Diagenesis and Porosity Evolution of the Pachmarhi Sandstones (Early Triassic), Satpura Gondwana Basin, Central India

Abstract

This study deals with the diagenetic history of the Pachmarhi Sandstones of Satpura Gondwana Basin, Central India. The sandstones are classified as quartz-arenite and sub-arkose type. Monocrystalline quartz grains dominate the detrital mineralogy followed by polycrystalline quartz grains, feldspars (more alkali-feldspar than plagioclase), rock fragments, detrital mica and heavy minerals. The diagenetic signatures observed in the Pachmarhi Sandstones include mechanical compaction, cementation (Fe-oxide, quartz overgrowths, calcite, matrix and clay minerals), replacement and dissolution of feldspar and calcite cement. The most commonly observed type of secondary porosity is dissolution of feldspars. The porosity loss is mainly due to cementation by pore occlusion and by early stage of mechanical compaction. In addition, several clay minerals occurred as pore-filling and pore-lining cements. The nature of various types of grain- to grain contacts suggests early cementation and consequent minor compaction. Among the various cements, calcite is the earliest followed by iron oxide while silica cementation occurred probably at a late stage. The carbonate cement formed during burial by dissolution and re-precipitation represents redistributed calcite which was buried with the sandstone. The iron cement was perhaps derived from weathering and leaching of ferromagnesian minerals of overlying Deccan traps. Silica cement was derived from the corrosion of quartz and feldspar grains. Types of grain contacts, minus-cement porosity and porosity reduction indicate a shallow depth of burial conditions for these sandstones.