Landform evolution and sedimentary environment of the riverine coastal plain around the Holocene/Pleistocene boundary.

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This research aims to clarify the landforms and sedimentary environments of the riverine coastal plain in the period around the Holocene/Pleistocene boundary.Detail analysis of the Holocece/Pleistocene boundary of the sediments was done on the Nobi plain. Regional defferences of the facies in the latest Pleistocene can be seen in the sediments. The differences are considered to be caused by the elandforms and sedimentary environments of each location at that time. Thick sandy sediments can be seen in the delta front, and the places where the thick oyster shell bed can be seen in the tidal flat at that time. Furthermore, in the places of the flood plain, fluvial muddy sediments develop widery. Shallow dissected valleys and channel fills are also seen in the contact of the Holocene/Pleistocene boundary. These regional differences of the sediments are the result of the slow transgression and the difference of the location in the deltas at that time.In the small riverine coastal plains at the Tsugaru and the Abashiri, muddy Holocene sediments cover the latest Pleistocene sediments which are consisted of sand or sandy gravels. Peat layrs are occasionaly seen in the lowest part of the muddy sediments. This shows that the sedimentary environment of the Pleistocene/Holocene boundary in these lowlands in mainly flood plain, and the abrupt change to the marshy condition can be seen in some places.