Enregistrement de l'eustatisme dans les sédiments quaternaires du bassin des Loyauté (Nouvelle-Calédonie, Sud-Ouest Pacifique)

Abstract : Periplatform and basinal Quaternary sediments sampled by piston coring off New Caledonia, consist of carbonate ooze interfugered with calcareous sands and silts deposited by gravity flows. d18O and CaCO3 fluctuations have been uses for correlating sediments. The cyclic downcore fluctuations of the carbonate mineralogy results from the behaviour of bank-derived sediment exports and of planktonic production,controlled by climatic and eustatic processes. During glacial periods of lowered sea level (18O enrichment), the carbonate content is low, and reversely during interglacial episodes and higher sea level. In the core KK21, peaks of aragonite and CaCO3 % correspond to ligh oxygen isotope values and thus to interglacial stage. This core allows a correlation between the carbonate percents variations and the growth of reefal-barrier off southwestern New-Caledonia. This chemostratigraphy calibrated with d18O curve gives useful chronological indications for other cores of Loyalty basin. Off Thio, the dilution by fine-grained terrigenous products during glacial episodes may constitute the dominant control of sedimentation in a basinal area close to mixed carbonate/siliciclastic sources; conversely off SW of Lifou island, a detailed echo-character map based on multiple 3,5 kHz echograms reveals a pure carbonate context, therefore altrenative flooding and exposure of the bank is probably the cause of carbonate cyclic variations observed through the cores (Bahamian model). The composition of sand and palynological fractions shows the relative importance of continental detritus and of biological debris issued from platform and from oceanic communities. Monospecific diatomaceous layers occur in core sections of low carbonate content, they indicate a variation in the pattern of oceanic circulation in the Loyalty basin during glacial ages. Terrigenous and carbonate flux variations have been determined for several cores. Siliciclastic detritus yielded by rivers and shelf sediments are major components of the terrigenous flux which controls the sedimentation in the western Loyalty basin. Reversely carbonate flux is the more important in the Eastern part of it. More over, the short-term changes in planktonic production, which presently shows considerable spatial and temporal variability in response to nutrient availability, depth of vertical mixing, circulation pattern and water transparency. This example shows that basinal deposition of hybrid siliciclastic / carbonate sequences is an originality, complicating depositional interpretations in the climatic-eustatic context.
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William Pannetier. Enregistrement de l'eustatisme dans les sédiments quaternaires du bassin des Loyauté (Nouvelle-Calédonie, Sud-Ouest Pacifique). Sciences de la Terre. Université Claude Bernard Lyon 1, 1990. Français. ⟨tel-01699845v3⟩

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