Petrography, diagenesis and depositional environments of Middle Jurassic Jaisalmer Carbonates, Rajasthan, India

Abstract

The Middle Jurassic carbonate rocks exposed around Jaisalmer are mainly wackestones, packs tones, grains tone and a few mudstones. The framework building materials are i) the skeletal fragments of algae, mollusc, brachiopod, echinoid, foraminifera, rare sponge and corals and ii) the non-skeletal particles of pellets, oolites, aggregate grains, intraclasts, extraclasts and terrigenous materials. The main authigenic constituents of these rocks are various pore filling cements and other later formed neomorphic material. Compaction and authigenesis are the main diagenetic changes observed in these rocks. The approximate sequence of diagenetic events include i) deposition of bioclasts and other sediments with Mg-calcite and aragonite, ii) micritization and formation of micritic coats over particles, iii) intergranular cementation including fibrous calcite and syntaxial overgrowths, iv) neomorphic inversions—aggrading and degrading, v) dolomitization and finally vi) silica replacement. The depositional environment of these carbonates characterizes an early littoral shallow marine condition influenced by fresh water influx, followed by the middle stable marginal marine environment and the final restricted lagoonal high energy environment marking the close of Jasialmer sedimentation.