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A study, involving mainly X-ray diffraction, of the mineralogy of Upper Cretaceous and Lower Tertiary deposits (Aroifilla, Chaunaca, El Molino, Santa Lucia and Cayara Formations) of the Andean basin in the Bolivian Eastern Cordillera is presented. This work refines environmental reconstructions based on previous sedimentological studies in the Potosi Basin. The results including the clay mineralogy show that the sediments were deposited in a lacustrine system which suffered large variations of; water level and groundwater influence. The siliciclastic sediments are characterized either by abundant feldspars or analcime, suggesting a perennial volcanoclastic influence. Tuff layers in the Santa Lucia Formation support this interpretation.

The great diversity of clay minerals identified reflects both detrital influences and diagenetic overprinting. Although diagenetic changes are common in this sedimentary series, it is still possible to identify the nature of detrital supply. Illite and illite- smectite mixed-layer clays constitute the background clay sedimentation and reflect predominantly arid climate. This had already been deduced from sedimentogical and geochemical investigations. However, detrital kaolinite occurs in a 40 m-thick interval of the El Molino Formation which probably embraces the K/T boundary and reflects more humid conditions. Further work is necessary to locate more precisely this boundary in the absence of accurate biostratigraphical markers. Authigenic clays are also recognized: in the sandstones in the basal part of the Cayara Formation kaolinite developed authigenically; in the tuff layers of the Santa Lucia Formation well-crystallized authigenic smectite characterizes the clay fraction. In addition, chlorite replaces smectite in limestones beds of the Fl Molino Formation which were deposited in evaporitic environments. The clay assemblages including illite and chlorite at base of the stratigraphic column (Aroifilla, and Chaunaca Formations) probably results from the replacement of smectitic minerals either by chlorite or illite where depth of burial reached more than 2000 m. (C) 2000 Elsevier Science B.V, All rights reserved.