GEOMORPHIC DEVELOPMENT OF NORTHERN RIKUCHU COASTAL REGION, NORTH-EASTERN JAPAN

Abstract

Coastal terraces developed along the coast of northern Rikuchu are classified into following 8 levels ; (1) Mizunashi Terraces (270 m.), (2) Hirono Terraces (220 m.), (3) Misaki Terraces (200 m.), (4) Samurai-hama Terraces (160 m.), (5) Mugyo Terraces (110 m.), (6) Uge Terraces (80 m.), (7) Taneichi Terraces (30 m.) and (8) Alluvial plains. The highest terraces in this region are Mizunashi Terraces, whose surfaces are composed of marine beds more than 10 meters in thickness in the northern part of the region. Hirono Terraces and Misaki Terraces are both flat rocky abraded terraces covered with thin marine beds, more than several kilometers wide. The initial form of those terraces will be inferred as the terraces formed during a period of slow rise in the sea level. Mugyo Terraces developed along the ria shoreline of Fuji Pay are composed of fluvial and marine beds more than 10 meters in thickness. Alluvial plains of comparatively large rivers also consist of marine and fluvial beds thicker than 20 meters. Judging from characteristics of the coastal terraces at these levels, the neighbourhood of Fuji Pay was deeply dissected and then submerged in relative. Samurai-hama Terraces, Uge Terraces and Taneichi Terraces without characteristic features of rise in sea level or submergence are also rocky abraded terraces and will be considered as terraces formed during the upheaval period. Relative changes in sea level in this region will be summarized as follows : After thedeposition of Neogene formation, submergence occured and consequently Mizunashi Terrace were formed. After the formation of Mizunashi Terraces, the region has risen generally. The cause of emergence was mainly crustal upheaval, but eustatic oscilation of sea level took place during emergence. During transgression due to eustatic rise of sea of sea level, Hirono Terraces Mugyo Terraces and Alluvial plains were formed. In the older Pleistocene (Hirono Period), since eustatic rise in sea level was slow, flat rocky abraded terraces were formed, but in later period (since Mugyo Period) embayments were formedat Fuji by rapid eustatic risesof sea level. Moreover, the formation of ria shorelines at Fuji in these periods represent remarkable dissection of the coastal region prior to transgressions. Eustatic oscilations of sea levels has influenced the geomorphical development of the coast and amplitude of relative changes in sea level were greater during the later Plesistocene Period than during the previous periods.