Porcellanite and Chert from Lodhwara Hill, Chitrakoot District, Uttar Pradesh: Insights from Field Observations and Petrographic Study

This study reports the presence of interbedded porcellanite and chert horizons within glauconitic sandstone at Lodhwara Hill (latitude: N 25°14’57’’ and longitude: E 80°54’59’’) of Chitrakoot District, Uttar Pradesh, where the Lower Vindhyan Supergroup (Semri Group) is well exposed. These porcellanite and chert bands deposited in the Semri Group are of the Mesoproterozoic age. Based on field appearance and mineralogical study, the characteristics of porcellanite and chert from the present study area are documented. The physical characteristics (field observations) reveal that the porcellanite appears as a creamy white colour band with low hardness, while chert occurs as a light brown to dark brown and grey colour band with higher hardness, a waxy lustre, and a conchoidal fracture. Petrographic analysis shows that porcellanite predominantly contains microcrystalline quartz and a few feldspar and mica minerals, whereas chert is composed of cryptocrystalline quartz. The associated glauconitic sandstone comprises quartz, rounded to subrounded pellets of glauconite, feldspar and mica. In glauconitic sandstone, glauconite minerals are found to have formed through the replacement of feldspar substrate, indicating their authigenic origin. The present study suggests that the cherts and porcellanites are deposited in shallow marine environments during the deposition of glauconitic sandstone in the Vindhyan basin. The deposition of these layers, along with glauconitic sandstone, infers an additional supply of silica. It is postulated that the siliciclastic input from volcanic activity in seawater facilitated chert and porcellanite formation. The silica dissolved in seawater is precipitated to form chert, while the precipitated silica becomes mixed with the intermittently supplied feldspar, mica and clay minerals as impurities, the layer of porcellanite is formed. Moreover, the transportation of tuffaceous materials and their deposition in seawater is suggested as an alternative mechanism for the formation of porcellanite.