濃尾平野北部のコア試料にみられた完新世中期以降の河成堆積物

摘要

濃尾平野内陸部の河成低地において採取した2本のコア堆積物を解析し，低地とその地下を構rn成する堆積物の特徴を検討した．2本のコアの採取地点は約280m離れているが，表層河成層のrn下位にみられる海成層は両コアともに類似しており，海成層が累重していた時期の堆積環境ははrnば同じだったと考えられる．河成層は約4,000calBP以降に堆積したが，ST1コアでは植物片のrn薄層を多く含む泥質堆積物で特徴づけられる氾濫堆積物が海成層を覆って厚く累垂する．それにrn対し，ST2コアでは砂礫からなる流路堆積物が下位の海成層を侵食し，この上にST1コアと同様rnの氾濫堆積物が積み重なる．また，両コアの氾濫堆積物は2つの層準に極細粒～細粒砂層を挟在rnし，これらの砂層はそれぞれ4,000～1,400 calBP,1,400 calBP以降に堆積したと考えられる．rnこれは，両地点が砂の堆積を被る規模の洪水を少なくとも2度経験したことを示唆する．%Two sediment cores named ST1 and ST2 were taken from the floodplain in the inner part of Nobi Plain to investigate sedimentary facies of the subsurface deposits. Brackish to marine deposits underlying fluvial deposits occur in both cores, and their characteristics, such as sedimentary facies and facies succession, are quite similar to each other, though ST1 is located approximately 280 m west of ST2. Sedimentary environment during the deposition of brackish to marine deposits would have been mostly the same in the two sites. Fluvial de-rnposits accumulated since ca. 4,000 cal BP. Over-bank mud characterized by abundant thin plant layers has been deposited at the ST1 site, while channel sand and gravel erodes the underlying brackish to marine deposits at the ST2 site. The channel deposits are overlain by overbank mud, as in the STL Two well-sorted very fine-to fine-sand beds, at approximately 1 and 5 m depth, are intercalated in the mud in both cores. This suggests that these sites had experienced floods, which deposited sands, at least twice.