NON- POLLEN PALYNOMORPHS IN THE FOSSIL RECORD OF MESOZOIC BASINS IN ARGENTINA: POTENTIAL TOOLS FOR BIOSTRATIGRAPHY AND PALEOECOLOGY

PRAMPARO, MERCEDES B

Amsterdam

Workshop; 5th Workshop on Non-Pollen Palynomorphs; 2012

Faculty of Science, University of Amsterdam

In fossil lacustrine sediments the algae represent the autochthonous vegetation in the water body and their presence in the palynological samples provides useful information such us: salinity, nutrients loading, chemical conditions, etc. Among the algae, certain species have a great potential to be preserved in the fossil record because they produce resistant outer organic walls and /or cysts during their life cycles. As an example, fossil representatives of Chlorophyta are reported in palynological literature since lower Paleozoic and zygospores of Zygnematalean algae are known from the upper Paleozoic, many of them persisted until today. However, some special forms like Syndesmorion and Plaesiodictyon have a restricted biocron. There are several records of palynofloras, with abundant or sometimes dominant algae, in the Mesozoic of Argentina. Syndesmorion stellatum (Fija³kowska) Foster and Afonin, a chlorophycean algae characterized by a high polymorphisms (great variety and sizes of coenobia), could be a very good estratigraphic marker for non-marine deposits as it is restricted to the Permian-Triassic boundary in the world. It is recovered from a late Permian lacustrine formation in central-western Argentina (Prámparo et al. 2009).The different types of coenobial shapes may represent either different stages of their development or, could reflect responses to various environmental stresses relating to changes in pH, salinity, nutrient, and light levels (Foster and Afonin 2006). That could be case of the Permian argentine basin as was indicated by the sedimentary facies (mudstones and hypersaline evaporitic deposits). In the Triassic, the lacustrine phytoplankton described from the Cuyana basin (western Argentina) consist of representatives of colonial chlorococcalean algae belonging to the Hydrodictyaceae (Plaesiodictyon) and Botryococcaceae (Botryococcus), and Zygnemataceae zygospores represented by Gelasinicysta?, Lecaniella, Mougeotia, Ovoidites, Peltacystia and Schizocystia. Plaesiodictyon is a type of algae that are known solely as fossils only from Triassic sediments. Based on the algae two different stages in the evolution of the lacustrine basin could be recognized (Zavattieri and Prámparo 2006). Continental fresh water sediments from different lower Cretaceous localities (Barremian to Albian in age), with Scenedesmus as an important component, associated with Crucigeniella, Tetrastrum, Ovoidites, Tetraedron, Leiosphaeridia, were analyzed (Prámparo 1999). The complete study of each group of algae and the interpretation of their relative abundance, allowed us to infer paleoenvironmental conditions of the basins.