CONICET | Buscador de Institutos y Recursos Humanos

The composition and textures of epiclastics rocks and the micromorphology ofpaleosoils of the Mercedes Formation, croping out to the west and south of Uruguay, havebeen studied and permited recognise provenience areas, tectonic framework of the sourceareas, paleoclimatic fluctuations and diagenetic processes that afected this unit of the UpperCretaceous of the Parana Basin.Compositionally from the epiclastic samples two areas with different proveniencehave been diferenciated with a transitional boundary between them. Towards the northvolcanic clasts form the Arapey Formation domain, and a secondary source from the GuichónFormation (sandstones) and Precambriam basement (metamorphic and plutonic rocks) weredocumented. Conversely, in the centre-south the Precambrian clasts are dominant, andfragments from the Arapey Formation are scarce, while Guichón Formation is not a sourceunit. The tectonic framework is cratonic, from interior to continental transitional.Stable grains are more abundant than metastables and instables which indicate thatsediments where deposited under warm and humid climates. The increasement ofmetaestables altered clasts in the centre-south, suggest a non distant source area.The diagenetic processes did not occur uniformly resulting in a varible porosity, with apredominance of secondary porosity by dissolution . The recognized processes are clay andiron oxides illuviation, clay neoformation and recristalization, carbonate and silicaprecipitation and disolution.The uppermost section of the unit contain paleosoils, related to the complete infillingof the basin and tectonic quiescence, that may suggest a climatic change, having in mind thepresence of palygorskyte and illuvial clays that point out a semiarid seasonal climate.