct trends in the distribution of Island on the more western side of the Seribu platform. In 266 | UTAMI ET AL.

FIGURE 8 Satellite‐derived environmental facies map generated by an image analysis algorithm for unsupervised classification. Sedimentary facies is shown for comparison as coloured dots. Ocean and land area have been masked. Classification results were subsequently validated with field observation

contrast, Panggang Island shows no pronounced morpho- apparent from the predominance of the coral grainstones logical asymmetry in its east–west sand apron (Figure 7) and the lack of coral–mollusc packstones in the sand and grain‐size distribution, which likely results from its apron without seagrass (Table 1). Three different environ- more sheltered position between the adjacent islands. mental facies can be distinguished on the sand apron, based on the density of seagrass cover (Figure 8). Grain‐ 5.2 | Correlation between water depth, size differences in areas without and with sparse seagrass sedimentary and environmental facies are statistically not significant, but areas with a dense seagrass cover are characterized by a significantly There is no close correspondence between the environ- (p < 0.05) finer sediment grain size compared to other mental and sedimentary facies in the sand apron. This areas on the sand apron (Figure 11). The dense seagrass lack of congruency partly arises from the low fine frac- meadows effectively reduce current velocities and attenu- tion content on the wave and tide swept sand apron. A ate bed shear stress (Hansen & Reidenbach, 2012; Koch similar discrepancy between sedimentary texture and envi- & Gust, 1999; Ward, Kemp, & Boynton, 1984) thus ronmental facies has been described for other mud lean resulting in a baffling effect which decreases sediment settings such as in the Holocene tidal carbonates from off- erosion and enhances particle deposition (Koch, Sanford, shore Al Ruwais in Qatar (Purkis et al., 2017). Despite Chen, Shafer, & Smith, 2006). Therefore, sediment in the absence of a direct correspondence between sedimen- vegetated areas is usually finer and more organic rich tary and environmental facies in the sand apron, the satel- compared to those in unvegetated areas (Fonseca & lite‐derived environmental facies map allow some Koehl, 2006; Kenworthy, Zieman, & Thayer, 1982). This predictions about the grain‐size distribution. This is baffling effect results in a slight increase in the fine UTAMI ET AL. | 267

FIGURE 9 Distribution of coral fragments in (a) Panggang Island and (b) Pramuka Island. Distribution of mollusc fragments in (c) Panggang Island and (d) Pramuka Island. Distribution of Halimeda fragments in (e) Panggang Island and (f) Pramuka Island fraction with denser seagrass cover and the generally finer sedimentary facies for the entire data set shows a large grain sizes in the sand apron environment with dense sea- overlap of the four facies in water depth from 0 to 22 m. grass cover (Figure 11). An even clearer relationship This results in a facies predictability of less than 40% and between sedimentary facies and satellite derived environ- therefore an apparently low degree of ordering (Fig- mental facies exists for the molluscs wackestone that ure 12a,b). If the data from the reef front are not taken occurs exclusively in the central lagoons of Panggang and into account, a high facies diversity is only observed in Semak Daun (Table 1, Figure 8). This correlation results shallow waters less than 7 m in depth (Figure 12c,d). On likely from the strong control of water depth on the satel- the other hand, a clear relationship between facies and lite derived map (Figure 8) and the abundance of the fine water depth, and therefore a high predictability can be fraction (Table 2). However, a distribution plot of the four observed for water depths below 7 m (Figure 12c,d). The 268 | UTAMI ET AL.

FIGURE 10 Distribution of the fine fraction (<125 μm) in (a) Panggang Island and (b) Pramuka Island. Mean grain‐size distribution in (c) Panggang Island and (d) Pramuka Island

FIGURE 11 (a) Box plot and arithmetic mean (x) for the sediment grain‐ size distribution within different sand apron environments. (b) Abundance of coarse and fine fraction (<125 μm) within different sand apron environments difference between the two scenarios basically results This in part random or mosaic‐like facies distribution is from the stronger wave and current exposure of the reef very different from the observation of Scrutton (1976b) front compared to similar depths in the protected lagoon. who stated that the boundaries between major facies units UTAMI ET AL. | 269

FIGURE 12 (a) The abundance of four delineated facies against water depth for the complete data set. (b) Shannon–Wiener evenness (E) of facies as related to water depth. (c) The abundance of four delineated facies against water depth excluding samples from the reef front and (d) Shannon–Wiener evenness (E) of facies as related to water depth are relatively sharp and also differs from the lithofacies indistinguishable from random (Purkis & Riegl, 2005; Pur- maps of Jordan et al. (1993) which suggest broad, kis et al., 2015; Rankey, 2004; Wilkinson et al., 1996), homogenous facies belts. deterministic order with respect to depth (Bosence, 2008; Recent studies on carbonate sediment, especially in Maloof & Grotzinger, 2012) and incorporating aspects of shallow water, have focused on whether facies distribution both random and deterministic (Gischler et al., 2017; Pur- is related to bathymetry, questioning the long‐held concept kis & Vlaswinkel, 2012). Study area scale and sampling in carbonate geology that changes in water depth can be density emerged to be a significant factor as well because recognized through analysis of the lithofacies (Bosence, large‐scale studies usually show a distinct relationship 2008; Gischler et al., 2017; Purkis et al., 2015; Rankey, between facies and water depth, whereas smaller areas and 2002, 2004; Wilkinson, Diedrich, & Drummond, 1996). depth range do not seem to represent a clear cut relation- These authors briefly outline that the understanding of ship (Harris, Purkis, & Ellis, 2011; Purkis et al., 2015; where and how carbonate sediments are produced and Rankey, 2004) and high‐resolution sediment maps are com- accumulated has evolved from the rather simple concept of monly more heterogeneous with facies mosaics instead of direct productivity–depth relationships to the recognition broad, homogenous facies belts (Kaczmarek et al., 2010; that carbonate depositional environments are influenced to Purkis, Harris, & Ellis, 2012). some degree by depth, and also by a complex suite of The results obtained here for the relatively small patch autogenic factors that lead to facies mosaics especially in reefs of the Seribu archipelago indicate elements of ran- shallow waters where sediments migrate and superimpose domness but also deterministic ordering (Figure 12). The on each other over short distances as a result of subtle lagoonal sediments in water depths of more than 7 m are environmental changes. Purkis et al. (2015) showed that distinct from the sand apron or reef front. The nearly com- observations on the relationship between facies and water plete lack of tidal inlets in the relatively wide sand aprons depth can be subdivided into three groups: prevents flushing of fine sediments from the interior 270 | UTAMI ET AL. lagoon. This is in contrast to other studies where currents similar water depth in the lagoon. The analysis shows that prevent deposition of fines in the relatively deep lagoons the shallow carbonate depositional environment (<7 m) is of atoll‐like carbonate systems (Betzler et al., 2015; Gis- influenced by a complex suite of autogenic processes, chler, 2006). Three factors were likely important for the resulting in an apparently random facies distribution. For formation of the continuous sand aprons. Firstly, the late water depths below 7 m, the sedimentary facies can be Holocene sea‐level fall is thought to have led to wide- predicted reliably if the reef front samples are excluded spread progradation of the sand aprons and partial infilling from the analysis. This facies dependence on water depth of the lagoons (Park et al., 2010). Secondly, bimodal mon- is in contrast to several other facies studies from atoll soonal winds lead to a relatively symmetrical distribution structures that showed random facies distributions at simi- of the sand aprons without pronounced lee effects. Thirdly, lar water depth. This difference can be explained by a the small size of the patch reefs of the Seribu archipelago lack of tidal inlets at this study site, leading to homoge- leads to a high ratio between the length of the productive neously low energy conditions in the lagoon. The rela- reef front and the area of the platform interior (Purdy & tively closed nature of the lagoons is interpreted to result Gischler, 2005). The sediments that are produced by the from a late Holocene sea‐level fall, leading to prograda- reef and transported by monsoonal winds and currents to tion of the sand apron under high energy monsoonal con- the sand apron therefore can fill in the available accommo- ditions and a partial infilling of the accommodation space dation space rapidly. Sea‐level fall, monsoonal climate and on these kilometre‐scale carbonate platforms. size therefore all contribute to the effective isolation of the lagoon and the development of a distinct facies. ACKNOWLEDGEMENTS We thank the anonymous reviewer and Sam Purkis for 6 | CONCLUSION their constructive and detailed comments on our manu- script. We are grateful to Lembaga Pengelola Dana Pen- Four sedimentary facies occur in isolated carbonate plat- didikan (LPDP) who awarded a scholarship to Dwi ‐ forms of Kepulauan Seribu: coral grainstone, coral pack- Amanda Utami Nasution (No: PRJ 4137/LPDP.3/2016), stone/grainstone, coral‐mollusc packstone and mollusc and to Geotechnology Research Center, Lembaga Ilmu wackestone. Six environmental facies could be distin- Pengetahuan Indonesia (LIPI) for DIPA 079.01.2.017152/ guished using statistical satellite image analysis. These 2015 Project no 3408.001.014.H who funded this field include the deeper lagoon/platform margin, shallower work. We thank Marfasran Hendrizan and Dudi Prayudi lagoon/platform margin and the subtidal reef margin/coral. for their assistance in the field work. Philipp Binger is The sand apron can be subdivided into three environmen- thanked for his assistance during sampling in the labora- tal facies based on the density of seagrass cover: sand tory. The authors thank Tubagus Solihuddin for his help in apron with dense seagrass, with sparse seagrass and with- providing satellite imagery as well as Ilham Arisbaya for ‐ out seagrass. The distribution of the environmental facies discussion during the making of the satellite based map. is mainly controlled by water depth, density of seagrass cover and coral abundance. Only one sedimentary facies ORCID (mollusc wackestone) can be directly correlated with satel- lite‐mapped environmental facies (lagoon). In contrast, Dwi Amanda Utami http://orcid.org/0000-0002-3500- there is no close correspondence between the density of 2693 seagrass cover and sedimentary facies due to the hetero- geneity and complexity of the environment. Although no close correspondence was found between the seagrass REFERENCES cover and sedimentary facies, there is a clear influence of Belde, J., Back, S., Bourget, J., & Reuning, L. (2017). Oligocene and seagrass density on the grain‐size distribution. 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