Investigação paleontológica do evento hidrotermal de Anhembí (SP) na Formação Teresina (Grupo Passa Dois, Permiano, Bacia do Paraná)

Abstract

In the Anhembi County, siliceous cones occur in the Teresina Formation (Permian) which are now interpreted as accumulations generated fìom a hydrothermal event that took place in shallow waters. The occurrence of such hydrothermal deposits is very rare in the geological record, making this deposit unique, especially when considering that more than 4,500 of these siliceous cones outcrop in less than 1,5 km2. The remarkable characteristics of this locality have made it an important object of scientific investigation, aside from suggest the comparison with the supposedly most ancient life forms ever found, fiom 3.5 Ga, deposited in similar context. This work consists of an analysis of the fossils found in intervals below, laterally continuous and above the place where such cones outcrop, an evaluation whether possible changes in the biota in this area are due to this hydrothermal event, and also to perform an analysis of the state ofthe controversy around Earth's most ancient microfbssils. Three diffcrent intervals (4, B and C) in which samples from three distinct levels (1,2 and 3) were collected were analyzed both macroscopically and pehographically. In the Interval A (Level l) we have observed bivalve shells, ostracod valves, oncoids and {ïagments of fish teeth including a big fragmented tooth of a dipnoi fish. In Interval B (Level 2) were observed only ostracod valves and oncoids. In Interval C (Level 3) were found ostracod valves, oncoids and bivalve shells. The silicification that took place in these 2 initial intervals difler from the third one, which suggests a diflbrent event for the silicification of the lower strata. The apparent dill'erence found in the biota fiom the three intervals strongly suggests a connection of the biota with the hydrothermal event that took place in that locality. The presence of 3.5 Ga microfossils is not entirely discarded by this work, though one must be cautíon about the reservations about the biogenicity of such microlbssils.