Halocinese e tectônica de rafts na bacia emersa de Benguela (Angola): dados de campo, modelagem física e restauração estrutural

Abstract

The present work was done with field data from Benguela Basin, which is located in the southern coast of Angola. At this site, an albian carbonatic platform, analogous to the hydrocarbon productive formations of the same age in Campos Basin, outcrops near the coast line , in the surroundings of the city of Benguela. Two major units are recognized among this section: The lower one is characterized by shallow water carbonates with sandstones and conglomerates interlayered at its basal levels, and a total thickness of 300 m. The upper section is composed of deep waters carbonate rocks in a 100 m thick package. This section occurs in two structural domains. The northern domain forms a 5 km wide and 35 km long platform elongated in the NE direction and close to the basement to the east. The southern part is represented by less than 10 km long and up to 5 km wide fragments, some of them displaced from the basement. Considering that evaporates underlie this platform, the main objective of this work is to investigate the influence of salt tectonics on the deformation of the albian rocks. The use of satellite images of public domain allowed the extrapolation of punctual information as well as the construction of the elevation profiles, utilized for the elaboration of the geologic sections. The field data indicate that the structuration at the northern domain, where there is less salt, is influenced by the basement structures. On the other hand, at the southern region, where the evaporitic section is thicker, the deformation is detached from the basement. One geologic section was constructed for each domain and their restorations indicated that the southern part of the platform was affected by an extension of 1,25 (125%) and that the extension at the northern part was 1,05 (105%). This variation is due to a thicker salt layer under the southern part, which conducted the albian package to extend more intensively in consequence of the downdip salt flow, when the basin was tilted toward the ocean. At this region extension was so intense that the faulted blocks lost contact with each other, forming the so called rafts. This effect was simulated in sand box models which indicated that the salt thickness plays a major role in the deformation of the overburden rocks. The variations on the original salt thickness was controlled by transfer faults in the basement and these are the same structures that controlled the boundaries of the basement blocks that responded differently to the Cenozoic uplift events