History of Indo-Pacific coral reef systems since the last glaciation: Development patterns and controlling factors

摘要

A significant body of new information about the development of coral reefs during the last 23 ka has been generated in the last three decades. In the Indo-Pacific province, structures from a variety of geodynamic settings have been investigated using subsurface drilling and submersible diving. This paper is based principally on the re-examination of the core dataset from the literature, with reconversion of many previously published radiocarbon ages into calendar dates.Seven framework and three detrital facies were identified on the basis of the nature and growth shapes of dominant framework builders, and on that of the texture of sediments, respectively. Framework facies in high-hydrodynamic energy settings were dominated by an association of coralline algae and robust-branching corals (Acropora robusta group, A. gr. humilis, A. palifera, Pocillopora damicornis) with locally encrusting coral forms (faviids). In moderate energy environments, these were replaced by domal (Porites), tabular-branching (Acropora gr. hyacinthus) and arborescent (Acropora gr. muricata), whereas sheltered areas included an association of arborescent, foliaceous (Montipora, Pavona) and encrusting coral species. Detrital facies comprise coral rubble, carbonate sand and mud. On compositional and textural bases, four main sand subfacies were recognized: coralgal rudstone to packstone; coral-molluscan grainstone/packstone; molluscan-foraminiferal grainstone/packstone; and green algal (Halimeda) grainstone/packstone. Despite some overlaps in the sand facies association, each subfacies can provide additional support to reconstruction of paleoreef environments.Three types of framework facies association were identified within entire reef-margin sequences: framework of homogeneous composition reflecting stability of environmental conditions through time; superimposition of two distinct frameworks, usually as deeper water corals overlain by shallower, higher energy ones, and recurrent alternations of shallower and deeper coral assemblages. The two last associations resulted probably from lateral displacements of coral communities in response to rapid changes in accommodation space. Such facies transitions also are described from backreef sediment piles: gravel graded into sand and mud successively as a result of upward shallowing. The degree of reef development seems to be linked to coral community structure. Communities consisting principally of branching and domal coral forms favoured substantial accretion and the formation of well-developed reefs, whereas assemblages comprising foliaceous and encrusting colonies produced only incipient reefs. Within reef systems, the proportions of detritus over framework tend to increase as hydrodynamic energy declines. The Indo-Pacific reef systems are classified into four anatomy types on the basis of dominant depositional patterns: balanced aggrading/onlapping, unbalanced aggrading/downlapping, prograding and backstepping types. Vertical accretion rates of frameworks are highly variable and are not directly dictated by coral growth habits. However, the highest rates recorded (up to 20 mm year(-1)) relate to tabular- and arborescent-acroporid rich sections. Abrupt variations in the aggradation rates of framework are recorded in sequences at the transitional zone between two distinct coral assemblages. In detritus-dominated sequences, accumulation rates range from 0.2 to about 40 mm year(-1), with higher values suggesting intense hurricane-controlled deposition. In addition, accretion rates also seem to depend on water-energy conditions. In high-energy environments, aggradation rates did not exceed 12 mm year(-1), but reached 25 mm year(-1) in more protected areas. By contrast, lateral accretion operated at an average rate of 90 mm year(-1) in agitated waters, while it did not exceed the mean rate of 55 min year(-1) in calm waters. Changes in accretion rates appear to be linked to reef growth modes. In the reef zones driv