Changes in space and time of Jurassic to Paleogene palynobiotas of lacustrine and distal fluvial fan deposits in Patagonia, the Neuquén Basin and the Salta Group Basin, Argentina: a review

VOLKHEIMER, W.; QUATTROCCHIO, M.E.; CABALERI, N.G.; PAULA LILIANA NARVAEZ

Barcelona

Congreso; 4º International Limnogeology Congress; 2007

International Association of Limnogeology (IAL)

The objective is to review and compare the composition and paleoenvironmental significance of the palynobiotas analysed through several case studies of: a) Middle to Late Jurassic lacustrine sequences of extraandean central Patagonia; b) Middle Jurassic deltaic swamp deposits and c) Aptian to Albian lacustrine beds of the Neuquén Basin; d) latest Senonian to early Paleogene palynobiotas of the Salta Group Basin and e) Danian lacustrine freshwater deposits in central Patagonia. The study is based on geologic field work in the areas mentioned, on palynologic sampling and on physical/chemical extraction of the palynomorphs in the paleopalynologic laboratory. Through taxonomic, palynofacial and statistic study, using optical microscopes and SEM, tendencies of biodiversity at the species level and those of suprageneric groups could be recognized, as well as paleoenvironmental tendencies, including paleoclimatic variables as temperatures, precipitations and pH. Results: For the Patagonian lacustrine basins and the Neuquén Basin, topographically elevated hinterlands with prevailing conifer vegetation (Araucariaceae and/or Podocarpaceae) are characteristic during the Jurassic. In the Salta Group Basin, the presence/absence and frequency of the coccal algae Botryococcus spp. and Pediastrum spp. is environmentally most significant and allows, in cases of very high frequencies of Pediastrum (for instance, more than 90% of the total spectrum in some levels of the Paleocene Tunal Formation), in combination with the data obtained by the study of organic matter, a clear identification of palustrine environments (vs. lacustrine). Since the Danian, the Ulmaceae (Verrustephanoporites) are dominating the arboreous vegetation giving origin to the previously characterized Ulmacean Paleophytogeographic Province, contrasting with a southern Nothofagidites Province. Conclusions: The Kingdom Protoctista is an important taxonomic group for characterizing the systems of freshwater bodies in the studied area: In Argentina, the Zygnematacean green algae are getting frequent since the Triassic and continue, with little change, through the Jurassic, Cretaceous and Paleogene, in lacustrine intervals of the sedimentary basins here considered, being valuable proxies for freshwater conditions. Since the latest Senonian, the high frequencies of water ferns like Azollopsis spp. begin to introduce changes in the freshwater lacustrine systems. Not less important is their probable symbiotic association with nitrogen fixing bacteria, living in cavities of their diminute leaves, as occurs in living Azolla spp.