Tertiary coals of Makum Coalfield, Assam, India: Petrography, genesis and sedimentation

Abstract

The two most important coal seams, viz., Seam no. 1 (18 m thick) and Seam no. 3 (6 m thick), of the Tikak Parbat Formation, Barail Group (Oligocene) are overlain by > 2000 m thick sediments of younger age. The coals are bright and non-banded in appearance. They are rich in vitrinite with subordinate amounts of liptinite and inertinite macerals. Early diagenetic pyrite and calcite alongwith clay and quartz are the main associated minerals. Under blue light excitation high amount of fluorescing macerals recorded are formed chiefly by perhydrous Vitrinite, liptodetrinite and resinite. Minor amount of sporinite, cutinite, suberinite and exsudatinite are also present with sporadic occurrence of f1uorinite and alginite. A comparison of Makum coals with the other Tertiary coals of India has also been attempted. The coals have low moisture and ash contents with high volatile matter and calorific value in relation to their rank by reflectivity measurements. The rank of the coal seams (R₀ max. 0.72-0.75%) corresponds to high volatile bituminous B stage.

From the biopetrological, palaeobotanical and geological evidences it has been concluded that the coal seams originated mostly from in situ mangrove-mixed angiospermous forest vegetation growing under humid to per-humid tropical climate. The vegetal accumulation took place in a rheotrophic swamp forming in a near-shore lagoon on a lower delta plain. The maceral and mineral associations in the coal seams indicate that the accumulated vegetal matter was mainly subjected to anaerobic microbial degradation under elevated swamp water pH (> 6). This facilitated the precipitation of early diagenetic pyrite, calcite and in situ release of plant-derived minerals in the peat. Under these conditions highly pyriteous and perhydrous coal seams were formed mainly by putrefaction. Whenever microbial degradation of organic matter was severe normal vegetal supply fell short to produce a peat layer, with the result minor and major authigenic partings within the coal seams were formed.