SEDIMENTATION ON RASDHOO AND ARI ATOLLS, MALDIVES, INDIAN OCEAN

With an area of 107.500 km2, the Maldives are among the largest reef and carbonate platform areas on earth. In contrast to other major tropical carbonate areas such as, e.g., the Bahamas, Belize, or the Queensland shelf, however, systematic studies on modern carbonate facies are lacking. Therefore, this study was designed in order to collect the first systematic modern facies data from this area. A study of composition, texture, and distribution of modern sediments in two Maldivian atolls reveals the predominance of skeletal carbonates with aragonite and high-Mg-calcite mineralogies. Fragments of corals, calcareous algae, mollusks, benthic foraminifera, and echinoderms are identified in the grain-size fraction >125 µm. Non-skeletal grains such as cemented fecal pellets and aggregate grains only occur in small percentages. Fragments of skeletal grains, aragonite needles, and nanograins (<1 µm) are found in the grain-size fraction <125 µm. Needles and nanograins are interpreted to be largely of skeletal origin. Five sedimentary facies are distinguished (1-5), for which the Dunham-classification is applied. Fore reef, reef, back reef, as well as lagoonal patch reef and faro areas in both atolls are characterized by the occurrence of coral grainstones (1), which also contain fragments of red coralline algae, the codiacean alga Halimeda, and mollusks. Skeletal grains in atoll-interior lagoons are mainly mollusks and foraminifera. On reef islands, coral-rich sediment is cemented to form intertidal beachrock and supratidal cayrock. The lagoon of Rasdhoo Atoll is covered in the west by mudstones (2), in the center by mollusk packstones (3) and mollusk wackestones (4), and by hard bottoms with corals in the east adjacent to channels through the atoll reef margin. The interior lagoon of Ari Atoll contains mollusk wackestones (4) in the center and mollusk-foraminifer packstones (5). Marginal lagoon areas are again characterized by hard bottoms with corals. Facies distribution is an expression of depositional energy, which decreases from atoll margins towards the center in Ari Atoll, and towards the west in Rasdhoo Atoll.