An Overview of Younger Kathmandu Lake, Nepal, during the Late Quaternary, with Special Reference to Ferruginous Structures in Carbonaceous Sediments

摘要

The Kathmandu Basin in Central Nepal is located in the Lesser Himalayas. Older Kathmandu Lake evolved during the Pliocene and Early Pleistocene; lounger Kathmandu Lake with its widespread carbonaceous rocks and ferruginous biogenic structures silted up during the Late Quaternary. Sedimentological analyses supplemented with palynological and chemical studies of carbonaceous sediments formed the basis of the present paleogeographic and paleoclimalic studies. The basin was covered by a perennial freshwater lake before 30,000 yr. B.P. It was silted up with alluvial and fluvial sediments. Diatomaceous earths, and carbonaceous mud- and siltstones were deposited in the center of the lake and small relict ponds. Toward the basin edge, lacustrine sediments gave way to deltaic deposits spread across the delta plain. The most proximal parts of this alluvial-fluvial sedimentary wedge also have debris flows interfingering with fine-grained floodplain deposits. Three lake highstands (> 30,000 yr. B.P., 28,000-19,000 yr. B.P., 11,000-4000 yr. B.P.(?)) have been recognized in the sedimentary record of Younger Kathmandu Lake. Second-order water-level fluctuations were triggered by damming as a result of tectonically induced landslides. First-order water-level fluctuations were due to climatic changes. During the last highsland, favorable conditions occurred for vertical to subvertical tubular ferricretes to develop. The tubular structures are likely to have been created by bottom dwellers. Three different types of mineralization have evolved during diagenesis, each of which is characterized by different Fe-bearing marker minerals. The overall silting-up process in Kathmandu Basin is manifested on a microscopic scale in the textural and mineralogical variations of Fe mineralization in these biogenic sedimentary structures.