Stratigraphic analysis and geodynamic evolution of the “Transitional Sequence” in the southern Sergipe-Alagoas Basin

Abstract

This paper presents a stratigraphic analysis and considerations about the tectonic activity and subsidence processes that controlled the deposition of the “Transitional Sequence” in the Sergipe Sub-basin (SBSE), southern Sergipe-Alagoas Basin, during the Neoaptian (upper Alagoas), the transition between the rift and drift stages in this basin. 
The stratigraphic analysis of selected wells led to the recognition of five 3rd order depositional sequences, that were deposited in decreasing conditions of water circulation, from continental siliciclastic systems to lagoonar-evaporitic systems and, finally, to restricted marine systems, indicating an increase of the base level. In terms of a 2nd order cycle, the “Transitional Sequence” represents the deposition in a stage of base-level rise and the beginning deposition of a transgressive system tract, whose transition to the overlying marine transgressive sequence is marked by flooding. In terms of a 3rd order cycle, this transition is marked by a local unconformity, which laterally changes to a correlative conformity. 
Such unconformity is interpreted as a breakup unconformity, of pre-Albian age in the SBSE, contrasting with the more expressive (especially for its angular character) of the Pre-Upper Alagoas Unconformity (DPAL), at the base of the “Transitional Sequence”, frequently regarded in the literature as the breakup unconformity. Even though, the Pre-Albian Unconformity is here favoured as the major change in depositional context and tectonic environment (rift to drift) in the SBSE. The deposition of the “Transitional Sequence” occurred close to the end of the Rifting Stage, with an epicontinental sea advancing along a segment of continental crust still in extension, con-trolled by a combination of thermal and mechanical subsidence, the latter already in pronounced decline.