An appraisal of coal petrographic facies in Lower Gondwana (Permian) coal seams of India

Abstract

Petrographic facies in Lower Gondwana coals of India are characterized by coal types that depend upon the paleogeographic and tectonic framework of the coal basins, paleoclimate, the flora and fauna of the coal-forming swamps, and coal-constituent characteristics. An appraisal of coal petrographic facies of these coals was made using these parameters. With time, Permian coal (peat) seams were deposited in India over a wide range of structural and sedimentary settings and formed suites of composite genetic coal types or, in other words, petrographic facies controlled mainly by temporal, climatic and floristic differences. At the first level of resolution of petrographic facies variation, the composite genetic coal types were controlled by stratigraphic sequences namely Karharbari, Barakar, and Raniganj Formations. Then, within the coal of each stratigraphic units, further differentiation in coal facies are discernible based on petrographic types in terms of restricted paleoenvironmental and structural setting. The coal seams of the Karharbari Formation are mainly characterized by high inertinite content although several local seams with high vitrinite content are present. In contrast, Barakar and Raniganj coal seams are generally characterized by high vitrinite content, although some coal seams are rich in inertinite content. The four main petrographic coal facies found in Permian strata are: (1) fusic, rich in inertinite group macerals over vitrinite; (2) trimaceric, rich in vitrinite group macerals over inertinite macerals (‘vitro-fusic’); (3) trimaceric, rich in inertinite-group macerals over vitrinite and liptinite (‘fuso-vitric’); and (4) vitric, rich in vitrinite-group macerals. The succession of these four main petrographic facies with time was governed mainly by changes in climate and peat-forming floras and paleoenvironmental conditions in the coal swamp. Petrographic composition differs within a restricted period of time as illustrated in the model given for the Bokaro coal basin. Such petrographic coal types forming coal-facies character are mainly controlled by the regional tectonic and sedimentary framework. A considerable diversity and overlapping of coal petrographic facies is discernible with most of the coal measures of India. Yet, the main petrographic facies or composite genetic coal types were controlled by evolving flora and changing paleoclimatic patterns. The delimitation of various petrographic facies of the Indian coal seams has implications for understanding the origin and evolution of Gondwana coals.