CONICET | Buscador de Institutos y Recursos Humanos

The K-T event and related floras in South America are poorly understood due to the almost lack of preserved terrestrial biotas. This is the case for Patagonia, affected by an extended K-T marine transgression. However, the Lefipán Fm. includes lower and middle horizons that preserve an end-Cretaceous macroplant assemblage. This unit represents a complex of tidally influenced deltaic deposits that show a shift from low to high stand sea level conditions. We report from here plant taphocoenoses with more than 1.000 collected specimens. In order to refine the age control an analysis of the associated palynological content, including pollen-spores and dinoflagellates was carried out. Likewise, a magnetostratigraphic study based on samples that retain a strong and stable remnant magnetization of mostly normal polarity, has also been performed. Geochemical analyses were also attempted in order to estimate possible shifts across the K-T boundary. Plant assemblages from the lower Lefipán Fm. at El Loro section, are Maastrichtian in age and occur in different horizons probably associated to upper delta plains. There, conifer twigs, seedcones and isolated seeds of araucarian alliance are dominant. Cutinized dicot leaves are also found, including aquatic nelumbonaceae. The associated palynological assemblage is entirely terrestrial suggesting fresh water environments. Plant assemblages from the middle Lefipán Fm., at San Martin section, occur 250 m above the base. These assemblages are considered latest Maastrichtian in age and occur in the uppermost 50 m of the Cretaceous part of the section, which probably represent the last 300 ky of the period. The plant record here shows allochthonous characteristics and occurs associated to lower delta plains with tidal influence but still under low stand sea conditions. Dicot leaves represent the dominant plant remains including at least 100 different morphotypes. Conifers (mostly podocarps), monocots, ferns and ginkgos complete the spectrum. Based on leaf margin and leaf area analyses the dicot plant assemblage suggests warm temperate or subtropical paleoclimatic conditions. Pollen and spores dominate the associated palynological record but dinoflagellates are also present suggesting mixed depositional settings. This event is suddenly interrupted by high stand level conditions as part of the K-T events, precluding envisaging major changes that affected the terrestrial vegetation. This can only be partially seen from the palynological terrestrial record that survives the boundary and shows significant changes.