The Maldives atolls, the very top of one of the largest modern carbonate platforms, occupy the central and largest part of the Chagos-Laccadives ridge located in the equatorial Indian Ocean. In the central part of the archipelago, the large atolls form two parallel north-south relatively continuous chains surrounding an internal basin, the Inner Sea, with water depths not exceeding 550 m. The Maldives carbonate system, uniquely evolved through a combination of global sea level fluctuations, subsiding history, and more regional seasonally varying monsoon circulation. Although the long-term evolution of this system is relatively well-established, the understanding of the detailed evolution of the Maldives carbonate edifice in the last 5 million years has remained limited. The latest phase of its stratigraphic evolution is explained by a shift from a well-developed Miocene-Pliocene progradational pattern to a mostly late Pliocene-Quaternary aggradational depositional signature. This last aggradation phase, forming the atolls the way we know them today, consists of stacked inner neritic limestone sequences, separated by a series of exposure horizons. The succesive periods of atoll exposure and re-flooding are recorded in the Inner Sea by late Pliocene-Quaternary glacial/interglacial clearly cyclic deposition of periplatform oozes. This cyclic sedimentary pattern also appears in the internal prograding geometry of carbonate drifts in the Inner Sea. A200 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, in a range of water depths from ~300 to 500 m. During the NEOMA 2007 research cruise on the RV Meteor lead by Universität Hamburg, the deep water sandy drift in the area north of Gaafaru Falhu atoll and an adjacent deeper muddy drift was extensively surveyed via 12 kHz multibeam bathymetry, a 4 kHz sub bottom profiler (Atlas Hydrographics), multi channel high resolution seismics, and three box and piston cores. My study focuses on understanding the Plio-Quaternary overall evolution of the set of adjacent sandy and muddy drifts, just north of Gaafaru Falhu Atoll. The sandy and muddy drift interconnected internal geometries observed in the available seismic data sets are integrated into a sequence stratigraphic framework. Analyses of two piston cores collected from the upper part of the muddy drift and a box core from the top of the sandy drift determine the overall downcore lithology variations and made possible the development of high-resolution chrono and cyclo-stratigraphies. In the muddy drift periplatform sequence, downcore cyclic variations in, (1) sediment coarse fraction, (2) Sr counts as proxy for atoll-derived fine aragonite, (3) planktic foraminifer oxygen stable isotope composition, in addition to carbonate preservation and biostratigraphic markers, were determined. These downcore lithologic and geochemical variations 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. Based on this aforementioned interpretation, the results of my research document the nature and timing of the longer-term evolution of the sandy and muddy drifts over multiple glacial-interglacial sea level cycles in the last 3 million years. Once the timing of the drift was determined, the prograding internal architecture of the sandy drift was examined and interpreted in the context of the relatively well-established Plio-Pleistocene sea level fluctuations and the bottom current variations
| Identifer | oai:union.ndltd.org:RICE/oai:scholarship.rice.edu:1911/71669 |
| Date | 24 July 2013 |
| Creators | Lopez, Karem |
| Contributors | Droxler, Andre W. |
| Source Sets | Rice University |
| Language | English |
| Detected Language | English |
| Type | thesis, text |
| Format | application/pdf |
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