钦州湾外湾水下地形地貌特征及沉积物来源

Characteristics of submarine topography geomorphology and sediments sources in outer Qinzhou Bay

人类对海岸带的开发利用活动改造了岸线形态,严重改变海洋区域性流场、水下地形形态及沉积物分布。利用单波束测深、沉积物采样分析等手段,查明研究区海底地形地貌和沉积物分布特征,探讨其沉积物来源。结果显示:①钦州湾水下地形复杂多变,水深为0~20 m,分为中部钦州湾外湾水下地形区、东部三娘湾水下地形区、西部防城港东侧水下地形区及南部湾外水下地形区等4个区域;②地貌分为潮控三角洲、潮流沙脊群、潮流冲刷槽和水下岸坡4个主要海底地貌单元以及潮滩、海蚀平台等潮间带地貌单元;③钦州湾砂质沉积分布广泛,占70%,自茅尾海口门向外海沉积物粒径逐渐变粗,由砂质泥变为泥质砂,再过渡为细砂和中粗砂。沉积物碎屑矿物主要由石英、长石、岩屑、钛铁矿、白钛石和电气石组成,粘土矿物以高岭石为主,其次为伊利石、蒙脱石,沉积物来源为近源河流的陆源输入为主。随着围填海和堤坝建设,钦州湾外湾海岸形态变化巨大,伴随着落潮流速大于涨潮流速的峡道效应,水下地形呈现出槽脊规模和高差加剧,近岸淤积加重的特点。

The exploitation and utilization of the coastal zone by human beings have transformed the shoreline morphology and changed seriously the regional flow field,underwater topography and sediments distribution of the sea.By means of single beam sounding,sediments sampling and data analysis,the characteristics of submarine topography and sediments distribution in the study area were ascertained and the sediments source was discussed.The results show that 1.the underwater terrain of Qinzhou Bay is complex and varies with water depth from 0 to 20 m.Four underwater topographic areas can be distinguished as middle and outer Qinzhou Bay,eastern Sanniang Bay,eastside of western Fangchenggang and southern outer bay.2.The landform is divided into four major submarine landform units including tidal-controlled delta,tidal and sand ridge group,tidal and scour trough and underwater bank slope,and secondary intertidal landform units such as tidal flat and marine erosion platform.3.The sandy sediments are widely distributed and cover 70%of the bay.They become coarser and change gradually from sandy mud to muddy sand before transformed to fine and medium coarse sand from the opening of Maowei Sea to the open sea.The detrital minerals in sediments are mainly composed of quartz,feldspar,debris,ilmenite,albinite and tourmaline.Clay minerals are mainly kaolinite,followed by illite and montmorillonite.The sediments are mainly terrigenous from near source rivers.With the reclamation and dam construction,the coastal morphology of outer Qinzhou Bay has changed greatly.Accompanying with the isthmic effect of the drop velocity,which is faster than that during flood tide,the underwater terrain shows characteristic aggravations of the ridge scale,elevation difference and nearshore deposition.