EVOLUÇÃO PALEOAMBIENTAL E SEQÜÊNCIAS DEPOSICIONAIS DO GRUPO BOM JARDIM E DA FORMAÇÃO ACAMPAMENTO VELHO (SUPERGRUPO CAMAQUÃ) NA PORÇÃO NORTE DA SUB-BACIA CAMAQUÃ OCIDENTAL

Abstract

The Camaquã Supergroup (Neoproterozoic III-Eopaleozoic) is composed of volcanic and sedimentary rocks that crop out in the south-central portion of the Rio Grande do Sul State. The Camaquã Supergroup units crop out in the Western, Central and Eastern Camaquã Sub-Basins. The segmentation in three sub-basins is the product of several events of syn and post-depositional tectonic activation, some of them also affecting the Paleozoic units of the Paraná Basin. The Bom Jardim Group, in the studied area, is 670m thick and can be divided into three litostratigraphic informal units: (i) Lower Rhythmites Unit, formed in a deltaic depositional environment; (ii) Intermediate Conglomerates Unit, deposited in alluvial environments; (iii) Upper Sandstones and Mudstones Unit, of fluvial and lacustrine environments. The Acampamento Velho Formation is approximately 700m thick and crops out only in the Western Camaquã Sub-Basin, where it is composed mainly of volcaniclastic rocks, including pyroclastic flow deposits (laminated and massive coarse-grained tuffs, lapilli tuffs and breccia tuffs) and pyroclastic deposits reworked by gravity flows (lapilli tuffs and tuff breccias). Rhyolithic volcanic rocks placed in subaerious environments occur at the top of the unit. The study also included the application of stratigraphic sequence concepts, with the recognition of the depositional sequences, systems tracts and major bounding surfaces.