Lacustrine and fluviolacustrine deposits of the Charo sequence crop out extensively along tens of kilometres, and they overlie the Miocene volcanic sequences of the Mil Cumbres Complex in an E-W oriented basin. Eastwards of Morelia city (central Mexico), layers of non lithified clayey sediments of about 3 m in thickness are overlaid by massive diatomite deposits developed during the Miocene-Pliocene in the Cuitzeo palaeolake. Major outcrops are located south and east of the village of Charo at the Las Pulgas, Lomas Blancas and Tzitzimeo sites; all having similar mineralogical facies and stratigraphic correlation. Field studies, petrography and X-ray fluorescence spectrometry (XRF) indicated that the parent rock was mainly of andesitic composition with minor dacitic-rhyolitic components. The source of the raw material was located in the upper topographic levels of the present horst-graben system. The diagenetic clays were identified as smectites. The central part of the Las Pulgas column shows a beidellite-nontronite (B-N) association. Likewise, 27 Al-NMR and Mossbauer spectroscopies confirm the B-N character. Interlaid and overlying diatomite strata indicate a lacustrine environment, favourable to the development of secondary clays. The stability of lacustrine conditions gave place to an incipient stratification (smectite-illite), better developed in the middle part of the clayey levels.