北上山地南西縁盆地群の侵蝕小起伏面

抄録

Undulating erosion surfaces were developed in the basins of the southwest piedmont of the Kitakami mountains. These basins are located along the east tributaries of the Kitakami river, i. e. Miyamori basin along the Sarugaishi river, Yanagawa and Hirose basins along the Hirose river, Maisato basin along the Hitokabe river, Ohara and Surisawa basins along the Satetsu river and Senmaya basin along the Senmaya river. They were formed in several rows under structural control and most are located on exposed granites intruding into Paleozoic rocks.
Undulating erosion surfaces in the basins are mostly less than 300m a. s. l. in height. The original forms of the basins are outlined before the deposition of upper Pliocene fluvial deposits, which are distributed widely along the Kitakami valley and its tributaries (Fig. 1). The fill top levels of Pliocene fluvial deposits are identified at 200 to 300m a. s. l. in most basins, and their levels are well explicable for each height of superposition by the rivers (mark in Fig. 1).
Underlying geology of the undulating erosion surfaces is thick unconsolidated Pliocene gravel bed in Miyamori, Hirose basins and northern half of Surisawa basin, and weathered granites in Yanagawa, Senmaya basins and southern half of Surisawa basin. However, patches of very thin Pliocene gravel bed remain along the ridge and the valley floor of the undulating erosion surface in the Yanagawa basin (Fig. 2).
The level of Senmaya basin is low enough to be drowned in Pliocene base level, though the Pliocene bed is not seen presently. The level of undulating erosion surface in the southern half of Surisawa basin, most of which underlain by granites with the exception of the lower part by a thin Pliocene bed, is lower than the top of the Pliocene bed in the northern half of Surisawa basin.
The base level in the periods of middle and late Pleistocene in the upstream basins is estimated only some ten meters lower than the top of the Pliocene beds. The base level in the upstream basins at the 140m terrace stage along the Kitakami main trunk is calculated to be at less than 100m below the top of the Pliocene beds in most basins (Tab. 1). The highest base level during middle and late Pleistocene is some ten meters higher than the 140m terrace along the Kitakami main river.
The Pliocene bed, therefore, was not removed in the duration of the earlier half of Quaternary, though it was not very thick originally. The formation of erosion surfaces in the basins is, at the latest, after middle and late Pleistocene, and it is not be said that the erosion surface controlled; by a long stable base level, but it is a rock- and base level-controlled topography which is a dynamic equilibrium of soft rocks under the condition of small potential height above the base level of erosion.
Deep weathering of granites under the undulating erosion surface may have occurred after the resurrection of the base form of Pliocene beds as their restored base form suggests the effect of selective erosion between Pliocene beds and granites.
Assuming that the undulating erosion surface on the mountain tops higher than 500m is a remnant of a peneplain, they are older than Pliocene because Pliocene beds are distributed lower than 400m a. s. l. in the level mentioned above, they are contemporary or older than Miocene because Miocene andesite, the basal member of the Green Tuff Group, is lower than 500m in the western piedmont of Kitakami mountains.