Panandhro lignite from Kutch (Gujarat), India: Petrological nature, genesis, rank and sedimentation

Abstract

Biopetrological investigation of four lignite seams and associated sediments of Lower Eocene (Ypresian) age from Panandhro Lignite field, district Kutch, Gujarat reveals that the lignite seams, under normal reflected light, are rich in huminite macerals with subordinate amounts of liptinite and inertinite macerals. Associated mineral matter (early diagenetic pyrite, calcite, clay and quartz) are low to moderately high. Under blue light excitation, they have quite high liptinite macerals formed chiefly of liptodetrinite and resinite. Fluorinite is sporadic and exsudatinite is rare. Alginite (Botryococcus and Pleurocapsa) are common to frequent. The rank of the lignite seams and associated sediments varies between R₀ max. 0.43 to 0.44 per cent probably in response to persistently low geothermal gradient since Eocene and also shallow depth of burial. The lignite seams were formed mainly from autochthonous mangrove-mixed angiospermous forest vegetation with prolific undergrowths growing under humid tropical climate. Nonperennial herbs and shrubs, especially the pteridophytes, were mostly responsible for the formation of in situ inertinite macerals during dry periods and/or periodic lowering of water table. The peat accumulation for Seam Nos. 1, 2 and 4 took place under highly anaerobic and elevated pH (> 6) conditions of the swamp water which favoured precipitation of high amount of syngenetic pyrite and calcite. With the result vegetal matter were subjected to high microbial degradation and a fair amount of plant-derived mineral matter were released in situ for incorporation in the ancient peat. On the other hand, formation of Seam No.3 was from a peat that accumulated under conditions favourable for vegetal preservation with relatively low pH of the swamp water. Its higher pyrite content is the result of leaching from the overlying highly pyriteous grey shales. It has been presumed that minor parting bands associated with lignite seams were formed in situ from severe degradation of organic matter. The formation of authigenic partings caused splitting of a single basal seam into three separate ones which may be considered as parts of as a single composite seam. The lignite seams were formed in a lagoon from rheotrophic peat swamp near the then existing shore-line; the brackish water influence appears to have been significant.

The petrographic composition suggests that the lignite Seam Nos. 1 and 4 can be utilized better for the manufacture of organic chemicals, and the high amount of pyrite in lignite seams and associated sediments can be used for the production of sulphuric acid or even fertilizers.