BIOMARKER ANALYSIS OF THE EASTERN HIMALAYA FORELAND BASIN (EHFB) AS A PROXY FOR PALEOCLIMATIC IDENTIFICATION

The deposits of the Eastern Himalayan Foreland Basin (EHFB) have recently spurred debate between a terrestrial fluvial and marginal marine setting. Previous studies have identified these deposits as fluvial sediments. Some recent studies of the Eastern Siwalik deposits from the Tista Valley, Bhutan, and Arunachal Pradesh have shown the presence of wave-generated sedimentary structures and marine trace fossils, indicating a marine deltaic depositional setting. Organic biomarkers are another helpful resource in determining and understanding the depositional environment. These geochemical analyses can differentiate between primarily aquatic or terrestrial plant assemblage. We analyzed n-alkane distribution, organic δ¹³C, and C/N ratio from 33 mudstone samples from the Neogene Siwalik deposits of Arunachal Pradesh, India. The samples were collected along the Siji River section in the eastern end of the foreland basin and spans across the Lower Siwalik Dafla (8 samples), Middle Siwalik Subansiri (4 samples), Upper Siwalik Siji (12 samples), and the Kimin formations (9 samples). Recent facies analysis identified Dafla as tidal, Subansiri as braided fluvial. Siji as shoal water fan delta and Kimin as alluvial fan or braided stream deposits. Seven Dafla and four Siji samples, show bimodal peaks centered on C19 and C31, indicating a mixture of marine and terrestrial matters. Our data set shows a strong correlation with the recently proposed facies distribution and supports a possible marine incursion in the Eastern Siwaliks.