Sedimentary Facies of the Early Pleistocene Alluvial Fans of the Uji Hills, Near Kyoto, Japan

Abstract

Fluvial facies of the lowermost to lower Pleistocene Osaka Group distributed in the southern part of the Uji Hills were investigated. Six sedimentary facies are distinguished in the sequence of the surveyed area. They are talus deposits (Facies 1), alluvial cone gravels (Facies 2), intramountain swamp silts and poured-in gravels (Facies 3), sinuous channel-fill river sands and gravels (Facies 4), sheet flood gravels and sands of the typical braided river systems (Facies 5), and backmarsh silts and inverse-graded finegrained sands (Facies 6). Among them, Facies 1, 2 and 3 are considered to represent intramountainous small sedimentary basins. Facies 4, 5 and 6 represent larger scale alluvial fan systems formed by the river which ran through the mountainous ranges northeast of the surveyed area. Three cycles of major coarsening-upward sequence up to 80 m thick, composed of Facies 3 or 6, 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 sequence of an intramountain basin which started from swamp or pond deposition and grew into a majada fan lobe.