GEOMORPHIC DEVELOPMENT OF THE TSUGARU PLAIN IN THE HOLOCENE PERIOD

Abstract

The Tsugaru Plain, about 60km long from south to north and 5 to 20 km wide from east to west, is located in the northernmost part of Honshu Island and is surrounded by hills and volcanoes except in its northern margin. The plain mainly consists of alluvial plains of the Iwaki and the other small rivers, which are classified into alluvial fans, a natural levees zone and a deltaic plain from south to north. In the natural levees zone, the alluvial plains are divided into two levels which intersect each other, the upper alluvial plain being dissected in its southern part and overlaid with deposits of the lower one in its northern part. Along the eastern and western margins of the deltaic plain are distributed terraces and landlocked sand spits 4 to 5 meters high above sea level which are correlated to the upper alluvial plain in the natural levees zone. Under the plain there are terraces of two levels and fluvial valleys buried with Holocene deposits. The Holocene deposits forming the plain are chiefly composed of sand and silt, and are locally different in composition in the natural levees zone and the deltaic plain. Accord-ing to results of diatomaceous analysis of the Holocene deposits, marine and brackish water species are dominant in the northern and western parts, whereas fresh water species predo-minate in the other parts of the plain. Generally speaking, the Holocene deposits were more remarkably influenced by fluvial action in process of deposition in the southern part than in the northern part, and a brackish water area seems to have most extensively occu-pied the plain when the middle silt bed was deposited. In conclusion, the geomorphological history of the Tsugaru Plain in the Holocene period was inferred as follows; (1) In the early Holocene, former fluvial valleys were drowned by transgression and were filled with sediments different in composition in each drowned valley. (2) With progress of transgression, an embayment had extended to the central part of the plain by 5, 500 y. B. P. Drowned valleys had been entirely submerged and the expan-sion of sedimentation area caused sediments to become fine. A delta composed of fine sedi-ments was formed at the head of the embayment. (3) A slight regression caused dissection of the deltaic plain and a new delta composed of rather coarse sediments was formed further downstream. (4) Around 2, 500 y. B. P. forest flourished on the dissected deltaic plain and peat accu-mulated on the new one. (5) After 2, 000 y. B. P., the lower alluvial plain was formed burying forest and peat lands.