宇治丘陵における前期更新世扇状地の形成過程

Abstract

Fluvial facies of the lowermost to lower early Pleistocene Osaka Group distributed in the southern part of the Uji Hills was investigated, and the lobe wandering model of alluvial fan system is discussed. The surveyed area is situated at the margin of the sedimentary basin of the Osaka Group. The fluvial sediments were formed by the Paleo-Seta river which ran through the mountainous ranges from the Kobiwako sedimentary basin, northeast of the surveyed area (Iida, 1980). These sediments are stratigraphically divided into Aodani clay and gravel, Myojogahara sand, Joyo gravel I, Hase clay, and lower, middle and upper part of Joyo gravel n members in ascending order. Six sedimentary facies are distinguished in the sequence of the surveyed area. They are talus deposits (Facies 1), alluvial cone gravels (Facies 2), intramountainous swamp silts (Facies 3), braided river sands and gravels with meandering channel-fill sediments (Facies 4), sheet flood gravels and sands of the typical braided river systems on the midfan (Facies 5), and backmarsh silts and inverse-graded fine-grained sands (Facies 6). Among them, Facies 1, 2 and 3, which are the Aodani clay and gravel member, are considered to represent intramountainous small sedimentary basins. Facies 4, 5 and 6 represent large-scale alluvial fan systems formed by the Paleo-Seta river. As a result, three cycles of major coarsening upward sequence up to 50 to 80 meters thick, composed of Facies 4 and 5 in ascending order, are recognized in the sediments of the surveyed area. It is considered that this vertical facies change provides a good sedimentary model of the lobe sequence of an intramountain-basin which started from swamp or pond deposition and grew into a majada fan lobe.