Geochemistry and Sedimentology of Jhill Limestone of Gaj Formation, in CapeMonze and Adjoining Area, Karachi

Among several lithostratigraphic subdivisions of the Ga Formation of Miocene age,the Jhill limestone is entirely different with respect to its colour, texture and structures. This limestone unit has been evaluated to elaborate its geochemical and sedimentological characteristics. The distribution of various elements in the acid-soluble fraction has been studied in order to determine their mineralogy, sedimentary environment, facies and diagenesis. Mineralogy,recrystallization and other diagenetic changes are the main factors affecting the distribution of trace elements and their mutual relationships in the limestones. Samples of the Jhill limestone show depletion in large-sized ions (Sr, Pb & K) and also in the ions that are not compatible with calcite space group. Elements (Fe, Mn, Zn, Cu & Co) having distribution eoefficient (D) above unity for natural calcites, are more enriched. Microscopic and X-ray studies revealed nearly complete conversion of aragonite into stable low-Mg calcite.An attempt has also been made to verify the reefal conditions for these limestones on the basis of geochemical studies. The plots of Sr and other facies-indicator elements show that the majority of the beds belong to forereef flank facies with some algal banks. Low Mg/Ca and Sr/Ca ratios suggest that a phreatic diagenetic environment prevailed after the deposilion. High concentrations of Cr, Ni, and Co in the Jhill limestone show a positive correlation with a higher amount of insoluble residue, which reflects a relatively high rate of influx of terrigenous material.