Depositional processes in the distal part of a large alluvial fan's feeder channel in Himalayan foothills, India

Abstract

This paper presents results of detailed facies and architectural element analyses of the sedimentary fill of the Ranibag valley in the Himalayan foothills of north-western India. Forming a buffer zone between the source and sink of the Ganga foreland basin's Gola alluvial fan, the Ranibag valley is filled with coarse gravels comprising five distinct facies. These are clast-supported massive gravel, horizontal-bedded gravel, crudely bedded gravel, sandy gravel and matrix-supported disorganized gravel facies. The specific assemblages and sequences of these facies, defined by fifth- and fourth-order bounding surfaces, belong to mutually associated channel (CH) and gravel bar and bedform (GB) architectural elements. Both the elements have accreted only during floods by combined fluvial and debris-flow processes, but dominantly by the fluvial processes under conditions of high water-to-sediment ratio. Depositional processes in the valley are akin to those of the proximal alluvial fans of the Ganga Basin. Nonetheless, voluminous alluviation of coarser gravels in this extreme distal feeder channel valley has affected the fan processes downstream. It reduced the sediment budget and thus increased water-to-sediment ratio downstream to favour fluvial processes on the Gola fan. Copyright © 2016 John Wiley & Sons, Ltd.