Modern fringing reef carbonates from equatorial SE Asia: An integrated environmental, sediment and satellite characterisation study.

Abstract

Fringing reefs of SE Asia may conservatively comprise ~30% of the world’s coral reef area, but remain almost unstudied (White, 1987; Tomascik et al., 1997). This study provides insights into the primary sedimentological and early alteration characteristics of an isolated fringing reef system (Kaledupa-Hoga) from the Tukang Besi Archipelago, SE Asia. A combined multispectral satellite imagery, field and petrographic study allowed for the generation of an environmental facies map, which acts as a model for the distribution of primary sedimentological characteristics in relation to the primary environmental facies. The islands of the Tukang Besi Archipelago are mesotidal (<2 m) affected by strong diurnal and oceanic tidal currents, as well as high wave energy influenced by the bi-directional southeast Asian monsoon. An environmental facies map generated from Landsat-7 imagery and utilising field observations defines ten environmental facies. The facies map generated has a >71% accuracy when compared with field and sedimentary data. With the exception of the reef crest and reef slope that commonly have widths on a sub-imaging resolution (<30 m), the facies map accurately demonstrates the heterogeneous nature of the carbonate system. Although field and satellite imagery observations reveal ten environmental facies, sedimentological characterisation results in a lower number of distinctive categories due to the similarity of many deposits. Foreshore/backshore and bare intertidal deposits are distinctive and are composed of reef-derived material that has been reworked shorewards. Seagrass-associated facies all show some fine silt-clay sized material (<8%) with common imperforate foraminifera and pervasive micritisation, but also contain high abundances of reworked coral and shell allochems. Coral-associated reef flat facies are typically low in imperforate but high in perforate foraminifera, and show lesser effects of bioerosion and very low silt contents. The reef slope and crest are characterised by high abundances of gravel-sized fragmented corals with the highest abundances of echinoderm material and alcyonarian sclerites. Sediment samples across all fringing reef environments from the Kaledupa-Hoga transects are characterised almost exclusively by grain-rudstone textures, with <2-5% silt and clay size fractions, and minor baffling of fines in seagrass-associated settings (grain-packstones). The paucity of fines across the fringing reef systems as a whole, and the degree of homogenisation of sediment characteristics across the different field- and satellite-identifiable environmental facies are attributed to: (1) high wave/current energies, (2) the small size of the islands rendering limited protection, (3) bidirectional monsoon winds and (4) the lack of reef rimmed margins built to sea level. Absent from these deposits are well developed high energy windward and low energy leeward deposit characteristics and/or an overriding hurricane influence that are commonly seen in fringing reef systems from other areas.