IANIGLA   20881
INSTITUTO ARGENTINO DE NIVOLOGIA, GLACIOLOGIA Y CIENCIAS AMBIENTALES
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Ordovician acritarch biodiversity of the western Gondwana active margin (Argentina): an overview
Autor/es:
RUBINSTEIN, C. V., SERVAIS, T., VECOLI, M & DELABROYE, A.
Lugar:
Lille, Francia
Reunión:
Congreso; Palaeozoic Climates - International Congress; 2008
Institución organizadora:
UMR 8157 Géosystèmes Lille 1-CNRS, Université Catholique de Lille
Resumen:
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The major Ordovician basins in Argentina were developed in a pericratonic area of
western Gondwana, which was an active margin since the Early Cambrian (Andean
Belt). Continuous subduction occurred along the Proto-Andean margin, partially
related to the development of wide retroforeland basins. The three main
sedimentary basins are the Central Andean Basin
(north-west Argentina,
extending into Chile, Bolivia and Peru);
the Precordillera Basin (central-west Argentina,
at the foothills of the south-central Andes), and the FamatinaBasin, situated between the northern
end of the Precordillera and the southern end of the Central Andean
Basin. The continuous link of
depositional settings and the gradual changes occurring across the Central Andean Basin evidence a foreland basin
system, characterized by a double feed system with sediment input from the Puna
arc, at the west, and an important imput at the east, related to major deltaic
complexes coming from the craton (Astini & Marengo, 2006). The Precordillera basin is
considered as a lithospheric block rifted from SE Laurentia in the early
Cambrian, and amalgamated to the Famatina arc (W Gondwana margin), in mid-late
Ordovician times (Thomas & Astini, 2003). The peri-Iapetus volcanic-arc depositional systems
represented in the Famatina
Basin were related to the
eastward subduction under the proto-Andean margin and the approach and subsequent accretion of the
Precordillera Terrane. In
light of this complex geodynamic scenario, it is difficult to compare acritarch
diversity, speciation and extinction trends in such different basins. Moreover,
sea level curves of the Proto-Andean basins can only be drawn up reliably by
basin, or even by different areas of the basins (i.e. Puna, Cordillera
Oriental, Sierras Subandinas), because of the intense tectonic activity. Nevertheless,
major and short-lived eustatic events, like glacially-induced Hirnantian one
may stand out from local fluctuations. The majority of palynological studies
have been focused on Tremadocian and Floian strata from the platform facies of
the Cordillera Oriental, rich in macrofauna,
where most of the acritarch assemblages have good independent age control.
Palynological analysis on the Cambrian/Ordovician boundary have not been
successful. Only sparse data from the late Cambrian and early Tremadocian,
corresponding to poorly preserved acritarchs, come from the Cordillera
Oriental. Acritarch diversity increases in middle to late
Tremadocian, flourishing during the Floian, also in Sierras Subandinas and
Famatina. Particularly in Famatina, an abundant and diverse acritarch
assemblage occurs at the uppermost section of the Lower Ordovician (Time Slice
2c). Acritarchs from the Middle Ordovician of Famatina are less diverse and the
lack of biostratigraphic markers prevent the restriction of the age. Middle Ordovician
acritarchs also occur in marginal marine facies, where their abundance and
diversity only reflect local palaeoenvironmental conditions. Low-diversity
acritarch assemblages may correspond to the Dapingian. Diversity and abundance
of acritarchs increase in the Darriwilian. In the Precordillera Basin
low-diversity acritarch assemblages have been found in levels independently
dated as Darriwilian to Sandbian. Late Ordovician glacial-related strata from Cordillera Oriental, Sierras Subandinas and Precordillera
yield poorly preserved and low diversified acritarch assemblages characterize
by abundance of reworked taxa from Early to Middle Ordovician sediments.