SEDIMENTOLOGICAL CONTROLS ON REEF ISLAND BUILDING: A DETAILED STUDY OF SUBSURFACE STRATIGRAPHY ON TWO LAGOONAL PLATFORM REEF ISLANDS IN THE MALDIVES

Reef islands are small, low-lying islands composed mainly of bioclastic sands and gravels from the late Holocene. These islands, which support around 700,000 people, are vulnerable to potential threats including land submergence and shoreline erosion. Thus, sedimentological controls are an important consideration for coastal maintenance but there is little research on the link between sediment supply and island building. This study provides the first detailed investigation of subsurface morphology of lagoonal platform reef islands in the Maldives using both quantitative sediment analyses and core sampling that extends below the level of live coral growth on the adjacent reef flats. Two islands in Huvadhu Atoll of the Republic of Maldives were used in this investigation. Sedimentological analyses (composition and texture) of contemporary reef sediments and surficial island sediments infer continued island-reef connectivity. Distinct facies in underground stratigraphy suggest compositional shifts that could be attributed to the effects of sea-level change on reef ecology. Islands will be more susceptible to erosion without a continuous supply of sediment or transport processes, and ecological change affects this sediment production. Island building is complex and relies heavily on sea-level driven reef growth, as this study has found. This research provides insights into how reef islands have formed and are maintained, which has implications for future island resilience in the face of sea-level rise.