Research paperHigher plant biomarker signatures of Early Eocene sediments of North Eastern India

•Biomarker data of Early Eocene petroleum source facies from NE India.•Oleanane index across section varies due to changes in the extent of diagenesis.•The tropical rain forest element in eastern India evolved in Early Eocene, similar to the pattern found in western India.

The biomarkers, molecular fossils of paleobiota, have been studied in detail from a coal-bearing sequence of Early Eocene Lakadong sandstone member of Sylhet Formation of Upper Assam Basin, NE India, using gas chromatography-mass spectrometry. Corresponding hydrocarbon source potential of the same unit has been studied using Rock-Eval pyrolysis. The Rock-Eval data suggest dominant terrestrial land plant contribution, i.e., Type III kerogen and the sediments of the section had entered the early stage of catagenesis. The alkane and isoprenoid distributions are characterized by odd over even preference (OEP) (1.1-1.2) of n-alkanes and high pristane/phytane ratio (2.9-7.6) which corroborate the Rock-Eval data. The distribution of aliphatic biomarkers is characterized by dominance of C24 tetracyclics over C23 tricyclics, high concentration of des-E hopane, moderate to high C29/C30 hopane ratio, extremely low oleanane index and low C35 homohopane index. The extremely low oleanane index is correlated with a high degree of aromatization of oleanane skeleton, favoured by acidic swampy depositional environment. The presence of diterpanes at the base of the Early Eocene section indicates contribution from gymnosperms (Coniferales order of land plants) whereas angiospermous biomarkers with oleanoid skeleton are also preserved. This pattern subsequently evolved to angiosperm dominated towards the top of the section with no gymnosperm signature indicating possible climatic variation during the Early Eocene. Towards the top of the section presence of bicadinanes indicates contribution of Dipterocarpaceae family of trees and advent of the evergreen tropical rain forest element in eastern India in Early Eocene, similar to the pattern found in western India, e.g., Cambay and Kutch basins.