南沙群岛“危险地带”腹地珊瑚礁的地貌与现代沉积特征

南沙群岛中央水道及南华水道两侧的珊瑚礁,大部分为环礁,分属开放型、半开放型、准封闭型、封闭型和台礁化型,反映了环礁向灰砂岛演变的不同阶段。每个环礁,从礁前斜坡向礁坪至潟湖,可相应划分出3种沉积相和细分9种沉积带。礁顶是全新世中期以来形成发育的。

GEOMORPHIC AND MODERN SEDIMENTARY FEATURES OF CORAL REEFS IN THE HINTERLAND OF "DANGEROUS GROUND", NANSHA ISLANDS

Most coral reefs on both sides of the Central Channel and Nanhua Channel, Nansha Islands, are atolls, except a few table reefs. According to their development indices, these atolls are divided into five types, i.e. open, half-open, quasi-closed, closed and tablelized types, corresponding respectively to the five stages in the evolution of atoll to sandy island and in the clog up of lagoon. The grain size and biological, mineral, chemical composition of the surface sediments in these atolls vary with regularity from reef front slope to reef flat and to lagoon. Several sedimentary facies are recognized: （1） Reef front slope facies, including an upper slope zone and a down slope zone; （2） Reef flat facies, consisting of reef fringe slope zone, positive zone, embossed spot zone, reef plain zone and reef puddle zone; （3） Lagoon facies, composed of a lagoon slope zone and a lagoon basin zone. The regularity depends on the distributions of predominate organisms in bioclastic sedimentary environment. In reef front slope there are mainly reef coral, molluseca and foraminifera; in reef flat there are reef coral and Corallinaceae, and in lagoon there are reef coral, mollusea and alg e （Halimeda and Corallinaceae）. Based on the ¹⁴C dating, the reef crests have formed since Middle Holocene,. an are transforming into table reef quickly or slowly.