Pedogenic Alteration in the Overbank Sediments, North Bihar Plains, India

The Quaternary successions exposed by excavation in the top 1.5-3.0 m of alluvial tract along a traverse in parts of the Indo-Gangetic plains are analyzed. The traverse extends across some 100 km of the interfan area between the Gandak and Kosi megafans of northern Bihar, eastern India. Most of the successions exhibit a fining upward grain size distribution, bottoming in sand or silt. though the overlying silts or clays are often interrupted by beds of coarse silt and sands. Radiocarbon dating suggests that all of the material was deposited during the last 2500 years and post-depositional pedogenic alteration has taken place. Micromorphological study of this subtle alteration shows (a) weathering of primary grains, (b) decomposition of plant and shell material, (c) carbonate dissolution and precipitation, (d) iron oxide/hydroxide accumulation and (e) illuviation of clays. The minor degree of alteration is contrasted with that reported in other areas of the Indo-Gangetic plains and it is concluded that the north Bihar plains, with an average accumulation rate of the order of 1 mm/a over 10³ years has been an area of exceptionally high sedimentation.