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 chlo rite are present in small amounts in the sedimentary rocks of Ariyalur Grou p. The samples belonging to Ariyalur Group exhibit LREE enriched and flat H REE patterns. These samples show high ratios of Sigma LREE/Sigma HREE (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 elastic samp les of four formations of Ariyalur Group distinctly implies a homogenous na ture of the source rocks. The Sillakkudi and Kallankurichchi Formations (Ca mpanian - early Maastrichtian) exhibit high content of illite which indicat e that the sediments were derived from pre-existing rocks (granites and gne iss), subjected to physical weathering in a temperate climate. A sudden cha nge in illite content was noticed in Ottakkovil Formation, which probably p oints out a change of climate from cold to warm during middle Maastrichtian . The abundance of pedogenic smectite in the Kallamedu Formation suggests w arm climate with dry/wet cycles.