Temporal and spatial variations of the type and rank of Gondwana coals of India

Abstract

Gondwana coals associated with Permian sediments in peninsular India vary widely but there is a distinct tendency of variation to be diagnostic both in time and space. The three maceral groups (Vitrinite, Liptinite and Inertinite) which are derived from diversified plant material exist in the Gondwana coals. However, maceral composition is controlled by the type of plant material available for accumulation as peat and by Eh and pH of the ancient swamp. These may all be in part controlled by the period when the peat was formed, climate and sedimentary environment. These variables seem to be inter-related rather than independent. The coals that occur in Karharbari, Barakar and Raniganj formations differ significantly in their nature and relative distribution of coal constituents.

The coals, deposited under wide range of tectonosedimentary settings of Permian Gondwana sequence, reveal fusic, trimaceric and vitric coal types and by and large associated with different lithologic sequences in ascending order. Provincialism can be discerned both in relation to lateral variation of individual seams and in differences between the modal composition found in different coal measures. Significant variation occurs within the individual seams, however, they retain some degree of characteristic petrological identity. Different seams in a given stratigraphic unit may differ widely in their petrographic composition but are characterized by their diagnostic modal composition. The coals of Damodar, Son-Mahanadi, Pench-Kanhan and Wardha-Godavari basins provide good examples for the temporal and spatial variation of the coal seams.

Coal rank assessed has shown significant variation in rank both in space and time ranging from high volatile sub-bituminous to low volatile bituminous stages including coking coal types. The wide range of coal rank recognized in the Gondwana of India is controlled by geothermal gradients and tectonic features. These have affected the extent of coalification by controlling the heat flow to which coals have been exposed. Tectonism may also have exercised some control over coal types (rank types) but only as a part of a complex mechanism which also involved sedimentary environment, the climate and the flora at any given time.