Ferrous biogenic structures in swamps of the Holocene Katmandu Lake, Nepal - Their implications concerning palaeogeography and physico-chemical disequilibria
Dill, Harald G.; Melcher, Frank
published: Jul 9, 2004
The Katmandu Lake, Nepal, was silted up during the Holocene with argillaceous and arenaceous sediments. Close to the SE margin of the basin, grey finegrained clastic rocks developed. They represent a fluvial-lacustrine environment of deposition, which offered favorable conditions for the development of vertical to sub vertical tubular ferricretes. Their siliciclastic host rocks predominantly contain illite, quartz, feldspar, chlorite, vermiculite and subordinate amounts of biotite. The tubular structures are likely to have been created by bottom dwellers such as worms. Three different types of mineralization have evolved during diagenesis each of which is characterized by different Fe-bearing marker minerals. Vivianite was precipitated during mineralization type I which is visible all across the fine-grained host rock. Stage-II mineralization is only encountered in and around the tubular structures. Sub stage II a has aggregates of framboidal pyrite, which during sub stage II b were replaced by goethite. The pH was almost constant lying around neutral throughout this Fe mineralization. The Eh value, however, drastically increased as the poorly drained swamps were converted into a well drained swamp about 4000 years ago as a result of a drastic lowering of the lake water level in the Katmandu-Banepa Basins (lake outburst at the southern rim). This sudden drop of the water table completed the conversion of the Katmandu Lake into a dry land what is called today the Katmandu Valley and has also preserved a disequilibrium manifested by the vivianite-pyrite-goethite mineralization. The overall silting-up process in the Katmandu Basin is manifested on a microscopic scale in the textural and mineralogical variations of Fe mineralizations in these biogenic sedimentary structures: lacustrine (vivianite) → poorly drained swamp (pyrite) → well-drained swamp (goethite pseudomorphs) → dry land (goethite matrix).