Sedimentation on Rasdhoo and Ari Atolls, Maldives, Indian Ocean

Abstract

A first systematic study of composition, texture, and distribution of modern sediments in two Maldivian atolls reveals the predominance of skeletal carbonates. Fragments of corals, calcareous algae, mollusks, benthic foraminifera, and echinoderms are identified in the grainsize 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 Dunhamclassification 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. On reef islands, coralrich sediment is cemented to form intertidal beachrock and supratidal cayrock. Skeletal grains in atoll-interior lagoons are mainly mollusks and foraminifera. 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 characterized by hard bottoms with corals. Facies distribution appears to be an expression of depositional energy, which decreases from the atoll margin towards the center in Ari Atoll, and towards the west in Rasdhoo Atoll. Predominant sediment mineralogies include aragonite and high-magnesium calcite. Mean aragonite content decreases from 90% in coral grainstone to 70-80% in mollusk packstone, mollusk wackestone, and mudstone, and to 50% in E. Gischler ( ) packstone. Stable isotopes of oxygen and carbon in bulk samples range from −3 to −1.5 (δ 18 O) and from +0.4 to +3.2 (δ 13 C). It is not possible to delineate facies based on O-and C-isotopes.

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“…Modern marginal reefs are composed of robust-branching corals and coralline algae, whereas the lagoonal reefs show domal corals and detrital sand and rubble facies (Gischler et al, 2008). Muddy sediments are present only in the smaller atolls' lagoons protected by a continuous marginal reef rim (Ciarapica and Passeri, 1993;Gischler, 2006).…”

“…Currents reach water depths of 200 m and more with only slightly reduced velocities (Tomczak and Godfrey, 2003). Within the modern atolls' passages, currents can exhibit velocities up to 2 m/s at the surface (Preu and Engelbrecht, 1991), accounting for winnowing in the passages and lagoons, where hard bottoms form (Ciarapica and Passeri, 1993;Gischler, 2006). …”