Contrasting Biomarker Signatures of Cretaceous and Early Paleogene Coal-Bearing Sediments During the Northward Flight of India

Abstract

The northward drift of the Indian plate from Early Cretaceous until it collided with the Eurasian plate in Early Eocene provides a unique opportunity to track its biotic evolution. The distribution of higher plant terpenoids of Early Cretaceous and Eocene coal- and lignite-bearing sequences of Kutch Basin, western India have been investigated to understand the change in floral evolution during the northward journey of the Indian plate. The higher plant terpenoids of studied Early Cretaceous samples are characterized by sesquiterpenoids and high abundance of diterpenoids. Angiosperm-derived triterpenoids are not detected here. Such terpenoid pattern clearly depicts the prevalence of gymnosperm, especially conifer dominated forests during that time in western India. The presence of diterpenoids such as norisopimarane, labdane, ent-beyerane, isopimarane, abietane, norabietane and phyllocladane in the aliphatic fraction and 16,17-bisnordehydroabietane, 16,17-bisnorsimonellite, dehydroabietane, 18-norabieta-6,8,11,13-tetraene, 1,2,3,4-tetrahydroretene, simonellite, C1 simonellite, totarane, sempervirane, retene and 2-methylretene in the aromatic fraction suggest that the Early Cretaceous coal forming plants in western India belonged to Araucariaceae and Podocarpaceae families of conifer. The terpenoids of Eocene lignite-bearing sequences are represented by the significant occurrence of angiosperm-derived triterpenoids and the complete absence of diterpenoids. The angiosperm-derived triterpenoids are amyrin-derivatives and the compounds are des-A-olean-12-ene; A-neo-oleana-3(5),2-diene; olean-2,13(18)-diene; olean-2,18-diene; ursa-2,12-diene; olean-12-ene; olean-18-ene etc. in aliphatic fraction and 24,25–dinoroleana-1,3,5(10),12-tetraene; 24,25,26,27-tetranor-oleana-1,3,5(10),6,8,11,13-heptaene; 24,25,26,27,28-pentanor-ursa-1,3,5(10),6,8,11,13,15,17-nonaene; C-ring cleaved aromatized triterpenoids etc. in aromatic fraction. Variation in biomarker assemblages from Early Cretaceous to Early Paleogene depicts a remarkable shift of floral association during the voyage of Indian continent. The reduction of coniferous vegetation and the proliferation of angiosperms during the Early Paleogene are observed in the present study.