Among several lithostratigraphic subdivisions of the Gaj 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 coefficient (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 deposition. 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.
Key words trace element - geochemistry - sedimentology - Jhill limestone - Gaj Formation - Cape Monze - Karachi