Petrography and Geochemistry of Pre-Ariyalur Sequence in Perambalur District, Tamil Nadu-Implications on Depositional Environment and Palaeoclimate

The Maruvattur quarry section (Dalmiapuram Formation) is located near the Archaean-sedimentary boundary at the western margin of the Ariyalur-Pondicherry sub-basin. The lithofacies observed in the quarry include boulder-algal limestone and clastic limestone alternating with siltstone and claystone. This amply reveals that the depositional conditions fluctuated between carbonate facies to fine clastic facies. The coarse clastic materials are not found in the litho-sequence suggesting that the source area reached a peneplanation stage. This stage released only fine clastic materials to develop silty clay and siltstone lithologies. The clastic deposition was fine grained, hence it is interpreted that the depositional basin experienced a calm environment. The limestone units are mostly bedded types pointing out that the older biohermal limestone underwent erosion and redeposited as clastic limestone to form biostromal limestone. The clastic limestone has a few impressions of trace fossils suggesting the slower rate of sedimentation in a comparatively deeper marginal basin.

The Anaipadi river traverse representing Garudamangalam Formation indicates a variable lithological association along the dip direction. The claystone is the bottom most unit of this traverse. The clastic deposition is followed by a muddy carbonate phase, which further indicates a calm neritic environment that existed for sometime before deposition of subarkosic-Arkosic lithofacies towards the top in the traverse. The varied distribution of clastics has been useful to synthesise information on tectonics and sedimentation.

The trilinear plots such as Q, F, L (quartz, feldspar and lithic fragments), and Qm, K, P (quartz monocrystalline, potash and plagioclase feldspars) derived through modal analysis suggest a granitoid source area. The petrographic study and modal analysis suggest that the bottom quartz rich sequence formed in a semi-humid type climate, which gradually became semi-Arid as evidenced by the subarkosic-Arkosic sands towards upper level along the Anaipadi river traverse.