Palaeopedogenesis and weathering in overbank sediments of the Siwalik Group: Link to Himalayan uplift and climate change during late Miocene

ABSTRACT The Himalayan uplift since the Miocene has caused large‐scale changes in relief, run‐off and sediment flux that influenced global climate. However, its linkage to palaeopedogenesis and weathering in overbank sediments of the Himalayan Foreland Basin (HFB) remains unknown. This study provides a high‐resolution record of palaeosols from a 2.74 km‐thick succession of the Lower Siwalik (LS), Middle Siwalik (MS) and Upper Siwalik (US) subgroups of the HFB, spanning from 12.77 to 4.48 Ma. This succession's LS and MS parts show incipient to strong palaeopedogenesis, while the US part shows no significant palaeopedogenesis. Field observations, petrography, geochemistry and clay mineralogy of 80 palaeosol profiles in the LS and MS parts were used to estimate the extent of chemical weathering, degree of palaeopedogenesis and palaeoclimatic conditions. Six major palaeopedogenic events were identified at (i) 12.0 to 11.0 Ma, (ii) 11.0 to 10.6 Ma, (iii) 10.6 to 8.7 Ma, (iv) 8.7 to 8.3 Ma, (v) 8.3 to 6.0 Ma and (vi) 6.0 to 5.4 Ma show varying intensities of weathering and palaeopedogenesis that correspond to global palaeoclimatic conditions during the late Miocene. For example, palaeopedogenic events (ii) and (vi) correspond to weak chemical weathering under semi‐arid to subhumid conditions and reactivation of major thrust sheets in the Himalayas. Event (v) is similar to (ii) and (vi), but with occasional strongly developed palaeosols. These three events correspond globally with consistent pCO 2 levels and GMST and a minor rise in sea‐level. In contrast, events (i), (iii) and (iv) show strong chemical weathering under subhumid to humid conditions and no major tectonic activity. These events correspond globally to a lowering of pCO 2 , GMST and sea‐level. Consistent δ 13 C values (avg. −10.8‰ ±0.03) in the pedogenic CaCO 3 of LS and MS palaeosols suggest the prevalence of C3 vegetation throughout 12 to 5.4 Ma.