ARCHITECTURE OF THE MIDDLE MIOCENE CARBONATE BANK MARGIN PROGRADING SEQUENCES IN THE MALDIVES, INDIAN OCEAN, AND THE DRIVING FORCE BEHIND IT

Margins of the individual carbonate banks of the giant isolated Maldives carbonate platform in the equatorial Indian Ocean exhibit spectacular progradation in the Middle Miocene. Prograding sequences are well-imaged by the industry 2-D seismic grid. Three individual bank margins, up to a 100 km apart, began prograding in the Middle Miocene, marking an abrupt change from aggradation and backstepping in the early Miocene.

Five prograding sequences are recognized within each prograding margin. Each sequence is composed of two primary packages – the lower strong-amplitude reflection package (SARP), and the upper weak-amplitude reflection (WARP). SARPs show a downward shift in facies, and sometimes exhibit downstepping geometries indicative of forced regression. SARPs are interpreted as deposits formed during the falling stage of relative sea-level. WARPs are typically composed of reflections that drape SARPs and onlap further up dip, with their topmost parts containing shingling reflections. They are interpreted as transgressive and high-stand deposits. Strike views show numerous small channels implying line source sediment supply. The five prograding sequences are interpreted as product of five complete sea-level cycles.

Miocene progradation in the Maldives is bi-directional, both from east and west towards the central part of the platform. This pattern suggests a system with a switching wind pattern, such as the monsoon. The origin of monsoons is related to the uplift of the Tibetan Plateau, and the timing of first monsoon remains controversial.

Seismic interpretation shows that commencement and cessation of progadation, as well as the timing of formation of individual sequences, was synchronous for the three banks, which implies regional, if not global control. The magnitudes of relative sea-level cycles that drove the progradation were in the order of tens of meters. The Maldives platform stratigraphic record appears to record the change from the dominantly green-house world with small frequency and magnitudes cycles to the ice-house world with high frequency/high magnitude cycles.