Evolution of Coastal Barriers in Japan during the Holocene

Abstract

The coastal barriers in Japan are of three different types, formed at three different times (Fig. 4). 1) The oldest barriers, named as “Tsugaru-type”, form an extensive marine terrace having originated as ancient bar and lagoon, probably during the Shimosueyoshi (Riss-Würm interglacial) stage. 2) “Akita-type” of the ancient barriers are believed to have been formed during some transgressional (interstadial) phases in Würm stage. The top of the deposits is separated from the succeeding Holocene soft sediments. 3) The youngest barriers, called as “Kanazawa-type”, are of the Holocene transgression. All of those types is often developed along a same stretch of coastal area, where the ancient barriers are partly incised with numerous valleys, buried by the Holocene shallow marine, lagoon or fluviatile deposits.
Based on the sedimentological analyses of subsurface data, it is generally established that the environment of deposition in the Japanese coastal area is changed from fresh or brackish to marine water during the early Holocene, in spite of the growth of sand spits or bars. Along the flanks of relatively large delta and fluvial fan, however, marine transgression was prevented its effect by the large supply of sediments.
During the late Holocene, three series of beach ridges built by a prograding shoreline are developed on the coastal barriers. The environment in the landward area is generally changed from marine to brackish or fresh water. The cause is attributed to the intermittent emergences of shoreline. It is remarkable that these emergences can be recognized even on the relatively sinking coasts in Japan.