Stratigraphy and structural evolution of the southwestern portion of the Rio do Peixe, NE of Brazil: Region of the Santa Helena Step
Rio do Peixe basin; early cretaceous rift; relações stratigraphic and strutura relations; Santa Helena Step
The Rio do Peixe Basin is located almost entirely in the State of Paraíba (Brazil), with small tracts in the neighboring states of Ceará and Rio Grande do Norte. The basin is part of the so-called Northeast Brazil Interior Basins, a system of rift basins running from the Araripe Basin to the Potiguar Basin. The basin compartmentalization consists of four half-grabens (or sub-basins) – from west to east: Icozinho, Brejo das Freiras, Sousa and Pombal, with their major axes following the trends of the shear zones that cut across the Precambrian basement (the Borborema Province). The Santa Helena Step comprises the region that connects the Brejo das Freiras and Sousa half-grabens, displaying structural trends with different directions (NE-SW and E-W), imprinting particular characteristics to this area. The previous maps of the Rio do Peixe Basin indicate that the relay ramp that corresponds to the Santa Helena Step is completely or almost completely filled by the Antenor Navarro Formation. In the present proposal, the region occupied by the Rio Piranhas Formation was considerably enlarged, whereas the portion related to the Antenor Navarro Formation was restricted to the vicinity of the nonconformity with the crystalline basement. Since both are arenitic-conglomeratic units of similar systems, some characteristics were determinant in the differentiation between them, such as the sense of paleocurrents and syndeposicional to synlitification deformation features that characterizes the Rio Piranhas Formation. This formation also displays sandstone dikes, fluidization effects along contacts between distinct lithotypes, and swarms of deformation bands which are almost always present in the outcrops of this unit. Regarding the relations between rock units, field data attest to the interdigitation relationships between formations of the Rio do Peixe Group. With acquisition of new seismic and well data and interpretations, this investigation can be extended to the subsurface. Selected sections from a 3D seismic volume will enable to individualize pre and syn-rift units in the study area and the recognition of their internal surfaces. Subsequently, the obtained results will support a construction of structural and stratigraphic models detailing the evolution of the Santa Helena Step.