INSTITUTO ARGENTINO DE NIVOLOGIA, GLACIOLOGIA Y CIENCIAS AMBIENTALES
Unidad Ejecutora - UE
congresos y reuniones científicas
THE EVOLUTION OF EARLY PALAEOZOIC MARINE PHYTOPLANKTON AT THE ACTIVE CONTINENTAL MARGIN OF WESTERN GONDWANA (ARGENTINA)
CLAUDIA VIVIANA RUBINSTEIN; VICTORIA JOSEFINA GARCÍA
Congreso; 4th International Palaeontological Congress. The history of life: A view from the Southern Hemisphere; 2014
The southern South American margin of Western Gondwana was active from the Neoproterozoic to the Mesozoic, until the break-up of the palaeocontinent. In this context, during the early Palaeozoic, this margin was strongly affected by successive episodes of subduction and accretion of terranes. Consequently, the basins related to this margin had a complex geodynamic history that should have influenced diversity trends of marine organic-walled phytoplankton. The oldest marine phytoplankton assemblage, low in abundance and diversity and poorly preserved, comes from the Late Cambrian of the Cordillera Oriental, Central Andean Basin. The Early Ordovician displays a gradual increase of diversity during the Tremadocian, with the remarkable presence of the Gondwanan characteristic messaoudensis-trifidum acritarch assemblage in the Late Tremadocian. The highest Ordovician diversity corresponds to the Floian platform facies of the Central Andean Basin. The Middle Ordovician shows a progressive decline of diversity. However, it must be taken into account that the Dapingian and Darriwillian marine phytoplankton comes mainly from estuarine-deltaic facies of the outermost part of the proto-Andean foreland basin. The diversity dramatically diminishes during the Sandbian and Katian in coincidence with thickening-coarsening facies related to the progradation of a deltaic system. The Hirnantian glacial-related deposits record a new peak in diversity throughout all the foreland basin systems of the Central Andean Basin. The peri-Gondwana volcanic arc deposits of the Famatina System yielded rich and diverse Lower to Middle Ordovician phytoplankton assemblages, even if they show noteworthy differences with coeval assemblages of the Central Andean Basin. The Ordovician of the Precordillera terrane, marked by its accretion to the Gondwana margin, yielded sparse and poorly diversified Middle to Late Ordovician phytoplankton assemblages. During the Silurian, the marine phytoplankton flourishes with an increase of diversity starting from the Llandovery in both the Precordillera and the Central Andean foreland basins. A new biostratigraphic scheme recently proposed for the Silurian deposits of the Precordillera allows the recognition of the Wenlock, highly questioned in this basin. The diversity of the marine phytoplankton during the Wenlock remains quite similar. The highest marine phytoplankton diversity of the whole Silurian is documented in the Ludlow of the Precordillera, particularly in shaly-predominant sediments that correspond to a low-energy open shelf environment. Towards the top of the Silurian sequence, the transition to an inner shelf exposed to storm activity and a shoreface, is accompanied by a decrease of the diversity in the Pridolian as well as in the Lower Devonian.