Revisiting the Quaternary development history of the western New Caledonian shelf system: From ramp to barrier reef

The present paper is based on the study of a new core (Tenia 3,151.20 m long) and revisitation of previous core data from the western New Caledonian barrier reef. Based on variations in lithology and stable isotope signatures, and the occurrence of unconformity surfaces, the Tenia 3 sequence is subdivided into ten units, regarded as deposited during interglacial, high sea levels. Five detrital and two framework facies are delineated using their biological and textural attributes, including coral growth forms and assemblages. These facies are interpreted as representative of outer to inner carbonate shelf (ramp to barrier reef) environments. Variations in the facies within each stratigraphic unit occur mainly as vertical transitions from lower energy (and probably deeper) to higher energy (probably shallower) settings. The succession of depositional events is reconstructed using all available age criteria including lithostratigraphy, magnetostratigraphy, uranium-series dating, and nannofossil biostratigraphy. Shallow-water, coral communities appear to have been developed as soon as carbonate deposition started at around 1.2 million years (Ma), but the initiation of typical coral reef tracts is likely to have occurred not prior to 0.40 million years during Marine Isotope Stage (MIS) 11. Carbonate deposition previous to MIS 11 is considered to have operated along an open shelf margin, i.e. a ramp or non-rimmed platform. Based on the re-examination of the core dataset previously acquired from the New Caledonian reefs, the coral communities, dominated by massive forms (mainly poritids) are assumed to have remained stable in composition over the past 1.4 Ma. Similarly, as indicated by a preliminary study of pollen assemblages in the core, the composition of terrestrial flora, dominated by Podocarpaceae and Araucariaceae, has not apparently suffered significant change over the past 1 Ma, and reflects climate conditions cooler and drier than today. Mean subsidence rates are derived tentatively over the past 1 Ma from the stratigraphic position at present reached by the top of depositional units; mean rates of <= 0.08 mm yr(-1) for the western New Caledonian margin are consistent with past sea-level estimates. The Quaternary depositional history of the New Caledonian shelf margin appears to have been controlled by both the amplitude of sea-level changes and by differing vertical tectonic movements. Comparison between the depositional shelf histories from western New Caledonia, north-eastern Australia, southern Belize and the Ryukyu Islands supports the assumption that coral populations over the early and mid Pleistocene developed as non-reefal communities along ramp systems, not forming continuous outer-shelf reef rims. At a global scale, Quaternary reef-rimmed platforms are likely to have initiated after the Mid-Pleistocene Climate Transition, from marine isotope stage 11, at about 0.4 Ma. (C) 2010 Elsevier B.V. All rights reserved.