The lacustrine section at Lukundol, Kathmandu basin, Nepal: Dating and magnetic fabric aspects

Abstract

The fluvial and lacustrine sediments of Kathmandu basin in central Nepal are a good archive to study the past environmental changes and the history of development of the SW Indian monsoon. A sequence of about 170 m exposed at the southern part of the basin is divided into two lithological units: Lukundol Formation and Itaiti Formation. The Lukundol Formation mainly comprises fine-grained sediments whereas gravel beds dominate the Itaiti Formation. In total 750 oriented samples were collected all along the accessible part of the sequence in 10 cm 3 plastic boxes with a sampling interval of ∼10 cm. Additional 128 samples were collected for pollen analysis with a sampling interval of ∼50 cm. Magnetostratigraphy is established based on four magnetic transitions indicating that the sampled section spans 0.75–1.1 Ma. Amino-acid dating on a fossil found in a lignite layer at 88 m yields an age of ∼0.8–1.0 Ma is consistent with the age derived from magnetic polarity stratigraphy. Anisotropy of magnetic susceptibility (AMS) measured in 250 samples spanning the whole section reveals a sedimentary fabric, with NW–SE oriented maximum directions. Lithological changes are significant at 67.5 m, above which thick gravel beds appear and imply significant change in the depositional regime. Mainly two magnetic components are identified: Magnetite – dominant all along the whole sequence, and hematite – relatively more important at depths with lower susceptibility.