AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Nature and Timing of a Late Quaternary Carbonate Sediment Drift, Inner Sea of the Maldives Archipelago
(1) Earth Science MS-126, Rice University, Houston, TX.
(2) Geologisch-Paläontologisches Institut, Universität Hamburg, Hamburg,, Germany.
(3) Centre for Marine and Atmospheric Sciences (ZMAW), Universität Hamburg, Hamburg,, Germany.
(4) Department of Sedimentology and Marine geology, VU University Amsterdam, Amsterdam, Netherlands.
A 150 m-thick deep carbonate sediment drift was first observed on a Shell E-W seismic line north of Gaafaru Falhu atoll in the NE corner of the Maldive Inner Sea, in a range of water depths from ~325 to 500 m. During the NEOMA 2007 cruise on the RV Meteor lead by Universität Hamburg, the area north of Gaafaru Falhu atoll was extensively surveyed via multibeam bathymetry, 4 kHz sub bottom profilers, and MC seismics. Based on the survey, the drift has been accumulating at the western exit of a deep channel located north of Gaafaru Falhu atoll. Observations of NEOMA/Shell 4 kHz profiles and MCS lines show that a large part of the sedimentary drift, down-lapping on a major unconformity, thins on the toe of its front into 5 to 6 distinct wedge-like subunits into a very recent sequence not much thicker than 15-25 m.
During the NEOMA cruise, one box core, M74-4-1121, was recovered on top of the carbonate drift itself and two 14 m-long piston cores were retrieved on the toe of its front, M74-4-1120 on a proximal location and M74-4-1144 on a more distal position. The box core recovered 20 cm of the very top of the drift. Two third of the carbonate drift sand, made mostly of skeletal angular shell fragments and benthic foraminifers, fell in a size range between 355-1000 μ. The other third of the sand, between 63-355 μ, is a mixture of planktic foraminifers/pteropods, skeletal fragments, and benthic foraminifers. Both piston cores display downcore a clear cyclic pattern in sediment size fraction variability. Interglacial intervals are expended by inclusion of fine (< 63 μ) bank derived material whereas intervening glacial stages are condensed and characterized by high coarse fraction (> 63 μ) proportions ranging between 60-90 % in both cores. Bio-stratigraphic markers help estimate that M74-4-1144 bottomed at the end of Marine Isotope (MIS) 11. The base of core M74-4-1120, displaying lower sedimentation rates than in core M74-4-1144, represents MIS 15. It is probably not a coincidence that the five glacial/interglacial couplets easily identified in M74-4-1121 (from MIS 5 to MIS 15) could correpond to the five small wedge sub-units forming the toe of the carbonate drift front. Based upon this observation, the accumulation of the large and thick carbonate drift north of Gaafahru Falhu atoll was most likely initiated only since the mid-Brunhes, when the atoll top became intermittently re-flooded, after a relative long period of exposure during the early Brunhes.