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

加莫湖是一个沿海泻湖，与龙津湾隔着海侵阶段形成的沙坝。为了明确卡墨湖的沉积和环境变化，首先研究了SH20和uniboom的回声测深剖面，并对岩心钻孔样品中的硅藻和花粉进行了二次分析。湖泊从地貌特征上分为南北两部分，即南部和北部。南湖的地貌呈船底状，是一大片边缘陡峭的平原。平原底深7 ~ 8米。东部边缘有一个长而窄的凹陷，深2米。另一方面，北部底部从北向南缓慢倾斜。南部底部覆盖着宽广的分散层。沿着湖岸在湖底挖出了一个测深底。北湖测深基底由东缘向中部延伸。单孔岩心和钻孔岩心回声测深剖面表明，该测深基底是末次冰期后发育的沙坝进积形成的。在湖底下37 m处观察到冲积沉积物的基底。冲积层下的陆相沉积被认为是上更新世阶地沉积。冲积层沉积物的硅藻组合分析显示了湖泊从咸淡水/淡水到咸淡水再到咸淡水的环境变化。

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.