X - ray diffraction analysis allowed us to compare the nature of the clay minerals of the Central Depression of Rio Grande do Sul (Southern Rosary Formation, Santa Maria Formation and Caturrita Member of Botucatu Formation) with those of Cenozoic rocks (Tertiary? partially cover the former. In the mesozoic rocks montmorillonite predominates, followed by the illite and the IM mixed layer (10-14M). In the Cenozoic sediments, on the other hand, there is a predominance of kaolinite, with small percentages of ilite, IM mixed layer (10-14M) and montmorillonite. This difference aids in the distinction of the two sequences, where the lithologies are similar. The predominance of montmorillonite in the Mesozoic and that of kaolinite in the Cenozoic can be explained by climatic difference and / or other factors. In case of climatic difference,
