ESTABLISHMENT OF MALé ISLAND ON PLEISTOCENE KARSTIFIED TOPOGRAPHY DURING THE HOLOCENE TRANSGRESSION AND ITS SLOPE STABILITY (REPUBLIC OF MALDIVES)

Malé Island, located on the southeast corner of North Malé Atoll, is part of the atoll discontinuous rim that grew on the edge of its lagoon ranging in depths today between 40-60 m. The modern atoll rim is mainly anchored on the remnant of the previous interglacial atoll rim (Marine isotope stage 5e?).

Samples from three 22-to-35 m-long boreholes, drilled across Malé Island by the Environmental Research Center are available for this study, in addition to information from multiple boreholes drilled throughout the island and available in several high rise pre-construction reports. Moreover, a detailed multi-beam bathymetric map is available for the deep surroundings and the margins of Malé Island. This map has clearly imaged eight slope failures along the northeastern margin of the island and an uncovered Pleistocene karstic surface, displaying a series of aligned sinkholes due to unusually high currents across the channel separating Malé and Hulhule Islands, removing any Holocene sediment in the middle of the channel and also on the toe of slope of Malé Island northeastern margin.

The Pleistocene karstified foundation, below the south and west part of Malé Island, consists of a continuous and elevated 35-40 m-high and 400 m-wide ridge, first encountered in several boreholes at depths between 10-13 mbsl. This Pleistocene ridge top and southern high energy edge became the substratum for a reef to establish itself ~8,000 yrs ago and grew close to sea level. The reef had to produce large amount of sand being shed towards the low energy back reef area, rapidly accumulating in thicknesses in excess of 35 m. Analyses of benthic foraminifer faunal assemblages in the borehole located just behind the active reef reveal reciprocal relation between two dominant species, Amphistegina sp. and Calcarina sp., whereas in the borehole located in the northeast part of the island where a thick sand sequence has accumulated no particular downhole trend in the foraminifer faunal assemblage is observed. The thick accumulation of sand, forming the majority of the foundation of the central and northern part of Malé Island has remained unconsolidated and could be at the origin of the slope failures observed in the northeastern margin of the island at the base of which Holocene sediment could not accumulate due to the channel high current velocity.