Print this pageA Proposal for a New Boundary Between Santos and Pelotas Basins, Southern Brasil
The Florianopolis Platform has been considered the boundary between the extensional Pelotas and Santos basins, Southern Brasil. This platform was formed in the Late Aptian and separates two different geotectonic domains: one that had a slower opening (from the Santos Basin to the north), with deposition of Aptian evaporites, and the other with a faster opening (from the Pelotas Basin to the south), without any salt deposition.
The Florianopolis Platform is a broad magmatic plateau, with a relatively homogeneous morphology. There is a progressive thinning of the sedimentary sequences deposited during the drift phase of Santos Basin towards a regional high currently located in the Pelotas Basin. The boundary between these basins has been traced as the SW limit of deposition of halite and / or the occurrence of halokinesis, which nearly coincides with the parallel 28 S. This criterion seems not appropriate to define the boundary between these two basins, since the well 1-SCS-3-B, drilled in an area currently considered as Pelotas Basin, sampled thin layers of anhydrite.
The absence of evaporites in Pelotas Basin provides an excellent seismic imaging of SDR (Seaward Dipping Reflectors) sequences. This fact allows not only to describe the characteristics of these sequences, but also to correlate them to similar events present in other volcanic margins of the world, such as the offshore margins of Greenland, Australia, Argentine and Namibia.
The present work proposes that the boundary between the Pelotas and Santos basins should be moved about 200 km towards SW, close to parallel 30 S, where a remarkable normal antithetic fault accommodates thick sections of SDR. On the hanging wall of this fault, the isopachs of the phase-drift-related-sediments reaches their minimum, becoming progressively thicker from its footwall towards the Rio Grande Cone (Pelotas Basin depocenter).
The change in the boundary between the two basins would increase the area of the Santos Basin in about 70.000 km2. The Aptian volcanic rocks of the Florianopolis Platform are seismically transparent. Thus, the chance of finding pre-salt leads is very low. On the other hand, the recognition of thick SDR sequences above a possible pre-rift basin in the Pelotas Basin, nearby the new boundary, creates new exploration targets, such as intertrap and pre-rift leads.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil