Análise de fácies da formação Antenor Navarro na seção-tipo, Bacia Sedimentar do Rio do Peixe (Paraíba/Ceará)

Abstract

The Rio do Peixe Sedimentary Basin (Early Cretaceous), is one of the most important interior Rift Basins of Northeast Brazil, it is located in the states of Ceara and Paraíba. Traditionally, it is divided in the Sousa, Rio Piranhas e Antenor Navarro formations. The Antenor Navarro (AN) Formation consists of one of the main hidric and oil reservoirs of the basin. Only a few published studies approach the syndepositional understanding of this formation and since the litologies are deformed by the intense activity of the Deformation Bands, it becomes important to characterize the lithological formations in their undeformed portions, based on this, the present work aims to characterize through facies analysis the AN formation in surface. The Antenor Navarro Sandstone has its type section located in Sítio Saco outcrop, a 790 m section was raised in this work, WNW-ESE oriented, with a total range thick of 7,04 meters. It consists in a succession of sandstones and fine-grained lithologies, 12 sedimentary facies have been summarized in this work, (Acr, Act, Adc, Af, Am, Ap, Apc, Av, Fc, Hl, S e S/amf). The analysis of the measured paleocurrents attested a S-SE dip-direction with an average dip in 21,3º corroborating with an underwater flux in the direction of the Brejo das Freiras sub-basin depocenter. Common sedimentary structures found in the sandstones are through cross-bedded, planar, tangential and festoon cross-bedded and horizontal bedded, in addition to siltstone intraclasts and igneous and metamorphic clasts. The architectural elements described in the sandstones were channel fill, downstream accretion, lateral accretion and bars. Structures described in the fine-grained rocks include mud cracks and plane-parallel lamination. Both shale and sandstone have carbonate cementation in some of the sedimentary facies. The presence of carbonate facies indicate a hypersaline environment in a dry arid climate, which corroborates with a braided fluvial system, where the dry environment implies few vegetation and poorly developed soils, reducing the stability of the margins (banks). The occurrence of Linsen type heterolytic structures, with the same paleoflow direction of the channel, suggest that the depositional system would be influenced by tidal activity. The facies analysis also included deposits of shales interspersed with the Antenor Navarro sandstone, attributed to flood plains. The depositional interpretation for the AN formation is a braided fluvial-estuarine system, with the Sousa Formation being the receiver lagoon.