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 al luvial tract along a traverse in parts of the Indo-Gangetic plains are anal yzed. 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 s and or silt, though the overlying silts or clays are often interrupted by b eds of coarse silt and sands. Radiocarbon dating suggests that all of the m aterial was deposited during the last 2500 years and post-depositional pedo genic alteration has taken place. Micromorphological study of this subtle a lteration shows (a) weathering of primary grains, (b) decomposition of plan t and shell material, (c) carbonate dissolution and precipitation, (d) iron oxide/hydroxide accumulation and (e) illuviation of clays. The minor degre e 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 a verage accumulation rate of the order of 1 mm/a over 10(3) years, has been an area of exceptionally high sedimentation.