Petrography, genesis and deposition of tertiary coals from Northeastern India

Abstract

The coalfields of Arunachal Pradesh, Assam and Nagaland are disposed along a linear belt of overthrusts where the coal-bearing Oligocene (Tikak Parbat Formation, Barail Group) strata are folded and dispersed into a number of thrust slices. Whereas, in Garo, Khasi and Jaintia hills of Meghalaya the coal seams are associated with an almost undisturbed and sub-horizontal Late Palaeocene Sylhet/Tura formations of Jaintia Group.

The coals are bright, non-banded, vitrinite-rich and have moderate to low amounts of liptinite and inertinite macerals. Pyrite and calcite are the dominant associated minerals, besides generally low proportions of clay and quartz. The coals have moderate to very high amounts of fluorescing macerals consisting chiefly of perhydrous vitrinite. liptodetrinite and resinite. Macerals, cutinite, sporinite, suberinite, exsudatinite, alginite and fluorinite are also present. The rank of the coals ranges between high volatile bituminous C and B stages, on the basis of vitrinite reflectance (Garo Hills: 0.54-0.62%: Jaintia Hills: 0.68-0.81 % and Assam: 0.70-0.74%).

The coals from northeastern India were derived essentially from tropical deciduous mangrove-mixed angiospermous forest vegetation. The coals, presumably, originated from autochthonous to hypo-autochthonous eutrophic peat accumulations in lagoons and/or near-shore back swamps in Assam. Arunachal Pradesh and Nagaland and in small isolated estuarine back swamps in Meghalaya. The vegetal maller, in general, was subjected mainly to anaerobic biodegradation under neutral to mildly alkaline subaqueous conditions and facilitated the precipitation of early diagenetic pyrite and calcite and in situ release of plant bound minerals in the peal. Seasonal freshwater ponds or lakes developed occasionally on the ancient-peat surfaces hosted the growth of aquatic and water-edge taxa whose remains are present in the coal seams. Especially pteridophytic herbaceous and shrubby vegetation growing in the vicinity of ancient peat swamps appear to be mainly responsible for the presence of structured and detrital inertinites. Occasional burning of the wet peat surfaces has been inferred to be the reason for relatively high inertinite contents in the coals from Meghalaya. It has also been presumed that whenever biodegradation of organic matter was severe the normal supply of vegetal matter fell short to produce a peat layer. instead some minor and major authigenic partings were developed within the pre-existing peal. The variation in the rank and thickness of the coal seams was presumably controlled by the prevailing geothermal gradient and tectonic conditions in different areas.