Clay Mineral Assemblages and Rare Earth Element Distribution in the Sediments of Ariyalur Group, Tiruchirapalli District, Tamil Nadu-Implication for Paleoclimate

Illite and smectite are the major clay minerals, whereas kaolinite and chlorite are present in small amounts in the sedimentary rocks of Ariyalur Group. The samples belonging to Ariyalur Group exhibit LREE enriched and flat HREE patterns. These samples show high ratios of CLREEIZHREE (7.65 to 19.42) which indicate that these sediments were derived from the felsic source rocks. The persistence of uniform REE patterns for the clastic samples of four formations of Ariyalur Group distinctly implies a hoinogenous nature of the source rocks. The Sillakkudi and Kallankurichchi Formations (Campanian - early Maastrichtian) exhibit high content of illite which indicate that the sediments were derived from pre-existing rocks (granites and gneiss), subjected to physical weathering in a tempcrate climate. A sudden change in illite content was noticed in Ottakkovil Formation, which probably points out a change of climate from cold to warm during middle Maastrichtian. The abundance of pedogenic smectite in the Kallamedu Formation suggests warm climate with drylwet cycles.