Changes in mineral composition and depositional environments recorded in the present and past basin-fill sediments of the Kathmandu Valley, central Nepal

Abstract

Surface and drill core sediments from the Kathmandu Basin were investigated to decipher their capability to record changes in the mineral composition and depositional environment. Spatial distribution of mineral assemblages in the terrigenous sand and mud reflects different provenance of the past and present basin filled sediments. Bulk mineral assemblages in the modern sediments are controlled by supply of terrigenous detritus from the source rocks in the adjacent surrounding mountains. This suggests the occurrence of granitic and gneiss source in the north and metasediments in the eastern, western and southern part of the Kathmandu Basin. X-ray diffraction (XRD) experiment was done for the mineralogical study of the present river sediments and RB core sediments. RB core was drilled at Rabhibhawan in the western part of the Kathmandu Basin (Sakai et al. 2001). All samples were prepared on the basis of rule adapted by Kuwahara (2001). XRD measurement were done by Rigaku X-ray Diffractometer RINT 2100V, using CuKα radiation monochromatized by a curve graphite crystal in a step counting time of 2 second. The profile fitting of obtained XRD patterns was performed with an Apple Power Macintosh computer and a scientific graphical analysis program XRD MacDiff (Petschik 2000). The result of the individual minerals obtained from the profile-fitting method was used for quantitative analysis. Relative amount of minerals in the sediments were determined by calibration curve obtained from integrated intensity ratio of the standard mineral to internal standard zincite. X-ray diffraction (XRD) analysis of greater than 2 mm fraction of the present river sediments from the Kathmandu Basin shows quartz, K-feldspar, plagioclase and mica to be the dominant bulk minerals (accounting over 80%). It shows the presence of low amounts of chlorite and calcite. Chlorite is relatively higher in the northern part than the east, west and south. Based on the study done, the mineral assemblages of the present river sediments within the Kathmandu Valley are divided into two groups. The first group is rich in mica, poor in quartz and has presence of K-feldspar, plagioclase and chlorite in all samples. This group of mineral originated from Shivapuri Lekh of the granite and gneiss complex. The second group is rich in quartz and poor in mica and has presence of plagioclase, K- feldspar and very poor chlorite. This group probably originated from metasediments in the eastern, western and southern part of the Kathmandu Valley. Some samples of this group also contain very low percentage of calcite (less than 5 %). Mineral assemblages in the RB core sediments from 7 to 40 m depth indicate the same as those encountered in the present river sediments. XRD-analysis of the greater than 2 mm fraction shows the presence of quartz (10-60%), K-feldspar (2-37%), plagioclase (2-8%), mica (4-19%), chlorite (2 -14%) and calcite (0-18%). Relatively high percentages of these minerals are