Geologia e petrologia da formação Irati (Permiano) no estado de São Paulo

Abstract

The results of geological, petrographic and sedimentologic studies on the Irati Formation (Permian of the Parana Basin, Brazil) are presented in this paper. It was possible to formulate some hypothesis concerning the environmental and climatic conditions of the formation, based on field and laboratory studies. The Rio Claro - Piracicaba - Laranjal region, State of Sao Paulo, afforded more detailed studies thanks to the occurrence of several dolomite quarries of Irati Formation. Macroscopic descriptions of this formation are presented from outcrops of Sao Paulo, Parana, Santa Catarina and Rio Grande do Sul States. Large and small structures were also studied. In Sao Paulo State the Irati Formation is commonly represented by a dolomitic bank at the base (or very near the base), with 2 - 3 meters thick. This bank is followed by a sequence of rythmic layers of dolomite and black shale 20 - 30 meters thick. Each one of the individual layers has about 10 to 20 centimeters thick. In the southern part of Brazil the lithology is the same, but with irregular distribution and varied thickness. Fine grained pyrite and chert nodules are present in the dolomites and black shales. The Irati Formation occurring in the central part of the Parana Basin, as known from drilling cores, has also the same lithology, and the thickness is between 10 to 50 meters. The beds dip slightly basinward, indicating the existence of one lake, whose former limits are not far from the outcrops of Irati Formation. This can be infered by the abundance of Gymnosperm bisaccate spores in the calcareous rocks. Between Limeira and Laranjal (Sao Paulo State) the general strike of the beds is N 35o E, dipping 1o 20' to NW. The main tectonic structures consist in normal faults, occuring however atectonic structures represented by small and rare fo ds due to underwater slumping. These are also responsable by the formation of one to theree layers (10 to 30 cm thick) of intraformational breccia in the basal dolomitic bank. The chert of Irati Formation was originated by several processes in different occasions. One of the processes occurred at the time of the diagenesis of limestones, so, it is syngenetic. It is probable that the origin of silica was related with dissolution and remobilization of spicules of sponges. The do omitization was in part probably early, soon after sedimentation. It is also possible that in other cases dolomitization also occurred as an epigenetic fenomena. Primary dolomitization was confirmed by some structural aspects of the intraformational breccia. The optalic metamorphism of carbonatic rocks, due to diabase intrusions, caused an increase of the granulation. The new formed minerals are: diopside, graphite, apophyllite and hidromagnesite. However, they are not common. The black shales changed the black color sometimes to greenish or to white colors and lost the fissibilit. The carbonatic rocks are generally a microgranoblastic in mosaic, and the crystals are commony 5 to 10 microns large. Sometimes the texture is typical of calcarenites, with oolithic or semispherical grains without internal structures. So, it predominates by far the tipes which caracterize the .ow grade energy environment. This agrees with the tectonic tranquillity of the deposition of Irati Formation. The insoluble residue of the carbonatic rocks consists mainly of fine grained chalcedony, sometimes coalesced in a spongy skeleton. Clay is very uncommon among the unsoluble residue. Sometimes abundant bisaccate spores of Gymnospermae occur, sometimes inside some black nodules of syngenetic chert associated with black share. Monaxonic spicules of silicisponge are extremely rare. Irati Formation was formed by the deposition of fine grained calcareous or clayey sediments, in a lake stablished in a low plane, tectonically stable area. Due to the water stagnation an euxinic environment was developed. The nickel content in ashes of betuminous material and the boron of shales, plus the presence of spicules of sponge led the Author to conclude that the lake had somewhere connections with the sea. It is admited that the climate was warm and dry during the first stages of the sedimentation, at least in the State of Sao Paulo. In the subsequent stages the climate changed to alternating phases of cold and rainy seasons to warm and dry ones.