Genesis of Indian Tertiary coals and lignites: a biopetrological and palaeobotanical view point

Abstract

Coal and lignite seams associated with the Tertiary sedimentary sequence of India are bright unbanded (coals) and sparingly banded (lignites) in appearance. They chiefly consist of vitrinite/huminite macerals with subordinate amounts of liptinites and inertinites. Desmocollinite/attrinite and densinite are the main vitrinite/huminite maceral groups. Sclerotinite (fungal remains) and inertodetrinite are the main inertinite macerals in the seams of northeastern States-Tamil Nadu, Rajasthan and Gujarat. Some of the coal and lignite seams of Meghalaya and Gujarat have predominance of structured inertinite macerals (semifusinite and fusinite). Main inorganics associated with the seams are syngenetic pyrite and calcite. Clay and quartz (argillaceous matter) are usually in low proportions except in certain seams of Gujarat and Meghalaya. The seams, under blue light excitation, are characterized by moderate to very high proportion of fluorescing macerals constituted chiefly by perhydrous vitrinite/huminite, liptodetrinite, bituminite and resinite. Other macerals of liptinite group, viz., cutinite, sporinite, suberinite, exsudatinite, alginite (Botryococcus) and fluorinite in order of decreasing abundance, are in subordinate amounts.

The coal and lignite-forming evergreen angiospermous forest vegetation flourishing under humid tropical climate comprised inland, coastal, beach, back mangrove to mangrove plant communities. Herbaceous and shrubby, including aquatic, angiosperms and pteridophytes formed prolific undergrowth. Some of the coal and lignite seams of Meghalaya, Rajasthan and Gujarat have definite marine and brackish water elements.

The Tertiary coal and lignite deposits of India originated from hypo-autochthonous to autochthonous rheotrophic peat accumulating in lagoons or near-shore back swamps in Assam, Nagaland, Tamil Nadu, Rajasthan and Gujarat and in small isolated estuarine or estuarine back swamps in Meghalaya. The vegetal matter experienced both aerobic and anaerobic microbial degradation under neutral to mildly alkaline subaqueous conditions. The coalification trends of the ancient peat were influenced more by putrefaction rather than purily by humification. Small and ephemeral fresh water ponds/lakes developed occasionally on peat surface were responsible for the presence of aquatic and water-edge taxa in the seams. Herbaceous and shrubby vegetation, especially pteridophytes, growing in the vicinity of ancient peat swamps were the main source of structured and detrital inertinites. Occasional natural cindering of the peat surface, e.g., in Gujarat and Meghalaya, was responsible for relatively high inertinite contents. Variations in the thickness and rank of coal and lignite seams were controlled by the existing tectonic conditions and geothermal gradients in different areas.

Keywords- Biopetrology, Coal and lignite genesis, Palaeoenvironment, India.