STRATIGRAPHY OF THE SERGIPE-ALAGOAS (BRAZIL), DOUALA, RIO MUNI, AND GABON (WEST AFRICA) SEDIMENTARY BASIN



 
 
 
INTRODUCTION
JURASSIC TIME 
   EARLY CRETACEOUS TIME
LATE CRETACEOUS TIME
SEA LEVEL
CONCLUSIONS

REFERENCES


EARLY CRETACEOUS
        The expansion of the African-South American rifting established the depositional character and controlled the accommodation of the sediments in the South Atlantic Ocean through the Cretaceous time. Variations of accommodation and clastic sediment rates occurred along the South Atlantic Ocean in the early Cretaceous. The northern part of the South Atlantic Ocean was always characterized by shallow water and was dominated by high depositional rate for the clastic sediments. The depositional character of the clastic sediments and the increase in depth of the South Atlantic Ocean were functions of the rifting. Seawater occupied new areas of the northern part of the South Atlantic Ocean, making its shoreline unstable. The destabilization of the shore caused the rapid accumulation of the clastic sediments deposited in the study area, and today they overlie the lacustrine sediments.
Fig. 2. Map showing the propagation of the rifting without Gondwana, Late Jurassic time about 144 Ma
        Through the Early Cretaceous time,  the prevailing sea level was relatively high in the South Atlantic Ocean(George and Baker, 1982). The sediments within this sequence are not coeval and they constitute high stand system tracts. By this time, the South Atlantic Ocean had not opened in all for, which guaranteed the continuity of the warm Cretaceous climate until the ocean had opened (Frankes, 1997).
        The depositional history of the Douala, Gabon, Rio Muni, and Sergipe Alagoas basins can be divided into three phases: the syn-rift, transitional, and the post-rift phases (Meyer et al, 1996). The syn-rift phase is divided into three sub phases in the Sergipe-Alagoas basin: a syn-rift I of Jurassic time, a syn-rift II of Neoconian, and a syn-rift III of Baremian (Mohriak et al, 2000). The orientation of the West African rift system is NW-SE in the Douala, Rio Muni and Gabon basins. On the other side of the Atlantic Ocean, the orientation of the same rift system is NE-SW. The deposition of the first sediments on both sides of the Atlantic Ocean (Brazilian and West African) was affected by the South Atlantic rift system, and particularly by the Reconcavo-Tucano-Jatoba rift system on the Brazilian margin. The formation of the South Atlantic and Reconcavo-Tucano-Jatoba rift systems began in the late Jurassic, and lasted until the Barremian period (Jayson et al, 1996). These rift systems not only influenced the deposition of the sediments, but also affected the structures of both sides of the South Atlantic Ocean dividing these into many transform zones. The basins on this margin are bound by basement highs and transform faults.

Fig.3. South Atlantic and Reconcavo-Tucano-Jatoba rift systems (Meyer et al, 1996)

Fig. 4. Fault system between Africa and South America (Meyer et al, 1996)

Fig. 5. Map of the Early Cretaceous (about 117 Ma) showing the expansion of the rifting and the formation of small lakes in the Douala, Rio Muni and Gabon margins
        The transitional sequence was the second stage of the basinís formation in the South Atlantic Ocean. The accumulation of the evaporite sequence confirms the warm climate of the Cretaceous period at this time in the South Atlantic Ocean. This sequence was formed in the shallow waters of the restricted South Atlantic Ocean in the early Cretaceous time, (about 114 Ma) . The South Atlantic Ocean remained shallow throughout the depositional period of the transitional Aptian salt sequence(Warren,1989). However, the depositional period and the subsidence were slow enough to maintain restriction on the margin (Jonh, 1989; Fig. 7). The sedimentary sequence is composed of Aptian salt formation and displays some differences along the South Atlantic Ocean. These include (1) the variation in thickness from the Gabon to Douala basins at which position this formation disappears completely from its subsurface of the West African coast (Dially, 2000); (2) the persistence of this formation in the Gabon sedimentary basin in the Albian interval (Jayson et al, 1996); (3) the chemical composition of the same formation in the Gabon sedimentary basin where the salt is thought to be grouped in two different depocenters, one with halite and sylvite, another with bischoffite (Alan and Selley, 1997); (4) the chemical changes in the salt formation of the Rio Muni margin where it is mixed with terrestrial sediments (Dailly,2000); and (5) the predominance of salt pillows in the northern portion of the South Atlantic Ocean and salt diapirs in the south.
Fig.6, Map showing the position of South America relative to Africa during the formation of the transitional sequence (Aptian salt formation, Early retaceous about 117 Ma)
        The transitional sequence constitutes a succession of Aptian salt formation that accumulated during the slow initial rise of the sea level or when the sea level was at a stand still. The South Atlantic Ocean remained shallow throughout the deposition of the Aptian salt sequence, confirming that the Aptian interval wasnít affected by tectonic movement (faulting, subsidence etc). The salt sequence in South Atlantic Ocean, in contrast, was deposited within a low stand system tract.
 


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