Bulletin of the Geological Society of Malaysia, Volume 19, April 1986, pp. 533 – 549
1Geological Institute, State University of Gent, Krijgslaan 281, B-9000 Gent, Belgium
2Department of Chemistry, University of Antwerp, B-2610 Wilrijk, Belgium
Abstract: Rhyolitic ash deposits, observed locally in north-west Malaysia, are studied with regard to their nature, origin and age.
Volcanic ash layers are found scattered along the axis of the Padang Terap river, in its most upstream section. The ash was deposited there, following its widespread deposition, subsequent erosion and transport to the contemporary drainage axis of the basin. The ash does not cover the young alluvium.
The pyroclastic materials contained in the deposits are composed of glass shards of different habits, pumice fragments and distorted phenocrysts (i.e. quartz, biotite, plagioclase, sanidine and hornblende).
The chemical analyses of the glass shards show their average composition corresponds to hydrous, slight peraluminous melts of rhyolitic composition, belonging to the high-K calc-alkaline rock series.
Based on its microscopical and chemical composition, the ash is thought to originate from the Toba area, north Sumatra, and can be correlated with either a 75,000 year old or a 30,000 year old eruption. A K-Ar dating of the glass shards is in progress.
The geomorphic position of the ashes and their approximate age suggest that older important landforms in the area have formed under drier Late Pleistocene conditions.
The stage of weathering of the soils developed on rhyolitic ash, characterised by an argillic horizon, is intermediate between the weathering stage of soils on young and on old alluvium. Differences in drainage conditions are thought responsible for differences in intensity of weathering of the material.
The present study deals with the geomorphological position of the ash deposits in the Padang Terap District and the description of their microscopical and chemical properties. The characteristics of the soils developed on the ash deposits are examined and compared to those of soils developed on similar on parent material in other localities. An absolute dating with the K-Ar method is in progress. The results will be communicated in a forthcoming paper.