THE IRATI FORMATION OUTCROP IN PASSO SÃO BORJA (UPPER PERMIAN, PARANÁ BASIN, SOUTHERN BRAZIL): PALEOBOTANY, PALYNOLOGY AND STRATIGRAPHY

The Passo São Borja outcrop is located to the west of São Gabriel City, on the right bank of Santa Maria River in Rio Grande do Sul State, southern Brazil. The outcrop is represented by a succession of bituminous and non-bituminous black shales and siltites, interbedded with light-colored carbonate levels, characteristic of the Irati Formation(Upper Permian) of Paraná Basin. Permineralized log fragments assigned to Barakaroxylon (Surange and Maithy) Kulkarni, Maithy and Surange 1970 were recovered from different stratigraphic levels in the outcrop. This morphogenus is related to the Barakar Stage (Damuda Series) in India. No Barakaroxylon logs have been recovered thus far from the mesosaur-bearing beds of the Whitehill Formation of South Africa, in contrast to the Irati Formation of southern Brazil where Barakaroxylon logs are commonly found in association with mesosaur remains.

Irati Formation palynomorphs recorded both in carbonate levels containing the fossil woods and in overlying black shales characterize a hydro-mesophylic assemblage dominated by gymnosperms (Coniferophyta and Pteridospermophyta), with subordinate pteridophytes (Filicophyta). The presence of distinct growth rings in the studied Barakaroxylon logs allows one to infer climatic conditions characterized by well-defined seasons, with cyclical variations from hot and dry summers (suggested by the relatively narrow growth rings) to rainy and humid winters of short duration (suggested by the narrow late logs). Such climatic variations are similar to those verified in present-day 30° to 45° N and S latitudes, comprising central and southern California, the Mediterranean zone, the western and southern coast of Australia, the Chilean coast, and southern Africa.