Abstract
The River Yoneshiro rising from the southern part of the Towada Volcano flows westward through Hanawa, Odate and Takanosu Basins into Noshiro Plain, and it finally pours into Japan Sea at Noshiro City, Akita Prefecture.
Terrace plains are well developed along the river course. The aim of this report is to make clear the subaqueous sedimentation of pumiceous fragments in the lower terrace deposits. First of all, the writer surveyed the distribution of the terrace plains and investigated their longitudial profil, micro-relief (figs. 1, 2, tab. 1). As to the result of these morphological observation, he presumed the distribution of the river agencies at the stage of the terrace plain formation. Then, he observed sedimentary facies of the terrace deposits, and classified them according to the lithologic kind and size of deposits (phot. 1_??_4) into such five types as pumiceous gravel layers (Ka-, Ge-facies), pumiceous sand layers (Od—), lithic sand layers (Ki—) and lithic gravel layers (Ot—) . And his survey was conducted to discusse the herizontal and vertical distrbution of each other (figs. 3, 4). So, it was worthy of his notice that pumiceous gravel layers (Ka-, Ge-) were always accumulated on pumiceouse sand layers, in reverse the case of lithic layers. Further, he measured the grain size, ratio of mixing of lithology, ratio of suspensive and floating material, bulk density and absorption of pumiceous gravels, and sheir sinking ratio (figs. 5, 6, tabs. 2, 3, 4).
The results of these field survey and laboratory analysis, are summerized as follow:
1. Enumerating these facies in order of the presumed strengeth of the river agencies, they arrange Ot-, Ki-, Od-, Ge-and Ka-facies, so Ot-facies show the strongest of all.
2. Pumiceous fragments are accumulated under the qualitive environments as compared with lithic fragments of same size.
3. In spite of their small grain size, pumiceous sand leyers are formed under stronger river agencies than pumiceus gravel layers. Ka-facies layers are accumlated in the dum up basin, and Ge-facies layers are formed on the top of the natural levees or sand banks.
