霞ヶ浦の地史 : 海水準変動に影響された沿岸湖沼環境変遷史  [in Japanese] Coastal lagoon evolution influenced by Holocene sea-level changes, Lake Kasumigaura, central Japan  [in Japanese]

霞ヶ浦は, その形成場所, 成因及び海域となって以降の環境変遷史において, 最終間氷期以降の海水準変動の影響を強く受けて現在に至っている。霞ヶ浦において行ったボーリングコアの解析結果, 1万年以降の海域となって以降の霞ヶ浦の環境変遷は6つの期間に分けることができる。(1)海進初期の狭い溺れ谷の時期の10,000-9,000 yBP, (2)主に海進期の海域の拡大及び湾口部の潮汐三角州の上方・潟側への発達期, 潟の水深の増大期の9,000-5,500 yBP, (3)潮汐三角州の潟側への前進の5,500-4,000 yBP, (4)湾口部からの砂の流入が認められなくなり, 堆積中心が湖心へ移動し, 閉鎖性の増大した4,000-2,500 yBP, (5)外洋氷塊との交換がさらに悪くなった2,500-500 yBP, (6)低鹹汽水・淡水化して以降の500 yBP以降である。

Lake Kasumigaura and its surrounding Holocene coastal lowland, central Japan, are situated with a valley formed during the fall of sea level of the last glacial maximum. This valley has been infilled by the latest Pleistocene fluviatile and Holocene marine sequences. The Holocene sequence is 4 to 8 km wide, about 60 km long, a maximum of 60 m thick, and consists mostly of bay mouth barrier, flood tidal delta, lagoon, and estuarine delta deposits. Their Holocene evolutions are closely related to sea-level changes. The detailed Holocene evolutionary history of Lake Kasumigaura was reconstructed using data from sedimentary facies analysis, micro fossil analyses, chronostratigraphic analysis using about 30 tephra layers and ^<14>C-age data of peat and shells from borehole and outcrop samples, and from the field survey of emerged Holocene sequences. The Holocene evolutionary history in this area is closely related to sea-level changes. During the period of 10,000-5,500 y BP, a coarsening-upward sequence is recognized in the bay-mouth area from Kashima to Ushibori. This sequence thickens and coarsens baymouthward from Kasumigaura lagoon. Sea-level change during this period shows a rapid rise at a maximum rate of 2 cm/yr. The sand body in the bay-mouth area formed and grew upward and lagoon ward as a result of sea-level rise. Sea level reached 5-6 m above the present level about 6,000 yBP. After this sea level fell gradually. At 5,500 yBP bay-mouth barriers and apart of the flood tidal delta in Kashima were abandoned and widely emerged. During the period of 5,500-4,000 yBP the sand body of the flood tidal prograded lagoon ward in response to a small sea-level fall. This sediment flowed into the lagoon through tidal channels in the bay-mouth area. Tidal delta front deposits are characterized by alternating parallel lamina of sand, 1-8 mm thick, and mud, 1-3 mm. Mud lamina consist of mud couplets. Sand lamina show a thick-thin alternation and rhythmical change vertically in thickness. At 4,000 yBP, the depositional pattern in the backbarrier was changed. The depocenter moved from the tidal delta front to the lagoon center and the bottom sediments became finer. Sand inflow into the lagoon through the bay-mouth channels is not clearly recognized. Sea level fell further during 4,000-2,500 yBP. The Ryugasaki area of the south lagoon was infilled and became a coastal plain by intra-lagoonal delta progradation. The change from a marine to brackish environment in Kasumigaura at about 500 yBP is related to a small sea-level fall and abundant rain during the little ice age.