九十九里浜平野の地形発達史

Abstract

The Kujukuri coastal plain, situated in the eastern part of the Kanto region, is one of the longest sandy ridge plains in Japan. This plain consists of sandy ridges, dunes, swales, abandoned barriers, abandoned lagoons and valley bottoms, which have been formed during the last 6, 000 years. In this paper, the evolution of those landforms is discussed based on radiocarbon dates, humus horizons, archeological sites and historical data as well as on landforms and deposits. The results are as follows.Sandy ridges are divided into three groups, named Sandy ridges I (abbreviated as SI), II (SII) and III (SIII) from older to younger, judging from the areal pattern and transverse profile. SI was formed about 6, 000 to 4, 000 years ago. SI is composed of abandoned barriers in the northern part, while of widely distributed sandy ridges more than 5km in width in the southern part. SII seems to have been formed about 4, 000 to 2, 000 years ago. The ridges of SII converge at the both northern and southern ends of the plain. SIII has been formed mostly during the last 1, 500 years, the width being 3 to 4km at every part.As to sand dunes, three stages of Older (Do), Middle (Dm) and Younger (Dy) are classified, on the basis of their distribution and humus horizons covering the sand dunes. Do, which is developed on the abandoned barriers of SI, is mainly found in the northern half of the plain. Do was formed at the culmination of Holocene transgression about 6, 000 to 5, 500 years ago. Judging from the distribution and the stratigraphical relation to humus, some dune sands of Dm were derived from sands of Do at Yokaichiba, Yokoshiba and Higata, and the other dune Sands of Dm developed along sides of rivers were supplied from river beds. It is considered that both of them were formed during a short time from the Yayoi period (about 2, 000 years ago) to the Kofun period (about 1, 500 years ago). The destruction of woods by man seems to have played an important role in their formation. The destruction affected the sand supply into lower courses of rivers and the erosion of Do, resulted in the formation of Dm on the river sides and on the Do respectively.As to humus, two horizons of Humus I (HI) and Humus II (HII) are classified. HI is black to dark-brown in color and is observed only on the SI and SII. The thickness of HI gradually becomes thinner seaward. It was formed about 6, 000 to 2, 000 years ago. HII being light-brown, and irregular in thickness and is not distributed on Dy. As this humus is poorly developed, it is not observed even in some places of SI and SII. It was formed during last 1, 500 years.Valley bottoms in the Shimosa upland were lagoonal environment until about 3, 000 years ago, then, they rapidly gave way to swampy condition.The relations among the above-mentioned landforms, humus and relative sea-level change, and paleogeographic maps are shown in Figs. 7, 8 and 9.1. ca. 6, 000-5, 500 years ago (early Jomon period)The Holocene transgression equivalent to the Flandrian transgression, attained to the culmination, and valleys in the upland were drowned. As barriers SI extended in the vicinity of the mouth of such drowned valleys, lagoons were formed. Do began to form on those barriers by sand supply from the shore. It may be estimated that the relative sea-level at that time was +6m above the present sea-level in the Kuriyama valley bottom.