Seasonal wind patterns influence the configuration and geomorphology of insular reef systems: Yongxing Island, Xisha Islands, China

Abstract

Yongxing Island is a coral reef island formed by the growth and accumulation of corals and other skeletal fragments on a reef platform. The reef flat is developed around the entire island. The southeast reef flat and the north and northwest reef flat are asymmetrical in proportions, showing significant differences in reef‐flat width, patterns of sedimentary and ecological zonation, fore‐reef slope gradients, and submarine geomorphic features. Through application of an underwater survey, measurements and statistics were recorded on the extensional pattern of the Yongxing reef platform, the reef pool, and the spur–groove systems incised on the reef crests and forereefs. It is demonstrated that the Yongxing reef platform is an irregular ovoid in form with a distinct NW‐SE elongation that includes gently and steeply sloping forereefs and fore‐reef slopes. A gradual transition to the deep sea basin occurs in the southeast part of the Yongxing reef platform, whereas a steep forereef and fore‐reef slope that rapidly transition to the deep‐water basin are more characteristic of the north and northwest sectors of the Yongxing reef platform. The width of the reef platform in the southeast direction is much greater than that in the northwest direction. Correspondingly, the developmental pattern of the north and northwest reef crest and forereef is different from that of the southeast reef crests and forereefs. The southeast reef crest and forereef are characterized by reef pools with various shapes, different water depths, and discontinuous tidal channels on the outer reef flat. In contrast, the north and northwest reef crest and forereef developed a typical spur‐and‐groove system. The former is related to coral reef growth and construction, whereas the latter is shaped by wave or tide flow scouring that contributes to substrate erosion. Similar geomorphic features are known from examples of Indian‐Pacific and Caribbean‐Atlantic reef crests and forereefs, but the occurrence of such features in different parts of the same reef platform provides a new example of reef crest and forereef development. The differences in slope gradient of forereef and fore‐reef slopes (gentle vs. steep in transition), the development of contrasting submarine platform widths in different directions, as well as other submarine geomorphic features on reef crests and forereefs around the reef flats of Yongxing Island may be related to the confluence of several factors. These include the tectonic uplift of reef platform, the island's initial underwater topography, the windward and leeward positions, the direction and strength of winter and summer monsoon, the sea‐water dynamics, and the substrate characteristics related to coral reef growth.