GEOMORPHOLOGY OF IWAI HILLS AND ENVIRONS

Abstract

The erosion surfaces are generally recognized as accordant summit levels or undulating surfaces, the origin or the development process of which, I think, is not clearley explained. The development process of Iwai Hills, which are in the Kitakami Valley area between Kitakami Mountains to the east and Ou backbone range to the west and are dividing the Kitakami Valley area into the northern middle reaches and the southern lower reaches, is discussed here as an example of erosion surfaces. The hills were once considered as a part of so called ‘Rikuzen peneplain’. According to previous researches on geohistory of the Kitakami Valley area, the valley area was determined by the distribution area of Lower Pliocene sediments in the inter-mountain basin between Kitakami Mountains and 0u backbone range which had been uplifting since Miocene. The Isawa-Yushima line of flexture runs north to south in the center of the valley area, and the distribution of Middle and Upper Pliocene sediments and Upland Gravels of Iwai Hills is delimited to the east of the line, and it appeared as the dislocation of the terraces of Isawa Upland, north of Iwai Hills. The formation of erosion surfaces is related with the change of base level around the area (Table 2), and the process is summarized as follows; Description of the erosion surfaces and the terrace surfaces The erosion surfaces of West Iwai Hills, western half of Iwai Hills, and of Hiraizumi Hills situated between Iwai Hills and Isawa Upland are between the highest terraces and the next ones in height. The highest terraces such as the Upper Ishuzaka terrace of Isawa Upland, the Wayama terrace of Hiraizumi Hills and the Nonodake terrace to the south of Sanuma Lowland are at the margin of Middle and Upper Pliocene sedimentary basin overlying the Miocene and Lower Pliocene sediments, and their height are 200 meters above sea level at the border of the Kitakami river and the lowland. The terrace deposits of these terraces were almost worn away. The first two are cutting into the east flank of the backbone range, and there Uppermost Pliocene and Lower Pleistocene sediments fringing the range were perhaps lost. The next terraces are the Lower Ishuzaka terrace (less than 150 m at the lowest) of Isawa Upland and the Suganodaira terrace (130 to 140 m) of East Iwai Hills, eastern half of Iwai Hills. The Lower Ishuzaka terrace has a wide surface and thick terrace deposits covered with volcanic ash and pumice fall. The Suganodaira terrace has no wide and flat surface, and the hill ridges of East Iwai Hills make an undulating erosion surface (Tomisawa surface) cut into the Nakayana Formation, the upland gravels and its bedrocks. The restored summit level (Suganodaira surface) shows that the accordant hill tops are depositional or erosional in origin according to the interpretation of the age of the Nakayama Formation, that is, the interpretation of the time gap of the contact between the formation and the underlying Pliocene Mataki Formation. The Suganodaira surface is concluded to be a fill top terrace surface based on the following facts; a) Accordance of the restored summit level (fig. 1). b) The coincidence of the Suganodaira surface with the distribution of the Nakayama Formation and partly with the low interfluves (fig. 1). c) The Nakayama Formation is consisting of fluvial sediments of 40 meters in thickness at maximum, sand and gravel (10 m) at the base and clay and cross-laminated sand at middle and upper beds (fig. 3). d) The base of the Nakayama Formation has valley form matched with the geological structure of bedrocks underlying it perhaps in discomformity (fig. 4). e) No deposits cover the Nakayama Formation, and the upper member may be flood plain sand and silt. f) The Nakayama Formation is sheet typed, which is as wide as the present Kitakami river plain (fig. 1).