浜名湖の起源と地史的変遷  [in Japanese] 0rigin and sedimentary history of Hamana-ko Bay, Pacific coast of central Japan  [in Japanese]

浜名湖内の4地点で完新世の基底に達するボーリングを行ない, 連続した柱状試料(総延長149.36 m)を得た。この試料と周辺の陸上試料を使って, 地質学, 堆積学, 地理学, 地球化学, 古生物学, 生物学, 考古学, 人類学, 土壌学などの分野から浜名湖の起源と史的変遷について総合的な研究を行なった。炭素14法による年代測定および堆積物の解析から, 浜名湖の環境は海水準変動と関連して6つのステージに区分される歴史を経て現在にいたったことが明らかにされた。1) 〜9000年以前:最終氷期の海水準低下期にできた谷地形に, その後の海水準上昇により海水が侵入し, 浅海成の泥が堆積する古浜名湾の原型が形成された。2) 9000〜6000年前:引き続く海水準の上昇により古浜名湾の湾口付近に砂堤が発達して, 粗粒堆積物が湾内に運搬された。湖内への海水の流入はよく, 底生生物の活動も活発であった。また湾中央部から南部の堆積速度は大きかった。3) 6000〜3500年前:湾口部の砂質堆積物による埋めたてによって, 湾の環境は汽水から淡水に変わっていった。古浜名湾中央部では極端に嫌気性の環境が生まれ, ラミナの発達した厚い泥質の地層が堆積した。湾口付近では砂丘が発達し, 海岸湿地帯が形成された。4) 3500〜2800年前:一時的な海水準の低下に伴い淡水の浜名湖が出現し, 湖内では均質な泥質堆積物が堆積し, 生物活動も活発であった。5) 2800〜1000年前:海水面が低く, 湖口付近は完全に陸化し, 湖水は狭い河川によって外洋に注がれていた。6) 1000年前〜現在:海水準が再び上昇し, 古浜名湖には海水が侵入して汽水化したが, 本格的な海水の流入が起こったのは1498年の地震に伴う湖口の決壊による。

An interdisciplinary study including geology, sedimentology, geography, geochemistry, paleontology, biology, archeology, anthropology, and pedology was conducted on the origin and the geological history of the Hamana-ko Bay, central Japan. Sedimentary columns were collected using handheld and rotary power drilling apparatus from the bottom sediments of Hamana-ko Bay and from the adjacent Holocene low-land sediments of the Hamana-ko Bay area. Four drillcores, reaching almost down to the basement, were recovered from the Hamana-ko bottom sediments and divided into 20 cm lengths. The columns were further subdivided every 5 cm in length for paleontological and sedimentological analyses. ^<14>C age determinations on the drillcores were precisely determined by the Tandem Accelerating method. The results of analyses of the drillcores of sedimentary sequences in and around the Hamana-ko Bay revealed the following six stages of its environmental history in relation to global sea level changes: 1) Before 10,000〜9,000 yrBP (period of gradual rise in sea level). The Paleo-Hamana Bay was formed by the flooding of sea water along the valley system developed during the last glacial age. Homogeneous mud sediments were deposited in a shallow depth environment during this period. 2) 9,000〜6,000 yrBP (period of rapid rise in sea level). Sand barriers were developed off the Paleo-Hamana Bay in the early stage of this period, and there was extensive biological activity in the whole bay area. An anaerobic sedimentary environment was developed in part and overwhelmed the baymouth area to the inner bay area in the later stages of this period in relation to the extensive development of sand barriers at the bay mouth area. Thick alternating sequences of silt and sand were formed towards the inner bay area. Consequently, the high sedimentary rate (11.1 cm/y) appeared in the drillcore 85H-3. 3) 6,000〜3,500 yrBP (period of stable high sea level). An extensive anaerobic sedimentary environment was formed in the central part of the Paleo-Hamana Bay at 85H-1 and 86H-4 drillsites. Thick laminated fine silty sequences were developed through out this period. Biological facies of a marine environment gradually changed to brackish or low salinity facies which indicate that the water was gradually diluted by fresh water. In the southern part of the Paleo-Hamana Bay, the 1st and 2nd sand dunes were also formed, An extensive marshy environment developed around the Paleo-Hamana Bay.