秋田県八郎潟の完新世地史  [in Japanese] Holocene geologic development of the Hachiro-gata lagoon, Akita Prefecture, Northeast Honshu, Japan  [in Japanese]

八郎潟は, 現在干拓平野となっているが, 秋田県男鹿半島と本州との間の潟湖である。地下の完新統(八郎潟層)は八郎潟域が内湾から約2200 y.B.P.に汽水域化する過程で堆積した。最大層厚は62mで非常に厚く, 完新統堆積中に沈降運動が同時進行していたことを物語る。一方, 日本海との間には北東部に能代砂丘, 南部に天王砂丘が存在し, 潟を外洋から隔てている。これら2砂丘は縄文海進以降の完新世に形成されたが, 砂丘地形は対照的で, 能代砂丘は累重砂丘, 天王砂丘は千列砂丘である。これら対照的砂丘地形は男鹿半島を含む八郎潟西方一帯が北東方向へ傾動することにより出現した。以上のように八郎潟地域では西方地域の北東への沈下と, 潟を中心とする沈降運動の影響が大きいことが判明した。また, 八郎潟の成立を直接的に示す汽水湖は, 砂丘の形成による内湾の閉塞度の強化と, 河川による湾内水の希釈によって出現したと考えられる。

The Hachiro-gata lagoon, with two-thirds of its bottom reclaimed presently, is situated east of the Oga Peninsula and is separated from the Japan Sea by tombolos on which coastal dunes are well developed. The Hachiro-gata Formation, the Holocene in the Hachiro-gata lagoon, unconformably overlies the uppermost Pleistocene Tenno Formation and includes three distinct interfingering sedimentary facies; northwestern to southern tidal delta, eastern fluvial delta, and central bay to lagoonal facies. The Hachiro-gata Formation, which has a maximum thickness of 62 m in the central part of the lagoon, is much thicker than the Holocene of other major coastal plains in Japan. Accumulation of the thick Holocene suggests that synsedimentary subsidence occurred in the Hachiro-gata lagoon. The Holocene evolutionary stages of the lagoon which are recognized on the basis of mollusca sequences contained in the Hachiro-gata Formation, are named the Ostrea Bay, Macoma Bay, Raeta Bay and Corbicula Lake in ascending order. The ^<14>C age of peat intercalated in the basal part of the Holocene, which can be correlated with the Ostrea Bay sediments, is 12,240±80 y.B.P. The lower boundaries of the Raeta Bay and Corbicula Lake sediments in the central bay to lagoonal facies lie at 29.5 and 12 m below the lake bottom, respectively, and their ages, estimated on the basis of the rate of deposition, are about 5,400 and 2,300 y.B.P. During the deposition of the Hachiro-gata Formation two contrasting coastal dunes, transversal Noshiro and longitudinal Tenno coastal dunes, developed on the northwestern and southern sides of the lagoon, respectively. The Noshiro coastal dune consists of three or partly four units of aeolian sand. The Tenno coastal dune, on the other hand, consists of many coast-parallel dune ridges. They are grouped into three sets; Younger dune I, II, and III from inland to shore. Both dunes developed episodically after the Jomon transgression, during which the highest stand of sea level in Holocene Japan occurred. The development of the above dunes had little influence on the formation of the brackish water Corbicula Lake. On the contrary, the lake was established by the blockade of channels connecting the shallow marine embayment (Raeta Bay) to open sea. The contrastive dune morphology stated above can be attributed to the Holocene northeastward tilting of the Oga Peninsula including the western part of the Hachiro-gata lagoon. This tilting is also indicated by the result of geodetic survey and the displacement of former shorelines.