Análise bioestratigráfica do intervalo Llandoveriano da bacia do Paraná no Paraguai Oriental

BIOSTRATIGRAPHIC ANALYSIS OF THE LLANDOVERY IN THE PARANÁ BASIN, EAST PARAGUAY.
A refinement of miospore and chitinozoan zones has been achieved based on assemblages in boreholes 269-R1 and 269-R2, east Paraguay. The sediments studied belong to Itacurubi Group of Paraná basin. The miospore zone Imperfectotriletes (Early Aeronian and Early Telychian) corresponds to the lower part of the chitinozoan zone Conochitina elongata (Aeronian/Eotelychian). The chituis-nanus miospore zone (Telychian/Wenlock), corresponds to the Zone of Salopochitina monterrosae – Conochitina proboscifera (Early Telychian and Early Sheinwoodian), and the upper part of the Conochitina elongata zone (subzone Conochitina proboscifera – Desmochitina cf. D. densa, transition of Early Aeronian to Early Telychian). The chitinozoan assemblage observed in the upper part of the boreholes made it possible to establish a more detailed zonation, involving the early Telychian. The diversity of species and the number of described individuals in the boreholes, have an inversely proportional relation between the trilete spore and envelope-enclosed cryptospores. While the diversity and the number of the trilete spores increase upwards the borehole, the envelope-enclosed cryptospores show an inverse relation. A reduction in diversity and abundance occur also upwards in the borehole. The tetrads identified in borehole 269-R1 have an anomalous size variation when compared with those earlier described. The peculiar characteristic of the species, observed in this study, could be associated with specific ecological factors in this part of the Gondwana occidental, which were situated in peri-glacial conditions during the studied interval.