Contemporary sedimentation in the Cocos (Keeling) Islands, Indian Ocean: interpretation using settling velocity analysis

Abstract

Textural parameters of grain-size distributions derived from sieve analysis and component analysis have been used to the exclusion of other methods to examine sedimentary processes in modern reef environments. However, because these methods do not reflect the hydraulic behaviour of bioclastic sediment this study uses settling velocity analysis (accounting for grain size, density, and shape variability), and component analysis of separated settling velocity fractions, to describe and differentiate bioclastic deposits and infer transport pathways in the Cocos (Keeling) Islands, an Indian Ocean atoll. Principal Component and Cluster analysis of settling (251 samples), and component (90 samples) characteristics, discriminated eight settling velocity and five component classes. A derived settling velocity classification, reflecting the hydraulic properties of the sediments, describes a lagoonward gradient from those with fast settling characteristics on the outer reef flat to those with moderate settling character on sand aprons. The active sediment transport system is constrained between these zones. Component analysis of bulk samples identifies two broad sediment types: (1) those with reef-derived components, found throughout the active transport zone (reef flat to sand aprons); and (2) those characterised by greater proportions of gastropod and Halimeda fragments which are produced and deposited in situ in the lagoon. Component analysis of settling fractions further resolves details of sediment movement in the active transport zone. Sediment is transported from the productive reef flat to passage entrances which form initial depositional environments. Slower settling material is selectively transported from the entrances, through the passage conduits, to passage exit environments. These form the main depositional areas under mean energy conditions. During moderate- to high-energy events sediment is flushed from the passage to sand apron zones, with slower settling grains deposited in the deep lagoon sinks. Results show that settling velocity analysis and component analysis of settling fractions can resolve the nature of sediment transport patterns in reef environments at a much finer scale than studies in which analysis is based on grain-size distributions.