SEDIMENTARY STRUCTURE AND DEVELOPING MODEL OF COASTAL DUNES ALONG THE NORTHEASTERN COAST OF HAINAN ISLAND, CHINA
Wu Zheng Wu Kegang (Department of Geography, South china Normal University)
Along the northestern coast of Hainan Island, Holocene coastal dunes and dune rock make a wide tropical coastal dune belt. The sedimentary structure and developing model of coastal dunes are discussed based on morphologic field work, and laboratary research in which the analysing of grain size, scanning of the surface characteristics of quartz sand, identifing of microfossiles, researching of dune-rock cement under the polaring microscope, dating of 14C age are experimented. The structures are typified by aeolian large cross-bedding with steep (30?35? winddown dipping foreset and gentle (normally less than 10? windward dipping topset. The structures are also typified by special features of coastal dune structure, such as the thick remaining topset.Syngentic sedimentary deformed structures are common. All conditions of the coastal wind regime, dune-sand resource, and basement topography are advantageous to the development of coastal dune belts. The coastal wind regime coincides with the distribution of dunes and dip of the stratum structure of dunes. All this reveals that the major dynamic source to form, coastal dunes is the Northestern wind that prevails during the dry winter from Sept. to March. And the results of grain size analysis in coastal dune sands are identified neither with the continental dune nor with the littoral sediments. The surface characteristics determined by the electronic microscope scanning of quartz sands are diplogenetic characteristics of both beach and dune. It is proved that the resources of coastal dune sands are beach sands. There are a few kinds of dunes found along the coast. They are coastal foredunes, transverse dunes, barchan, parabolic dunes, longitudinal dunes, sand sheets and the shadow dunes which are superposed on others. The foredunes and sand sheets are particular types of coastal dunes distinguished from continental dunes. The other types are also different from continental dunes. The developing model of coastal dunes is as follows (from the coast to inland). According to the angle between the direction of the coastline and the direction of on shore wind, the types of dune may be divided into three types of dune coasts. The age of 14C dating illustrates that the coastal dunes began to develop 3000 years ago.