Biomarker Based Palaeohydrology Reconstruction of the Neogene Eastern Siwalik Group: A Look into Possible Marine Influence

The depositional environment of the Neogene Eastern Siwalik Group of the Eastern Himalayan Foreland Basin (EHFB) has recently been debated. This group has four formations: the Dafla, Subansiri, Siji, and Kimin formations. These formations were previously characterized as fluvial deposits. However, recent studies identified the Dafla Formation as a tidal location, and the Siji Formation, as a river delta setting. The new interpretation is based on marine trace fossils and the presence of wave-generated structures. The Subansiri and Kimin Formations have been characterized as braided fluvial environments. Our new research is using geochemical proxies to address the debate. Preliminary leaf wax n-alkane data from these formations show that most (13/15) samples from the Dafla and Siji formations have bimodal n-alkane distribution. Given the marine depositional facies classification for the Dafla and Siji formations, these samples should show clear distinctions from the Subansiri and Kimin formations in n-alkane chain length. However, we see the presence of bimodal distribution in a few Subansiri and Kimin samples. We are analyzing compound-specific stable isotopes of the n-alkanes to better understand the paleohydrological condition. Modern-day δD values for rivers in the Eastern Siwalik region are between -122.8 ‰ and -88.5 ‰. For marine water in the area, values are between -15.0 ‰ and - 25.0 ‰. The difference in these values can be used to determine the water source during deposition, which can ultimately give a better understanding of the depositional environment of each Formation.