Provenance and paleocurrents of conglomerates and sandstones of the Santa Barbara Group (Ediacaran) in the Piquiri Valley, Eastern Camaquã Sub-basin, Rio Grande do Sul, Brazil: tectonic implications

Abstract

The Eastern Camaquã Sub-basin comprises exposures of Eastern border, as the Piquiri Valley, belonging to the Santa Barbara Group (Ediacaran). Stratigraphic studies of facies, depositional systems, provenance and paleocurrentes led to the subdivision of the Santa Barbara Group on Eastern Camaquã Sub-basin in two units: (i) Passo da Capela Formation, formed by turbidites of subaqueous fans; (ii) Rincão dos Mouras Formation, that consists of sandstones and conglomerates of alluvial fans and fluvial braided systems. This article proposes a model of structural highs evolution during the filling of a rift basin. Subaqueous fan conglomerates of Passo da Capela Formation feature framework composition derived the basin (sandstones), indicating autophagy, and rocks of the basement (quartzites, schists, metarhyolites) also occur. Paleocurrent analyses indicate dispersal of sediments toward NNE, compatible with lithotypes of the Porongos Complex (quartzites, metarhyolites, mylonites). Northward, provenance analyses of conglomerates indicate derivation mostly from Syenite Piquiri, but also from other lithologies. This fact suggests a local contribution (clasts of Syenite Piquiri), with little or no transportation. Alluvial conglomerates of the Rincão dos Mouras Formation have diverse provenance. Provenance analysis suggests denudation of the Porongos Complex (basement highs), exemplified by metarhyolites, and of the own basin (sandstones). Middle portion presents clasts of granites/muscovite leucogranites correlated to the Pelotas Batolith, and lithologies of the Porongos Complex. Syenites and trachytes appear in small proportion, as well as sandstones. Provenance and paleocurrent analyses indicate that areas to the east were being uplifted and eroded, such as Pelotas Batolith and Porongos Complex, behaving like basement highs, supplying sediments to the basin. At the top fragments of the Porongos Complex predominate (marbles). Paleocurrent analyses in the Rincão dos Mouras Formation suggest important tectonic control during the filling of the unit, with shifts in sources of debris to the basin and uplift of internal highs. Provenance analysis carried out in all units (i) indicated always adjacent sources to the sedimentary deposits, (ii) demonstrated the sources of debris, and (iii) assisted in the differentiation between Passo da Capela and Rincão dos Mouras formations. The occurrence in this last unit of marble fragments and the considerable increase in the proportion of clasts related to the Encruzilhada do Sul Granite, located ENE of the region of the Piquiri Valley, were two important criteria used to differentiate both units, especially when stratified conglomerates of both are contiguous.