Successive reef depositional events along the Marquesas foreslopes (French Polynesia) since 26 ka

2008

Article référencé dans le Web of Science WOS:000258809900002

Cabioch Guy, Montaggioni L., Frank N., Seard C., Salle E., Payri Claude, Pelletier Bernard, Paterne M.

Marine Geology, 2008, 254 (1-2), p. 18-34 ISSN 0025-3227

Bathymetric surveys of submarine platforms and terraces of the tropical Marquesas archipelago (French Polynesia), coupled with sedimentological and paleoecological analyses and radiometric dating of dredged rocks, reveal the occurrence of several distinct intervals of reef deposition during the last deglacial sea-level rise (i.e. the last 26,000 yr or 26 ky). Five facies related to reef, shallow-water and deeper depositional zones have been identified, on the basis of their respective biological assemblages. Facies 1, 2 and 3 consist of in situ scleractinian corals (mainly Porites) associated with thick crusts of coralline algae species, Hydrolithon onkodes, H. reinboldii, Lithophyllum pustulatum group, and widespread vermetids and victoriellinid foraminifers. These assemblages are typical of high-energy, shallow-water reef settings (i.e. upper forereef, reef crest and outer reef flat, and inner reef flat respectively). Facies 4 is characterized by the prevalence of Halimeda plates, mixed with debris of Amphiroa and Amphistegina spp.; these algae generally inhabit the inner parts of the reef flat and backreef zones but also the relatively protected, deeper parts of outer reef slopes. Facies 5 is composed of large broken mollusc shells (oysters), embedded in fine-grained sediments with significant numbers of deep-living coralline algae Mesophyllum sp., and planktonic foraminifers. Locally, mollusc shells are covered with microbialite crusts. The replacement of shallower assemblages by deeper ones is interpreted as an upward-deepening sequence. Identification of morphological and biological features combined with coral dating provide evidence of 4 reef generations (RG) between 26 and 9 ka, now ranging from 138 to 58 m in depth: 1) RG 0 from 26 to 19 ka in the 125-115 m depth interval; 2) RG 1 from 18 to 15 ka between 110 and 95 m: 3) RG 2 from 14 to 11.5 ka between 80 and 68 m; and 4) RG 3 younger than 11.5 ka, at around 55 to 60 m. The first three units exhibit clear zonation, from upper reef slope to reef flat. Their development was probably interrupted by abrupt rises in sea-level and/or drastic climate changes. Such a scenario is similar to observations reported in Barbados and Hawai'i where rapid rises in sea-level, interpreted as reflecting major melting events of continental ice, caused the drowning of reefs. The younger unit refers to coral banks similar to those in modern Marquesas environments. The replacement of typical coral reefs by coral banks may reflect an increase El Nino-Southern Oscillation (ENSO) frequency and intensity inimical to reef development in the last 12 ka.

Géologie et formations superficielles [064]

Fonds IRD [F B010042762]

fdi:010042762