GEOLOGICAL INVESTIGATIONS IN THE LASSI GRANITE MASS (CENTRAL SUMATRA)

• Main Author: A. Katili, John
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/775
• Institution: Institut Teknologi Bandung

The Lassi mass southeast of Lake Singkarak is a complex rock assemblage of Permo-Carboniferous volcanics and sediments and Triassic deposits, intruded by plutons and partly overlain by Tertiary sediments. During Permo-Carboniferous time this area was occupied by an elongated sea basin, in which thick sequence of bathyal and neritic sediments were deposited. Pelitic sediments dominated but contemporaneous volcanic products were also deposited. These Late Paleozoic Early Mesozoic volcanic deposits (the Silungkang formation) have been studied petrographically and the results of this study, combined with the field relations, has led to the conclusion that they are autochtoneous deposits. The petro graphy of the granitic rocks has also been studied and the mode of emplacement of the granites discussed, based on both microscopical and field data. The genesis of the Lassi granites can be explained best by a combination of magmatic and metasomatic processes. The age of the different granitic rocks in the investigated area and in other parts of Sumatra are studied in relation to the tectonics, indicating a distinct zonal arrangement of the granites in South Sumatra according to age. However this zonal structure can not be used conversely as a base to establish the age of the granites or the age of the folding in the investigated area or in other parts of Sumatra. The unraveling of the geological structure in this area is of great importance, because many geologists have assumed the existence of a nappe structure similar to the presumed nappe at Djambi. The problem of this so called Umbilin nappe hypothesis has been treated in this paper and the present author comes to the conclusion that no nappe structure exists in the area southeast of Lake Singkarak.