PALEOPEDOGENESIS IN OVERBANK SEDIMENTS OF SIWALIK GROUP, NW HIMALAYA: WEATHERING AND CLIMATE CHANGE DURING 12 TO 5 MA

The Himalayan Foreland Basin (HFB) formed in response to the collision of the Indian and Tibetan plates. The HFB has been explored extensively in the past for its fluvial sedimentology. But details regarding weathering and paleopedogenesis in overbank sediments of HFB in response to tectonics and climate change are not known. This study presents a high-resolution record of the paleosols developed in overbanks sediments of the Lower and Middle Siwalik groups during 12.0 Ma to 5.4 Ma in a 2.25 km section of the Kangra Basin, NW Himalaya. Eighty representative paleosol profiles studied for field characteristics, micromorphology, clay mineralogy, major elements geochemistry, and stable isotope geochemistry indicate presence of 4 pedofacies i.e., Type-A, Type-B, Type-C, and Type-D showing strongly developed pedofeatures to incipient stages of pedogenesis. The 0.5 km part of the section exposed as Lower Siwalik (LS) sediments is dominated by Type-A pedofacies and strongly developed paleosols, whereas Middle Siwalik (MS) sediments in remaining 1.75 km part of the succession contains moderate to weakly developed paleosols with Type-B and Type-C pedofacies. The paleopedological details of the LS and MS paleosols indicates six major events: (i) 12.0 Ma – 11.0 Ma, (ii) 11.0 Ma – 10.6 Ma, (iii) 10.6 Ma – 8.7 Ma, (iv) 8.7 Ma – 8.3 Ma, (v) 8.3 Ma – 6.0 Ma, and (vi) 6.0 Ma -5.4 Ma, which are marked by stable or unstable landscape, gradual to abrupt exhumation of hinterland, arid dry to warm-humid climates, weak to strong chemical weathering conditions. For example, (ii) and (vi) events demarcate arid to semiarid climate, weak chemical weathering, poorly developed paleosols, and dominance of smectite-rich clay mineral assemblage and reactivation of major thrust sheets of the Himalayas. In contrast, (i), (iii), and (iv) events indicate no major tectonic activity, subhumid to humid climate, moderate-strong chemical weathering, well-developed paleosols, and dominance of kaolinite-rich clay mineral assemblage. On the other hand event (v) is somewhat similar to (i) and (vi) events, but it is marked by occasional presence of well-developed paleosols forming during 8.3 Ma to 6.0 Ma in humid conditions. The consistent values of the δ¹³C in LS and MS pedogenic carbonates (-11.3‰ avg.) suggests prevalence of C3 vegetation and no major input of the C4 vegetation in the Kangra Basin of NW Himalaya.