Early Eocene equatorial vegetation and depositional environment: Biomarker and palynological evidences from a lignite-bearing sequence of Cambay Basin, western India

Abstract

An investigation of palaeofloral distribution with depositional environment and palaeoclimatic condition of western Indian subcontinent during Early Eocene period has been carried out from a lignite-bearing succession of Cambay Basin, western India. The study of biomarkers, palynological remains and petrography is taken as proxies for providing supportive evidence for our inference. The petrographic results reveal that the sediments are thermally immature and coalification was extended up to the lignitic stage (low rank B). The palynological results show the dominance of angiosperms, mainly members belonging to the Arecaceae family, throughout the studied section. Triterpenoids with oleanane, ursane and lupane structure in extractable organic matter are derived from angiosperm wax. Pollen grains (Spinizonocolpites, Proxapertites, Dipterocarpuspollenites, Albertipollenites, Retipollenites, Arengapollenites, Lakiapollis and Longapertites) with pteridophyte spores and fungal fruiting bodies confirm the prevalence of tropical to subtropical climate during the deposition of the sediments. Sesquiterpenoids like longifolene-12, methanoazulene, cadinane and δ-selinene together with amyrin derivatives are the indicator of Dipterocarpaceae family that grows in the extant tropical rain forests in SE Asia. Appearance of dinoflagellate cysts and the predominance of lower molecular weight part in n-alkane series with the signature of submerged/floating vegetation in the shale horizon indicate marine influence in the younger part of the mine section. The presence of mangrove pollen such as Spinizonocolpites, Proxapertites and Acanthotricolpites further suggests that the lignite-bearing sequence was deposited in marshy marginal marine environment.