Occurence of dolomitic limestones from a restricted and structurally complex region of Kallankurichchi Formation, Ariyalur Group, South India is being documented in this paper. Field, petrographic, mineralogic and geochemical characteristics of the rocks suggest that these dolomites are diagenetic in origin and are developed during the late stage in a partially-closed system under oxygenated and partially-confined meteoric waters. Due to semi-closed system of diagenesis, stabilization of high magnesian calcite into low magnesian calcite concomitant with expulsion of Mg2+ to diagenetic waters, source for magnesium is local and available nearby. Poor dolomitic conditions in terms of weak diagenetic fluids, less bulk chemical difference between diagenetic components and susceptibility of micritic mud to alteration have led to the dolomitization of only the matrix portion of limestone.
The Kallankurichchi Formation of Ariyalur Group forms a major carbonate unit of Cauvery basin. This formation had been studied extensively in terms of its faunal content and biostratigraphy (Rao, 1956; Sastry et al., 1968, 1972; Bhatia, 1984; Ayyasamy, 1990), fauna and ecology (Guha, 1987; Radulovic and Ramamoorthy, 1992; Petrovic and Ramamoorthy, 1992; Ramkumar and Chandrasekaran, 1994; and Hart et al., 2000) and depositional environments based on taphonomy, gross lithology, sedimentary structures, and geochemistry (Sastry et al., 1972; Nair, 1974, 1978; Sundaram, 1977, Ramkumar and Chandrasekaran, 1994; Ramkumar, 1995, 1996 a, b, c, 1997, 1999a, b, 2001, 2004; Guha and Mukhopadhyay, 1996; Ramkumar et al., 1996a, b; and Fürsich and Pandey, 1999). However, the reports on its diagenetic characteristics are scarce. This paper attempts to unravel the mode and conditions of dolomite precipitation in an otherwise exclusively calcitic skeletal limestones.
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