Cenozoic Neritic Carbonates in the Maldives Controlled by Sea Level and Ocean Currents

Abstract

The Maldives carbonate platform is subjected to two major controlling factors: sea-level changes and ocean currents. The impact of these factors on sediment accumulation on the atolls and in the Inner Sea is elucidated by the correlation of seismic facies to cores and downhole logs acquired at eight sites during Expedition 359 of the International Ocean Discovery Program. The Maldives architecture with two N – S running carbonate banks was established during the Oligocene (Aubert & Droxler, 1992, Bull. Cent. Rech. Expl.-Prod. Elf-Aquitaine, 16; Belopolsky & Droxler, 2004, AAPG Stud. Geol., 49). Shrinking of these banks preceded an apparent restriction of the Inner Sea area in which sapropel layers alternating with chalks formed between ~ 24 and ~ 21.5 Ma. From ~ 21.5 to ~ 17.2 Ma, the western carbonate banks increased in width through progradation of the bank edge into the Inner Sea. The eastern banks during this time did not widen, but were segmented, opening the Inner Sea to the Indian Ocean. At ~ 17.2 Ma, i.e. the beginning of the Miocene Climate Optimum (MCO), the aggrading/prograding platform growth changed to a predominantly aggradational mode. A turning point in platform reorganization occurred at ~ 15.1 Ma with prograding sequences reflecting platform growth under an overall lowered sea level. Tracing of the global sea level evolution ended at ~ 12.9 Ma when ocean currents started to control the sedimentary system. This is linked to the establishment of the modern Indian monsoon and the coeval demise of parts of the Maldivian platforms. Partial platform drowning seems to be the combined product of change in sea level and ocean circulation. As sea level started to fall from the height of the MCO, platforms were repeatedly exposed. During the subsequent sea-level rises strong currents swept the platforms, preventing the re-establishment of shallow-water ecosystems and causing the demise of several platform segments. A concomitant higher particulate organic matter accumulation shows initiation of increased upwelling. Hence, since late middle Miocene times the Maldives show a configuration of bank development of persisting growth, which is accompanied by partial platform drowning and associated deposition of drifts just some kilometers apart. The juxtaposition of current deposits and shallow-water edifices is probably more frequent in the geological record than previously thought and expressed in carbonate sequence-stratigraphic models.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017