SEDIMENTATION AND ENVIRONMENTAL CHANGE FROM SEDIMENTOLOGICAL ANALYSIS OF BOTTOM SEDIMENT OF LAKE KAMO,SADO ISLAND

卡莫湖是与凉津湾隔开的一个海岸泻湖，在最后一次海侵阶段形成了一个砂坝。为了弄清卡莫湖的沉积作用和环境变化，首先利用SH 20和Uniboom等仪器对卡莫湖的回声测深剖面进行了调查，并对沉积物中的硅藻和花粉进行了分析，从地貌特征上将卡莫湖分为南、北方两部分。南湖地貌呈船底状，为边缘陡峭的宽阔平原。平原底部深度为7至8米。沿东缘沿着观察到一个2 m深的狭长凹陷。另一方面，北方底由北向南倾斜平缓，南方底被宽的分散层覆盖。湖底沿着湖岸出露一个测深基底。在北方湖泊，测深基底由东缘向中部延伸。单杆测深剖面和钻孔岩芯测深剖面表明，测深基底是末次冰期后发育的砂坝冲淤作用形成的，湖底37 m处为冲积物基底。冲积层下的陆地沉积物被认为是上更新世阶地沉积。通过对沉积物硅藻组合的分析，揭示了湖泊环境由半咸/淡水→海相→半咸的变化过程。

Lake Kamo is a coastal lagoon separated from Ryotsu Bay by a sand bar formed at the transgressional stage after the last. To make clear the sedimentation and environmental change of Lake Kamo, echo sounding profiles by SH20 and uniboom were firstly investigated, and diatom and pollen in core dorring samples were secondarily analyzed.The lake is divided into two parts from the geomorphological feature, that is, the southern and northern part. The geomorphology of the southern lake shows the form like a bottom of the ship, which is a broad plain with steep margin. The plain bottom is 7 to 8m in depth. A long and narrow depression with 2 m depth is observed along the eastern margin. On the other hand, the northern bottom slopes gentlly from north to south.The southern bottom is covered by broad dispersive layrs. A sounding basement is cropped out on the lake bottom along the lake coast. In the northern lake the sounding basement is extended to the central part from the eastern margin. The echo sounding profiles by uniboom and boring cores showsthat the sounding basement was formed by progradation of sand bar, which developed after last glacial stage.The base of alluvium sediments is observed at 37 m in depth under the lake bottom. Terrestrial sediment underlying the alluvium sediment is considered to be Upper Pleistocene terrace deposits. The analysis of diatom assemblage about the alluvium sediments show the environmental change of the lake from brackish/fresh through marine to brackish.