Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Grande Terre of New Caledonia is enclosed by one of the longest barrier reefs in the world. For the first time, the fore-reef slopes of this barrier reef have been sampled by dredging, from 40 to 320 m deep, in order to analyze their sedimentological and biological characteristics. The rocks and sediments can be divided into seven sedimentary facies: bindstones dominated by coralline algal crusts, bindstones dominated by foraminiferal crusts, bindstones dominated by bryozoan crusts, coral framestones, bindstones and interstratified packstones rich in skeletal debris, packstones/wackestones and grainstones rich in rock gravels. Radiocarbon dating performed on encrusting organisms (coralline algae and acervulinids) and corals provide ages relatively young. These ages confirm that the encrusting organisms are modern and the corals mainly dated of Holocene are reworked due to the instability of the fore-reef slopes, especially during storms. Because the biological assemblages are distributed according to a bathymetric range depending on light intensity, a model of distribution of modern encrusting calcareous organisms can be proposed from our observations and analyses. From the upper reef slopes to approximately 90 m, thick coralline algal crusts are dominant and distributed in three groups. Group C, the shallowest parts of the fore slopes, is mainly characterized by mastophorids (Hydrolithon reinboldii, H. cf. munitum, Lithoporella melobesoides, Aethesolithon cf. problematicum, Neogoniolithon sp. and undetermined species) and lithophylloids (Lithophyllum sp., L. pustulatum). Group B, composed of lithophylloids (Lithophyllum sp., L. cf. kotschyanum, L. cf. moluccence, L. pustulatum), Mesophyllum sp. and Peyssonnelia sp. occurs from 15 to 40 m. Group A, rich in Mesophyllum sp., M. cf. mesomorphum, Peyssonnelia sp. and Sporolithon sp. is characteristic of deep reef slopes up to 90 m. Below approximately 90 m, when the light intensity decreases, the encrusting foraminifera acervulinids progressively replace the coralline algal crusts. Such a model is particularly useful to interpret and reconstruct the past Quaternary reef environments rich in crusts of coralline algae and/or foraminifera.
ISSN: 0025-3227

hal-00393705
https://hal.archives-ouvertes.fr/hal-00393705
DOI : 10.1016/j.margeo.2007.12.002

Éditeur(s) : Elsevier
Résumé : In situ microbialites occurring in reef rocks dredged between 80 and 130 in water depth on the modern fore-reef slopes of Tahiti and the Marquesas islands yield ages ranging from 17,100 2900 to 4410 2250 years BP, suggesting that they played a prominent role during the last deglacial sea level rise. Microbialites developed in both shallow and deep water depositional environments where they characterize various zones of the reef tracts (reef crests, upper reef slopes, deep fore-reef slopes), reflecting contrasting scenarios of microbialite development involving ''reefal microbialites'' in shallow-water settings and ''slope microbialites'' that formed in environments deeper than 1020 in and extending down to more than 100 m. Reefal microbialites correspond to a late stage of encrustation of the dead parts of coral colonies, or more commonly, of related encrusting organisms (red algae and foraminifers), thus forming surface crusts. Slope microbialites generally form the ultimate stage of a biological succession indicating a deepening sequence, whereby shallow water corals and associated encrusting organisms are replaced by deeper water assemblages of red algae and foraminifers before microbialite growth. The precipitation of phosphatic-iron-manganese crusts and the deposition of planktonic micritic limestones on the microbialites characterize a deepening-upward sequence. The widespread development of microbialites in reef sequences from the Last Deglaciation characterizes a period of environmental degradation consequential oil the rapid sea-level rise and abrupt climatic changes of that time. The reported biological succession reflects changes in water quality, and especially an increase in nutrients. In shallow-water settings, increased alkalinity and nutrient availability in interstitial waters were related to surface fluxes and terrestrial groundwater seepage while slope environments were exposed to continuous upwelling of nutrient-rich deeper waters during the last deglacial sea level rise. The age differences between corals and overlying slope microbialites range from 1600 to 8400 years, based on high-precision U-series age measurements of both corals and microbialites, and indicates that a significant time (several thousand years) elapsed between the development of the coralgal frameworks and the growth of slope microbialite crusts. Microbialites cannot be considered as part of the drowning event some 14,000 years ago that resulted in the demise of reef frameworks in the 90-110 m present depth range, but are Substantially younger. (c) 2006 Elsevier B.V. All rights reserved.
Sedimentary Geology (0037-0738) (Elsevier), 2006-03 , Vol. 185 , N. 3-4 , P. 277-295

Droits : 2006 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/2006/publication-4204.pdf
DOI:10.1016/j.sedgeo.2005.12.018
http://archimer.ifremer.fr/doc/00000/4204/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The lower Pleistocene Kritika Member of the Rhodes Formation is composed of predominantly siliciclastic deposits that are sometimes intercalated with hardbottoms formed by drowning of former beachrock beds. A wide variety of sclerobionts occur in this environment, very different from the poorest soft‐bottom populations occurring in other levels of the Kritika Member. The most characteristic, abundant and frequently diverse are the encrusting forms: coralline algae, bivalves, serpulid worms, and bryozoans. Numerous other groups of sessile and vagile organisms are associated with these crust‐forming communities. A number of skeletal organisms (sometimes including zooxanthellate coral colonies) formed small carbonate buildups on top of several beachrock beds. Boring organisms also occur. Evidence for a two‐step deepening succession over the previously formed beachrock is provided by palaeontological and sedimentological data. The rapidly cemented sandy‐conglomeratic beds were first colonised by a relatively shallow‐water (around 20 m) biocoenosis of mostly encrusting organisms. Another association, with encrusting and erect organisms, was later established in a deeper environment (20-40 m). A comparison with Holocene beachrock occurring on the coast of Rhodes was also undertaken. Their submerged part provided habitat for poorer but similar biotas to those occurring in the Kritika Member. The lower Pleistocene beachrock and associated organogenic fossil assemblages indicate repeated drowning episodes during deposition of the Kritika Member and provide help in sequence stratigraphic interpretations of these siliciclastic deposits. Notably, they can be used as indicators of relative sea‐level changes.
ISSN: 0031-0182

hal-00795591
https://hal.archives-ouvertes.fr/hal-00795591
DOI : 10.1016/j.palaeo.2012.09.007