INVESTIGADORES
BARREDA Viviana Dora
congresos y reuniones científicas
Título:
The K-T boundary and the associated floral event in South America. The case for Patagonia
Autor/es:
CÚNEO, R.; JOHNSON, K.; SCASSO, R.; BARREDA, V.; BRINKHUIS, H.; CLYDE, W.; GANDOLFO, M.A.; WILF, P.
Lugar:
Bonn
Reunión:
Congreso; 12th International Palynological Congress; 2008
Resumen:
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.