Abstract
The five lagoons along the coastline of the Tokachi region, Hokkadio, sporadically open by breaking each sandbar from the overflow of lagoon water at the lowest part of the sandbar. In order to explore formation processes of the lagoons, sediment cores ca. 10 m long were sampled on the sandbars of Oikamanai Lagoon and Yudo Lagoon, and the C14 dating for the cores was performed. The origin of gravels in the cores was then identified by specifying the rock species. Meanwhile, the hydrological function of the sandbars was investigated by obtaining the hydraulic conductivity of the cores by permeability tests and grain size analyses. All the analyses for the cores gave the following findings: 1) The gravels in the cores were transported into the sea by fluvial erosion of the Hidaka metamorphic zone, deposited by flood tides or tsunamis, 2) the prototype of Oikamanai Lagoon was produced ca. 6,000 yrs ago followed by the gravel layer formation ca. 5,400 – 4,800 yrs ago and the buildup of the nearly present sandbar by ca. 4,800 yr ago, and 3) under closed condition, the lagoon water is likely to outflow into the sea as confined groundwater through the pebble layer in sandbar.
