Geologia do grupo Maricá e do grupo Bom Jardim na região norte do Distrito de Joca Tavares, Bagé/RS

Abstract

The Camaquan Basin presents exceptional records of Ediacarano deposits not affected by regional metamorphism, that appear in the South riograndense Shields, and are reunited stratigraphically in Supergroup Camaqua, whose basal unit are: i) Maricá Group, sedimentary sequence composed by fluvial deposits and turbidites deposits, that recovers in non-conformity metamorphic rocks related to Brasiliana Orogeny; and ii) Bom Jardim Group, volcano-sedimentary sequence composed by turbidites deposits and volcanic rocks predominantly intermediate, besides vulcanoclastic rocks. In the present research, the Maricá and Bom Jardim groups were studied from geological detail mapping of an area of 20 km² in the northern region of the District of Joca Tavares, Bagé/RS, being carried out integrated studies of stratigraphy, petrography, sedimentary petrology, analysis facies, and structural geology. The study region presents variety and geological complexity due mainly to structural constraints, which correspond to strike-slip fault and normal fault, among them, it is worth noting The Porteiras Fault and The Província Fault. Within this context, the following units were identified in the mapping área: 1) metasedimentary rocks, schists and filitos of low grade metamorphic, grouped in the Metamorphic Complex Arroio Marmeleiro, that constitute the metamorphic basement of the region; 2) sandstone with trough cross stratification and scattered pebbles rounded of metamorphic rocks and/or plutonic, grouped in the Passo da Promessa Formation; 3) rhythmic interleaving of massive sandstone arcosean, sandstone with plan-parallel stratification, sandstone with ripple cross-lamination and mudrocks, constituing a classical sequence of Bouma turbidites, grouped in the São Rafael Formation; 4) rhythmic intercalations of massive sandstone, stratified sandstone and mudrocks, grouped in the Mangueirão Formation, and sandstone medium to coarse massive or stratified and conglomerates with intraclasts, deposited in distributive channels, grouped in the Vargas Formation. Both units presents a large volcanic contribution and are interpreted as consisting of deposits generated by turbidity currents; (5) andesitic with porphyritc texture and phenocrysts of plagioclase not infrequently oriented, basalts and subordinately pyroclastic rocks, grouped in the Hilário Formation; and 6) sub-volcanic intrusions of porphyritic andesite with plagioclase and amphibole phenocrysts, here defined as Andesitic Porteira. About the questions raised by many authors concerning volcanism syn-Maricá, the work of Geologist Maurício Ribeiro have proven correct up to the present time. However, the recent finding of pyroclastic levels will enable from tefrochronology to discover the absolute age of the unit. Within this context, it is worth noting the lithological and faciological similarity of the São Rafael Formation with the Mangueirão Formation, fact that has generated numerous confusions recorded in scientific articles and geological maps. Therefore, advances in petrological studies of the units will provide a better understanding of them.