| The Weizhou Island in Guangxi is located in the northeast of Beibuwan bay.It is the largest volcanic island in the Beibuwan bay. The island consists of volcanic detritus, terrigenous deposits and the carbonate coral reef surroundings. The study of the geomorphology and the development of the island is of great practical value.This study is based on the field investigation and the previous data-collection. The grain sizes of beach sediments were measured, and the beach rock and the volcanic detritus at some specific geomorphologic locations were dated. All data collected in the field, in the lab and from the previous research were integrated into the geographic information system. Based on the integrated data, the Endogenic-exo genic forces and the evolution of the coast around the Weizhou Island were studied and the suggestion about the coastal development of the Weizhou island were proposed. Main results of the study are as follows:The topography height of the Weizhou Island decreases from the southwest to the northeast, the base of the basalt and volcanic rock had consisted of since the early Pleistocene Period. The coast type of the Weizhou Island can be divided into four types in this paper. ① Marine deposited sandy coasts. most are distributed at the Shiluokou in the west, Nanan bay in the south. Most are sandy beaches developed in this coast with rare gravel beaches and no muddy flat absolutely. The larger mean grain size and steep slope gradient of the beaches is distributed in the west, but smallest one in the east.Largest sand ridge located in the north and smallest in the west. ② Bedrock coasts, are mostly distributed along west coast except the Shiluokou, the Eyuzui and the east side of the south coast. The marine-abrasion geomorphology develops best in the west where the cliff collapses are common and modern and relict marine-abraded cave are visible. In the south relict marine-abrasion cliffs are found in the Nanwan Bay and gravel beaches developed in the front of the modern cliffs. ③Coral reef coast. The coral reefs develop offshore in the water depth of -2～-4m and are widely distributed in the east and the north. The living coral bends are 660m wide and the coral platforms are 2250m wide. In the caves on the marine-abraded platform at the Eyuzui plenty of honeycomb coral were observed.④ Artificial coast, include harbour docks, sea walls, spur dikes, ports and breakwaters.The evolution history of the geomorphology of Weizhou Island could be summarized as follows:①During the early-middle Pleistocene Period, the volcano erupted in the northwest to form the Henglu hill and the volcanic basalt built up the base of the Weizhou Island.②In the late Pleistocene, the volcano of Nanwan erupted and produced a large volum of volcanic detritus. The volcanic detritus were removed by wave processes and deposited as a thick layer of volcanic debris.③the period of marine and biological transformation. In the southwest after the volcano eruption, wave erosion and to form the retreated coast line as steep cliff. The detritus produced by coastal erosion and the biologic detritus were washed sorted and removed to the north, to the east and to the west building up the sand ridges. The oldest ridge might be formed at 6900a BP. In the shield from wave action the coral reef developed well. About 1830a BP the beach rock formed along the highest tide level when the sea level was stable. Based on the field investigation evidence of the marine-abrasion caves, marine ridges and wave platforms, the relative sea level around the Weizhou Island went through 5 times changes.The main suggestions about the coast utilization are proposed as:the main development should be harbour and airport construction fist, the consummate transportation can bring more and more visites for sightseeing of the volcanoes and the bedrock coastal landscape, marine diving, leisure fishing etc. As a result can improve the Cultured and Economic level of the island. |
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