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Timing and Evolution of a Deep-Water Carbonate Sand Drift: Inner Sea of the Maldives Archipelago, Equatorial Indian Ocean

Abstract

A 200 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 Maldives Inner Sea. During the 2007 NEOMA expedition on the R/V Meteor, piston cores (M74-4-1120/M74-4-1144) were collected on multi-beam and high resolution multi-channel seismic lines. Based on the coupled interpretation of lithology variations and seismic stratigraphy, the overall Plio-Quaternary evolution of the juxtaposed sandy/muddy deep water carbonate drifts is better understood. Analyses of cores 1120/1144, and a box core 1121, collected on the muddy drift and on the sandy drift, respectively, determine their overall downcore lithology variations and chrono/cyclo-stratigraphies. In the muddy drift, downcore cyclic variations in sediment coarse fraction, Sr counts as proxy for atoll-derived fine aragonite, planktic foraminifer oxygen isotope, carbonate preservation, and biostratigraphic markers, were determined. Depth/age models were developed in cores 1120/1144, comparing their planktic foraminifer oxygen isotope records with the well-dated stacked oxygen isotope LR-04 curve. Because each one of the last 18 Marine Isotope Stages (MIS) are identified along core 1120, its 12 m-long sediment section represents a continuous 750 ky-long record. The 12.86 m-long core 1144 displays also a continuous high resolution record, because it bottoms within the second half of MIS 11, an estimated 391 ky age. The downcore lithologic analyses and chronostratigraphies in the muddy drift were tied to the seismic lines imaging the sandy-muddy drifts to resolve the timing of the carbonate sandy drift establishment and its overall evolution. Model for the sandy drift evolution was developed based on the interpretation of multi-channel seismic lines. Unit I, II and III, identified in the sandy drift based on strong-amplitude reflectors bounding these units, were tied to the litho and chronostratigraphies developed in cores 1120/1144 in addition to ODP Site 716, drilled in close proximity of the drifts. The ages of the lower Unit I and upper Unit II/III are estimated to be late Pliocene and Pleistocene, respectively. The youngest prograding main unit III, represents the last 740 ky (∼Bruhnes) and its internal five subunits would then correspond to the main interglacial highstand intervals (MIS 17, 15-11, 9, 7, and 5). The deep carbonate drift at Gaafaru Falhu atoll was most likely initiated during the late Pliocene.