Exotic conglomerates of the Neogene Siwalik succession and their implications for the tectonic and topographic evolution of the Western Himalaya

Published information on the Siwalik Group (Neogene) conglomerates in the Sub-Himalayan belt of Himachal Pradesh, India, and the Potwar Plateau, Pakistan, is used as the basis of a model that recognizes two types of conglomerates with different relationships to the linear Himalayan mountain front. The two types are: (1) foothills-fed conglomerates, widespread along the mountain front, that mark the appearance in the basinal succession of sediments derived from within the Sub-Himalayan fold-and-thrust belt or the neighbouring Lower Himalaya; these Upper Siwalik conglomerates provide information about relatively young frontal fault activity and the growth of local river systems that transferred the sediment generated; and (2) high-mountain-fed conglomerates, with restricted lateral extent along the front, that mark the presence of large trunk rivers, draining the Lower and Higher zones of the Himalaya; these Middle and Upper Siwalik conglomerates provide information about the extent of older incision in the high mountains, and the position in the basin of the major drainage paths. Our work on the Janak (formerly Indus) Conglomerate Formation, outcropping near the present Indus River, shows that it is a high-mountain-fed conglomerate, and was deposited by a large braided system (channels up to 15 m deep) that drained southwards from the High Himalaya of northernmost Pakistan. The conglomerate was deposited between 9 and 1 Ma, and is preserved as a formation 1.5 km thick and 25 km wide, perpendicular to flow. This conglomerate marks the position of the Palaeo-Indus in this part of the foreland basin. The syntaxial position of the present Indus in the mountain front, and the remarkable route of the upper Indus, have resulted from drainage evolution during the later stages of transpressive indentation of the western margin of the Indian block. Our interpretation of the Janak Conglomerate implies that this special role of the Indus must have already started by 9 Ma.