Coral Reef Development as an Indicator of Seal Level Fluctuation: a Preliminary Study on Pleistocene Reef in Bulukumba, South Sulawesi

DOI:10.17014/ijog.3.1.53-66Pleistocene reefs in eastern part of Indonesia are abundantly found along the coast as raised reef. They express highly tectonized and/or uplifted area characterized by several terraces. Therefore the reef size is narrow. This research is located at Bira District of Bulukumba Regency or it lies at the southern tip of South Sulawesi Peninsula. The objective of this research is to define depositional environment based on coral development. Several methods were applied such as intersect lines which were perpendicular to the cliff, geochemical and petrographic analyses, as well as paleoenvironment interpretation. Three facies are described at the Pleistocene reef, namely 1) Reef Front Facies, 2) Reef Core Facies, 3) Back Reef Facies. Based on facies association and organism accumulation, the depositional environment of Pleistocene reef is interpreted to be developed in a small reef complex on an unstable basement. The reef has experienced at least 3 (three) times of sea level fluctuation.


Introduction
Quaternary reefs are well exposed along coasts in the eastern part of Indonesia. One of the reefs is found at the southern tip of South Sulawesi and it is mapped as Selayar limestone. The Selayar limestone spread out from Selayar Island in the south through Bulukumba Regency in the north. The ages of the rocks range from Upper Miocene to Pleistocene (Van Bemmelen, 1949) including raised Quaternary reef. In studying the Selayar limestone in Bulukumba Regency, Imran and Koch (2006) explained that the limestone developed as narrow coral reefs. Furthermore, the rocks are differentiated by terraces in which the highest terrace consists of the oldest rock in the northern part and the lower terrace is composed of the youngest one in the southern part. Imran (2000) divided the rock into four rock units, namely early Late Miocene reef of knollreef unit, Late Miocene -Early Pliocene reef of upper terrace unit, Late Pliocene reef of middle terrace unit, and Early Pleistocene reef of lower terrace unit. The development of reef terraces was driven by the uplifting that intensively affected the region of the southern arm of Sulawesi since Neogene (Sukamto, 1975). The Pleistocene reef growth shows a small reef complex as a fringing reef found along the coast of Bira area of South Sulawesi. Kennedy and Woodroffe (2002) discuss that sea-level fluctuations are important in the development of fringing reef. Furthermore, they propose seven reef growth models which are: model A, the fringing reef is established at a depth and primarily accretes vertically towards the sea surface; model B initiates at sea level and due to the lack of vertical accommodation space grows laterally; model C has a similar morphology to model B; however, the reef progrades over a nonreefal sediment wedge; and model D where the reef occurs episodically lateral and vertical growth with a stepwise progradation of the fore reef. The remaining models are characterized by a seaward reef framework behind which unconsolidated sediments accumulate. In model E, reef-crest growth forms a barrier leading to the development of a backreef lagoon. Model F has a similar morphology to model E, except that the reef crest is formed by hurricane rubble accumulation rather than a framework accretion, and is periodically reworked.
Coral skeletons give an excellent record of climate archives of tropical and subtropical areas (Eakin and Grottoli, 2006). Changes of sea condition such as temperature, salinity, upwelling, and ocean circulation are well preserved within coral skeletons. Indonesia as a tropical country has a lot of coral reefs from ancient to recent and is susceptible to a climate change. In the period 1997 -1998, Indonesia experienced high damage of coral reef due to the changing of El Nino characterized by global warming (EUSAI, 2001). In the last 300 years, there is a significant increase in CO 2 concentration from about 280 ppm in 1700 to 360 ppm in 2000 ( Figure 1).
De Klerk (1982) has studied the development of coral reefs (based on 14 C dating) in Spermonde Platform, South Sulawesi. He found that 4500 years before present, seawater was as high as 5 m above the present sea level. Hantoro et al. (1994) describeed an evolution of geodynamic processes in Alor Island, Indonesia, is affected by an uplifting with reference to Quaternary reefs. The indication of the uplifting was also found on Figure 1. Historical data of rising CO 2 concentration during 300 years (1700 -2000). Sources: Joosa et al. (1999).  (Sumosusastro et al., 1989) and in Bulukumba (Imran and Koch, 2006). Mann et al. (2016) have studied the Spermonde Island and suggest that there was a sea level fluctuation ca.
5600 cal. yr BP and reached the present sea level that was at around 4000 cal. yr BP. The studied area is predominantly made up of Plio-Pleistocene reef, Pleistocene reef, and modern reef. They are the member of Selayar limestone, Walanae Formation (Sukamto and Supriatna, 1982). The Selayar limestone in Bulukumba area formed at least four terraces ( Figure  2) indicating an active tectonism during and after the reef development. According to Imran (2000) and Farida (2002), the studied area experienced rapture, forming terraces of reefs as well as the presence of notches (cliff abrasion results). The purpose of this study is to determine the depositional environment and the sea level fluctuation based on the reef development. At the end of this  G study, a paleoclimate change as base data will be constructed to predict the future climate condition.

Methods
The study is focused on the Pleistocene reef cropping out in a lower terrace. Several methods were applied in this study, namely field survey, morphology and petrography analyses. The field survey was taken on four line transects which perpendicular and parallel to the shoreline (Figure 2). In order to get the geometry of the reef, a morphology view was done from the highest level surrounding the studied area. Petrography analyses on 21 thin sections were applied on matrixes for microscopic needs such as microbiofacies and fossil analysis. Thin sections from samples were done in Bandung. Furthermore, a petrography analysis was done in Petrography Laboratory of Geology Department, Hasanuddin University. The analysis was done under a polarized microscope to take the type and textural components of the rock. The lithology nomenclature used the classification of Dunham's (1962) as well as Embry and Klovan (1971). The paleontology analysis was done for identifying organism contents, especially foraminifera in Paleontology Laboratory, Geology Department, Hasanuddin University.

Geological Setting
Walanae Formation formed in the Walanae Depression trending north -south of South Sulawesi. It consists of volcanic rock, sandstone, and carbonate rock including Selayar Limestone (Sukamto and Supriatna, 1982). During the Middle Miocene, the growth of carbonate production in the western part of South Sulawesi was interrupted by volcaniclastic materials which buried the shallow water carbonate and allowed a new carbonate production in other sites (Wilson, 2000). Bromfield and Renema (2011) argued that the Selayar Limestone developed contemporaneously with volcanic activity in a protected area of distal environment during Late Miocene to very Early Pleistocene. The Plio-Pleistocene event was accompanied by a general uplift of the region as indicated by the subaerial nature of most Quaternary deposits. The local appearance of thin coal layers at the upper part of the Walanae Formation may indicate the beginning of this uplift. Raised Pleistocene coral reefs in the Walanae Depression and in North Bone probably lie unconformably upon the gently folded Neogene rock (Van Leeuwen, 1981). Imran (2000) mentioned that the stratigraphy of the studied area is composed of foraminiferal Selayar Limestone overlain by Walanae Formation in some places. The Selayar Limestone has an age 5.8 -1.4 million years or Late Miocene to Early Pleistocene and correlated with Taccipi Limestone Members of Walanae Formation (Bromfield, 2013). In the southern tip of Sulawesi, the limestone exposes several terraces, similar to limestone in the Selayar Island (Figure 3). The lower terrace has been described as a lower Pleistocene reef (Imran, 2000) and in Selayar Island is dated as very Early Pleistocene or 1.6 to 1.4 Ma (Bromfield, 2013). Through the observation of the reef terraces, it is determined that the higher terraces are located leeward and the lower terraces seaward. This geographic position is proportional to their age; the higher terrace is older. Similar characteristics were also studied in Alor Island, eastern part of Indonesia by Hantoro et al. (1994). His observations documented that the reefs were developed under the control of tectonics and sea level fluctuation and, therefore, the oldest terrace was developed in the highest part leeward.
Based on larger foraminiferal assemblage and lithologic characteristics supported by other organisms, the Selayar limestone is informally divided into four units (Imran, 2000;Imran and Koch, 2006). The units are: a. foraminiferal limestone unit; b. coral reef unit in the Bontotiro area and upper terraces on the Bira area of Bulukumba; c. coralgal reef unit on the middle terrace; and d. raised coral reef unit in the lower terrace. This study is focused on the raised coral reef and mapped as Quaternary raised coral reefs I J O G those are: a. reef front facies, b. reef core facies, and c. back reef facies. The reef was initiated to develop within the protected shallow marine. It unconformably overlies siliciclastic rock of Walanae Formation. The second period of the reef development is characterized by the growing of branching, delicate, robust, and massive corals associating giant Tridacna sp. The third period is the formation of bafflestone dominated by branching coral. During its development, the reef has experienced at least three times of sea level fluctuation indicated by beach abrasion (notch).
The notches occurred during the reef development which abraded the Pliocene reef, during the first sea level fall as the second notch at the reef crest and back reef and finally the recent sea level as the third notch.

Reef Colonization or Establishment (Model A)
Reef colonization of Pleistocene reef in Bulukumba is well exposed in the south coast of Bira area. It unconformably overlies the older rock of Walanae volcanics (Figure 4a). At the lower part, the reef is dominated by soft branching coral of Pleistocene age (Van Leeuwen, 1981;Van Bemellen, 1949). The morphological survey shows that there are three subterraces consisting of Pleistocene reef in the west coast. The terraces indicating sea level fluctuations during and after the development of the reef can be interpreted as a result of a short term climate change and/or tectonic activities. Another indication of the sea level fluctuation is the presence of notches. There are three notches found in the Bira area, one notch at the Pliocene reef and two notches at Pleistocene reef. The Pleistocene reef is separated from the older Pliocene reef by a notch indicating sea level rise. The condition allowed the development of Pleistocene reef. The sea level fall occurred after the development of the reef marked by notches at the reef crest and back reef. The sea level fall continues to the present sea level and is marked by a cliff along the present coastline.

Pleistocene Reef Development
Three facies are described at the Pleistocene reef of Bulukumba Regency (Imran et al., 2015),  Sukamto and Supriatna,1982;Imran, 2000).  Under a microscope, matrix of the reef is characterized by bioclastic-packstone -grainstone. The bioclasts of the reef matrixes are commonly composed of coral fragment, red algae, gastropods, and foraminifera (Figures 5a and b). On the other hand, a thin section of tuff shows rich planktonic foraminifera (Figures 5c and d). Imran et al. (2015) suggested that the reef developed in the front reef zone as a reef front facies. The facies corresponds to the fore slope microfacies zone of Flügel (2010) and Wilson (1975). The presence of burrowing structures and branching corals indicates a low energy regime and is interpreted as a shallow subtidal and intertidal environment. It agrees to the theory of Garrett's (1977). The growth of the reef at the upper part from siliciclastic rock of Walanae Formation at the lower part separated by an angular unconformable relationship indicates a sea level rise environment. Such coral reef growth is the initiation to grow isolated coral colonies (Kennedy and Woodroffe, 2002). This first stage of reef development grew vertically to approach the sea level rise. This stage of Pleistocene reef in Bulukumba continues to form reef diversification.

Reef Diversification or Reef Model D
The reef diversification or model D developed not only vertically but also laterally. The reef model has a thick zone and diversifies coral organisms. The diversification of coral is characterized by the presence of massive coral with a  (Figure 6d), small pelecypods, and gastropods as well as algae are commonly found as a reef builder within this reef model.
Laterally, the reef developed from reef slope the leeward, and formed back reef lagoon. Rhodolith channel-like deposits composing rhodolithic rudstone (Figure 7a and 7b) developed within this reef model. It has graded bedding structure with fining upward and contains foraminifera of Amphistegina sp. (Figure 7c), Calcarina sp. (Figure 7d), and Heterostegina sp.
The association of organisms and rock texture suggests a shallow marine environment with high energy. This type of environment appropriates to a reef front zone as suggested by James (1983) and Veron (2002). The presence of rhodolith channellike is probably influenced by an upwelling current from Makassar Strait. Fragments of red algae (rhodolith) and rock fragments contained in this facies interprete that they experienced transport and accumulated in the channels at the diversification zone. The presence of Tridacna sp. in modern environments found up to 10 m of water depth (Rosewater, 1965) designates a shallow marine environment. In general, this reef zone, based on faunal compositions, developed in depth of about 10 -20 m.

Reef Domination or Model E
The reef is the uppermost part of the reef complex consisting of reef-crest and backreef lagoon. This reef model is dominated by branching coral of Acroporidae (Figure 8a) extending from fore reef to back reef facies (Figures 8b -d).

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To the north, branching corals are more common and form beach cliff as high as 5 -7 m (Figure 8b) Most of the corals are highly dissolved ( Figure  9a) due to a diagenetic process since the rock exposes. They are also found as fragmented sediments and form bioclastic grain-rudstone texture at the reef slope towards lagoon or closed to the reef crest (Figure 9b). The other organisms found in this reef are red algae (Figure 9c) and foraminifera such as Calcarina sp. (Figure 9d), as well as green algae (Figure 9e). These organisms are commonly found within the matrix.
The presence of corals of genus Acropora as a fragile organism suggests a low energy environment with high sedimentation rate. Such environmental deposition corresponds to back reef or at reef slope in reef front. The reef is interpreted to develop as back reef and reef crest in the shallow marine. This reef zone (back reef and reef crest) experienced seawater abrasion marked as a notch. The environment of Acropora agrees with the Quaternary counterpart in southeast Sulawesi studied by (Crabbe et al., 2006). Furthermore, they find that the reef crest and reef flat deposits are dominated by branching Acropora and developed in very shallow waters.

Sea Level Fluctuation During the Pleistocene Reef Development
The development of Pleistocene reef characterized by burrowing structures at the base of reef It is marked by abundant burrowing structures. The sea level rise occurred to allow the reef community to grow and diversify in the diversification zone. This zone is designated by several types of coral such massive, branching platy, and head corals. This transgression continued to develop the domination zone of branching coral.
These coral types are found from front reef to back reef. The environment development can be inferred from modern counterpart in Indo-Pacific region which has a 6 -15 m depth (Cabioch et al., 1999). Chappell et al. (1996) suggested that the Late Quaternary sea levels in Huon Peninsula represent a global pattern. They measured the sea level fluctuation in the reef terraces and to be in an agreement throughout the last glacial cycle. Bromfield and Renema (2011)

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G with reference to Selayar Island ranging from 10 -20 m depth in the reef slope. Furthermore, they argue that the presence of reef slope channel with reverse graded bedding in the upper Pleistocene reef represents a reef slope channel a higher energy environment. In the diversification reef, such channel deposits dominated by rhodolite imply a high energy regime.
A superimposed marine notch is identified on the older Pleistocene reef and Pleistocene reef. The notch spread out parallel to the recent coastline at the latitude of around 15 m which abraded the Pliocene reef (Imran, 2000;Imran et al., 2015), 7 m at reef crest and back reef ( Figure  11) above the present sea level. The notch at the Pliocene reef indicates an abrasion process during the development of Pleistocene reef (or sea level rise). On the other hand, the notches at the back reef and reef crest indicate a sea level fall after the reef development ( Figure 12). The sea level fall continues to the present sea level and abrades the lower part of the Pleistocene reef.
The study of sea level fluctuation related to the presence of notches has also been done by Hantoro et al. (1994) on Quaternary reef in Alor Island, eastern Indonesia. They found two superimposed marine notches at about 5.0 m and 8.6 m respectively above the present MLWST level. They interpreted to be corresponding to a glacial interstadial and to the Holocene sea-level peak. Chappell et al. (1996) calculate that the sea level  (Bromfield and Renema, 2011), as a reference to the Pleistocene Selayar reef in Bulukumba, the reef development does not match the interglacial sea level. Nevertheless, it has two superimpose marine notches (as in Quaternary reef in Alor).

Conclusion
Based on the facies distribution and textural dominant, the studied area is a small fringing reef complex. It developed in three stages, namely coral colonization or reef model A, coral diversification or reef model D, and coral domination or reef model E. The reef started to develop from a protected shallow marine to a reef front slope seaward. Laterally, the top the reef developed from the reef front seaward to the back reef with narrow lagoon with depth of <20m.
Sea level fluctuation played an important role in reef terraces. Three terraces and two notches are identified in the Pleistocene reef. The reef has experienced at least three times of sea level fluctuation. Sea level rise allowed reef development from colonization to domination. This sea level is identified by a line of notch within Pliocene reef. After the reef development, sea level fall occurred, indicated by marine abrasion at reef crest and back reef. This continued to the present sea level.

Acknowledgements
The authors would like to thank DIKTI which funded this research through Higher Education Research Program in year 2015. Profusely with condition in reef of Bira area, Bulukumba).

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G thanks are addressed to Prof. Mary Elliot and Prof Laurant for academic discussion during the fieldwork. Gratitude is also to students for helping in the field and laboratory works.