Sandstone petrography of the Mesozoic Jhuran Formation: implications on provenance and evolution of the Kutch rift basin.

This paper investigates the provenance of Late Jurassic to Early Cretaceous sediments in Kutch basin based on mineralogical investigations using results from thin section petrography of sandstones (QFL plot), ZTR index and mineral chemistry of heavy minerals. This study also examines the compositional variation of sandstone in the Mesozoic of Kutch during the evolution of Kutch basin. Apart from the conventional techniques, this paper presents quantification of heavy minerals in sandstones using X-ray micro-tomography, which is much simpler than conventional technique using heavy liquid. The Kutch basin at the western continental margin of India originated as a rift basin during the Late Triassic and evolved into a passive margin basin by the end of Cretaceous. Predominantly siliciclastic deposition continued from the Oxfordian to Albian age, while carbonate sedimentation dominated the Callovian. In this study. This study analyses sandstone samples of Middle Jurassic (Jhumara Formation), Late Jurassic (Jhuran Formation) and Early Cretaceous (Bhuj Formation) are analysed. While the older two formations are shallow shelf-originated, the Bhuj Formation records a transition from marine to fluvial depositional conditions. Sandstone composition records a compositional evolution from arkosic to subarkosic as the feldspar content decreases from 68% in Jhumara Formation to 27% in the Bhuj Formation through intermediate values in the Jhuran Formation. These sandstones comprise sub-rounded and moderately sorted grains with mainly monocrystalline quartz, feldspars, heavy minerals, polycrystalline quartz and lithic fragments in decreasing order of abundance. Feldspar grains are partially or completely replaced by carbonate cements, making its identification difficult in places. Quartz grains are replaced by carbonate cement occassionally. Sub-rounded to rounded heavy mineral grains indicate a multi-cyclic sedimentary provenance. Heavy mineral separation of these sandstones reveals the presence of zircon, tourmaline, rutile, garnet, apatite, monazite and opaque minerals. ZTR index calculated for these samples are 25%, 30% and 50% approximately for the Jhumara Formation, Jhuran Formation and Bhuj Formation respectively. Mineral chemistry of opaque minerals reveal ilmenite, rutile, hematite/magnetite and pyrite in decreasing order of abundances. Mineral chemistry of garnets indicates the presence of Fe-rich (Almandine) with a few Mg-rich (Pyrope) grains. X-ray microscopic study provides the percentage of heavy minerals in sandstone which ranges from 4-7%. Integration of results from petrography and heavy mineral analysis suggest a felsic to intermediate source rock of medium to high grade of regional metamorphism. The petrographic data points to evolution of the basin from an active rift to passive margin basin during Jurassic time.