Provenance of sandstones at Campos Basin (Alagoas Stage) through garnet composition

Abstract

In this article, a compositional study of garnets was performed in order to identify the main source-rocks involved in the provenance of sandstones from Campos Basin (Alagoas Stage). In the analysis of 17 offshore boreholes, the presence of garnets is remarkable, identified as types A, B and C. Among these, the predominant type belongs to the B type. The individualization of source-rock types in this classification allows classifying the garnets into the Bi type, which is derived from acidic and intermediate igneous rocks, mainly granitoids. The absence or minor incidence of inclusions and zonations in the samples support this classification. D-type and E-type garnets do not appear. Interpretation of the results of approximately 4,000 analysis of the 65 samples indicates two main contributions to the provenance of these sandstones: those from the southern region of the basin, derived from Cabo Frio terrain (metamorphic rocks of amphibolite to granulite facies, mainly aluminous metapelites and secondarily metabasites and granites); while those from the northern part of the basin, where there were substantially more samples analyzed, derived from the eastern terrains of Ribeira Orogen (high-grade metamorphism terrains and granitoids). The main direction of sedimentary supplement is from southwest to northeast, interpreted as a result of the identification of this major contribution of sediment supply. The sandstones located in the northern wells, with garnets derived from B and Bi fields, indicate mainly the presence of granitoids in the source-area. These source-rocks produce quartz or quartz-feldspathic sands, which may be potentially high quality sandstone reservoirs.