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A hypothetic scenario for the Middle Jurassic in the southern part of the Neuquén Basin, is proposed on the basis of sedimentary facies and palynological data obtained from the study of eight stratigraphic sections. During the Middle Jurassic in the study area, the Los Molles, Lajas and Challacó Formations of the Cuyo Group constitute an overall prograding system deposited in a distal to shallow marine deltaic environment, with correlative fluvial deposits. The marine environment was characterized on the basis of sedimentary facies and paleomicroplankton contents (30 species). The analysis of the terrestrial palynological assemblages (102 species of sporomorphs) shows that certain plant groups dominated the scenario, in particular the Cheirolepidiaceae, Araucariaceae and Podocarpaceae and to a lesser degree Cyatheaceae, Osmundaceae, Marattiaceae, Dipteridaceae, Lycopodiaceae, Schizaeaceae, Anthocerotaceae, Ricciaceae, Cycadales/Bennettitales, Caytoniaceae and Gnetales. In addition to the conventional approach using percentage values, principal component analysis proves to be an excellent and complementary tool, because reduces the data set into definitive groups of representative taxa. Modern analogues - the Chilean-Argentinean forest and Planalto of southern Brazil- are used to explain the paleoclimatic and paleoenvironmental conditions during the Middle Jurassic. In the Planalto (about 1000 m above sea level) the Araucariaceae and Podocarpaceae grow together, but the latter disappear with an increase of temperature and humidity farther north in Brazil. The influence of acid efusives of the Serra Geral Formation conditioned the low pH of the soils. In the Neuquén Basin, rhyolites of Choiyoi Group (Permo-Triassic) and other acid rocks of the basement (Colohuincul and Huechulafquen formations) could have been the substratum of the ancient Araucariaceae. According to the above assessment, a plateaus scenario, with warm to temperate conditions and variable precipitation rates through the Middle Jurassic, is proposed. KEY WORDS. Palynology. Sedimentology. Paleoenvironment. Middle Jurassic. Cuyo Group. Neuquén Basin. Argentina.

Abstract. Based on palynological studies, paleoclimatic trends were inferred for the Paleocene-Miocene of the marginal part of the Colorado Basin, Argentina. The data show warm and humid conditions during the Late Danian (Paleocene), a decline of temperature and humidity from the ?Mid Eocene to the Oligocene. Warm, humid conditions at the end of the Oligocene and Early Miocene were recorded; dry-temperate periods alternated with warmer humid ones during the Mid and Late Miocene. Temperature and humidity decreases coincide with sea level drops at Eocene/Oligocene, Early/Late Oligocene and Miocene/Oligocene boundaries. These paleoclimatic interpretations are compatible with those derived from calcareous microfossils of Colorado Basin, fossil plants from South America and global paleoclimatic changes inferred by other authors. KEY WORDS. Argentina. Colorado Basin. Cenozoic pre-Pliocene. Palynology. Paleoenvironment. Paleoclimatical trends. Resumen. FLUCTUACIONES PALEOAMBIENTALES DEL CENOZOICO (PRE-PLlOCENO) EN BASE A PALlNOMORPHS EN LA CUENCA DEL COLORADO, ARGENTINA. A partir de estudios palinológicos realizados en el sector marginal de la Cuenca del Colorado, se determinaron las tendencias paleoclimáticas durante el lapso Paleoceno- Mioceno. Para el Daniano tardío (Paleoceno) fueron inferidas condiciones cálidas y húmedas; para el Eoceno medio, templado-cálidas y húmedas; posterior disminución de la temperatura y humedad hasta el Oligoceno temprano; un mejoramiento climático para el Oligoceno tardío y Mioceno temprano; alternancia de períodos húmedos y cálidos con episodios relativamente más secos y fríos, durante el Mioceno medio y tardío. Las principales inflexiones en las tendencias de temperaturas reconocidas presentan una notable correspondencia con los límites estratigráficos convencionales. Los límites Eoceno/Oligoceno, Oligoceno inferior/Oligoceno superior y Mioceno/Plioceno coinciden con disminuciones de temperatura y humedad relacionadas con caídas relativas del nivel del mar. Las tendencias paleoclimáticas propuestas en este trabajo son compatibles con la información disponible acerca de microfósiles calcáreos de la Cuenca del Colorado, tafofloras de América del Sur y con las fluctuaciones paleoclimáticas de carácter global. PALABRAS CLAVE. Argentina. Cuenca del Colorado. Cenozoico pre-Plioceno. Palinología. Paleoambientes. Tendencias paleoclimáticas.