Environmental and tectonic influence on growth and internal structure of a fringing reef at Tasmaloum (SW Espiritu Santo, New Hebrides island arc, SW Pacific)

Subduction of the Australian Plate has caused rapid uplift of the central New Hebrides island arc (15°S, SW Pacific). The d'Entrecasteaux ridge system, a prominent bathymetric feature on the downgoing plate, is underthrusting the central part of the New Hebrides arc. The coastlines of most islands are characterized by emerged Holocene coral reef terraces. A maximum uplift rate of 6 mm yr−1 occurs along the south-west coast of Espiritu Santo, near the plate boundary. To investigate the Late Quaternary neotectonic and environmental evolution of the uplifted fringing reefs, we drilled the emerged Holocene reef at Tasmaloum (SW Espiritu Santo) to depths as great as 40–45 m. Coral samples from various levels were dated by 230Th and 14C, as described elsewhere, and the internal structure of the reefs was studied. Preliminary palaeoecological and sedimentological data indicate the following. First, the coral reef colonized a substrate and began to grow by 24 ka on weakly indurated calcareous sand beds, which probably formed during the Late Pleistocene and are possibly as old as 30 ka or more. These sand levels could represent the deep fore-reef area of an older reef, at present behind the uplifted Holocene terraces. Second, the biofacies and coral ages from the reef sequence, which is continuous from the last glacial maximum (LGM), provide a view of the internal structure: between 24 and 12–10 ka, coral levels, composed mainly of Acropora gr. hyacinthus and gr. cytherea (accompanied by a few Galaxea gr. fascicularis), constitute medium- to high-energy assemblages, reflecting relatively deeper and more protected environments. However, at a few levels, acroporid build-ups of Acropora gr. danai/robusta indicate high-energy environment alternation. Between 12–10 ka and the present, assemblages of acroporids (Acropora gr. danai/robusta), scarce poritids, numerous encrusting coralline algae, vermetid gastropods and encrusting foraminiferids indicate high-energy environments, probably corresponding to the upper part of the exposed reef slopes. This biofacies succession indicates changes in wave energy, related to fluctuations in local bathymetry controlled by the net effect of variable rates of sea-level rise, reef growth and tectonic uplift. After 6 ka, the replacement of coral, coralline algal and stromatolite assemblages by coral and coralline algal frameworks implies the establishment of a new hydrological and oceanographic regime. The biofacies and age structure of the reef show that the uplift rate has varied since 24 ka.