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An active barrier system is present in Akkeshi Bay and a lagoonal area along the Pacific coast of eastern Hokkaido, Japan. This rare feature consists of a so-called lagoon (the Akkeshi-ko lagoon), a flood tidal delta, a barrier, and a tidal inlet that leads into the outer sea of Akkeshi Bay and the Pacific Ocean. Large oyster reefs were living on the flood tidal delta until their sudden extinction in the early 1980s. The processes involved in the formation of this barrier system during a postglacial transgression period are not well understood. Here, we use sedimentological, paleontological, and radiocarbon dating methods to analyze Holocene drillcore obtained from the Akkeshi fishing port in 2009 and borehole log data previously obtained from public works around the Akkeshi lowlands. The data obtained from both drillcore and borehole logs indicates that: (1) postglacial transgression occurred in this area some 11,400 years ago and reached 50 m below the present sea water level, with the barrier system in the study area established 8800 years ago. (2) The preservation of this barrier system is due to the stable sea level in this area. The sea level has been constant for the past 5500 years, and has led to the development of oyster reefs on the tidal flood delta. (3) The current topographically distinctive barrier system has been affected by sudden uplifts associated with giant earthquakes. These events occur every few hundred years, with the barrier undergoing prolonged but relatively rapid post-earthquake subsidence at about 1 cm/year, as observed after a giant 17th century earthquake.

石狩平野の深度50 mのSSC-1，同44 mのH16B-7，同40 mのH16B-3，同18 mのMHR-1および同28.5 mのYUB-1の5掘削井の後期更新世～完新世地質試料について14C年代測定，珪藻分析，火山灰分析を行い，層序や古環境を検討した．珪藻分析ではH16B-7とH16B-3で「大量な海水の流入」が認められ，完新世の中期にバリア（砂州）を海水が乗り越えたためと推定した．海生種の最大割合を示す層準は約6,000年前の縄文海進高頂期と考えられ，この時期には紅葉山砂丘より東に大きさ東西30 km，南北20 km程度の汽水湖が形成されたと推定される．火山灰分析ではSSC-1の深度42.38～42.25 mに約11.3万年前降灰のToyaが確認され，最終間氷期の堆積面が石狩平野下に埋積していることが示唆された．

札幌市市街域の北海道大学構内で，1981年の掘削で得られた150 mコアの第四系層序を明らかにするために，14C年代測定，堆積相解析，火山灰分析，花粉分析，珪藻分析，放散虫分析を行った．孔内地質はHU-1層～HU-9層に区分され,分析結果に基づく野幌丘陵との地層対比では，HU-1層～HU-3層（深度150～75 m）は前・中期更新世の下野幌層，HU-4層～HU-5層（深度75～35 m）は後期更新世のもみじ台層，HU-6層（深度35～34 m）は後期更新世の小野幌層，HU-7層～HU-8層（深度34～7.8 m）は後期更新世の札幌扇状地堆積物，HU-9層（深度7.8～0 m）は完新統に相当することが明らかになった．最終間氷期堆積物であるHU-4層～HU-5層からは，寒冷期や温暖期が確認され，今後の石狩低地帯南部の第四系との詳細な対比の可能性が生じた．既存井との地層対比では，150 mコアの深度130 m（標高－119 m）付近に挟在する下野幌層相当層中の泥炭は，隣接する2本の井戸の標高－110 m付近に認められる亜炭質層と連続し，更には石狩海岸平野下の下野幌層相当層の標高－102～－126 mに挟在する泥炭に対比される．