Abstract
The Izumo Plain is one of the most extensive alluvial plains in the San'in District, Western Japan; it stretches about 20 km east-west and 5 km north-south. The plain is composed of alluvial deposits supplied by R-s. Hii and Kando. The geomorphic development of the alluvial lowland has been studied in the Izumo Plain with special reference to the microrelief of the deltaic plain and the stratigraphy of the alluvial deposits. The geomorphological map of the Izumo Plain is shown in Fig. 1.
On the basis of the tephrochronology and the existence of iron slag grains which were produced by the Japanese traditional iron industry called Tatara which flourised during the Edo period (17th to the 19th century), the alluvial deposits have been divided into four groups: Lower Gravel Bed and Hikawa, Kandogawa and Hiikawa Formations. The Lower Gravel Bed is composed of fluvial deposits directly overlying the bedrock. It can be subdivided into lower gravel and upper sand layers.
The Aira Tn ash (AT), one of the Japanese representative marker tephra, erupted at 24, 000 to 25, 000 y. B. P., lies between the two layers. The Hikawa Formation is composed of marine silt and clay deposited during the Postglacial transgression. The Kandogawa Formation is composed mainly of dacite fragments which were provided by Sambe Ohirayama pyroclastic flow (SOh). The SOh erupted about 3, 600 y. B. P. from Sambe Volcano, which is located in the upstream area of H. Kando. The Hiikawa Formation is the latest fluviatile deposit. The iron slag grains which are often observed in the lower or the middle layer of this formation suggest that a considerable amount of sandy deposit of the Hiikawa Formation was laid during the modern age. The relation between the tephras and the stratigraphy of alluvial deposits is shown in Fig. 4, and the schematic cross-sections of alluvial deposits are drawn in Fig. 5.
The geomorphic development of the Izumo Plain is summarized as follows:
(1) The bedrock topography buried by alluvial deposits is characterized by flat-topped ridges and deep valleys like gorges. The deep valleys were formed in accordance with the lowered sea level, at least 80 m below the present sea level, during the Last Glacial period. The bottom of buried valley is covered with the Lower Gravel Bed. The rather fine materials in the upper sand layer of this Bed suggest an ascending stage of sea level. This transgression probably started between 24, 000 and 15, 000 y. B. P., on the basis of the tephrochronology.
(2) During the Postglacial transgression a deep inlet extended from west to east, and it isolated the northern Shimane Peninsula from the mainland. The sea level was 3 to 4 m above the present sea level at its maximum stage, which occurred between 6, 000 and 5, 000y. B. P.. A large sand bar was formed at the western margin in correspondence with this higher sea level.
(3) The Sambe Ohirayama pyroclastic flow, which erupted about 3, 600 y. B. P., made a sudden change in this inlet. A large quantity of dacite gravel provided by this pyroclastic flow rapidly formed a fan-like delta at the mouth of R. Kando. Soon after, the original surface of this fan-like delta was slightly dissected, while the delta advanced further. Thus, delta surface I was formed. The pre-existing inlet was interrupted by this delta I and was separated into two lagoons: the western “Kando-no-mizuumi” and the eastern L. Shinji. The absolute age, about 2, 700 y. B. P. was obtained from a peat layer which overlies the Kandogawa Formation. These facts suggest that delta surface I was formed between 3, 600 to 2, 700 y. B. P., when the sea level descended. At the maximum stage of this regression, the sea level stood at least 2 m below the present level, which was estimated on the basis of geomorphological observation carried out in the lu Plain, close to the southwestern end of L. Nakaumi.
