Geological study of carbonate band in Upper Bhuban Formation, Mizoram, India

摘要

The main carbonate band in Upper Bhuban unit in Bhuban Formation of Surma Group in Mizoram is exposed in an N-S trending hill range about 5 km northeast of Muthi village in Aizawl district. Representative samples from the study area were prepared for rock thin sections and were also analyzed for their major oxides by X-ray fluorescence to find out the mineral content and dolomitisation process. The limestone, which constitutes angular grains of calcite detritally accumulated with shell fragments and organic structures. The shell fragment belongs to echinoids, bryozoans and oysters in a sparitic matrix. Several foraminifera are also found in rock thin sections and it is the first time Index microfossil has been identified in Carbonate band of Upper Bhuban formation. Calcite is colourless under plane Polarized light showing distinct twinkling and low relief. Between cross-nicols, it shows high order interference colour red, blue. It exhibits perfect rhombus grains having two distinct cleavages. Organic shell structure is observed and is wholly made up of calcite. The non-carbonate constituents of the Upper Bhuban carbonate rocks comprise of quartz, microcline, biotite, phosphorite, glauconite with augite and also an opaque mineral of limonite in considerable amount. Quartz is the most abundant non-carbonate constituent mineral in the Upper Bhuban carbonate band. It is characterized by low relief between cross-nicols. It shows first order grey colour, wavy extinction and exhibits angular to sub-angular grained. The petrological studies suggest that the limestone was formed by accumulation of shell fragments with inclusion of detritus sediments. The concentration of CaO ranges from 30.812 to 43.48% and the ratio of Mg/Ca is low varying from 0.03 to 0.07 indicating that the process of dolomitization is not significant.