Abstract: The Opening of the South Atlantic Ocean: Palynological and Geochemical Evidence from the North Gabon Sub-basin

Wood, Gordon D.; Miller, Merrell A.; Krebs, William N.; Sofer, Zvi - Amoco E&P

Studies of outcrops within the suture zone between the Congo and the São Paulo cratons are important in understanding the early history of the South Atlantic Ocean. The Cocobeach Group consists of conglomerates, sandstones, siltstones, shales (varved, bituminous) and thin carbonates representing fluvial-lacustrine deposition in a rift-valley setting. The Madiela Formation consists of calcareous shales and shallow marine carbonates (e.g., reefs and oolites). The intervening Ezanga salt represents a restricted marine setting. The Cocobeach palynomorphs are dominated by Classopollis and ephedroid pollen, but contain Afropollis jardinus and Uesuguipollenites callosus and chlorococcalean algae (e.g., Botryococcus, Scenedesmus). This suggests an arid tropical-subtropical climate with probable monsoonal rains. The Cocobeach organic geochemistry indicates intervals with high total organic carbon values (TOC) and hydrogen indices (HI) are dominated by palynofacies of amorphous kerogen or fluorescing sheets of possible algal or cuticular origin (Type I kerogen ? liquid prone). Low TOC/HI samples usually contained more abundant wood and inertinite (Type III ? gas prone and Type IV kerogen).

The Madiela Formation palynomorphs consist of pollen, spores, foram linings and dinocysts. Geochemical data indicate Type I/II kerogen, however, TOC values from these samples were very low (0.4%). The Cocobeach Group microflora is devoid of marine palynomorphs whereas the Madiela Formation contains marine dinocysts and foram linings. The former represents rift-valley non-marine environments and the latter the first open marine transgression related to the rifting of Gondwana.

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