Clay-mineral distribution patterns in late Neogene fluvial sediments of the Subathu sub-basin, central sector of Himalayan foreland basin: implications for provenance and climate

Abstract

The late Neogene (6–0.5 Ma) fluvial succession of the Subathu sub-basin, a part of the Himalayan foreland basin, comprises a 2.4-km-thick pile of conglomerate, grey and buff sandstone, and mudstone, representing Middle and Upper Siwalik subgroup. This basin is filled mainly by major trunk and piedmont drainage, which are nearly perpendicular to each other. The clay-mineral assemblages of this sedimentary succession have illite (7–82%), smectite (0–90%), chlorite (2–23%) and kaolinite (1–13%). The grey sandstones have moderate to abundant smectite (23–90%), whereas the buff sandstones have abundant illite (66–79%) and low to absent smectite (0–14%). The mudstones that dominates the succession (>50%) have clay-mineral assemblages similar to grey and buff sandstones, or intermediate proportion. The temporal distribution of clay minerals in mudstones shows occasional intense zigzag pattern with either smectite (3–81%) or illite (15–82%) abundance. The smectite-rich grey sandstones and mudstones are deposited by trunk drainage, and the illite-rich buff sandstones and mudstones are deposited by piedmont drainage. The intense zigzag distribution pattern of clay minerals in mudstone indicates interfingering of floods from trunk and piedmont drainages. The interfingering was severe, ranging between 4.8 and 3.36 Ma and between 2.60 and 1.77 Ma, related to tectonic activity. The association of smectite (>36%) bearing mudstones and piedmont source-derived buff sandstone and conglomerate towards the upper part of the section (above 1.77 Ma) suggests either floodwater of trunk drainage over spill on the fringe of piedmont alluvial fan or derivation from smectite bearing Middle Siwalik rocks, exposed due to the activity of an intra-foreland thrust (IFT) in the piedmont zone. The occurrence of smectite and its variable proportion with time suggests its probable derivation not only from the sparsely exposed basic rock in the catchment area but also from siliceous and metamorphic rocks under favourable climatic conditions between 6 and 0.5 Ma.