CHRONOLOGICAL STUDY OF THE MICRORELIEF ON THE FLUVIAL LOWLAND (2)

Is the microrelief on the flood plain such as natural levees, point bars and abandoned river channels growing during the recent floods? Some of them seem to have been formed under the control of eustatic base-level changes since the last transgression (so-called “Jomon transgression”), when relating the depth of archaeological sites and remnants to microreliefs on the fluvial lowland. The height of the last transgression in the Jomon period, the peak of which was during the late stage of the Proto Jomon to the early stage of the Early Jomon, about 8, 000 to 6, 000 years B. P., is estimated to be either 2-3 or 6 meters above the present sea-level. The level of succeeding minor regression in the Yayoi period, B. C. 300 to A. D. 300 years, is estimated to be 3 meters below the present sea-level in Japan.<br> Among the three characteristic sections composing the river plains, the lowermost deltaic plain section was once an offshore part in the former estuary during the Jomon transgression. The other two sections in the upstream side, i.e., the flood plain section characterized by natural levees and back-swamps and the fan plain section which are distributed in the upper side from the former river mouth, also may not have been independant from the sea-level fluctuation. Responding to those base-level changes, a sequence of filling up and cutting down above and below the present flood plain level and subsequent refilling up to the present flood plain level have occurred in the upstream side from the former river mouth (denoted as Type A). The spreading of fluvial deposits and the growth of deltaic plains have occurred responding to the regression in the former estuaries (TypeB). Type A -the Yoshinogawa plain. The Yoshinogawa plain is located in the tectonic graben along the Median Tectonic Line, where the late Pleistocene fans and terraces have been dislocated by the mosaic-like block faulting. The amount of dislocation during the Holocene is estimated to de on the order of several meters fine heights of the fault scarps cutting through the alluvial fans and elevation of wave-cut notches formed during the, Jomon transgression. The height of the Jomon transgression is 2.5 meters above the present sea-level in the Tokushima deltaic plain.<br> The geomorphic surfaces on the flood plain are classified into three; one space the up-per, middle and lower surfaces. The upper and middle surfaces distributed between Ikeda and Miyoshi, 75 to 65km from the sea, have scarps of more than 10 meters high above the present river floor(Figs. 2-a, b). The upper surface has usually remained as point bar during floods, but has been drowned twice during the last 50 years. Therefore, it may be considered as natural level now under construction. The middle surface is abandoned river channel or strath in shape and drowned frequently during floods.<br> At Ogaki, Miyoshi town, the ruins of ancient hamlets from the Yayoi to Mediaeval Age have been found on the upper surface, and the depth of the ancient cottage floor is within the horizon of paddy field soil, only a few tens of centimeters below the ground surface. A celamic fragment of the Late Jomon period was picked up on the upper surface, but it could be not related to any archaeological site. The remnants of the Yayoi period were found at 2 meters below the top of the middle surface which was composed of deposits 5 meters thick filling up the former river channel, and at 1.2 m below the surface the fragment of Sue celamic (5th to 12th C.) was containd (Fig. 3). Hence the upper surface had emerged at a certain time before the Yayoi period during the last regression.<br> The Joni field system, the rectangular lot system established in 7th and 8th C. on many Japanese plains, can be recognized on the upper and middle surfaces at Ogaki.