International Geologiical Congress - Oslo 2008

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SDD-01 Scientific drilling

 

A new view of the Neogene to Quaternary evolution of the Maldives carbonate platform (Indian ocean)

 

Christian Hübscher, University of Hamburg (Germany)
Betzler Christian, University of Hamburg (Christmas Island)
Luedmann Thomas, University of Hamburg (Germany)
John Reijmer, Free University Amsterdam (Netherlands)
Andre Droxler, Rice University (United States)
Sebastian Lindhorst, University of Hamburg (Germany)
 

 

The Maldives carbonate platform in the Indian Ocean is the second largest isolated carbonate platform in the world oceans. It has been the subject of several studies highlighting the role of global sea-level changes for its evolution during the last 60 Mio years. New geophysical and geological data recorded during the Meteor cruise M 74/4 (project "NEOMA") in December 2007 introduce new aspects which challenge this model. The data complement an existing data set for IODP Proposal 514 Full 6, and new site survey data will allow to expand, sharpen, and revise the concepts forwarded in this proposal.

The Maldives consist of two N-S oriented rows of atolls enclosing the up to 500 m deep Inner Sea. Seismic and hydroacoustic data measured in the Inner Sea reveal that the atolls are lined by active giant drift bodies separated from the atolls by a current moat and covered by migrating submarine dunes. Dune and moat facies can be traced back into time, thus allowing reconstructing the signatures of bottom currents in the sediments back for the last 5 or possibly even 8 Mio years. Therefore, these strong currents were a major controlling factor of platform slope sedimentation and of platform evolution. It is proposed that currents not only shape the carbonate platform slopes, but that they are also responsible for the so-called empty bucket geometry of the atolls, because shallow water carbonate produced in the inner platform was continuously exported out of the atolls and re-distributed in the drift bodies. Ultimately this implies that the Maldives are a current-controlled carbonate platform and that its peculiar geometry is directly linked to its oceanographic setting. The new data show that the Maldives carbonate platform is dissected by a series of deeply rooted faults. The most spectacular expression of these faults on the seafloor are strings and clusters of giant pockmarks with diameters of up to 1500 m and depths of up to 180 m. Pockmarks correlate vertically with faults and partly with pinnacles previously interpreted as more than 25 Mio years old patch reefs. To our knowledge this is the first record of giant pockmarks in isolated carbonate platforms far away from any continental margin. Hydroacoustic surveys of the pockmarks and sediment sampling (box cores and piston cores) indicate that they are possibly not active and that at least some of them serve as sediment sinks which contain the record of past events. In one of the pockmarks, for example, the tsunamite layer generated by the 2004 Indian Ocean tsunami was recovered.

 

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