Holocene geomorphic development of coastal barriers in Japan

Most coastal lowlands in Japan are characterized by the presence of barriers or beach ridges which are elongated sand or gravel ridge complexes usually parallel to the shore. Coastal lowlands with barriers or beach ridges can be classified into six types: a) barrier-lagoon complexes; b) barrier-backswamp complexes; c) beach ridge plains; d) valley plains; e) delta-beach ridge complexes; and f) others. The first purpose of this paper is to clarify the geomorphic development of coastal barriers during the Holocene, and to investigate both common and variable processes in the development of coastal barriers. The areas studied represent typical coastal lowlands with barriers or beach ridges in Japan: Lake Hamana (barrier-lagoon complexes); the Tokoro and the Ukishimagahara Lowlands (barrier-backswamp complexes); the Shimizu Lowland (beach ridge plain); the Matsuzaki and the Haibara Lowlands (valley plains); and the Sagami River Lowland (delta-beach ridge complexes). The processes of enclosure of bays by barriers are reconstructed based on analysis of the fossil foraminiferal assemblages in bore hole cores. In addition to these studies, the results of previous studies on the geomorphological development of coastal barriers in other areas are reviewed. The second purpose of this paper is to discuss the relationships between the development of coastal barriers and human activity. The periods of the enclosure of coastal barriers are compared with the periods when humans began to settle on the barriers. The Kano River, the Ukishimagahara, and the Shimizu Lowlands are studied. The results of this study can be summarized as follows: 1) Coastal barriers began to form in the stage of sea-level rise during early to middle Holocene, without reference to present landforms of coastal lowlands. 2) Variability in the development of coastal barriers occurred in both the initial and final periods of enclosure of bays by barriers. This variability was affected by the basal landforms of coastal barriers, sediment supply and crustal movements. These factors also influence present landforms of coastal lowlands. 3) The final periods of enclosure by coastal barriers correspond to stable or lowered stages in the Holocene relative sea level changes. 4) Humans began to settle on the coastal barriers thousands of years after the final stages of barrier or beach ridge formation.