The record of Oligocene - Middle Miocene paleoenvironmental changes in a carbonate platform (IODP Exp. 359, Maldives, Indian Ocean)

Abstract

The carbonate edifice of the Maldives has an exceptional record of the global climate and ocean current reconfigurations during the Neogene that set the path for the establishment of the present-day global climate system. The study of seismic profiles, sedimentary facies, microfacies, and ichnofabrics of the Oligocene-Miocene platform slopes allows reconstructing the different stages of basin evolution. The focus is on the characterization of paleoenvironmental conditions along the slope at IODP Expedition 359 drill sites, as well as subtle changes of the slope depositional regime controlled by variations in the frequency and intensity of the platform shedding and the basin oxygenation. Natural gamma radiation and ichnofabric logs are used as a proxy for sea-floor ventilation in the Inner Sea area, which is today surrounded by atolls. From the Oligocene (~25.5 Ma) to the end of the Middle Miocene Climate Optimum (~15 Ma) the Maldives carbonate platform slopes were poorly oxygenated. The opening of the eastern passages through the Maldives carbonate platform at 17.5 Ma led to an improvement of the sea-floor oxygenation. Sea-level fluctuations controlled the sediment input from the platform onto the slope that resulted in changes of the fossil and trace-fossil assemblages. After the evolution from a ramp to a flat-topped platform with well-defined bank edges, the platform developed hanging shoulder and more sigmoidal clinoforms. This correlates to the global decline of coral reefs during the Middle Miocene Climate Transition (~15 Ma) and a general sea-level lowering. The associated slope deposits show abundant channels and cyclic steps, are rich in reworked bioclasts, and have ichnofabrics pointing towards a good oxygenation of the sea-floor. The example of the Maldives carbonate platform therefore shows how integrating seismic geometries and compositional changes, i.e. microfacies and ichnofabrics, allow identifying changes in sea level and in the current pattern as the main controlling factors on carbonate platform evolution.