Proposta litoestratigráfica e considerações paleoambientais sobre o Grupo Maricá (Neoproterozóico Iii), Bacia Do Camaquã, Rio Grande Do Sul

Abstract

The Marica Group (Rio Grande do Sul State, Southern Brazil) contains the first sedimentary successions deposited after the denudation and tectonic subsidence of the basement units of the Gaucho Shield that was formed during the Brasiliano Orogeny. This subsidence event lead to the generation of the Camaqua Basin (Neoproterozoic III / Early Paleozoic), which posteriorly evolved to a rift system characterized by volcanosedimentary successions (Bom Jardim group) and coarse-grained siliciclastics (Santa Barbara and Guaritas groups). The Marica group consists of 2000-2500 m of arkosean sandstones and, in smaller proportion, conglomerates and pelites. Three lithostratigraphic units are here defined: Passo da Promessa formation (fluvial sandstones and conglomerates), Sao Rafael formation (marine sandstones and rhythmites) and Arroio America formation (fluvial sandstones and conglomerates). Stratigraphic and provenance analyses of the alluvial units, as well as paleocurrent data, show that: (i) sedimentary transport was towards the North during the evolution of these units; (ii) source areas were composed of granitic rocks, both foliated and unfoliated; (iii) there was an important contribution of a volcanic source, comprehending mainly rhyolitic and pyroclastic rocks, which frequency of occurrence increase towards the top of the succession, also including rare andesites in the Arroio America formation. The stratigraphic relationship among the Marica formation and the basement units shows that the basin was not related to the Brasiliano Orogeny, as revealed by the fact that the Marica group overlies both the cratonic foreland regions and the collisional suture, represented respectively by paleoproterozoic granulites (Rio the La Plata Craton) and a neoproterozoic subduction complex (Rio Vacacai Terrane).