ANÁLISE ESTRATIGRÁFICA DO GRUPO SANTA BÁRBARA (EDIACARANO) NA SUB-BACIA CAMAQUÃ ORIENTAL, RS

Abstract

In the Eastern Camaquã Sub-basin the Santa Bárbara Group (Ediacaran) is over 6,000 m thick. The group is represented by a succession of marine to coastal sandstones and rhythmites and alluvial conglomerates and sandstones, disposed in progradational-retrogrational cycles. Detailed stratigraphic studies of paleoenvironments, facies and depositional systems have lead to the subdivision of the Santa Bárbara Group, in the Eastern Camaquã Sub-basin, into two formations: (i) Passo da Capela Formation: unit composed of subaqueous fan turbidites and (ii) Rincão dos Mouras Formation : alluvial fan and pebbly braided river unit. The Passo da Capela Formation is the thickest unit of the Santa Bárbara Group, reaching a thickness of about 4,000 m in the eastern sub-basin. The main lithofacies is composed of conglomerates and coarse-grained sandstones deposited by subaqueous gravity mass flows, and sandstones and rhythmites deposited by turbidity currents, in subaqueous fans. These fan deposits are intercalated with marine sandstones and rhythmites (indicated for glauconita minerals) dominated by storm waves. This formation also contains two main levels of seismic deposits which indicate the action of syn-sedimentary tectonic events. The Rincão dos Mouras Formation, up to 2,000 m thick, is present in all sub-basins, and is composed of conglomerates and conglomeratic sandstones accumulated mainly in alluvial fans and braided rivers. Stratigraphic analyses show that basement highs must have been the source areas for these alluvial deposits, which suggest that these areas were up-lined during the course of sedimentation of this unit. The Rincão dos Mouras Formation therefore records the separation of the Camaquã Basin into sub-basins through uplift or internal highs. Three depositional sequences were recognized in the Eastern Camaquã Sub-basin. The Sequence 1 is composed mainly of turbidite sandstones which represent the transgressive and highstand tracts. The Sequence 2 is formed by subaqueous fan systems, representing the lowstand tract, and is overlained by marine succession with shallow-water deposits - transgressive and highstand tracts. The Sequence 3 registers the tectonic reorganization of the Camaquã Basin which becomes separated into three sub-basins by the elevation of the Serra das Encantadas basement high. This uplift lead to the installation or alluvial fans and flood plains which are typical of this sequence. The integration of the data also suggests that the Santa Bárbara Group and, by extension, the entire Camaquã Supergroup, was deposited in an extensional rift, whose border faults had normal or oblique throw without large slip movements, and whose sedimentary infilling was controlled mainly by tectonic subsidence, the clastic supply and the sedimentary transport under the influence of sea level changes.