Chemical Weathering Record of the Himalayan Foreland Basin in Response to Changing Monsoon Climate During the Late Miocene

The Late Miocene was a period of extensive environmental changes and declining CO2 levels, which likely had a large impact on monsoon strength as well as on the weathering and erosion intensity in the South Asian monsoon domain. To improve our understanding of these feedback systems, we have analyzed fine grain sediments (<63 um) of Himalayan foreland basin from the Jammu Region, NW Himalaya. In the present study, we have used major elemental compositions, trace and rare earth elements concentrations and clay mineralogical assemblages between 12.5 to 5.6 Ma to identify provenance and weathering regime related to monsoon climate. The elemental ratios of La/Sc, Th/Sc, Th/Co, Eu/Eu* suggest felsic source rock composition, which is further supported by chondrite-normalised REE pattern consisting enriched LREE and flat HREE, along with negative Eu anomaly. The detrital sediments deposited in the Himalayan foreland basin possibly originated from the Higher Himalayan Crystalline and Lesser Himalayan regions. A combination of weathering indices such Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA) and trace elemental Rb/Sr ratio indicate that the source rocks has undergone variable degree of chemical weathering and indicate sharp change from low to high weathering around 9.8 Ma. The relatively decrease in kaolinite abundance and increase in smectite/(illite+chlorite) ratio during the Late Miocene cooling suggests increased aridity and intensitifed seasonality. This trend in relative aridity are not apparently linked to changes in Tibetan and Himalayan elevation, but more closely linked with global cooling.