A FACIOLOGIA DO GRUPO PASSA DOIS NO RIO GRANDE DO SUL

Abstract

The study of geological sections in the outcropping area of the Passa Dois Group in Rio Grande do Sul, complemented with the study of cores from drillings for coal, showed the relations in the State, which are related bellow. 1) The Passa Dois Group is constituted by the Irati and Estrada Nova Formations; the Rio do Rasto Formations is lacking. 2) The thickness of the group is about 100 m. 3) The contact with the subjacent formation is characterized by the occurrence of limestone concretions immersed in a grey silty shale, very similar to the Palermo Formation. 4) The Irati Formation has two facies. One characterized by the occurrence of two beds of pyrobituminous shale associated with fossiliferous limestone lenses. These two beds are intercalated with a grey silty shale with concoidal fracture, and with yellowish limestone concretions immersed in it. This facies, when present, occupies the lower part of the formation and was called Tiaraju facies. The second facies, partially synchronous with the first one, is constituted by grey silty shales with limestone concretions and West of Pantano Grande, is cut by oblique veins of silica. Toward the east it is predominantly siltic, showing cross-lamination. This facies ends the Irati deposition in the State and was called Valente facies. 5) The Estrada Nova Formation also has two lithologic facies, one being pellitic; the other, sandy, which, when present, occupies the top. The pellitic facies was called Caveiras facies and is characterized by the development of pink and red colours alternating with grey. The sandy facies, also reddish, is characterized by lenticular bodies of very fine sandstone, by occurrences of limestone lenses with irregular shapes, and also by large limestone concretions. This facies was called Armada facies. 6) The upper limit of the group was set on the contact of the above mentioned lithologies with a pink medium sanstone, with planar and channal cross stratifications, and with intraformational conglomerates.