Environmental and Tectonic Influence on Growth and Internal Structure of a Fringing Reef at Tasmaloum (SW Espiritu Santo, New Hebrides Island Arc, SW Pacific)

Abstract

Subduction of the Australian Plate has caused rapid uplift of the central New Hebrides island arc (1 5"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 y r ' 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 I4C, as described elsewhere, and the intemal 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 ofkroporu gr. hyacinthus and gr. cytlzereu (accompanied by a few Gulaxeu gr. fmciculuris), constitute medium-to high-energy assemblages, reflecting relatively deeper and more protected environments. However, at a few levels, acroporid build-ups of Acroporu gr. dunui/robustu indicate high-energy environment altemation. Between 12-10 ka and the present, assemblages of acroporids (Acroporu gr. dunui/robustu), 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 vaned sinze24-ka.

Holocene and Pleistocene fringing reef growth and the role of accommodation space and exposure to waves and currents (Bora Bora, Society Islands, French Polynesia)

“…The vertical accretion rate of the windward fringing reef in Bora Bora amounts to 3Á12 m kyr À1 on average and is somewhat lower than the reported rate of the leeward fringing reef of 5Á65 m kyr À1 . This observation is similar to the data of Cabioch et al (1998) in New Caledonia where accretion rates of the leeward fringing reef exceed those of the windward fringing reef. The lateral accretion of both windward and leeward fringing reefs in Bora Bora is estimated to be some 80 m in 6 kyr, i.e.…”

“…This observation is similar to the data of Cabioch et al . () in New Caledonia where accretion rates of the leeward fringing reef exceed those of the windward fringing reef. The lateral accretion of both windward and leeward fringing reefs in Bora Bora is estimated to be some 80 m in 6 kyr, i.e.…”

“…In post-glacial fringing reef development that experienced high rates of sea-level rise following the last glacial maximum, retrogradation has been common also until the modern level was reached (e.g. Cabioch et al, 1998).…”

Geomorphology of carbonate platform‐marginal uppermost slopes: Insights from a Holocene analogue, Little Bahama Bank, Bahamas

“…At a more global scale, this protection could possibly explain the regional paucity of evidence in the Bahamas for a notch or terrace representing the last glacial maximum, even though it is evident on many South Pacific, open‐ocean atoll systems (for example, Mayotte –Dullo et al. , 1998; Vanuatu –Cabioch et al. , 1998; Marquesas –Cabioch et al.…”

Évolution des paléoenvironnements dans le Pacifique lors de la dernière déglaciation : exemples en Nouvelle-Calédonie et au Vanuatu

“…Les informations relatives à la paléocli-matologie sont plus récentes (Beck et al, 1992(Beck et al, , 1997Cabioch et al, 1998 ;Corrège et al, 2000 ;Quinn et al, 1996). L'étude des environnements terrestres quaternaires reste à dévelop-per.…”