Abstract: A Three-phase Early Cretaceous Rift History of the South Atlantic Salt Basins and its Influence on Lacustrine Source Facies Distribution

Norvick, Martin S. and Schaller, Hannfried - BHP

Detailed maps are presented illustrating three phases of rifting that occurred during the Early Cretaceous in the South and Equatorial Atlantic (Fig. 1). The first phase began in the late Tithonian or Neocomian (Rio da Serra Brazilian stage, fig.2) and resulted in the formation of a rift valley complex, which ran obliquely across the future Atlantic from the Tucano-Reconcavo basins to northern Angola. A transcurrent fault, the Trans-Brazilian Shear, controlled the northwestern limit of the rift valley. In northeast Brazil, a second group of rift basins, the Potiguar, Araripe and Jatoba basins, formed parallel to this transcurrent fault, which may have extended as far north as the Hoggar Mountains in Algeria. Meanwhile, flood basalts were extruded in the Parana and Etendeka regions and these may have been continuous with areas of basic vulcanism in the Espirito Santo, Campos, Santos and southern Kwanza basins. Kerogen-rich lacustrine shales accumulated in the central, shielded part of the rift valley complex, especially during the Hauterivian and Barremian. These source rocks were probably responsible for the generation of the large oil fields in Reconcavo, Cabinda and onshore southern Gabon. Oxidized, coarse clastic sediments without source potential dominated the northern extremities of the rift complex (e.g., northern Tucano and Jatoba) and some of the lateral branches (e.g., interior basin of Gabon).

The rifting direction rotated about 20 degrees clockwise during the Late Barremian (Jiquia stage), resulting in the abandonment of the Trans-Brazilian Shear and the development of a new, second set of broader rift basins parallel to the future Atlantic margin from Cameroon to Santos. The onshore basins in northeast Brazil were uplifted and became inactive. The second rift phase lasted until the Early Aptian. Extensive organic-rich shales accumulated under freshwater lacustrine conditions in the north of the rift system and in slightly saline lakes further south. Much of the oil in the Campos basin, and some of hydrocarbons in southern onshore Gabon, Congo and Cabinda, are probably derived from these source rocks.

A third phase of extension occurred during the Early Aptian to earliest Albian (Alagoas stage), characterized by initial emplacement of oceanic crust and thermal sagging, followed by evaporite accumulation. Thick salt was deposited, from Gabon and Sergipe-Alagoas in the north to Kwanza and Santos in the south, in a basin approximately similar in size, shape and climatic position to the modern Red Sea. There may have been a contribution of oils from hypersaline kerogen-rich shale underneath the salt. A continuing debate exists whether the salt was formed in the final stages of rifting or by desiccation of a small ocean basin. At the same time as salt deposition, rifting and transcurrent faulting propagated eastwards and westwards through the Equatorial Atlantic basins, linking the Jurassic oceanic crust off Senegal with the new oceanic crust in the South Atlantic. Additional strike-slip faults, including the Romanche Fracture Zone, became active between northeast Brazil and the Central African basins. A string of small transtensional rift basins formed at this time, including the Barreirinhas, Mundau and outer Potiguar basins in Brazil, and the Benue, Bornu and Doba basins in Africa. There was minor kerogen-rich lacustrine sedimentation in the Mundau and Potiguar basins. This final rifting phase was completed when oceanic crust began to be emplaced in small ocean basins between Nigeria and Senegal during the Late Albian.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil