Journal - Palynological study of the drilled sediments from the Kathmandu Basin and its palaeoclimatic and sedimentological significance

Palynological study of the drilled sediments from the Kathmandu Basin and its palaeoclimatic and sedimentological significance

Rie Fujii and Harutaka Sakai

Department of Earth Sciences, Kyushu University,

Ropponmatsu, Fukuoka, 810-8560, Japan

Jour. Nepal Geol. Soc., 2001, Vol. 25 (Sp. Issue), 53–61


Abstract

Palynological study of a series of slimes obtained from a 284-m-long drill well in the Kathmandu Basin revealed palaeoclimatic records and depositional environmental changes in the Kathmandu Valley during the last 2.5 myr. The slimes consist of fluvio-deltaic and lacustrine sediments comprising 218 m long muddy beds and 66.3 m of sandy beds. As a result of palynological study on the 189 pieces of slime, a totaxl 43 types of fossil pollen and spores were identified. They comprise arboreal pollen of one family and 20 genera, non-arboreal pollen of 10 families and 10 genera and spores. In the pollen diagram, Quercus and Cyclobalanopsis are predominant, with frequencies exceeding 70%. Pinus, Alnus and Gramineae are the next dominant taxa. Three fossil pollen zones, which reflect major climatic change, were discriminated: Zone I indicates a cool climate from ca. 2.5 to 2.1 Ma; Zone II shows a warm and relatively dry climate without remarkable fluctuation from ca. 2.1 to 1.0 Ma; and Zone III is characterized by seven cycles of warm-and-wet and cold-and-dry climate from 1.0 Ma to present, which reflect the repetition of glacial and interglacial periods. The last cold maximum at 11 m depth seems to correspond to the last glacial age around 20 kyr B.P.

 

The sedimentation rate of the drill-well was estimated from an approximate age-depth plot based on approximate ages inferred by comparison between the palaeoclimatic curve drawn by pollen analysis and d18O curve obtained from planktonic foraminifers of deep-sea sediments in the Indian Ocean. Consequently, the average sedimentation rate of the uppermost part from 115 m to 11 m in depth is estimated at 104 mm/kyr.

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