PETROGRAPHY AND DIAGENETIC HISTORY OF NAGRI FORMATION SANDSTONE IN DISTRICT BAGH AND MUZAFFARABAD, PAKISTAN

The petrographic analysis of Nagri Formation sandstone based on outcrop study and interpretation of thin sections with special emphasis on the diagenetic history from widely separated areas was accomplished. The calcite concreations, hematite leaching, petrified wood fossils, spheroid weathering, quartz filled joints and the meso scale drag folding has few geological significances. Interpretation of thin sections revealed that the sandstone is lithic arenite and lithic graywackes. The concavo-convex, sutured contacts and muscovite deformations indicates that the compaction of sand under lithostatic stress during early diagenesis. The pore water has widened the intergranular distance in some rocks by dissolving the fractured surfaces of carbonates and other grains, enhancing the concentration of carbonates in water at high pH. On the other hand, the silica dust formation can be attributed to the abrasion of quartz grains during compaction, alteration of feldspar to sericite and overgrowth on quartz by calcite under low pH of solution and pressure solution. The leaching and precipitation of hematite along fractures and thin coating around the grains indicates iron oxides paragenesis from chlorite and biotite in the area. The clay mineral authigenesis includes replacement along the cleaved and fractured feldspar and sericitation. Further elaborations indicate plastic deformations of hydrolytically weak quartz grains and deformations by tectonic upliftment, enhancing the permeability and reservoir quality of sandstone.