1987 年 60 巻 12 号 p. 821-834
It has been indicated that littoral shelves are generally developed widely at the coastal lakes in Japan. The author takes notice that the littoral shelves are not a continuous flat plain but partly consist of two terraces, and considers that its formative processes have close relativity to the Holocene sea level changes. So, in this paper, the author investigated the southeast part of Lake Kasumigaura, which is the biggest coastal lake in Japan, where littoral shelves are developed in width of about 600m.
Two transections of littoral shelves were obtained at Tennouzaki and Ukishima by using an echo-sounder (Fig. 4). Twelve surface deposit samples on the lake bottom were collected at two cases with a Ekman Birge sampler. Each sample was split to 200 g, sieved to the -2 to 4 phi fraction and analysed for particle sizes (Fig. 5). On the basis of these data, the detailed structure of the littoral shelves in Lake Kasumigaura was clarified and the origin of wide littoral shelves in the coastal lakes in Japan were considered as follows.
(1) There are two kinds of littoral shelves in Lake Kasumigaura (Fig. 2). Littoral shelf I is developed along the shoreline continuously at the depth of 0.5_??_2.0m. On the other hand, littoral shelf II is found partly in the offing of littoral shelf I at the depth of 2.0_??_3.5 m. Littoral shelf I is considered to be a modern surface but the formation of littoral shelf II has high possibility to be a relict surface which was formed in the past at a low water level.
That is to say, the wide littoral shelves in Lake Kasumigaura are made up of a modern surface (littoral shelf I) and a relict surface (littoral shelf II). And the former seems to be partially superposed on the latter.
(2) At other main coastal lakes in Japan, for example, Lake Saroma, Lake Notoro, Lake Abashiri, Lake Ogawara, etc. (Table 2), we can also observe that littoral shelves are partly consist of two terraces. And these two terraces are correlative with the littoral shelf I and the littoral shelf II in Lake Kasumigaura respectively (Fig. 2). So the author assumes that the formation of the wide littoral shelves at the coastal lakes in Japan are also due to the superposition of littoral shelf I on littoral shelf II, and the low water level, when littoral shelf II was formed, is estimated to have appeared at almost the same time in those lakes, affected by the sea level changes in Late Holocene. It is known that the regression occurred at least three times in Late Holocene in Japan; middle Jomon period (about 5, 000_??_4, 000 y. B. P.), Yayoi period (about 3, 0002000 y. B. P.) and early Edo period (about 300 y. B. P.). The author supposes that littoral shelf II was formed during one of these periods, or during two or three periods with plural affections.