Petrography and Diagenesis of the Panchet Sandstones, Raniganj Coalfield, West Bengal

The Lower Tirassic Panchet sandstones of the Gondwana Supergroup in the Raniganj coalfield are chiefly immature arkose to sub-Arkose. The clastics are derived from the adjacent Precambarian granitic gneissic terrain.

Iron oxides as primary grain coat, pore filling silica and quartz overgrowth, epimatrix formed of kaolinized feldspar and biotites. kaolinite filling up the primary pore spaces and that altered at places to montmorillonite are the episodes related to early diagenesis compaction and oxy acidic pore water would be playing important part during early lithification. Orthomatrix ncoformed primary si1ica serialized feldspars and illithized montmorillonites representing the late diagenetic products assumed to have formed under burial depth around 3 km. Chlorite also a late diagenetic product and mostly confined to the lower level of the Panchet Formation was formed through combining Fe and Mg with kaolinite under reducing evironment chlorite at places transfomed as vermiculite with its probable interaction with ca²⁺ rich groundwater.

Carbonate volumetrically important next to iron oxide mostly occurs as pore filling cement or sometimes filling up the fissures developed over micritic calcite cement and both being related to the late part of the late diagenesis precipitated from the meteoric water penetrated through the post depositional fault and fractures planes. Mica peridotites too could be the sink supplying ions required for calcite precipitation. Multistage emplacement of allogenic and authigenic cements implies varying concentration of ions and fluctuating pH of the pore water during diagnesis.