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Proveniência de arenitos da Formação Cerro da Angélica (Grupo Bom Jardim, Ediacarano do RS) na porção sul da sub-bacia Camaquã central e suas implicações tectônicas

2007; Sociedade Brasileira de Geologia; Volume: 37; Issue: 4 Linguagem: Inglês

10.5327/rbg.v37i4.1305

2317-4889

Alexsandra Costa de Freitas Leitão, Liliane Janikian, Renato Paes de Almeida, Antônio Romalino Santos Fragoso-Cesar, Felipe Torres Figueiredo,

Geological and Geochemical Analysis

This paper presents provenance data from the lowermost sandstone deposits of the Cerro da Angelica Formation (~605 Ma) of the Bom Jardim Group (Camaqua Basin), which crops out in the Casa de Pedra region, Bage, Rio do Sul State, southern Brazil, integrated with sedimentary facies analysis, based on detailed description of columnar sections. This approach aims to determine the tectonic context and the sense of displacement of the basin-border faults during the deposition of the unit. Four facies associations were identified: (i) distal alluvial fan deposits, (ii) ephemeral stream deposits, (iii) proximal alluvial fan deposits and (iv) eolian dune field deposits. Microscopic provenance analysis revealed stratigraphic changes in the proportion of granitic and basic volcanic sources. In the case of the alluvial fan deposits, there is a progressive increase of the granitic contribution, which is interpreted as the result of a progressive denudation of the volcanic cover and ensuing exposure of the plutonic basement. In the distal alluvial deposits, there is an alternation between the two main sources, which is interpreted as a consequence of change in the contribution from proximal, border-fault derived detritus related to transversal alluvial systems and from distal sources fed by axial alluvial systems. The presence of basic or intermediate volcanic rocks as detritus in the basin reveals volcanic activity before the deposition of the Cerro da Angelica Formation. This interpretation corroborates the model of active rifting for the evolution of the Camaqua Basin, as the main extensional phase (Bom Jardim Group) begins with volcanic activity. The absence of provenance variations in the alluvial fan successions suggests that there was no sin-depositional strike-slip displacement between the source and the deposits, thus confirming previous models of normal faulting during the deposition of the Bom Jardim Group.