Paleopedological evolution of Siwalik succession from Kangra sub-Basin, NW Himalayan: Implications for climate change and weathering conditions

In the present study we report paleopedology of the fluvial sequences of the Siwalik Group in the Himalayan Foreland Basin, NW Himalaya that formed during ~12 Ma -5.5 Ma. The paleosols formed during this time period are critical to understand weathering and paleopedogenic processes during the evolution of foreland basin over the entire Himalayan range. This work highlights field-characteristics, micromorphology, clay mineralogy, and geochemistry of the ~0.5 km thick Lower Siwalik and ~1.7 km thick Middle Siwalik successions along the Katilu Khad, Kangra sub-basin. In the field, the paleosols are characterized by 1-2 m thick Bw, Bt, Bk, Bss, and BC horizons, blocky and wedge-shaped pedogenic structures, root traces, color mottling, Fe-Mn oxide concretions, slickensides, pedogenic CaCO₃ (PC), and bioturbation features. 

Micromorphological observations show the dominance of moderate to well-developed paleopedofeatures in paleosols of the Lower Siwalik in contrast to the moderately to weakly-developed paleopedofeatures in paleosols of the Middle Siwalik. The comparative analysis of various pedogenic features i.e., PC, illuvial clay, mottles, Fe-Mn concretions, microstructures, and bioturbation features confirmed varying degree of the paleopedogenic maturity in the paleosols at different intervals of the Siwalik successions.

Clay mineralogy of the total clay (<2 μm) and fine clay fraction (< 0.2 μm) of the Lower and Middle Siwalik paleosols suggests varying chemical weathering of silicates and change of paleoclimatic conditions during paleopedogenic processes during this time period. The clay mineral assemblage of the total clay and fine clay fraction show the varying distribution of illite, chlorite, kaolinite, smectite, vermiculite and interstratified clay minerals in these paleosols. Large amounts of smectite together with pedogenic carbonates in part of the Lower Siwalik at 12.0 Ma, and at 10.9 Ma and in Middle Siwalik at 9.2 Ma, and at 5.5 Ma suggest arid to semiarid dry climatic conditions Whereas, dominance of kaolin, illuvial features, and dissolution of pedogenic carbonates suggests sub-humid to humid climatic condition at 11.6 Ma, 8.5 Ma, 7.1 Ma, and at 6.5 Ma.

The bulk geochemistry of the paleosols also confirmed varying degree of pedogenic weathering showing high CIA and CIA-K (CIW) values and ~ 800 mm to 1400 mm MAP for paleosols of the Lower and Middle Siwalik. The high MAP (~ 1200 mm to 1400 mm) at ~11.6 Ma, ~8.5 to 8.0 Ma, and 7.1 to 6.5 Ma in paleosols of the Lower Siwalik and Middle Siwalik correspond to increased chemical weathering and paleopedogenesis. While the intervening periods correspond to less MAP (~800 mm to 1100 mm) with large amount PC and less chemical weathering. Based on micromorphology, clay mineralogy, and geochemical characteristics of the paleosols it is interpreted that climate change during ~12 Ma to 5.5 Ma is characterized by humid (11.6 Ma) to semiarid (11.0 Ma to 8.5 Ma), and humid-subhumid (8.5 Ma to 6.5 Ma) in response to Himalayan orogeny and its linkage to regional and global atmospheric conditions.

Keywords: Himalayan Foreland Basin, Paleosols, Siwalik, Micromorphology, Clay mineralogy, Geochemistry