Abstract
The Quaternary evolution of an alluvial fan system in the Himalayan foothills reveals two distinct phases. The Early Quaternary Siwalik system (I) and the Late Quaternary intramontane piggy-back system (II) have been studied in detail in the Subathu sub-basin of NW Himalaya (with system I followed by system II). Sedimentary architecture and facies analysis from chronologically constrained sections (using magnetostratigraphy and TL/OSL) indicate that systems I and II, although developed in similar hinterland-basin settings, indicate contrasting aggradation and entrenchment. System I is characterised by predominant fan aggradation, in contrast to the variable aggradation—entrenchment response in time and space for system II. System I is time transgressive laterally from east to west with the central part remaining as the uplifted inter-fan domain. Further confinement of system I along the basin margin indicates its syn-orogenic evolution linked to the intra-foreland thrusting. This continued with the formation of the piggy-back basin of system II. Glacial–interglacial cycles influenced the evolution of both alluvial fan systems. However, greater sediment yield and larger accommodation space favored aggradation during system I. In contrast in System II, insufficient accommodation space relative to sediment yield and ongoing upliftment (and reduced subsidence) resulted in aggradation at the fan head during incessant precipitation, followed by entrenchment during low precipitation. The latest phase, Late Quaternary to Recent, is characterised by two level terraces (at ca.16 and 5 ka) within the entrenched streams, due to variation in water budget and sediment load governed by glacial–interglacial cycles. This study thus demonstrate the variable importance of accommodation space, base-level change and magnitude of tectonic and climatic forcing as controlling factors on aggradation and entrenchment in Quaternary alluvial fan systems of the Himalayan foreland basin.
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