Faciologia das unidades vulcano-sedimentares nos afloramentos Passo do Moinho e Fervor, município de Santana da Boa Vista – RS

Abstract

In Passo do Moinho (Arroio Pessegueiro) and Fervor (Rio Camaquã) outcrops southwestern region of the Santana da Boa Vista municipality, there are important basic volcanic rocks of basaltic composition and various features of volcanosedimentary interactions, representing contemporaneity between lava and sediment at the end of filling the Camaquã Basin. The present work carried out integrated studies of geological mapping, petrography and faciology of volcanic and sedimentary rocks. In this region they are rocks of the last volcanic cycle of filling of the Camaquã Basin, denominated Rodeio Velho Member. They are volcanic rocks formed under pahoehoe regime and secondarily a'a 'regime. They are aphiric and hypocrystalline to porphyritic rocks, with phenocrysts of plagioclase. The features of vulcano-sedimentary interaction are represented by clastic dykes (injetites), peperites and stretch marks of flow. They are rocks with large amounts of vesicles and amygdalas, filled by quartz, chalcedony, calcite, zeolite, celadonite and fluidized clastic sediments. In some cases, the clastic sediments converge by narrowing fractures, evidencing the filling character. The description of the numerous features of vulcano-sedimentary interaction allowed us to conclude that the depositional environment of the Pedra Pintada Aloformation has a humid rather than desert character as suggested by bibliographical works.