江苏小庙洪牡蛎礁的地貌特征及演化

GEOMORPHOLOGY AND EVOLUTION OF THE XIAOMIAOHONG OYSTER REEF OFF JIANGSU COAST,CHINA

江苏岸外的小庙洪牡蛎礁在海岸地貌学上是一稀有的生物沉积体。这种位于蚀、淤交替岸段潮间带、古礁体和活体共存的牡蛎礁体是我国所仅见。现场观察了牡蛎礁体的地貌特征,并使用GPS和全站仪进行了主要礁体分布与高程的测量,表明礁区地貌面不是一个水平面,相差可达2m以上。礁体顶高主要分布在两个高程上,一是与平均潮位相当,二是比前者低约30cm,略高于小潮平均低潮位。礁体起伏面高出礁坪面多在0.5—1.5m。小庙洪牡蛎礁正处于退化状态,除物理因素造成的侵蚀外,化学溶蚀及人类活动也是侵蚀退化的主要因素。对于礁区内部的礁体,其退化过程为连片礁→环礁→带礁→斑礁→礁坪的顺序。礁体沉积体有三种:原生礁体、与原生礁体脱离的礁块以及已离开原生地的松散堆积体。浪蚀礁坪分布在礁体的迎浪面上,而溶蚀礁坪则分布在礁体密集区。在礁体稀疏区,则覆盖约20—30cm厚的粉沙或淤泥,形成次生的潮坪。

Xiaomiaohong Reef, recently found oyster reef bodied off Jiangsu coast of eastern China is a rare bio-deposition body in coastal geomorphology. The reef, situated between erosion and progradation zones of tidal area, was investigated in-situ in geomorphology and GPS-mapped for the dimensions against recent correspondent satellite images. The result of the study shows that the surface of the reef body was a gently domed body with 2m difference in elevation. There are two dome tops, one is leveled at the average tidal height, and the other is about 30cm lower than the former or slightly below the mean low water neaps. The tops of the wavy reefs are generally 0.5-1.5m above the reef flat. The study also shows that the reef is currently under erosion, resulted by not only physical and chemical factors but also by human being activities. Reef bodies were degenerated in a sequence from reef sheet, ringed reef （atoll）, dispersed reef, to reef flat in turn. Three types of oyster-reef deposits were classified： proto-reef, reef masses disintegrated from the proto-reefs, and incompact deposits far from their native places; and two erosion types were recognized： wave eroded reef that generally distributed on the side that against wave direction, and dissolved reef flats that appeared with dense reef bodies. Away from dense reef bodies were sparse reefs covered by 20 to 30-cm-thick silt or mud, which may then developed into secondary tidal flat.