IANIGLA   20881
Unidad Ejecutora - UE
congresos y reuniones científicas
Ordovician acritarch biodiversity of the western Gondwana active margin (Argentina): an overview
Lille, Francia
Congreso; Palaeozoic Climates - International Congress; 2008
Institución organizadora:
UMR 8157 Géosystèmes Lille 1-CNRS, Université Catholique de Lille
<!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} @page Section1 {size:612.0pt 792.0pt; margin:70.9pt 70.9pt 70.9pt 70.9pt; mso-header-margin:35.45pt; mso-footer-margin:35.45pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> 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.