Abstract
The sandy coast of Japan is characterized by large-scale dunes which were formed during the Holocene and the late Pleistocene periods. Coastal dunes are composed of eolianite of aqueous origin, noncalcareous sand, and volcanic sand. Eohanite is widely distributed in the southern part of Japan, while non-calcareous sand is more common in coastal areas. Dunes consisting of volcanic sand are developed along the coasts near volcanos (Fig. 1).
The results of the present research may be summarized as follows:
1) Data presented in a study of Holocene dunes by Endo (1969) indicate that older dunes (Do) were formed during 6, 000-4, 000 y. B. P., while younger dunes (Dy) were formed during 1, 800-500 y. B. P. In some coasts, dunes were formed during 3, 500-3, 000 y. B. P.
Holocene dunes developed mainly as a result of Holocene sea level oscillations. As eolian sand blew up from expanded sandy beaches in the initial stage of regression following transgression, coastal dunes were formed on the beach ridges. The sequence of events that led to the formation of dunes was repeated at least four times during the Holocene age. As a consequence, three types of dune lines, as shown in Fig. 5, have been established in conformity with the shape and gradient of the coastal plain.
Since 2, 000 y. B. P., however, the increase of sand supply on the coast has resulted in changes in the dune formation mechanism, depending on land transformation in the inland area (Fig. 6). In particular, extensive land transformation since 1, 300 y. B. P. accelerated sand supply on the coast. Moreover, tree-cutting for salt-making and forest fires on the dunes have resulted in large-scale formation of secondary dunes. Consequently, the orientation of the Holocene secondary dunes coincides with the direction of prevailing winds stronger than 4. 5m/sec (Fig. 3). Landesburg's method can be adopted satisfactorily to show the relation between prevailing winds and dune orientation in Japan.
2) During the last Glacialage, paleodunes developed extensively along the coasts of the Japan Sea and the East China Sea. These paleodunes contain several datable paleosols which divide individual dunesand members into four parts. The four major periods of dune formation are as follows: 18, 000 y. B. P., 30, 000 y. B. P., 50, 000y. B. P., and 70, 000 y. B. P. The latter two periods, which saw dune formation over a wide area, are associated with the early stage of regression during the last Glacialage. The distribution of dunes of 18, 000 y. B. P. and 30, 000 y. B. P. is limited to a few coastal areas.As coastlines in these two periods were located more than 50km from the present coastline, sandcould not have been transported by prevailing winds to the position of the present coast line.
Lithified dunes with several sequences of eolianite and brown paleosol associations are found in the Ryukyu Islands. It is recognized that lithified dunes developed all over the world during the last Glacial age. Those of the Ryukyu Islands, were formed after 30, 000 y. B. P.
3) Kurosuna paleosols of the Holocene age were buried in the Holocene dunes. Thick modernhumus layers appear on stabilized dunes which are covered with shrubs at the back of foredunes along the coast of northern Hokkaido (Fig. 7). By analogy with the present condition of soil development in dunes in northern Hokkaido, kurosuna developed in stabilized inland dunes if the foredune could hold eolian sand from the sandy beaches.
4) Five paleosols of the last Glacial age are observed in the paleodunes. The clay assemblage of paleosols consists mainly of quartz, 14A minerals, and illite which originated in eolian dust from Continental China, dried sea floors, and sandy beaches. The oxygen isotope composition of fine quartz in the paleosols, as well as kurosuna, also reveals that fine quartz originated in eolian dust (Fig. 8).
