Alluvial Fan Model for the Himalayan Piedmont Deposits

The Ganga-Brahmaputra fluvial plains are separated from the Himalayan foothills by the extensive piedmont belt. Four longitudinal large-scale terraces that have sequentially developed are recognised ill the interfluvial region of the Tista and the Jaldhaka rivers. These terraces are composed primarily of Boulder Beds and Gravel Beds, and of subordinate Sand Beds, and show rapid facies variations. These deposits are dissected by streams into cut and fill type transverse small-scale terraces. The evolution of the Himalayan piedmont tJelt is consistent with a dynamic alluvial fan model in which the sediments are considered to have been derived through upliftdependent episodic debris-flows and rrtass emplacement. The vertical and lateral accretion of the fans is in consequence to shifting of the mountain front, and to flattening of the stream gradient in the mountains. The hinge-lines of the fans migrated toward south with time, while the transitional zones between the fanglomeratic and the floodplain facies, toward north. Several empirical relationships in the magnitude-frequency matrix between uplifting, terracing and fan-formation are suggested.