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The Himalayan foreland basin originated through the India–Asia continent–continent collision driven by downward flexing of the Indian plate. It forms an ideal assemblage to delineate the role of process-controlled sedimentary fill in basin evolution. This paper provides the first comprehensive account of the lithofacies associations and architecture of the Middle Siwalik succession of the Himalayan foreland basin in the south-central Himachal Pradesh sector, India through detailed sedimentological analysis. Eight lithofacies form three distinct sand, gravel, and silt-mudstone lithofacies associations pertaining to specific depositional environments. Architectural elements have been defined together with bounding surfaces separating them. Sedimentological investigations reveal that the Middle Siwalik sequence represents a flash flood dominated piedmont fan and proximal to middle megafan. The fan expansion cycles indicate tectono-climatically controlled sedimentation in the Middle Siwalik foreland basin. Frequent flooding and a high sedimentation rate were influenced by the tectonic uplift along the reactivated thrusts in the hinterland coupled with phases of monsoonal intensification. • The studied Middle Siwalik belongs to a yet unattended Himachal sector of the Indian Himalaya. • Architectural documentation reveals different orders of fluvial hierarchy. • Sedimentation took place in the foreland basin setting solely by continental processes. • Contemporaneous piedmont fan and megafan sedimentary sequences are identified and interpreted. • The Middle Siwalik Foreland Basin of the present study is compared with foreland basins of similar tectonic settings globally. [ABSTRACT FROM AUTHOR]